SUMMARY The mechanisms that govern the voluntary transition from walking to running as walking speed increases in human gait are not well understood. The objective of this study was to examine the hypothesis that plantar flexor muscle force production is greatly impaired at the preferred transition speed(PTS) due to intrinsic muscle properties and, thus, serves as a determinant for the walk-to-run transition. The plantar flexors have been shown to be important contributors to satisfying the mechanical energetic demands of walking and are the primary contributors to the observed ground reaction forces (GRFs) during the propulsion phase. Thus, if the plantar flexor force production begins to diminish near the PTS despite an increase in muscle activation, then a corresponding decrease in the GRFs during the propulsion phase would be expected. This expectation was supported. Both the peak anterior/posterior and vertical GRFs decreased during the propulsion phase at walking speeds near the PTS. A similar decrease was not observed during the braking phase. Further analysis using forward dynamics simulations of walking at increasing speeds and running at the PTS revealed that all lower extremity muscle forces increased with walking speed, except the ankle plantar flexors. Despite an increase in muscle activation with walking speed, the gastrocnemius muscle force decreased with increasing speed, and the soleus force decreased for walking speeds exceeding 80% PTS. These decreases in force production were attributed to the intrinsic force–length–velocity properties of muscle. In addition, the running simulation analysis revealed that the plantar flexor forces nearly doubled for similar activation levels when the gait switched to a run at the PTS due to improved contractile conditions. These results suggest the plantar flexors may serve as an important determinant for the walk-to-run transition and highlight the important role intrinsic muscle properties play in determining the specific neuromotor strategies used in human locomotion.