Development of dynamic knee stability after acute ACL injury

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Abstract

Recently, a training program that includes perturbation of support surfaces has been shown to allow most active individuals with ACL injury who pass a screening examination to successfully return to high level activities. The purpose of this study was to identify the effect of this rehabilitation program on involved side muscle activation during walking in subjects with acute ACL rupture and to determine if the activation changes were coincident with improved function. Nine subjects with an acute, unilateral ACL injury or rupture of an ACL graft, who met the screening examination criteria, received ten sessions of rehabilitation that included perturbation training. Motion analysis of five self-paced walking trials were performed before and after training. Electromyographic (EMG) data were collected during stance. After training during walking, the vastus lateralis (VL) integral of activity increased, and relationships between muscles were significantly altered. During walking, VL activation variables were dependent on lateral hamstrings (LH) and/or the soleus (SOL) activation, while no relationships were found before training. Function improved after training, and all subjects returned to their pre-injury activities without experiencing instability. The relationships formed between muscles post-training suggests that perturbation training enhances dynamic knee stability by inducing a well-coordinated strategy among muscles that affect tibial translation.

Introduction

Individuals have different abilities to compensate for complete anterior cruciate ligament (ACL) rupture. More than half of those with ACL rupture who regularly participated in high-level activities such as jumping, pivoting and cutting sports before injury have significant knee instability, even during daily activities (“non-copers”) [1]. There are some individuals, though, who are able to return to these high level activities without episodes of giving way (“copers”) [2], [3]. Copers have been shown to have stance phase knee kinematics and kinetics during walking and jogging that are virtually identical to uninjured subjects, however their quadriceps and hamstrings muscle activity are altered [3]. Non-copers, on the other hand, decrease the injured knee motion and internal extensor moment, have higher soleus activity, altered gastrocnemius and hamstring timing and use significantly more muscle co-contraction [3]. Other investigators have reported changes in quadriceps, hamstrings, gastrocnemius, soleus and tibialis anterior activity during walking after ACL injury, but the relationship to knee stability is unclear since the subjects were not sub-classified [4], [5], [6]. Muscle activity alterations after ACL rupture are reasonable since dynamic mechanisms are required to compensate for the loss of a static restraint; however, only those muscle activity alterations that improve knee stability and function can be considered successful.

Theoretically, successful muscle activation compensations after ACL injury reduce anterior translational forces on the tibio-femoral joint, resulting in improved dynamic knee stability. The specific compensation for each muscle is likely dependent on the role the muscle plays in knee stability. The quadriceps can contribute to knee instability by drawing the tibia anteriorly in relationship to the femur. Isolated contraction of the quadriceps has been found to increase ACL strain [7], [8], [9]. Conversely, the hamstrings are a potential knee stabilizer because of the ability to draw the tibia posterior in relationship to the femur. Contraction of the hamstrings has been found to reduce strain on the ACL [7], [8] and decrease anterior tibial translation [10]. The gastrocnemius may aid knee stability by increasing knee stiffness. The soleus, although it does not cross the knee joint, has the potential to stabilize the knee during gait through an indirect effect. The soleus is typically activated near the completion of the loading response, controlling forward progression of the tiba [11]. In this way, the soleus aids the quadriceps in extending the knee and may counter the destabilizing effect of the quadriceps. A variety of muscle activity alterations, through altered timing or magnitude, are therefore possible to produce dynamic knee stability. It is not surprising, then, that no single activation strategy has been shown to be optimal to produce dynamic knee stability in all patients [3], [12]. Consequently, rehabilitation to induce dynamic knee stability should focus on activities that challenge knee stability, allowing individuals to develop their own compensation strategies to maintain knee stability during destabilizing activities [3].

Developing strategies to elicit appropriate muscle responses is challenging, but the proposed “force feedback” hypothesis of Nichols [13], based on a series of experiments in decerebrate cats, suggests one mechanism by which appropriate muscle strategies may be trained. They found that when a perturbing force is applied to a joint, muscles that resist the perturbation are stretched and activated to resist the perturbation. Simultaneously, muscles that act in the same direction as the perturbation are reflexly inhibited to reduce the unwanted motion [13]. The net result is a coordinated co-activation of muscles to stiffen the joint and maintain dynamic stability. Force-dependent reflex inhibitory neural pathways are linked to muscles that directly influence torques produced at a given joint. Force-dependent inhibitory pathways are also linked to muscles whose mechanical actions influence torques about adjacent joints [14], [15], [16]. Coordinated, compensatory muscle activity patterns that provide dynamic knee stability in patients with ACL rupture involve several muscle groups in the lower extremity that could be mediated by these pathways. Repeated movement experiences during treatment may refine the protective neuromuscular responses because spinal mediated pathways are influenced by input from higher motor control centers in the central nervous system [17], [18]. In addition, treatment programs should provide practice in the context of functional and sport specific tasks.

