Mono- versus biarticular muscle function in relation to speed and gait changes: in vivo analysis of the goat triceps brachii.

The roles of muscles that span a single joint (monoarticular) versus those that span two (biarticular) or more joints have been suggested to differ. Monoarticular muscles are argued to perform work at a joint, whereas biarticular muscles are argued to transfer energy while resisting moments across adjacent joints. To test these predictions, in vivo patterns of muscle activation, strain, and strain rate were compared using electromyography and sonomicrometry in two major elbow extensors, the long and lateral heads of the triceps brachii of goats (Capra hircus), across a range of speed (1-5 m s(-1)) and gait. Muscle recordings were synchronized to limb kinematics using high-speed digital video imaging (250 Hz). Measurements obtained from four goats (25-45 kg) showed that the monoarticular lateral head exhibited a stretch-shortening pattern (6.8+/-0.6% stretch and -10.6+/-2.7% shortening; mean+/-s.e.m. for all speeds and gaits) after being activated, which parallels the flexion-extension pattern of the elbow. By contrast, the biarticular long head shortened through most of stance (-16.4+/-3.4%), despite elbow flexion in the first half and shoulder extension in the last half of stance. The magnitude of elbow flexion and shoulder extension increased with increasing speed (ANCOVA, P<0.05 and P<0.001), as did the magnitude and rate of active stretch of fascicles in the lateral head (P<0.001 for both). In all individuals, shortening fascicle strain rates increased with speed in the long head (P<0.001), and, in three of the four individuals, strain magnitude increased. Few independent effects of gait were found. In contrast to its expected function, the biarticular long head appears to produce positive work throughout stance, whereas the monoarticular lateral head appears to absorb work at the elbow. The biarticular anatomy of the long head may mitigate increases in muscle strain with speed in this muscle, because strain magnitude in the second phase of stance (when the shoulder extends) decreased with speed (P<0.05).