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Introduction:
The implications of ACL injuries in athletes have been profoundly associated with contemporary references to the use of plyometric exercises. However, it is essential to present a critical perspective on the implementation of plyometric exercises for the rehabilitation of athletes with ACL injuries (Chmielewski et al., 2006). The essential factor that should be taken into account for reviewing the benefits of plyometric exercises in ACL rehabilitation would include the evaluation of anatomy pertaining to ACL injuries, the impact of ACL injuries on athletes, existing concepts pertaining to plyometric exercise and the physiological implications of plyometric exercise for rehabilitation of athletes with respect to recovering from ACL injuries.
Anatomy of ACL injuries:
ACL or Anterior Cruciate Ligament reconstruction is followed by a rehabilitation process in order to help in strengthening muscles surrounding the knee. The scope of rehabilitation is profoundly observed in the objective of regaining complete functionality in terms of movement. However, it is essential to understand the anatomy of ACL in order to obtain credible insights into potential measures that could assist in rehabilitation (Davies, Riemann & Manske, 2015). The knee is primarily composed of the connection between the thigh bone (Femur) and the shin bone (Tibia) as well as the joint between the kneecap and the front side of the thigh bone. The role of ligaments in the surrounding of the knee is explicitly directed towards stabilization of the joint and some of them include the cruciate ligaments and the collateral ligaments among which the former are found within the joint whereas the latter is observed at the side of the knee.
ACL
ACL is a cruciate ligament that prevents the forward shearing of the shin bone (Tibia). The ligament is found in its origin at the femur and is connected to the tibia. The supporting muscles of the knee are the hamstrings, the calf muscle and the quadriceps (Kvist, 2004). The identification of an ACL injury could be validated on the grounds of the tear or stretch in the ligament beyond the expected or normal range. The common causes of ACL injury could be identified as contact and non-contact causes. The contact causes of injury could be identified in the case of direct impact on the exterior of the knee or lower leg. The non-contact causes of an ACL injury are identified in excessive strain on the knee due to bending or straightening as well as a combination of suddenly stopping and changing in direction during running, landing from a jump or pivoting.
Impact of ACL injuries
As per Nyland, Brand & Fisher (2010), the impact of ACL injuries on athletes could be profoundly observed as the feeling of instability that leads to their limited participation or complete rest from sports. The initial reaction is observed in extensive swelling and extreme pain following the injury albeit with the delay in swelling in certain cases (Nyland, Brand & Fisher, 2010).
Movement of the knee is restricted especially in terms of complete extension alongside the identification of tenderness at the medial side of the joint that could be reflective of a cartilage injury. ACL injuries are also subject to the influence of associated injuries that could pose substantial pitfalls for managing the reconstruction and rehabilitation of the ACL injuries. Some of them are identified in meniscal lesions, bone contusions, medial collateral ligament injuries, posterolateral corner injuries, tibial plateau fractures, popliteal cysts or chondral injuries.
Common Occurrence of ACL
Apart from considering the anatomy of ACL injuries, it is essential to focus on the common occurrence of ACL injuries among football players and the impact of the injuries on the sports performance and specific muscle performance of athletes. As per Hoover, VanWye & Judge (2016), these factors have been accounted in the literature pertaining to long-term consequences of ACL injury as well as clinical commentaries regarding the considerations for ACL rehabilitation in order to improve athletic performance and prevent risks of re-injury (Hoover, VanWye & Judge, 2016).
ACL injuries are accountable for more than half of all knee injuries sustained by athletes and it is estimated that the frequency of ACL injuries is 6.5 among 10,000 athletic exposures with the expenditure of almost one billion dollars every year on ACL rehabilitation in the US. It is also observed that almost 90% of patients that experience the common ACL injury, ACL tear are likely to be subjected to surgical reconstruction (Milewski & Nissen, 2018).
According to Snyder-Mackler, et al (2017), the primary objectives of patients, in this case, could be identified in the continuity of sports participation and engagement in recreational activities at the pre-injury level. However, various evidence has profoundly indicated that the rates of return to sport and continuation of participants in sports are considerably limited as compared to the expectations of athletes. The estimates indicate that rates of return to the sport after 12 months post ACL reconstruction is within 33% to 92% (Snyder-Mackler, et al., 2017). The instances of secondary injury as well as re-tear in the lower extremity that is uninvolved in the ACL tear could be observed profoundly after returning to high-level sporting activity. Approximately one-third of young female athletes are liable to experience injury in the contralateral lower extremity after returning to sports specificity following ACL rehabilitation.
Furthermore, it was observed that particular functional deficits following the ACL reconstruction were not related to the time period after surgery. Therefore athletes could require substantial periods of time in order to restore to functional performance as compared to the suggested time frames for restoration to acceptable levels of functional performance. The results are also reflective of setbacks in rehabilitation standards followed after the reconstruction surgeries as well as in the criteria for testing the feasibility of returning to the sport.
As per Fältström, Hägglund & Kvist (2017), the rehabilitation protocols are profoundly emphasized on musculoskeletal and biomechanical factors including lower extremity strength, graft laxity, knee range of motion and metrics for the ability of power production as compared to uninvolved side in order to validate the preparedness of athletes for returning to sports specificity and rehabilitation advancement (Fältström, Hägglund & Kvist, 2017).
The foremost implication that should be observed in ACL injury-related literature is focused on movement alterations and risks of injury. Various authors have recognized that ACL reconstruction causes alterations in movement patterns and biomechanics through comparison of the involved limb and the unaffected one. Lower extremity biomechanics are altered in the case of patients with ACL injuries as compared to uninjured athletes. The movement alterations are profoundly observed during multiple instances of sport-specific manoeuvres such as lateral hopping, jogging, single leg jumping, side-step cutting and sagittal double leg jumping and landings (Snyder-Mackler, et al., 2017).
The alterations in movement could be observed profoundly over a period ranging from six months and exceeding two years after the reconstruction surgery and can be identified in the case of athletes that are allowed for complete participation in sports. There are various factors that are responsible for inducing probabilities of second injuries to ACL in athletes that have undergone ACL reconstruction. The factors include prominent references to single-leg postural stability, increased knee valgus, opposite hip rotation movement and asymmetry in the internal knee extensor moment at the instance of initial contact. These risk factors could also be complicated further by the references to particularly targeted neuromuscular training. ACL injury has been found to induce alterations in movement patterns for the involved as well as unaffected lower extremities and it is similar to the cases which are predictive of injuries in the lower extremity (Lynch, Cummer & Joreitz, 2017). The reformed movement patterns were not resolved after the ACL reconstruction surgery and subsequent rehabilitation thereby implying the requirement of changes in conventional paradigms of ACL rehabilitation programs. Rehabilitation personnel is often concerned with the musculoskeletal as well as biomechanical issues arising from ACL injury alongside the potential effects that can be imposed on higher levels of the neuromuscular system. It is also essential to focus on investigations and refinement of targeted interventions that could be used to improve the quality of movement during sports-specific tasks so that secondary injury rates and risks of re-tear could be addressed effectively.
