PURPOSE: The purpose of this study was to determine whether similar patterns of quadriceps dysfunction are observed when people with anterior cruciate ligament (ACL) deficiency perform static and dynamic tasks. METHODS: EMG data were collected from 15 subjects with an ACL deficient knee and 15 uninjured subjects as they performed static and dynamic tasks that were isolated to the knee and presented no threat to joint stability. The dynamic task was cyclic flexion and extension in the terminal 30 degrees of knee extension; the static task was an established isometric target-matching protocol. The muscle activity patterns observed during the tasks were evaluated and compared. RESULTS: The subjects with ACL deficiency exhibited quadriceps muscle control strategies that were significantly different from those of the uninjured subjects. This was true in both the dynamic and the static tasks. The findings were most noteworthy in the vastus lateralis muscle. Good agreement (r = -0.73 to -0.75) was observed in subjects' static and dynamic VL results; more moderate agreement was observed in results of the other quadriceps muscles. CONCLUSION: Diminished quadriceps control was observed when people with ACL deficiency performed static and dynamic tasks. The most striking feature of this impaired control was failure to turn the quadriceps "off" when performing flexion tasks in which the knee extensors are usually "silent." Our findings suggest that quadriceps dyskinesia after ACL injury is relatively global. Changes in neural function and muscle physiology after ACL injury are put forth as the most likely source of the observed dyskinesia.
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