Fitzgerald and colleagues developed a “perturbation training program” based on the force-feedback principle. Perturbation training involves activities that challenge knee stability in a safe and progressively more difficult manner [19], [20]. Patients with unilateral, acute ACL rupture who were previously active in high-level sports and who were classified as being good candidates for non-operative treatment by a screening exam [1] were randomly assigned to receive either strength and agility training or the same treatment with the addition of perturbation training. Fitzgerald found that 93% of subjects who received the additional perturbation training were able to return to high-level activity without episodes of giving way [19]. Only 50% of those treated with strength and agility training alone returned to high-level activities [19]. The mechanism underlying the success of rehabilitation with perturbation training was not studied.

The purpose of this study was to determine if rehabilitation augmented with perturbation training results in predictable changes in lower extremity muscle activity during walking and if the changes are coincident with improved function. Subjects were individuals with acute ACL rupture who had passed a screening examination designed to identify those with good potential for non-operative management. We hypothesized that rehabilitation that included perturbation training would result in a change in muscular responses. Specifically, we expected that muscles that can mitigate anterior tibial translation on the femur (i.e. hamstring, gastrocnemius and soleus) would change activity to allow for greater quadriceps activation during walking.

Section snippets

Subjects

Nine subjects (seven males, two females) with an acute, unilateral ACL tear who were regular participants (≥50 h/year) in level I or II activities (e.g. jumping, pivoting and cutting sports) [21] prior to injury and who passed an ACL screening examination [1], participated in this study. These subjects represented consecutive subjects over 18 months (comprising five athletic seasons) who both passed the screening examination and elected to participate in the training and testing in an effort to

Screening examination

The quadriceps index was high prior to training (90.9±12.5%) and was unchanged at the end of training (91.3±5.8%). The mean timed hop test score also showed no change after training (97.3±7.9% pre-training, 96.0±4.9% post-training). The pre-training mean Knee Outcome Survey-Activities of Daily Living Scale score was 91.8±6.6% and significantly increased to 97.3±2.3% after training (p=0.037). The Global Rating, which measures overall function, significantly increased from 83.7±13.6% to 94.3±4.3%

Discussion

We investigated the effect of rehabilitation with perturbation training on lower extremity muscle activity and function in ACL deficient individuals who pass a screening examination. We specifically chose this subject population because they have previously been shown to successfully compensate for their injury and return to high level activities short-term after completing training [19]. This investigation was undertaken to help explain the mechanism which allows for improved dynamic knee

Acknowledgements

This research was supported by the National Institutes of Health (training grant no. 5T32HD7490, grant no. 1RO3HD3554701). The authors would like to acknowledge Mike Lewek, MPT, for his assistance with data collection and Tara Manal, MPT, OCS, for her assistance with subject training.

Katherine S. Rudolph earned her Master of Science degree in Physical Therapy from Boston University in 1989 and her Doctor of Philosophy in Biomechanics and Movement Sciences from the University of Delaware in 1994. She received post-doctoral training in the Center for BioDynamics in the Department of Biomedical Engineering at Boston University and the Biomechanics Laboratory in the Department of Mechanical Engineering at the University of Delaware. Dr Rudolph is currently assistant professor

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    Katherine S. Rudolph earned her Master of Science degree in Physical Therapy from Boston University in 1989 and her Doctor of Philosophy in Biomechanics and Movement Sciences from the University of Delaware in 1994. She received post-doctoral training in the Center for BioDynamics in the Department of Biomedical Engineering at Boston University and the Biomechanics Laboratory in the Department of Mechanical Engineering at the University of Delaware. Dr Rudolph is currently assistant professor in the Department of Physical Therapy, University of Delaware, where she teaches and performs research into dynamic control of the knee in persons with anterior cruciate ligament injury and persons with knee osteoarthritis using automated motion analysis and EMG technology. She also performs research involving the development and testing exercise devices for use in rehabilitation.

    Lynn Snyder-Mackler is a professor in the Department of Physical Therapy at the University of Delaware. Dr Snyder-Mackler is a graduate of the Johns Hopkins University and received a Certificate in Physical Therapy from the University of Pennsylvania. She received a Master of Science degree from the University of Pennsylvania and her doctorate from Boston University. She maintains an active Sports Physical Therapy practice at the University of Delaware and serves as a rehabilitation consultant to several professional teams. She is a Board Certified Sports Physical Therapist and concentrates her clinical practice and research in the areas of knee, back and shoulder rehabilitation, and electrical stimulation of muscle. She has authored a textbook on electrotherapy and many research publications in the areas of knee, shoulder and back rehabilitation and neuromuscular electrical stimulation. She was the recipient of the 1994 Eugene Michel’s New Investigator Award and the 1995 Golden Pen Award from the American Physical Therapy Association, as well as the 1996 Rose Excellence in Research Award. She served as Head Trainer for the beach volleyball venue at the ‘96 Olympic Games in Atlanta.

    Terese L. Chmielewski received a Master of Science degree in Physical Therapy from the College of St Scholastica in 1993. She completed a Sports Physical Therapy Fellowship at HealthSouth Sports Medicine and Rehabilitation in Birmingham, AL, in 1996. She is a Board Certified Sports Physical Therapist and is currently a doctoral candidate in the Biomechanics and Movement Science Program at the University of Delaware where she is investigating neuromuscular control in ACL-deficient knees.

    This study was approved by the University of Delaware’s Human Subjects Review Board.

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