Document Title: Shields-AJSM-Oct05.shtml
Article Title: Neuromuscular Control of the Knee During a Resisted Single-Limb Squat Exercise
Authors: Richard K. Shields, PhD, PT, Sangeetha Madhavan, PT, MA, Emy Gregg, PT, Jennifer Leitch, PT, Ben Petersen, PT, Sara Salata, PT and Stacey Wallerich, PT
Publication: American Journal of Sports Medicine, Baltimore, Maryland
Date: October 2005
Volume 33, pages 1520-1526
Keywords: proprioception, neuromuscular control, single-leg squat, exercise, antagonist-agonist co-contraction, quadriceps-hamstring co-activation, quadriceps-hamstring ratio reduction, closed-kinetic-chain exercise.

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Comments: This study shows that adding resistance to the standard single-limb squat exercise brings a number of benefits, most notably improved co-contraction of the quadriceps and hamstring muscles -- henceforth engendering reduced anterior-posterior shearing across the ACL-injury-history knee undergoing rehabilitation, as well as better hamstring control and improved dynamic (muscle-based) stabilization of the knee joint (and thus reduced likelihood of future knee injury).

ABSTRACT

Background: Closed kinetic chain exercises such as single-limb squats are preferred for knee rehabilitation. A complete understanding of the neuromuscular control of the knee during the single-limb squat is essential to increase the efficiency of rehabilitation programs.

Hypothesis: Performing a controlled single-limb squat with resistance to knee flexion and extension will increase the coactivation of the hamstring muscle group, thus reducing the quadriceps/hamstrings ratio.

Study Design: Descriptive laboratory study.

Methods: A total of 15 healthy human subjects (7 women, 8 men) performed controlled single-limb squats in a custom mechanical device that provided resistance to both flexion and extension. Subjects performed the task at 3 levels of resistance, set as a percentage of body weight. Surface electromyographic recordings from 7 muscles (gluteus medius, rectus femoris, vastus medialis oblique, vastus lateralis, biceps femoris, semitendinosus, and medial gastrocnemius) were collected during the task.

Results: Biceps femoris activity during knee flexion increased from approximately 12% maximum voluntary isometric contractions during low resistance (0% body weight) to approximately 27% maximum voluntary isometric contractions during high resistance (8% body weight). Although the quadriceps had greater activity than the hamstrings at all levels of resistance, the quadriceps/hamstrings ratio declined significantly with resistance (F2,27 = 29.05; P = .012) from 3.0 at low resistance to 2.32 at the highest resistance.

Conclusions: Performing controlled resisted single-limb squats may help to simultaneously strengthen the quadriceps and facilitate coactivation of the hamstrings, thus reducing anterior tibial shear forces. The coactivation may also increase the dynamic control of the knee joint.

Clinical Relevance: The typical single-limb squat exercise performed in the clinic does not usually control for bidirectional resistance and knee joint excursion. As seen in this study, controlled single-limb squats at increased levels of resistance help to increase the coactivation of the hamstring muscles, which is essential to optimize neuromuscular control of the knee.

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