Reflection on medical literature
Reflection on medical literature depicts considerable indications towards the impact of ACL injury on the performance of isolated muscle group, joint stability, and biomechanics pertaining to lower extremities. However, it is imperative to observe the lack of investigation into the detrimental neuroplastic consequences of ACL injury and reconstruction that can be accounted as one of the notable factors influencing the functional performance and return of athletes to sports specificity. According to Davies, Riemann & Manske (2015), the impact of disruption of ACL is commonly identified in the modifications of mechanical stability of the knee albeit with profound indications towards the disruption of mechanoreceptors in the ligament that can lead to alteration of neuromuscular control (Davies, Riemann & Manske, 2015). The disruption of mechanoreceptors could be liable to induce prominent insufficiencies in afferent input to the central nervous system by causing alterations in somatosensory signals.
The combined impact of increased nociceptor activity alongside pain and effusion as well as the decrease in joint position sense and kinaesthesia can lead to profound impairment of motor control. ACL injuries are also accounted as prominent contributors to the reorganization of CNS thereby leading to changes in activation patterns of sensorimotor cortical areas in comparison to similar controls in the case of unaffected ACL. As per Anderson & Anderson (2108), the changes identified in the neurophysiologic function could not be resolved with ACL reconstruction since the afferent pathway arising from mechanoreceptors could not be restored feasibly to the previous state. It has also been observed that movement tasks in the knee after ACL reconstruction are likely to depict higher levels of cortical activity in comparison to uninjured knees (Anderson & Anderson, 2018).
Effective regulation of the neuromuscular control in event of a reduction in somatosensory input available after ACL injury is completely dependent on improvement in conscious cortical involvement and visual feedback. On the other hand, the effectiveness of visual feedback and conscious cortical involvement for neuromuscular control could be identified in the case of simple tasks albeit with implications for a reduction in efficiency when compared to the complexity of tasks involved in an athletic environment which creates possibilities for increased risks of injury. Therefore it can be identified that conventional models of rehabilitation which are focused on the improvement of biomechanical function over a range of varying dynamic tasks, muscle strength, restoration of range of motion and improvement in endurance could not be considered sufficient for resolution of risks of secondary injury and re-injury.
Impact of ACL Injuries
It is also imperative to identify the impact of ACL injuries on the muscle function of athletes which has been noted in various instances in the literature. The implications of pain, as well as other notable factors, have been noted prominently for obstruction of optimal performance levels in the knee and lower extremity muscles for patients that undergo ACL reconstruction. On one hand, the ankle plantar flexors, as well as hip adductors and hip extensors, are able to depict complete recovery after the ACL reconstruction while there is a clear identification of residual weaknesses in the knee flexors and knee extensors albeit with notable concerns drawn towards the lack of investigation into the impact of ACL reconstruction on the strength of hip external rotators and hip abductors.
Weakness in core muscle groups and hip has been identified as a notable determinant of injury risks for lower extremities. According to Davies, Riemann & Manske (2015), mechanics of the lower extremities are profoundly influenced by proximal lower extremity muscle function in athletes which when subject to weakness leads to other complicated pathologies of the lower extremity that include ankle sprain, patellofemoral pain syndrome and iliotibial band syndrome (Davies, Riemann & Manske, 2015). The concept of regional interdependence is profoundly validated on the basis of the interplay between lower extremity pathology and proximal hip weakness. Despite the limitations in studies on core musculature strength in the case of ACL reconstruction the discrepancies in neuromuscular control, as well as core proprioception, are identified as notable indicators of risks of knee injury (Hoover, VanWye & Judge, 2016).
Studies on female athletes subjected to ACL reconstruction have depicted that despite the lack of changes in external rotation strength and hip abduction there are instances of notable differences in trunk neuromuscular control as compared to healthy and unaffected subjects. It is also essential to note the limitations in the literature pertaining to the investigation of changes in core muscle and hip activation patterns with respect to ACL injury and reconstruction surgery. The possibilities of the presence of activation differences or weakness in hip and core muscle groups prior to or after ACL reconstruction and their impact on movement function and mechanics cannot be undermined (Hoover, VanWye & Judge, 2016).
As per Lynch, Cummer & Joreitz (2017), the restoration of hamstring strength and torque is a widely investigated aspect in the context of hamstring graft harvest and relevant tendon regeneration and morphology with the majority of studies inferring the occurrence of hamstring weakness post ACL reconstruction which is profoundly observed at higher knee flexion angles. On the contrary, limited studies have emphasized the investigation of possible neuromotor influence on activation and weakness of hamstring after ACL reconstruction.
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Hamstring graft harvest in ACL reconstruction has been associated with implications towards possibilities of electromechanical delay in activation of hamstring (Lynch, Cummer & Joreitz, 2017). It is also identified in the medical literature that inter-limb hamstring activation patterns were distinct from that observed in the case of healthy control subjects and in the case of ACL reconstruction subjects there was a profound increase in the lateral hamstring activity as compared to medial hamstring activity. During hamstring exercise, ACL reconstruction subjects were also identified with explicit alterations in medial and lateral hamstring activation among lower extremities.
The improvement in the medial hamstring activation has been found to be a profound influence on the knee valgus thereby leading to limitations on the subsequent ACL loading. Depreciation in the semitendinosus pre-activation with cutting was also identified as a potential indicator of improvement in probabilities for non-contact ACL injuries. The modification in lower extremity mechanics found in ACL reconstruction subjects has been found to be affected by modification in neuromotor activity of hamstrings. Therefore targeted neuromuscular interventions have been identified as functional influences on the modification of hamstring activity subsequently leading to influence on positioning of the knee valgus during sports performance (Davies, Riemann & Manske, 2015). The influence of ACL reconstruction on the function of quadriceps is also identified as a notable element in medical literature with various studies inferring persistent activation discrepancies and weakness in quadriceps muscle and knee extensor in the initial as well as later stages following the ACL reconstruction surgery.
Quadriceps inhibition in the initial stages following the ACL operation has been identified as the resultant of improvement in knee effusion in the concerned stage that is also responsible for alterations in afferent feedback. However, recent studies have depicted that quadriceps inhibition in the initial post-operative stages is not directly influenced by the knee effusion. It has been identified that inflammation, inactivity, and pain could be responsible for such deficits alongside considering the presence of arthrogenic muscle inhibition that is identified after ACL injury on the quadriceps bilaterally (Anderson & Anderson, 2018).
