Muscle volume is a major determinant of joint torque in humans

Muscle force (MF) is linearly related to physiological cross-sectional area (CSA), which is obtained from muscle volume (MV) divided by fibre length. Taking into account the fact that joint torque (TO) is determined by MF multiplied by the moment arm, the maximal TO would be a function of MV. This proposition was tested in the present study by investigating the relationship between MV and TO for elbow flexor (EF) and extensor (EE) muscles of 26 males. The MVs of EF and EE were determined from a series of muscle CSA by magnetic resonance imaging (MRI), and pennation angle (theta) and FL by ultrasonography (US). Maximal isometric TO was measured at right angle of elbow joint for EF and EE, There was a highly significant correlation between MV and TO both for EF and EE (r = 0.95 and 0.96 respectively) compared with that between muscle CSA and TO, suggesting the dependence of TO on MV. Furthermore, prediction equations for MV (MVULT) from muscle thickness (MT) measured by US was developed with reference to MVMRI, by the MRI on 26 subjects, and the equations were applied to estimate MV of healthy university students (CON; 160 males) and sports athletes (ATH; 99 males), There were significant linear relationships between MVULT and TO both for EF (r = 0.783) and EE (r = 0.695) for all subjects (n = 259). The MVULT was significantly higher in ATH (by 32% for EF and 33% for EE, respectively) than in CON. Similarly, significantly greater TO was observed in ATH (by 35% for EF, 37% for EE, respectively). The theta for EE showed no difference between both groups (17.8 degrees for CON and 17.5 degrees for ATH). On the other hand, the TO to MV ratio were identical for CON and ATH. The results reveal that the muscle volume of the upper arm is a major determinant of joint torque (TQ), regardless of athletic training.