The effect of joint velocity on the contribution of the antagonist musculature to knee stiffness and laxity

The electromyographic (EMG) coactivation patterns of the knee flexors and extensors when acting as antag onists were studied as a function of limb velocity to assess their contribution to joint stiffness and laxity. Normalized antagonist coactivation patterns devel oped from surface EMG recordings from the hamstrings and quadriceps during maximal effort isokinetic exten sion and flexion, respectively, demonstrated character istic variations as the joint velocity increased from 15 deg/sec up to 240 deg/sec. The two-tailed t-test (P < 0.1) was performed on the data obtained from eight normal knees. The results indicate that both hamstrings and quadriceps demonstrate a significant increase (>100%) in their antagonist coactivation pattern during the final 40° of fast extension and flexion movements, respectively, as limb velocity increases. A minor de crease in antagonist activity of the hamstrings (24%) and quadriceps (8%) was evident during the initial phase of the extension and flexion movements, respec tively, as joint velocity increased. We concluded that as limb velocity is increased, there is a substantial reflexive (unintentional) increase in the contribution of the antagonist musculature to joint stiff ness and reduction of laxity. The results also suggest that strength training of the hamstrings (rather than quadriceps) should be considered as a modality for conservative treatment of ACL deficiencies, as well as an adjunct to surgical reconstruction. Such training can also reduce the risk of high performance athletes in a reflexive manner by increasing joint stiffness.