The bilateral activation deficits reflect on the complexity of involvement of central nervous system as compared to the focus on locally mediated neurologic response identified at the knee. Neuromotor deficits are identified commonly at the higher CNS levels in the case of patients that undergo ACL reconstruction. The alterations in the muscle mechanics and resultant weakness were identified post ACL reconstruction as compared to the unaffected lower extremities thereby implying the prominence of changes at local quadriceps muscular level as well as the neuromotor levels. The changes are reflective of depreciation in strength at slower speeds and increased lengthened positions identified in higher knee flexion angles through isometric and isokinetic testing (Snyder-Mackler, et al., 2017).
ACL reconstruction
ACL reconstruction patients are also likely to depict profound discrepancies in quadriceps activation, cortical activation and quadriceps strength in the surgical limb. The discrepancies in cortical excitability are found to be persistent even six months after surgery and could also be observed in extended periods following return to recreational and sporting activity. Comprehensive studies in this context have also reflected on the presence of decreased corticospinal excitability and spinal reflexive excitability prior to and after ACL reconstruction surgery which was associated with documentation of activation deficits and weakness of quadriceps as well as depreciation in central activation ratio. The bilateral deficits identified in the context of central activation ratio have also been found as notable detrimental influences on the restoration of strength and activation in the quadriceps and existing models of ACL rehabilitation are not substantially helpful for the resolution of such neuromotor deficits (Hoover, VanWye & Judge, 2016).
The comprehensive investigation of the central activation ratio could be responsible for developing considerably promising predictions regarding restoration of activation and strength which could contribute to rehabilitation advancements and decision making frameworks for returning to sports.
The use of healthy knee related outcomes derived from knee-related function, pain and physical activity level suggested that the central activation ratio exceeding 89.3% were most likely to be a formidable unilateral indicator for the affected limb. On the other hand, the quadriceps index for ACL affected athletes was found to be less than 85% thereby reflecting on poor functional performance in hop test as compared to quadriceps index above 90% in the case of unaffected individuals. The observation of quadriceps index lesser than 85% was reflective of alteration in lower extremity landing forces and mechanics while scores more than 90% indicated healthy performance of lower extremity (Hoover, VanWye & Judge, 2016).
Athletes with ACL injury with quadriceps index less than 85% are also identified with peak loading rate and peak ground reaction force at the unaffected limb. The notable impact on landing mechanics is profoundly observed considering the evidence that suggests the interplay between reforms in movement mechanics in ACL reconstruction patients and performance deficits, improvement in joint reactive forces and risks of re-injury. The rate of development of force is considered as a crucial metric for neural drive and explosive muscle action.
The reduction in rate of force development in the case of specific muscles after ACL reconstruction has been associated with impacts on athletic performance including muscle weakness. Despite the favourable estimates of restoration of maximum voluntary isometric contraction levels in the affected limb, it is observed the persistence of depreciated rate of force development at the involved lower extremity and restoration to pre-injury levels was not identified for 12 months after the reconstruction (Lynch, Cummer & Joreitz, 2017). The deficits in maximal strength and hamstrings and quadriceps between affected and unaffected lower extremities as well as rates of force development are noticeably identified six months after the operation. The rate of development of force could be accounted as a formidable significant factor for athletes since it is needed for changing direction, acceleration, and deceleration. Therefore, the implications of studies pertaining to the influence of ACL injuries on muscle performance and movement patterns have been found to be crucial for determination of time frames for returning to sports specificity as well as overall athletic performance of athletes that undergo ACL reconstruction.
One of the prominent highlights that could be identified from literature pertaining to anatomy of ACL injuries and reconstruction as well as subsequent conditions is that neuromuscular function of the affected lower extremity could be potentially impaired till later stages of the ACL rehabilitation course after reconstruction. A comprehensive investigation into the possibilities of persistent deficits in movement patterns, neuromuscular activity, and tissue response could be integrated into the existing ACL rehabilitation frameworks for accomplishing precision in development of athletic performance and rapid restoration of sports specificity of athletes (Snyder-Mackler, et al., 2017).
Concepts of plyometric exercises:
Plyometric training as evolved profoundly in terms of description of relevant terminology that has also led to notable concerns regarding its inconsistent usage. It can be observed that the basis of plyometric exercises in the shock method has implied the requirements of maximum effort activities such as high-intensity depth jumps. However, plyometric exercises are also defined on the grounds of movements that involve the stretch-shortening cycle without any explicit indications towards the amount of effort invested in the exercise. The use of the stretch-shortening cycle can be described in physiology literature to illustrate activities such as throwing, jumping and running.
The investigation into rehabilitation and conditioning literature is reflective of the nature of plyometric activities as obtaining support from the stretch-shortening cycle to improve force production and performance (Akinleye, Sewick & Wells, 2013). Another profound ambiguity that has been identified in the context of literature pertaining to plyometric exercises is identified in the categorization of different phases of plyometric activities.
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While the most commonly identified stages are the eccentric and concentric phases with the inclusion of a transition phase between these two stages, there are notable references in literature which suggest the addition of momentum phases at the initial and end phases of the three stages of plyometric exercise. It has also been observed there are notable ambiguities related to the amortization phase used frequently in the description of plyometric activity. The literal meaning of the term amortization is indicative of gradual decline, extinguishing or extinction. With relation to a death jump, amortization could be accounted as time difference between initial ground contact and the reversal of motion or the transition period between muscle lengthening and shortening and the time gap between initial ground contact and take-off. The term amortization has been used liberally thereby leading to its consideration as the time period between concentric actions related to antagonistic muscle groups (Anderson & Anderson, 2018).
The formidable expansion of the scope of literature in context of rehabilitation of athletes from musculoskeletal athletic injuries could be profoundly associated with drastic reforms. The reforms are primarily focused on the final stages of rehabilitation and are aligned with the objectives of power development, safe return to athletic activity and improvement in performance.
Rehabilitation Programs
The variety of strength and conditioning programs followed for rehabilitation programs could be validated on the grounds of contributions of different clinicians in the development of individual programs (Kvist, 2004). However, plyometric exercises are recognized as integral aspects of rehabilitation programs irrespective of the individual objectives of the program such as performance improvement, conditioning or strength. Plyometric exercises could be effective contributors to the work of physical therapists in prevention of injuries, rehabilitation of injuries, improvement in conditioning and strength of athletes and provide specificity in the performance of athletes. It is specifically beneficial considering the fact that athletes are subject to diverse extremities thereby implying the requirement of developing power of athletes during rehabilitation.
