Influence of training history and contraction velocity on hamstring muscle coactivation during maximal effort knee extension

Abstract

When a muscle produces voluntary force, muscles on the opposite side of the joint, the antagonists, are also activated. While coactivation of the knee flexors during knee extension is presumed to increase joint stability by decreasing anterior shear force of the tibia on the femur, the coactivation of the hamstrings also produces what is called the antagonist torque. Systematic exercise in the form of resistance training can reduce antagonist muscle coactivation in healthy young adults. However, the mechanical consequence of this neurological adaptation is unclear. We thus hypothesized that previously strength-trained individuals would exhibit less antagonist coactivation, resulting in a reduced antagonist torque, and that with an increase in contraction speed there would be an increase in antagonist coactivation to slow the movement but there would be less of an increase due to speed in the trained compared with untrained individuals. Therefore, the purpose of this study was to determine the effects of training status on coactivation, i.e., antagonist torque, and on the speed-sensitivity of coactivation. Subjects for this study were fitted with surface EMG electrodes on their thigh muscles, and performed maximal effort knee extensions on a dynamometer, using shortening (concentric) and lengthening (eccentric) contractions at 30, 90, and 150°/s. As expected, trained individuals produced ~44% less coactivation at all contractions speeds. Against the hypothesis, coactivation did not increase in either group as velocity increased, as there was less than 10% difference in coactivation levels between the 3 speeds. Also against the hypothesis, as determined with an EMG-driven mathematical model, antagonist torque did not decrease with decreasing coactivation; in fact we see a trend towards the opposite for trained individuals. A borderline greater antagonist torque was noted in the trained compared to the untrained subjects even with decreased coactivation of the trained. These data suggest that antagonist muscle coactivation is less in trained healthy young adults but this reduced neural activation does not manifest itself in lower levels of antagonist torque. Therefore, leg strength training may increase muscle strength in part by reducing antagonist muscle coactivation without compromising joint stability.