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Neuromuscular Control of the Knee During a Resisted Single-Limb Squat Exercise

From the University of Iowa, Graduate Program in Physical Therapy and Rehabilitation Science, Iowa City, Iowa

^* Address correspondence to Richard K. Shields, PhD, PT, Graduate Program in Physical Therapy and Rehabilitation Science, University of Iowa, 1-252 Medical Education Building, Iowa City, IA 52242-1190 (e-mail: Richard-shields{at}uiowa.edu).

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 (F_2,27 = 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.

Key Words: single-limb squat • quadriceps/hamstrings ratio • neuromuscular control • closed kinetic chain exercise