Therefore the notable concepts that have been ascertained in literature pertaining to the use of plyometrics in the ACL rehabilitation programs are identified in the definition, various phases as well as physiological implications of plyometrics (Davies, Riemann & Manske, 2015).
Plyometrics
Plyometrics are explicitly based on the stretch-shortening cycle (SSC) that implements eccentric movement for extension that is followed by concentric movement for shortening. The eccentric movement phase is also described as pre-loading, preparatory, counter-movement, readiness or pre-setting phase. It is associated with the stretching of the non-contractile muscle tissue known as Serial Elastic components (SEC), the parallel elastic components (PEC) and the muscle spindle found in the muscle-tendon unit.This phase in the plyometric exercise can be profoundly associated with the improvement of prospects for the concentric muscle contraction (Nyland, Brand & Fisher, 2010).
The three variables
The three variables which are considered crucial for this phase include the rate, duration, and magnitude of the stretch are responsible for determining the amount of energy that is stored in this phase. The period of rest between the eccentric and concentric movements is considered as rebound time or amortization phase and is also known as the electromechanical delay phase in plyometrics. The amortization phase is considered as significant in the plyometrics exercises. Concentric shortening phase is the last stage in a plyometric exercise and is associated with various interactions involving biomechanical responses through leveraging the elastic properties in pre-stretched muscles (Davies, Riemann & Manske, 2015). The three phases are incorporated in a plyometric exercise thereby facilitating plausible improvement of power performance of muscles.
The expanded illustration of the physiology associated with each of the phases in plyometric exercises could be helpful for contributing to the existing understanding alongside providing plausible indications towards their implementation for ACL rehabilitation.
Loading Phase
The first phase is commonly known as the loading phase reflecting on eccentric movements and is also known by various names as illustrated above in this section of the review. The loading phase of the plyometric exercise involves stretching of synergists and muscle-tendon units of the prime movers are observed as an outcome of applying loading or kinetic energy to the joint. Synergists could be typically identified in the lower extremity in the form of anti-gravity muscles. As per Arundale, et al., (2017), the kinetic energy is derived from preceding actions from an external source in which the preceding action could be reflective of flight from a preceding jump and the external source could include countermovement facilitated by concentric action of the antagonistic muscle group or an approaching medicine ball. The stretching of the muscle-tendon unit in this phase is responsible for initiating the stretch-shortening cycle that is responsible for improvement in performance and production of force as compared to the lack of stretching (Arundale, et al., 2017).
The negative work performed by the muscle-tendon unit is responsible for beginning the loading phase of plyometric activity. It is imperative to consider that the terminal stage of the loading phase is subject to variable interpretations. According to studies which are focused on evaluation of entire body movement reflect that the terminal point of the loading phase is identified when the ground reaction force curve initiates reversal of direction, centre of mass reaches the lowest position or the reduction in velocity of centre of mass to zero (Bien & Dubuque, 2015).
The involvement of multiple joints in movements of the entire body and the possibilities of different amplitudes and timings of angular changes in joints imply that the loading phase could be further delineated through application of a combination of individual joint angular velocity and ground reaction force. The delineation would be responsible for precise application and a comprehensive understanding of the mechanisms at play in context of a specific joint. The stretching of active muscles during the loading phase is responsible for initiating two distinct mechanisms that can be related to the stretch-shortening cycles such as stretch reflex and muscle potentiation (Capin, et al., 2017).
The muscle potentiation is realized through modification of contractile properties of the muscle resulting in production of higher force. The stretching of the active muscle is responsible for depreciation in the cross-bridge detachment rate and an improvement in the proportion of cross-bridges that are attached to actin which is also responsible for stimulation of the muscle spindle. Sensory information originating in the muscle spindle is transmitted through the monosynaptic reflect loop in order to facilitate excitatory feedback to the muscle thereby resulting in reflex muscle activity characterized by short latency which is also denoted as the stretch reflex or myotatic reflex.
The stretch reflex output is dependent on the rate and magnitude of loading thereby implying that higher magnitudes and faster rates of loading could be responsible for improvement in stretch reflex. It has been found that the duration for completion of the stretch reflex is approximately 30 to 40 milliseconds and the time gap between the production of force and initiation of reflex is approximately 50 to 55 milliseconds by taking the electromechanical delay into account in the case of lower extremity muscles (Cavanaugh & Powers, 2017).
Furthermore, it has also been reported that the loading phase in diverse plyometric jumps could exceed 100 milliseconds thereby suggesting that stretch reflex could be responsible for augmentation of muscle activity in the loading phase. It is also imperative to observe that stretch reflex could not be initiated in all muscles that are stretched in a plyometric exercise. The factors of the specific activity, as well as the number of crossed joints, would be responsible for determining the muscle response. The example for this aspect could be presented in the form of apparent reflex muscle activity in the soleus while in the case of the biarticular gastrocnemius the reflex muscle activity is characterized by inconsistency.
The disparities in muscle reflex activities for biarticular and monoarticular muscles could be based on the differences of changes in muscle length observed during loading. Davies, Riemann & Manske said that in vivo ultrasonography results have depicted that all stretch-shortening cycles do not have lengthening impact on the fascicles of the biarticular muscles. It has been observed that in specific activities, the fascicles are associated with isometric behaviour (Davies, Riemann & Manske, 2015). On the other hand, the lengthening of tendon is found to be influential on the lengthening of the gastrocnemius muscle-tendon unit in such activities.
The inconsistency in reflex activity of the biarticular gastrocnemius could be explained on the basis of lack in muscle fascicle lengthening that leads to lack of stimulation in the muscle spindles. Therefore monoarticular muscles are found to be susceptible to higher benefits as compared to biarticular muscles in terms of improved work output due to stretch reflex force improvement.
The stretch-shortening cycle also involves storage of elastic potential energy which is a crucial mechanism and is identified in the series elastic component. The tendon is identified to be the prime contributor to the changes in length of muscle-tendon unit and storing elastic potential energy among the other series elastic components such as actin and myosin filaments during the stretching that is observed in loading of the joint (Eckenrode, et al., 2017).
The stretching of the tendon is responsible for stimulation of the Golgi tendon organ embedded in the tendon that is transmitted through an interneuron to the spinal cord which sends inhibitory feedback to the contracting muscle. Previous assumptions were prominently indicative of the function of the Golgi tendon organ as the protection of muscle from excessive tension thereby interfering with the forces involved in plyometric activities. These assumptions have been challenged on the basis of the Golgi tendon organ’s responsiveness to submaximal forces and its roles in developing excitatory reflexes during locomotion that can be accounted as a plyometric activity. It is also essential to observe that various complex reflex mechanisms could also be developed during the plyometric exercise that could complement joint stability and motor coordination (Elias, et al., 2015).
