IN-VIVO MUSCLE FORCE-LENGTH BEHAVIOR DURING STEADY-SPEED HOPPING IN TAMMAR WALLABIES

Moderate to large macropodids can increase their speed while hopping w ith little or no increase in energy expenditure. This has been interpr eted by some workers as resulting from elastic energy savings in their hindlimb tendons. For this to occur, the muscle fibers must transmit force to their tendons with little or no length change. To test whethe r this is the case, we made in vivo measurements of muscle fiber lengt h change and tendon force in the lateral gastrocnemius (LG) and planta ris (PL) muscles of tammar wallabies Macropus eugenii as they hopped a t different speeds on a treadmill. Muscle fiber length changes were le ss than +/-0.5 mm in the plantaris and +/-2.2 mm in the lateral gastro cnemius, representing less than 2 % of total fiber length in the plant aris and less than 6 % in the lateral gastrocnemius, with respect to r esting length. The length changes of the plantaris fibers suggest that this occurred by means of elastic extension of attached cross-bridges . Much of the length change in the lateral gastrocnemius fibers occurr ed at low force early in the stance phase, with generally isometric be havior at higher forces. Fiber length changes did not vary significant ly with increased hopping speed in either muscle (P>0.05), despite a 1 .6-fold increase in muscle-tendon force between speeds of 2.5 and 6.0 ms(-1) Length changes of the PL fibers were only 7+/-4 % and of the LG fibers 34+/-12 % (mean +/- S.D., N=170) of the stretch calculated for their tendons, resulting in little net work by either muscle (plantar is 0.01+/-0.03 J; gastrocnemius -0.04+/-0.30 J; mean +/- S.D.). In con trast, elastic strain energy stored in the tendons increased with incr easing speed and averaged 20-fold greater than the shortening work per formed by the two muscles. These results show that an increasing amoun t of strain energy stored within the hindlimb tendons is usefully reco vered at faster steady hopping speeds, without being dissipated by inc reased stretch of the muscles) fibers. This finding supports the view that tendon elastic saving of energy is an important mechanism by whic h this species is able to hop at faster speeds with little or no incre ase in metabolic energy expenditure.