We studied force-sharing behavior between the cat medial gastrocnemius (MG) and soleus (SOL) muscles by direct measurement of the muscle forces and electromyographic activities (EMGs), muscle lengths, speeds of contraction, joint kinematics and kinetics, for a variety of locomotor conditions. Previous studies suggested that the modulation of MG force and activation is associated with movement demands, while SOL force and activation remain nearly constant. However, no systematic, quantitative analysis has been done to evaluate the degree of (possible) modulation of SOL force and activation across a range of vastly different locomotor conditions. In the present study, we investigated the effects of speed and intensity of locomotion on the modulation of SOL force and EMG activity, based on quantitative, statistical analyses. We also investigated the hypothesis that MG forces are primarily associated with MG activation for changing movement demands, while SOL forces are primarily associated with the contractile conditions, rather than activation. Seven cats were trained to walk, trot and gallop at different speeds on a motor-driven treadmill, and to walk up and down different slopes on a walkway. Statistical analysis suggested that SOL activation (EMG activity) significantly increased with increasing speeds and intensities of locomotion, while SOL forces remained constant in these situations. MG forces and EMG activities, however, both increased with increasing speeds and intensities of locomotion. We conclude from these results that SOL is not maximally activated at slow walking, as suggested in the literature, and that its force remains nearly constant for a range of locomotor conditions despite changes in EMG activity. Therefore, SOL forces appear to be affected substantially by the changing contractile conditions associated with changing movement demands. In contrast, MG peak forces correlated well with EMG activities, suggesting that MG forces are primarily associated with activation while its contractile conditions play a minor role for the movement conditions tested here.