Meeting Abstract
The long forearm and handwing bones of bats are moved by antebrachial muscles via tendons that often exceed half of the total length of the muscle tendon unit (MTU). One such muscle, extensor carpi radialis longus (ECRL) crosses both the elbow and wrist. It has been proposed that its biarticular architecture, coupled with a strut-like MTU behavior lets the ECRL relay elbow extension by triceps brachii to handwing extension. We probed this idea during hovering flight of Carollia perspicillata (n = 3). We calculated MTU length (Lmtu) and strain from in vivo measurements of joint kinematics and ex vivo measurements of muscle moment arms. Extension of the elbow and wrist occurred at the same time and at similar rates during downstroke. Measured moment arms were similar at each joint (0.016 ± 0.001 mm degree-1; mean ± S.D.) and remained constant across the operating ranges of both joints. The MTU underwent a modest lengthening of 4.0 ± 0.2% during wing extension, consistent with the idea of strut-like MTU behavior. A strut-like MTU could result from length change of both muscle and tendon in a compliant system, or alternatively from isometry of both muscle and tendon in a stiff system. To test between these possibilities, we measured muscle length (Lm) using fluoromicrometry and calculated tendon length (Lmtu – Lm). During wing extension, the muscle shortened by 0.12 ± 0.04 cm and the tendon lengthened by 0.25 ± 0.04 cm. These results suggest that a strut-like function of the ECRL results not from isometric muscle action, but from the balancing influence of muscle shortening and tendon lengthening.