The complex reflex mechanisms are generally developed through the neural signals originating in the muscle receptors and are provided as feedback to the origin muscle or other muscles. The reflex mechanisms include force feedback which refers to signals developed through muscle force and length feedback refers to signals developed by stretching of muscles. As per Fältström, Hägglund & Kvist (2017), length feedback could be observed along the same time period in which the stretch reflex is observed and is responsible for linking of the synergist muscles by excitatory feedback and using reciprocal inhibition in case of muscles with opposite actions (Fältström, Hägglund & Kvist, 2017).
Length feedback is also responsible for linking of monoarticular muscles using excitatory feedback and is observed in the case of muscles such as soleus and vastus lateralis. Force feedback is originated from the stimulation of the Golgi tendon organ and implements inhibitory feedback for connecting muscles that exert torque in different directions alongside connecting muscles crossing different joints. The impact of force feedback is identified in regulation of coupling between the joints while length feedback is responsible for joint stiffness. The combined impact of force and length feedback during the loading phase of plyometric exercises is identified as a promising contributor to the improvement in neuromuscular control.
Coupling Phase
The coupling phase is the second stage in the triphasic description of plyometric activities commonly illustrated in rehabilitation literature. The coupling phase is also referred to as amortization phase as well as transmission phase in varying interpretations of rehabilitation literature. However, amortization could not be aptly used for describing this phase as the definition of the term and the activities of this phase are not aligned. It has been observed that description of plyometric activities as biphasic framework involves consideration of coupling phase as a part of the unloading phase that subsequently leads to overgeneralization of the physiological basis of plyometric exercises (Gobbi & Whyte, 2017).
The coupling phase is often considered as the period of quasi-isometric muscle activity alongside the utilization of movement variables for defining the initial and terminal instances of the coupling phase. In most of the instances in the coupling phase, the length of muscle fascicle is not subject to change especially at the time when the body’s centre of mass, joint angle or the vertical ground reaction force undergoes a change in direction. The stability in the length of muscle fascicle and accomplishing transition between lengthening and shortening at the instance when vertical ground reaction force and joint angle are subject to reversal of direction has been validated through studies on the medial gastrocnemius muscle while subject to saggital plane ankle movements in the standing or supine positions (Gokeler, et al., 2015). On the same grounds, it has been observed that the fascicle length of the vastus lateralis muscle is also stable when drop jumps are performed on a sledge apparatus during the tie when vertical ground reaction force and sledge velocity are associated with reversal of direction.
On the contrary, it has also been identified that during a counter-movement jump, the medial gastrocnemius muscle is in the middle of a shortening phase at the point where the body’s centre of mass and the joint angle are subject to reversal of direction. Therefore, these studies clearly reflect on the variability in the behaviour of muscle fascicle during the coupling phase that is defined by the characteristic of the muscle and the nature of the task.
The coupling phase can be accounted as a significant element in plyometric activities and it is essential that the transition should be continuous in order to ensure that the benefits of the stretch-shortening cycle are not lost thereby realizing the actual purpose of a plyometric activity (Grindem, et al., 2015). It has been found that loss of stored potential elastic energy is initiated in coupling phases which are extended more than 25 ms and the ideal duration of the coupling phase is identified to be 15 ms. Physical activities involving the stretch-shortening cycle associated with a pause in joint movement could provide benefits for strengthening of muscles albeit failing to be classified as plyometric activities since they would be associated with prolonged energy dissipation and prolonged coupling.
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Unloading Phase
The unloading phase is the final stage of a plyometric exercise that is identified soon after the coupling phase and is associated with the depreciation in length of the muscle-tendon unit. The phase could also be characteristically denoted as the propulsion phase, rebound phase or push-off phase. In the case of a single lower extremity joint, the initiating point of the unloading phase could be identified at the instance when the joint angle curve is subject to reversal of direction and the terminal instance is defined when the ground reaction force is reduced to zero (Hildebrandt, et al., 2015).
The initiating point could also be identified in the instance when the shortening of the muscle-tendon unit is started and the ending point is defined at toe-off. The consideration of plyometric activities as biphasic in nature suggests that the beginning of the unloading phase is observed in the initiation of upward movement of the centre of mass while the ending is identified in the cessation of ground contact (Hoover, VanWye & Judge, 2016).
The unloading phase is commonly identified as the resultant or payoff phase since the mechanisms introduced in the loading phase are responsible for improvement in force production in this stage of the plyometric activities. Evaluation of data is clearly reflective of the fact that improvement in performance efficiency and production of force is not possible through any isolated mechanism. On the contrary, the improvement of performance through a plyometric activity is profoundly dependent on the combined impact of storage and utilization of potential elastic energy, contribution of the myotatic stretch reflex and muscle potentiation.
Benefits of plyometrics:
From a theoretical perspective the training benefits of plyometric exercises can be identified in the case of upper and lower extremities reflecting profoundly on the increased peak force and velocity of acceleration, energy stored in SEC, ability to respond to stretch reflexes, increased average velocity and power, improved levels for muscle activation and increased time for the development of force (Nyland, Brand & Fisher, 2010). In order to obtain a comprehensive understanding of the benefits of plyometric activities in ACL rehabilitation and the references to rehabilitation’s role in helping athletes regain their sports performance specificity, it is essential to anticipate the factors associated with the implementation of plyometric exercises.
First of all, the criteria of the candidates under ACL rehabilitation should be taken into account for returning to specificity (Joreitz, et al., 2017). The principle of specificity suggests that the training should be closely aligned with performance thereby plyometric exercise should be implemented in case of patients that have a profound will to return to sports activities involving explosive movements such as in the case of forwarding strikers in soccer.
The conventional rehabilitation exercises are generally implemented at slower speeds that are characterized by low and moderate forces and exercised in single planes of motion. The exercises are intended to accomplish the objectives of improvement in muscle endurance, promotion of muscle recruitment as well as increment in muscle strength.
On the other hand, the conventional rehabilitation exercises could not be accounted as productive contributors to stimulation of forces, planes of movement and speed that are associated with competitive sports alongside the limitations on opportunities for reacquisition of skills (Lepley & Palmieri-Smith, 2016). Therefore plyometric exercises could be considered as the appropriate entity for resolving the gap between traditional rehabilitation exercises and sports specificity.
The contraindications that must be considered for beginning plyometric exercises could be identified in immediate postoperative status, acute pain and inflammation or instability of joints. Joint pathologies including chondral injuries, arthritis or bone injury are also identified as contraindications and are dependent on the tolerance of the tissue for joint loading and high forces that are common in plyometric exercises. Another significant contraindication that should be taken into account for plyometric exercises is identified in musculotendinous injuries alongside the period taken for the tissue to endure rapid and high forces that are common in the case of plyometric exercises (Lindström, et al., 2015).
