MCGOWAN, C.P.*; BIEWENER, A.A.; Concord Field Station, Harvard University: Proximal muscle function during level and incline hopping in tammar wallabies.

MCGOWAN, C. P.* and BIEWENER, A. A. Concord Field Station, Harvard University, cmcgowan@oeb.harvard.edu Proximal muscle function during level and incline hopping in tammar wallabies. Kangaroos and wallabies are exceptional in their ability to minimize locomotor costs during hopping. Ankle extensor muscle-tendon units in wallabies have been shown to act primarily as springs, storing and returning elastic strain energy during steady speed hopping on the level as well as on an incline. However, this raises the question of what muscles contribute net positive work during incline hopping and during acceleration. We hypothesized that the vastus lateralis (VL, knee extensor) and biceps femoris (BF, hip extensor) may serve such a role. To test this hypothesis and explore regional differences in muscle behavior, sonomicrometry and EMG electrodes were implanted at two locations in the VL and BF of five tammar wallabies. Animals were hopped at 0 and +10&deg on a treadmill at steady speeds. Under both conditions, proximal and distal sites in the VL were actively stretched (+22% and +13% respectively) more then they shortened (-13% and -7%), though slightly less on a 10&deg incline (3.0% and 2.5%). Similar patterns of strain were observed at both VL locations. The inferior BF showed the greatest adjustment to incline hopping with over a two-fold increase in net fascicle shortening (level: 3.7% vs 10&deg: 8.4%). The superior site, where the BF fascicles have little flexor moment about the knee, underwent small net strains (< 2%) under both conditions. These results suggest that the inferior, bi-articular portion of the wallaby BF plays a major role in modulating the work necessary for incline vs level hopping, while the VL acts to absorb energy under both conditions.