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Abstract & Commentary
Synopsis: Following a laboratory-induced stretch of the ACL, a significant neuromuscular change is induced in the knee. This work supports the neuromuscular role of the ACL and the subsequent neuromuscular changes that result from injury. These findings substantiate the theory that injury to the ACL results in a risk of instability not only due to a mechanical increase in translation, but also due to a temporary neuromuscular disorder initiated by a stretch of the ligament.
Source: Chu D, et al. Neuromuscular disorder in response to anterior cruciate ligament creep. Clin Biomech. 2003;18(3):222-230.
Various authors have investigated the sensorimotor role of the ACL in the last decade ranging from histological to clinical studies. This study attempted to show that the application of prolonged static stretch to the ACL would induce creep and have a significant effect on the reflexive activation of the associated musculature in a way that would decrease knee stability and increase the risk of injury.
Twenty healthy subjects (10 men and 10 women) participated in the study. Patients were seated in a chair with arms crossed and EMG was affixed to the rectus femoris and biceps femoris. The knee was placed at the reference angle (90° or 35°), and a load cell was applied to measure force. A linear placement potentiometer was fixed to the thigh to measure ACL creep via anterior tibial displacement. During testing, a maximum effort isometric extension and flexion contraction was executed. This was followed by the application of the anterior tibial load (150N for women and 200N for men) × 10 minutes. Max effort isometric contraction was repeated following removal of the load. EMG and force measures were reported as changes pre- and postcreep.
Chu and associates reported a significant increase in quadriceps EMG activity with isometric knee extension following ACL creep (P < .01) without increase in hamstring coactivation. A trend toward increased quad force was noted but not significant. However, females had significantly greater increase in quad torque than males (P = .04). There was also a significant increase in hamstring EMG activity with isometric knee flexion following ACL creep (P < .02). There was no increase in quad coactivation and only a trend of increased hamstring torque. An average of 3-6 mm of creep was noted following the application of the load. ACL displacement following creep was significantly greater with the knee flexion angle at 35° than 90° (P < .05), and a trend of greater translation in women compared to men (P < .09) was noted.
Comment by Timothy E. Hewett, PhD, and Mark V. Paterno, MS, PT, SCS, ATC
Overall, Chu et al have done a good job advancing the current knowledge base on the neuromuscular function of the ACL and potential neuromuscular deficits that can result from prolonged static stretch to the ligament. The most notable findings included the increased EMG activity in the agonistic muscle and the failure to demonstrate a subsequent coactivation of the antagonistic muscle group following creep in the ACL. These results suggest that stretch to the ACL will result in a transient neuromuscular disorder causing an inability to generate cocontraction of the thigh musculature, which is believed to protect the ACL in functional activities. The article also provides support for the belief that the ACL has a significant mechanical and neuromuscular role in the stability of the knee joint.
There are several weaknesses in the study. From a methodological standpoint, the question arises regarding the decision to provide different amounts of anterior tibial stress to males and females. With respect to the results and discussion, the intuitive leap is made that the ability to contract in an open kinetic chain position correlated to closed-chain functional activities. Considering the extensive support in the literature that success in open kinetic chain activities fails to equate to success in closed kinetic chain activities, there needs to be some link between knee extension and closed kinetic chain positions, which is where most injuries occur, to make these statements regarding injuries.
Dr. Hewett, Director and Assistant Professor, The Sports Medicine Biodynamics Center, University of Cincinnati College of Medicine, Cincinnati, OH, is Associate Editor of Sports Medicine Reports. Dr. Paterno is a Research Associate in Sports Medicine at the University of Cincinnati.