Criteria for Beginning Plyometric Exercises
The criteria for beginning plyometric exercises should also be reviewed prior to the evaluation of the benefits associated with the exercises. Early phases of ACL rehabilitation could not be considered appropriate for initiating plyometric activities since even at low intensities the plyometric exercises are characterized by the exposure of joints to substantial movement speeds and forces. Therefore in order to initiate plyometric activities in ACL rehabilitation, it is essential for the patients to tolerate the demands of daily activities without any pain or swelling of joints.
Implementation of plyometric exercises in the initial phases of ACL rehabilitation could be responsible for the worsening of the patient’s condition. Other notable conditions that must be verified in the case of implementation of plyometric exercises in the case of ACL rehabilitation include the adequate based level of endurance, neuromuscular control and strength as well as a complete range of motion so that plyometric exercises could be performed without the development of any symptoms (Lynch, Cummer & Joreitz, 2017).
It is also imperative to consider that majority of guidelines for the implementation of plyometric exercises in ACL rehabilitation is comparatively underdeveloped. The majority of the criteria established in plyometric activities for rehabilitation are generally associated with high-intensity exercises which are suitable for athletes without any injuries and it is also essential to note these criteria are based largely on opinion with minimal concern for research. One of the examples can be presented in the form of the condition that plyometric activities should be initiated only if the minimum strength levels are accomplished that include the ability to squat 60% of body mass 5 times in a duration of 5 seconds or perform a complete free-weight squat that is 1.5 to 2.5 times of the body mass (Makhni, et al., 2016).
In the upper extremities, the ability to perform a free-weight bench press equal to body mass or 5 hand clap push-ups is considered as the minimum criteria for initiating plyometric activities for ACL rehabilitation. However, it is impractical for athletes in rehabilitation to comply with these minimum requirements. Furthermore, these criteria are also reflective of the exclusion of female athletes and younger athletes from participation in plyometric exercises despite the fact that they have to participate in competitive sports in which activities are responsible for developing ground reaction force that is equivalent to 5 to 7 times the body mass (Milewski & Nissen, 2018).
Despite the availability of clinically validated guidelines for initiation of plyometric exercises in ACL rehabilitation, it is essential to observe empirical evidence that is reflective of feasibility for initiating plyometric exercises when the patient is able to perform functional movement patterns inappropriate form or tolerate moderate loading during conventional strengthening activities. Considering the clinical criteria for beginning plyometric activities, it is imperative to conclude that plyometric exercises should be implemented in the later stages of ACL rehabilitation (Moran, et al., 2017).
The capability of Plyometric Exercises
The capability of plyometric exercises to contribute efficiently to ACL rehabilitation to sports specificity can be ascertained on the basis of compliance with guidelines for implementation of plyometric activities. The careful implementation of plyometric exercises could deliver similar benefits as other forms of sports training and competition that would help athletes in adapting flexibly to the rigorous implications in competitive sports. It has been found that if an athlete is not able to adapt to plyometric activities in ACL rehabilitation could be less likely to return to sports specificity. Myers said that the performance of lower extremity injury prevention programs comprising of plyometric activities has suggested that this type of physical training could be associated with prophylactic effects thereby facilitating prospects for reduction in possibilities of re-injury (Myers, 2015). On the other hand, it is also essential to focus on the application of plyometric exercises with caution in order to prevent adverse reactions that include references to swelling or pain in joints that can subsequently lead to reduction in the progress of the rehabilitation process.
Appropriate implementation of plyometric exercises is also responsible for avoiding complications associated with novel exercises or high-intensity eccentric movements that result in DOMS (Delayed Onset Muscle Soreness). It is imperative to observe that like the criteria for initiating plyometric activities in ACL rehabilitation, the guidelines for plyometric activity training variables are tailored for uninjured athletes who are capable of engaging in high-intensity plyometric exercises.
The insufficiency of these guidelines can be explicitly ascertained from the lack of emphasis on patient variables such as technical performance and tissue response which are accounted as formidably crucial aspects in rehabilitation settings (Nyland, et al., 2015). Therefore it is essential for clinicians to consider moderation of the patient variables such as technical performance and tissues response and training variables such as intensity, volume, progression, recovery, and progression in order to ensure effective implementation of the rehabilitation programs. A discussion on the general guidelines for moderation of the relevant variables could be helpful in obtaining critical insights into the benefits that can be derived from plyometric activities in ACL rehabilitation.
Frequency of an exercise
Frequency of an exercise is defined by the number of times the exercise is conducted within a training cycle. High-intensity plyometric exercises are generally implemented on twice per week basis in the training cycle for facilitating a full recovery period between sessions amounting approximately between 48 to 72 hours. The low intensity of plyometric activities in rehabilitation period is reflective of the fact that athletes could be able to cope with frequent bouts amounting to 3 times every week without depicting any complicacies such as profound muscle soreness or joint irritation. Intensity is another training variable that accounts for efficiency of plyometric exercises in rehabilitation to help athletes return to sports specificity. Intensity can be defined as the effort invested in performance of an exercise and is related to the loading force (Pfeiffer, 2016).
It can be observed that the increment in stretch load is responsible for improving the intensity of a specific plyometric activity. Furthermore, it is essential to consider the inversely proportional relationship between intensity and frequency in plyometric training programs. The increment in intensity of plyometric activities is reflective of explicit reduction in frequency for facilitating appropriate recovery between the bouts. The ideal intensity for plyometric activities could be profoundly based on the capability of the patient to perform an activity appropriately with the relevant technique and the capacity of the healing tissue to tolerate the loading.
The variation in intensity of plyometric activities should be similar to that of other types of training and rehabilitation activities which are reflective of gradual improvement from low to high-intensity activities so that adverse responses can be avoided (Robey, 2014). One of the promising approaches for reducing the intensity in the case of lower extremity activities could be identified in the use of equipment of initiation that can facilitate unloading of body weight. The subsequent activities could include performance of full body weight plyometric activities against gravity for increasing intensity that can be followed by increasing bounding and the height of jumps then leading towards single-leg activities.
Impact of Loading Phase
The impact of loading phase on the joints could be reduced through the use of gymnastic mats albeit with concerns for extension of the coupling phase. In the subsequent phases, the athlete has to be subjected to highly rigid surfaces that are similar to sports environments that can help in promotion of adaptability to the mechanisms involved in plyometric exercises (Snyder-Mackler, et al., 2017).
