Changing the demand on specific muscle groups affects the walk-run transition speed

It has been proposed that muscle-specific factors trigger the human walk-run transition. We investigated if changing the demand on trigger muscles alters the preferred walk-run transition speed. We hypothesized that (1) reducing the demand on trigger muscles would increase the transition speed and (2) increasing the demand on trigger muscles would decrease the transition speed. We first determined the normal preferred walk-run transition speed (PTS) using a step-wise protocol with a randomized speed order. We then determined PTS while subjects walked with external devices that decreased or increased the demand on specific muscle groups. We concurrently measured the electromyographic activity of five leg muscles (tibialis anterior, soleus, rectus femoris, medial and lateral gastrocnemius) at each speed and condition. For this study, we developed a dorsiflexor assist device that aids the dorsiflexor muscles. A leg swing assist device applied forward pulling forces at the feet thus aiding the hip flexors during swing. A third device applied a horizontal force near the center of mass, which impedes or aids forward progression thus overloading or unloading the plantarflexor muscles. We found that when demand was decreased in the muscles measured, the PTS significantly increased. Conversely, when muscle demand was increased in the plantar flexors, the PTS decreased. However, combining assistive devices did not produce an even faster PTS. We conclude that altering the demand on specific muscles can change the preferred walk-run transition speed. However, the lack of a summation effect with multiple external devices, suggests that another underlying factor ultimately determines the preferred walk-run transition speed.