Morphology of the brachial biceps muscle and elbow flexion in man

This study was undertaken to determine whether skeletal muscle fibre characteristics could be demonstrated to be of significance for muscle function in voluntary contraction in man. 4 male and 4 female adult subjects were studied. During elbow flexion force and velocity was measured at the hand with the forearm in a 100 degree position. A motor-driven heavy flywheel guaranteed a constant velocity or movement at the time of measurement. Force was registered by a straingauge dynamometer, and velocity by two sets of photocells. Cross-sectional area of the brachial biceps muscle was determined by computerized tomography scanning. Muscle fibre composition and fibre cross-sectional areas were assessed histochemically on needle biopsy samples obtained superficially from the brachial biceps muscle, the more superficial of the two large elbow flexor muscles. At contraction velocities from 2 to 7 radians per second (rad/s) a close relationship existed between the relative force output and the relative area of fast-twitch fibers (p less than 0.01). Maximal voluntary isometric contraction force averaged 189 N (120 to 309 N), and showed a close relationship with total cross-sectional area of the brachial biceps muscle. The specific tension (maximal isometric tension) of the muscle averaged 33 N/cm2 with no demonstrable difference between subjects of widely different fibre compositions, suggesting that maximal tetanic tension is similar in fast- and slow-twitch fibres in man.