The plyometric exercises for upper extremities could be initiated through reduction of the force of gravity or the use of lightweight medicine balls. The intensity could be improved in the case of upper extremity plyometric exercises through utilization of higher resistance, use of higher speeds and movement of the body or the medicine ball across larger distances. Subsequently, the athlete could promote single-arm activities from double-arm activities in order to increase intensity.
The volume in the case of plyometric activities could be ascertained from the total work that is performed in a single session comprising of sets and repetitions. The volume of exercise is generally defined on the grounds of number of contacts with objects or the ground and the recommendations for volume in an exercise are based on a single variable. One of the examples could be identified in the volume recommendations based on experience level that imply 80 to 100 contacts for athletes with limited experience, 100 to 120 contacts every session for athletes with favourable amount of experience and 120 to 140 contacts for every session in the case of experienced athletes (Thomeé & Kvist, 2015).
On the contrary, the volume recommendations derived on the grounds of exercise intensity are reflective of 400 contacts of low-intensity exercises, 350 contacts in the case of moderate intensity exercises, 300 contacts in context of high-intensity exercises and 200 contacts in the case of very high-intensity exercises. However, it is essential to observe that recommendations for volume of plyometric exercises for ACL rehabilitation should not be based on isolated variables. It is mandatory to consider the implications of patient variables such as patient responses and technical performance alongside the intensity of exercise and athlete experience level while determining recommendations for volume of exercise.
The clinician should implement an increase in volume only if the technique can be applied consistently without the probabilities of adverse impacts (Timothy Carey, et al., 2013). On a general basis, patients should be able to demonstrate tolerance for activities characterized by low intensity and high volume prior to promotion towards activities with low volume and high intensity. Furthermore, it is also essential to consider the implications of plyometric activities conducted in the external context of the rehabilitation in order to make adjustments in the volume of exercise in rehabilitation.
Recovery could be identified as the period of rest that is allowed between sessions, repetitions or sets of plyometric activities. The ratio of work to rest in a plyometric activity could be profoundly influenced by the nature of energy system implemented for the exercise and the intensity of the exercise. The recommended work-rest ratio in the case of high-intensity plyometric activities is estimated to be within the range of 1:5 to 1:10 for providing appropriate rest in order to execute the exercise productively.
The example of the maximum effort drop vertical jump can be considered for identifying work-rest ratio in high-intensity exercises which is reflective of 5 to 10 seconds allowed for rest between repetitions. On the other hand, low-intensity exercises which are commonly used in rehabilitation settings have been recommended with smaller work-rest ratios such as 1:1 or 1:2 (Weir, et al., 2016). The example for low-intensity exercises could be identified in the case of line jumps that are associated with performance for 10 seconds and 10 to 20 seconds of rest. Furthermore, it is also essential to consider the period of rest that is allowed in between the plyometric training sessions that are estimated to be 48 to 72 hours (Snyder-Mackler, et al., 2017).
The recovery time in between plyometric exercise sessions could be influenced by the identification of DOMS. In the case of occurrence of DOMS after plyometric exercise, the prominent impact could be identified 24 to 48 after completion of the exercise and is most probably reduced within 96 hours. DOMS developed through maximal intensity eccentric movement is responsible for depreciation of maximal voluntary force for duration of 48 hours post the exercise session.
The depreciation is found to be limited in terms of severity in case of high-intensity exercises while in the case of low-intensity exercises voluntary force depreciation could be identified 24 to 48 hours after completion of the exercise. Suitable recovery time is considered significant for availability of adequate muscle force in order to accomplish optimal performance in plyometric exercises (Timothy Carey, et al., 2013).
Technique
Technique could be accounted as another significant training variable that determines the benefits obtained from plyometric exercises for ACL rehabilitation. The prime objective of implementing plyometric exercise in the case of ACL rehabilitation could be identified in providing assistance to the athlete for reacquisition of sports specificity and establishing a biomechanically appropriate and safe technique that contributes to accomplishing optimal performance for the athlete (Milewski & Nissen, 2018). It is also essential to focus specifically on undesirable techniques that can possibly arise from the injury or with prominent relation to the cause of injury.
The inappropriateness of techniques in plyometric exercises can be clearly identified in scenarios where the athletes are allowed to practice plyometric exercise manoeuvres in the improper approach (Nyland, et al., 2015). The initial implementation of plyometric exercise shall be associated with the mandatory initiatives on behalf of the clinicians to provide consistent and immediate verbal feedback to the athlete during and after the exercise bouts for improving the latter’s awareness regarding appropriate techniques and positions that are inappropriate as well as capable of delivering detrimental consequences (Nyland, et al., 2015). Athletes could be provided with visual feedback by leveraging the use of television mirror or video camera as well as recommending the athlete to perform exercises in front of a mirror.
The clinician’s capability for recognizing an appropriate technique in case of a specific exercise as well as motivating the athlete for maintaining a productive technique for a substantial period of time would be prolific contributions to the improvement in the outcomes of plyometric activities for ACL rehabilitation. If the athlete depicts profound fatigue alongside degradation of the technique and drastic decline in the level of proficiency then the activity should be terminated immediately. The objective in this context should be primarily directed towards improving the volume or intensity of plyometric activity alongside sustaining the practice of proper form (Pfeiffer, 2016).
Similar to other forms of rehabilitation exercises, plyometric activities should be initiated at the level of highest demand that can be tolerated by the patient and the progress is dependent on the completion of activities with appropriate form without any potential improvement in symptoms. The level of progression is also accounted a formidable determinant of the effectiveness of plyometric exercises in ACL rehabilitation and is dependent on the coordinated moderation of the variables of volume, recovery, frequency, and intensity associated with exercise (Robey, 2014).
Factors of Empirical Evidence
The factors of empirical evidence, patient response, and clinical experience are responsible for selection of appropriate training variable for modification. On a general basis, the volume of plyometric activities is increased initially for development of suitable levels of endurance and neuromuscular control prior to improvement in frequency or intensity of exercise or depreciation in recovery time. Even though the occurrence of DOMS is identified as a potential adverse reaction to plyometric activities, it is not considered mandatory (Snyder-Mackler, et al., 2017). Eccentric movement exercises that are performed initially at low intensities in rehabilitation and subsequently improved over the course of the rehabilitation program are not liable to induce any evidence of muscle injury or formidable amount of muscle soreness.
On the other hand, it can be identified that when performed in high volume, low-intensity exercises are liable to produce similar levels of DOMS as that of high-intensity exercises. Hence it is essential to comply with careful modification of volume and intensity in order to ensure tolerable levels of DOMS during ACL rehabilitation (Thomeé & Kvist, 2015). The progression of plyometric exercises should also be based on identification of adverse responses that may include joint swelling or joint pain which could be used to guide the activities. The recovery period should be extended till the period of resolution of the identified adverse consequence.
