A motor and a brake

during locomotion or under in vivo conditions, individual locomotor muscles have been found to have functionally diverse roles. Pectoral muscles in flying birds, the abductor muscle in swimming scallops, and myotomes in fish clearly generate power like a motor (Abraham and Loeb, 1985; Johnston, 1991; Biewener et al., 1992; Marsh et al., 1992; Rome et al., 1993), as do the ankle extensors of cats during running (Walmsley et al., 1978; Gregor et al., 1988; Prilutsky et al., 1996). In contrast, the leg muscles of hopping wallabies and running turkeys generate force nearly isometrically to perform little work during level running (Roberts et al., 1997; Biewener et al., 1998). The mechanical function of the turkey gastrocnemius changes depending on the incline of the running surface (Roberts et al., 1997). While the turkey leg muscle contracts isometrically when generating force on the level, it shortens considerably to generate mechanical work and power when the animals runs up a 12 ° incline. Moreover, a hindlimb extensor muscle in the cockroach may play a role in control by generating larger forces during lengthening and actively absorbing mechanical energy to slow leg movement during running (Full et al., 1998). Energy absorption, storage, and transmission also occur in flying and swimming animals, despite the large overall power demands of these activities. For example, locomotor muscles in flies absorb energy to control steering (Tu and Dickinson, 1994). In some fish, the cranial muscle fibers shorten early in a tail-beat to produce power that is transmitted by more caudal muscle fibers that do not themselves generate net mechanical energy (van Leeuwen et al., 1990; Altringham et al., 1993). These studies demonstrate the challenge in predicting the mechanical function of an individual muscle (for a review, see Dickinson et al., 2000). The realization that individual muscles can function as motors, brakes, springs or struts during locomotion presents the possibility that muscles of a single anatomical group may not necessarily share the load or even share a common mechanical function. It is most often assumed that synergistic muscles share the work or load because muscles within a multiple muscle group are capable of executing the same action. In vivo studies show that the multiple ankle extensors in the cat and the wallaby operate together to share the force and power demands during locomotion (Walmsley et al., 1978; Abraham and Loeb, 1985; Prilutsky et al., 1996; Biewener et al., 1998). Numerous studies have modeled the functions of multiple muscles using assumptions of equal load-sharing (Seireg and Arvikar, 1975; Crowninshield, 1978; Dul et al., 379 The Journal of Experimental Biology 205, 379–389 (2002) Printed in Great Britain © The Company of Biologists Limited 2002 JEB3995