Effect of dynamic stability on a step task in ACL deficient individuals

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Abstract

Stair ascent and descent requires large knee motions and muscle forces that can be challenging for people with anterior cruciate ligament (ACL) deficiency. Movement and muscle activity patterns were compared in two groups of ACL deficient subjects and a group of uninjured subjects. The ACL deficient subjects were prospectively classified according to functional ability. “Copers” were defined as individuals with complete ACL rupture and no symptoms of knee instability and participated in high-level sports without difficulty. “Non-copers” were defined as ACL deficient individuals who had instability with low-level daily activities and were not able to participate in sports. Sagittal plane kinematic and kinetic data from the hip, knee and ankle and electromyographic data from the vastus lateralis, lateral hamstring, medial gastrocnemius, and soleus were collected as subjects stepped up and over a 26 cm high step. Both coper and non-coper subjects had altered movement patterns as they controlled the rapid movement from step ascent to descent with their involved limbs. Only non-copers used significantly different movement patterns on their involved limb compared to controls after they had descended from the step and their involved side accepted the weight of the body. Classifying subjects by functional ability resulted in more pronounced differences in movement patterns between non-copers and copers. Copers moved more like uninjured subjects.

Introduction

Passive knee laxity increases after anterior cruciate ligament (ACL) rupture, however, the ability to stabilize the knee during dynamic activities varies widely. Some ACL deficient individuals have symptoms of knee instability with daily activities (non-copers) while a small percentage are able to perform high-level sports involving jumping and pivoting without difficulty (copers). Investigators have observed reduced external flexion moments in subjects with ACL deficiency and hypothesized that the subjects are attempting to reduce knee instability by avoiding strong quadriceps contraction [1]. They postulated that in the absence of an ACL a strong quadriceps contraction may lead to an anterior shift of the tibia that would destabilize the knee. The large knee motion and quadriceps muscle force required during stair climbing make it an activity that could reveal differences in knee stabilization strategies between copers and non-copers. Studies of movement patterns of individuals with ACL deficiency during stair climbing are difficult to interpret because of inadequate descriptions of the activity. It is often impossible to determine from the reports which limb (involved or uninvolved) is accepting weight or moving the body to the next step. Studies of the movement patterns of people with ACL rupture during stair ascent and descent, like studies of walking and jogging, find inconsistent and contradictory results. Some investigators have reported differences in knee motions and moments during stair climbing [1] while others show no differences [22]. Alterations in muscle activity have shown some differences in ACL deficient limbs but interpretations are incomplete because kinematic data are absent [2]. One possible explanation for the inability of the investigators to uncover consistent alterations in the movement patterns of people with ACL deficiency is the failure to classify subjects by functional ability. People with ACL rupture, who have substantial difficulty with knee stability during low-level activities, would likely show more alterations in movement patterns than ACL deficient people with excellent dynamic knee stability. A comprehensive evaluation of the movement and muscle activity patterns in ACL deficient people, who are classified by their ability to stabilize their knees during dynamic activities, should provide more insight into successful and unsuccessful stabilization strategies.

The purpose of this study was to compare movement patterns and muscle activation during ascent and descent of a single step between healthy controls and two groups of ACL deficient individuals: one group with excellent knee stability (copers) and the other with objective signs of knee instability during daily activities (non-copers). We hypothesized that when subjects were classified according to functional ability, differences in movement and muscle activity patterns would emerge between non-copers and copers. We expected that copers would move more like uninjured subjects and non-copers would have altered movement and muscle activity patterns compared to copers and control subjects. We also expected the step activity to be challenging to the non-copers both during the transition from step ascent to descent as well as during weight acceptance following step-down.

Section snippets

Subjects

Thirty-one active subjects, who regularly participated in Level I (involving jumping, pivoting and hard cutting) or II (involving lateral motions) sports [3] gave informed consent to participate in this study. Eleven subjects were copers, 10 were non-copers and 10 were control subjects with healthy knees. ACL rupture in the coper and non-coper subjects was confirmed arthroscopically or by MRI and all subjects had at least 3 mm difference in side-to-side laxity which Daniel et al. [3] confirmed

Results

No differences were observed between the three groups in age (F=0.774, p=0.471) or leg length (F=0.012, p=0.98). The copers and non-copers were no different in the amount of passive joint laxity; average joint laxity in the copers was 5.1 mm (range 3.0–7.0 mm) and the average laxity of the non-copers was 6.0 (range 3.0–12.0 mm) (t=0.894, p=0.382). Quadriceps index was significantly lower in the non-copers (75.3%; ± 11) than in the copers (97.1%; ± 12.7) (t=4.033, p=0.001). No differences were

Discussion

The results of this study support some of our hypotheses, most notably that when people with ACL rupture and very poor dynamic knee stability use different movement patterns than healthy controls and from ACL deficient persons with excellent knee stability. The step activity in this study was chosen to provide several challenging components: a larger than expected step, 26 cm as opposed to the standard step height of 20 cm; and a rapid transition from step ascent to descent. The novelty of the

Conclusions

The stratification of ACL deficient subjects into copers and non-copers helped to delineate differences in a step task, which have likely been masked in other studies where subjects within a group had widely ranging functional abilities. Copers and non-copers appear to have some similarities in the manner in which they negotiated the step movement. Both groups appear to alter their movement patterns by flexing their hips more at the transition from step-up to step-down. Both groups also

Acknowledgements

This work was supported by the National Institutes of Health (Training Grant #5T32HD7490, Grant #1RO3HD35547-01), the National Athletic Training Association (Grant #396-E001) and the Foundation for Physical Therapy (Doctoral Research Award #97D-12-RUD-01).

Dr. Katherine S. Rudolph is an Assistant Professor in the Department of Physical Therapy and the Interdisciplinary Program in Biomechanics and Movement Sciences at the University of Delaware. She is a graduate of Syracuse University and received her MSPT from Boston University and her Ph.D. from the Interdisciplinary Program in Biomechanics and Movement Sciences at the University of Delaware. Dr. Rudolph has received numerous grants from the National Institutes of Health and has an active

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    Dr. Katherine S. Rudolph is an Assistant Professor in the Department of Physical Therapy and the Interdisciplinary Program in Biomechanics and Movement Sciences at the University of Delaware. She is a graduate of Syracuse University and received her MSPT from Boston University and her Ph.D. from the Interdisciplinary Program in Biomechanics and Movement Sciences at the University of Delaware. Dr. Rudolph has received numerous grants from the National Institutes of Health and has an active research program studying movement and muscle activity patterns in persons with knee osteoarthritis and persons with neurological injury.

    Dr. Lynn Snyder-Mackler is a Professor in the Department of Physical Therapy and Director of the Sports Physical Therapy Residency at the University of Delaware. She is a graduate of The Johns Hopkins University (undergraduate), the University of Pennsylvania (PT) and received her doctoral training at Boston University. She maintains an active Sports Physical Therapy practice at the University of Delaware. 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 textbooks on electrotherapy and sports physical therapy as well as more than 50 research publications in the areas of knee (ACL and osteoarthritis), shoulder (baseball and softball) and spine rehabilitation and neuromuscular electrical stimulation. Her research is funded by grants from the National Institutes of Health and several private foundations. She was the recipient of the 1994 Eugene Michels’ New Investigator Award and the 1995 Golden Pen Award from the American Physical Therapy Association, as well as the 1996 Rose Excellence in Orthopedic Research Award. She was recently (2003) named a Catherine Worthingham Fellow of the APTA. She served as Head Trainer for the beach volleyball venue at the 1996 Olympic Games in Atlanta.

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