The resuming of plyometric activities should be associated with realignment of the volume and intensity of the exercises to the levels identified prior to the progression (Weir, et al., 2016). Another significant concern that should be taken into account is the experience of joint swelling or pain after the exercise albeit with resolution of symptoms in the next rehabilitation session or after warm-up. This is essential to continue with similar level of progression rather than improving it in order to identify the possibilities for reoccurrence of the symptoms. According to clinical evidence, the recommended number of sessions at a particular intensity without the experience of any detrimental response before improving the intensity of the program is 2 to 3 sessions (Timothy Carey, et al., 2013).
Considering the effectiveness of compliance with the above-mentioned factors, it is possible to leverage the maximum possible positive impacts on the ACL rehabilitation and return to sports specificity for an athlete. The plyometric training is considered as a promising method of choice when the objective is to improve leg muscle speed, power, and strength as well as the vertical jump ability. Reaction time and agility are substantially improved through plyometric exercises that account for reduction in ground reaction time. The peak torque strength ratio for hamstring to quadriceps is considerably improved through plyometric training that can be accounted as a promising benefit of the activity in helping athletes to return to competitive sport. Plyometric training is responsible for decreasing the peak landing forces from volleyball block jump as well as medially and laterally directed torque in abduction moments and knee adduction. It has been imperatively observed from the empirical analysis that abduction moments and knee adduction are promising contributors to the determination of peak landing forces. In the case of females, the lower landing forces are identified as compared to male counterparts alongside reduction in abduction moments and adduction in the period following the training session. Plyometric training could also contribute to ACL rehabilitation through improving knee stability as well as preventing concerns of serious knee injury in athletes that accounts for sustainable results. The prospects of training in season rather than preseason for athletes could also contribute to the improvement in sports specificity of athletes in ACL rehabilitation through prevention of injury.
Plyometric activities are responsible for facilitating neural adaptations that are responsible for improvement in kinaesthesia, muscle performance characteristics, and proprioception. The practice of plyometric exercises over a longer duration of time could be accounted as a promising factor for improvement in bone mass that can contribute to effective ACL rehabilitation especially in the case of younger athletes. The plyometric exercises are responsible for desensitization of the Golgi tendon organ thereby promoting the generation of force in muscle through subjecting the musculoskeletal system to increased workloads without the excitation of GTO. Plyometrics are also responsible for expediting ACL rehabilitation through improvement in neuromuscular coordination by training of the nervous system alongside automation of movements during the session that can be counted as the training effect.
The training effect is responsible for inducing reinforcement of a motor pattern and activity automation that contributes to improvement in neural efficiency and subsequently neuromuscular performance. It is inevitable to observe the lack of an associated increase in morphological changes in a muscle alongside the improvement in performance of the athlete that is reflective of minimal concerns for re-injury. The training effect is accounted as dominant entity in the initial six to eight weeks of any training or rehabilitation program followed by hypertrophic changes in the muscles after several weeks following the initial phase.
Conclusion:
The literature review focused on significant concepts pertaining to plyometric exercises and their role in ACL rehabilitation with profound emphasis on discussions for causes of ACL injuries, the related anatomical and physiological implications of plyometric exercises and the phases involved in the exercises. The studies pertaining to the benefits of plyometrics for ACL rehabilitation could be improvised through considering the insufficiencies in existing ACL rehabilitation programs and identifying the possibilities for integrating plyometrics activities for addressing the concerned pitfalls. The return to sporting activities and complete restoration of athletic performance has been recognized as the formidable objectives for ACL reconstruction and rehabilitation. However, the success of returning to sport after reconstruction of ACL could be influenced by a diverse range of factors. The explicit indications towards profound rates of re-injury as well as undesirable outcomes reported by majority of patients are reflective of barriers to optimal performance that have to be recognized and resolved efficiently. The most profound limitation pertaining to research on plyometric exercise with respect to its efficiency in ACL rehabilitation is identified in the lack of investigation related to trunk and upper extremity. It can be observed that a large share of literature is devoted to studies on positive adaptations in neuromuscular function following plyometric activities, documentation of performance improvement and physiology of muscle-tendon unit during the stretch-shortening cycle that is profoundly applicable to lower extremities. Therefore the future direction of research should be aligned with the objective of determining the extent of similarity of mechanisms of the stretch-shortening cycle for upper extremity, trunk, and lower extremity. Research should also be directed towards establishing comparison of the benefits of plyometric training in the case of upper extremities and trunk to that of lower extremity. Other significant avenues that shall be explored through research in this context are reflective of references to validating the assumption pertaining to the efficiency of plyometric exercise in promoting return to sports alongside evaluation of the significance of plyometric activity in preventing possibilities of re-injury. The findings of such research could be accounted as pertinent if the participation period in plyometric training is within the range of 6 to 15 weeks for obtaining the desirable benefits.
The limitations of financing for formal therapy extension alongside the substantial time period required for restoration of functional performance in the case of majority of musculoskeletal injuries should also be taken into account for deriving precise and productive insights into the problem. Another promising area of research that can be explored further refers to the observation of additional benefits induced by plyometric activities for ACL rehabilitation apart from the traditional rehabilitation interventions such as proprioception, balance, interval sports activities, and strength. It would also be credible to investigate the implications of lack of consideration for empirical evidence in the determination of frameworks of plyometric rehabilitation programs for ACL as well as lack of references to clinical feedback which can lead to recognition of novel opportunities for reforming the existing rehabilitation programs.
On the other hand, it is imperative to observe that despite the profound lack of evidence suggesting the influence of plyometric activities in ACL rehabilitation the clinical outcomes of effectively moderated and implemented plyometric exercise routines are responsible for validating the use of plyometrics as a preferred training course for athletes after ACL reconstruction in order to gain sports specificity and restore their overall athletic performance. The scope of future research in the context of the benefits of plyometric exercises for ACL rehabilitation should also comprise references to an assessment of movement quality with respect to dynamic activities resembling sports-specific activities and the requirements of speed and force development for competitive sport.
The most notably neglected area in the context of ACL rehabilitation is identified on the neurophysiological basis of ACL injuries which cannot be resolved completely by plyometric exercises. Therefore, it is essential to collate the findings of psychological implications of ACL injuries on athletes and their competitiveness for returning to the sport with respect to the application of plyometrics in order to identify the possibilities of leveraging plyometric exercises to develop confidence in athletes thereby contributing to the development of pre-injury levels of athletic performance and sports specificity. Finally, it would be imperative to focus on the implications of patient variables such as technical performance and tissue response for determining appropriate frameworks for plyometric exercises that can facilitate productive results from ACL rehabilitation.
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