The ankle plantar flexor muscles, gastrocnemius (GAS) and soleus (SOL), have been shown to play important roles in providing body support and forward propulsion during human walking. However, there has been disagreement about the relative contributions of these muscles to these functional tasks. In this study, we examined the relative contribution of the individual plantar flexors to support and propulsion using independent manipulations of body weight and body mass. We hypothesized that both GAS and SOL contribute to body support while SOL is the primary contributor to forward trunk propulsion. We tested this hypothesis by measuring muscle activity while experimentally manipulating body weight and mass by 1) decreasing body weight using a weight support system, 2) increasing body mass alone using a combination of equal added trunk load and weight support, and 3) increasing trunk loads (increasing both body weight and mass). The rationale for this study was that muscles that provide body support would be sensitive to changes in body weight, whereas muscles that provide forward propulsion would be sensitive to changes in body mass. GAS muscle activity increased with added loads and decreased with weight support, but showed only a small increase relative to control trials when mass alone was increased. SOL muscle activity showed a similar increase with added loads and with added mass alone, and decreased in early stance with weight support. Therefore, we accepted the hypothesis that both SOL and GAS contribute to body support whereas SOL is the primary contributor to forward trunk propulsion.