Scaling of forearm muscle architecture in primates

Relative mass of primate forearm flexors and extensors has been predicted to relate to locomotion. However, we previously reported that the weight of forearm muscles scales isometrically in primates. However, variation in humeral epicondylar morphology has been shown to correlate with locomotor patterns, and if muscle mass does not explain this variation, then physiological crossectional area (PCSA) and fascicle length (FL) of the finger and wrist flexors may. To test this, we increased our sample to include six strepsirrhines, six platyrrhines and seven catarhines. We dissected each antebrachial muscle, measured wet mass and linear dimensions. Through chemical dissection, FL was measured and PCSA calculated. Muscles were grouped into flexors, extensors and others (that do not cross the wrist) and studied using RMA regression (alpha = 0.05). As expected, total forearm (TFor) MM highly correlates isometrically with flexor and extensor MM. TFor PCSA is tightly correlated with TFor MM across the whole sample and within each suborder and is slightly, though significantly, positively allometric across the whole sample and strepsirrhines and catarhines (but not platyrrhines). Similar correlations and allometry between MM and PCSA are found within the flexor and extensor compartments. FL is not highly correlated with total MM variables and appears to relate to locomotor patterns. Thus primate forearm muscles have relatively consistent (though slightly positively allometric) crossections but vary according to FL, suggesting locomotor adaptations in stretch and flexibility, but not force production; variation in epicondylar anatomy likely relates to leverage adaptations and not to muscle force.