THE EFFECT OF MUSCLE RECRUITMENT STRATEGIES ON KNEE STABILITY AFTER ACUTE ACL RUPTURE

Abstract

Patients with acute anterior cruciate ligament (ACL) rupture can be categorized by their potential for dynamic knee stability (potential copers = good potential, non-copers = poor potential). Demonstrated differences between these groups in walking and jogging suggest differences in knee stabilization strategies; however, muscle recruitment has not been compared. Recent studies show that lower-functioning subjects with ACL rupture recruit their quadriceps first in response to a mechanically applied, anterior-directed force on the tibia. Quadriceps contraction may induce anterior tibial translation, potentially destabilizing the ACL deficient knee. PURPOSE To differentiate the knee stabilization strategies of potential copers and non-copers by examining lower extremity muscle recruitment in response to a postural disturbance. METHODS Nineteen subjects with ACL rupture (9 potential copers, 10 non-copers) and 10 highly active people without lower extremity injury participated in the study. Subjects stood in unilateral stance (ACL deficient subjects = injured leg; uninjured subjects = randomly assigned leg) on a custom-built platform that was manually triggered to translate horizontally in an anterior direction (40 cm/s, 5.8 cm). Surface electromyography was collected from the lateral hamstrings, quadriceps, and gastrocnemius muscles. Approximately 2 seconds of data were collected before and after the random application of platform movement. Ten trials were collected. Subjects maintained unilateral stance during data acquisition. The threshold for muscle onset was when muscle activity exceeded 2 times the average level of activity in the 100 ms prior to platform movement. Muscle recruitment was ordered by onset during the interval from 60–180 ms after platform translation. RESULTS The majority of potential copers (7/9) and uninjured subjects (8/10) recruited the hamstrings, whereas the quadriceps was recruited first by most non-copers (6/10). CONCLUSIONS Recruitment of the quadriceps first in response to an anterior-directed destabilization of posture is a strategy adopted by those with poor potential for dynamic knee stability and gives insight into why their knee stabilization strategy fails.