Muscle architecture, moment arms, and locomotor movements in the distal limb segments of the procyonids Nasua (coati) and Procyon (raccoon) are analyzed with reference to patterns of muscle fiber length. This study addresses the hypothesis that relative fiber lengths among muscles in a muscle group can be predicted on the basis of correlates of muscle tension. The results include the following: consistent patterns of fiber length of muscles in a muscle group exist within and between the two genera. Differences in fiber length between muscles can be accounted for by two principal correlates of muscle excursion--length of a muscle's moment arm about a joint and joint-angle excursion. Muscle fiber pinnation permits increased tendon excursion, but this effect is relatively small in comparison to the effects of moment-arm length and joint-angle excursion. Corollary action between two or more joints (or lack thereof) is an important factor in determination of fiber lengths.