Cycling has become a popular model in recent years for the study of muscle mechanics. The purpose of this study was to manipulate bicycle seat heights in order to perturbate muscle lengths and contraction velocities of three lower leg muscles, the soleus, medial gastrocnemius, and tibialis anterior, and to measure subsequently the effects on the muscles’ contribution to the cycling task as measured by EMG.

Two groups of female subjects, riders (n=7) and non-riders (n=6), rode a bicycle mounted on a Schwinn Velodyne® at 200 Watts and cadence of 80 rpm. Individuals rode at a self-selected seat height, a 10% lowered, and 5 % raised seat position. It was hypothesized that responses in muscle EMG would differ on the basis of cycling experience but results showed this to not be true. It was also hypothesized that because the three muscles would operate at decreased contraction velocities at the low seat, the integrated EMG would be less for the lowest seat position. This hypothesis was based on the force-velocity relationship of muscle, where muscles operating at the lower end of the velocity spectrum can produce greater forces and thereby require the recruitment of fewer fibers to perform the same task. Although the soleus and medial gastrocnemius muscles did show a decrease in integrated EMG value with decreases in seat height, the tibialis anterior showed unexpected results, with the chosen seat height resulting in the lowest integrated EMG. This suggests either that EMG responses occurred independently from the calculated muscle lengths and contraction velocities or that electromyography may not be a sensitive enough tool to reflect changes in muscle force induced by altered muscle lengths and/or contraction velocities.

As seat height decreased, it was also observed that the absolute muscle lengths increased for both the soleus and medial gastrocnemius muscles, but decreased for the tibialis anterior muscles. The absolute peak muscle lengths did not show a large variation across seat heights, with only an average difference of 2 %. Therefore, it is unlikely that the changes in muscle lengths affected the force production of the muscles to a significant extent.

Although the muscle length and contraction velocity variables add considerably more insight into the mechanics of muscle action than either EMG or kinematic analyses of limb motion alone, their use in predicting muscle forces and muscular contributions to movement remains very limited. Furthermore, the choice of using the cycling model for investigating certain aspects of muscle mechanics should be examined further.