The purpose of this study was to examine which of the following exhibited greater advantageous in achieving greater mechanical outputs, on the condition that muscle mass (thus the muscle volume) is identical: (1) to have a greater fascicle length, or (2) to have a greater physiological cross-sectional area (PCSA). (1) Corresponds to a thin and long muscle, whereas (2) corresponds to a thick and short muscle. Three simulation models of the human m. soleus in the region of the ankle joint were constructed. (A) The default model muscle parameter values were derived from the literature. (B) The optimal length of the contractile element (CE) was changed to 1.1 times and the maximal isometric force of the CE was changed to 1/1.1 times the original values. (C) The optimal length of the CE was changed to 1/1.1 times and the maximal isometric force of the CE was changed to 1.1 times the original values. The total muscle volume was unchanged through these modifications. Maximal-effort isokinetic plantarflexion/dorsiflexion activities were simulated at a variety of angular velocities. In concentric actions excluding those at very slow angular velocities, greater mechanical outputs were obtained with model (B). During eccentric actions, the mechanical output was consistently greater with model (C). The findings of this study raise suggestions related to assessing the aptness of individuals for athletic activities in which the competitors are classified by their body mass (such as weight lifting and judo). Results suggest that it is more advantageous to have a greater fascicle length (tall-and-slender physique) for concentric activities, whereas it is more advantageous to have a greater PCSA (short-and-thick physique) for eccentric activities.