The purpose of this study was to determine in vivo moment arm lengths (MAs) of three elbow flexors at rest and during low- and relatively high-intensity contractions, and to examine the contraction intensity dependence of MAs at different joint positions. At 50°, 80° and 110° of elbow flexion, MAs of the biceps brachii, brachialis and brachioradialis were measured in 10 young men using sagittal images of the right arm obtained by magnetic resonance imaging, at rest and during 20% and 60% of isometric maximal voluntary elbow flexion. In most conditions, MAs increased with isometric contractions, which is presumably due to the contraction-induced thickening of the muscles. This phenomenon was especially evident in the flexed elbow positions. The influence of the contraction intensities on the increases in MAs varied across the muscles. These results suggest that in vivo measurements of each elbow flexor MA during contractions are essential to properly examine the effects on the interrelationships between elbow flexion torque and individual muscle forces.
Ryota Akagi (Corresponding Author) is with the Department of Sports Sciences, Japan Institute of Sports Sciences, Tokyo, Japan. Soichiro Iwanuma is with the Graduate School of Sport Sciences, Waseda University, Saitama, Japan, and with the Japan Society for the Promotion of Science. Satoru Hashizume is with the Graduate School of Sport Sciences, Waseda University, Saitama, Japan. Hiroaki Kanehisa is with the National Institute of Fitness and Sports in Kanoya, Kagoshima, Japan. Toshimasa Yanai and Yasuo Kawakami are with the Faculty of Sport Sciences, Waseda University, Saitama, Japan.