Co-contraction during static and dynamic knee extensions in ACL deficient subjects.

Co-contraction of the muscles is proposed in the literature as one of the strategies that anterior cruciate ligament deficient (ACLD) subjects can use to compensate the loss of ACL function. This study examined the response of ACLD and control subjects to different shear forces in isometric and slow-dynamic knee extensions. Twelve chronic ACLD and 10 control subjects performed submaximal positioning and slow-dynamic knee extensions (between 45 degrees and 5 degrees of knee flexion) with two external flexion moments both applied at two distances on the lower leg. The shear force was controlled by changing the moment arm without changing the moment. Electromyographic data were collected from knee flexor and extensor muscles. In the analysis of variance, no significant effect of subject group was found in positioning or slow-dynamic tasks across all muscles. The effect of knee angle was significantly different between the subject groups for biceps femoris in positioning and for rectus femoris in slow-dynamic tasks, but these effects were very small and will not have a great impact on the resulting shear forces. There was no interaction between moment arm and subject group. Therefore, the hypothesis that ACLD subjects increase co-contraction in situations with an increased shear load in positioning and slow-dynamic knee extensions could not be confirmed.