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First published online March 16, 2007
Journal of Experimental Biology 210, 1255-1265 (2007)
Published by The Company of Biologists 2007
doi: 10.1242/jeb.02742

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Modulation of proximal muscle function during level versus incline hopping in tammar wallabies (Macropus eugenii)

C. P. McGowan^1,*, R. V. Baudinette^2, and A. A. Biewener¹

¹ Concord Field Station, Department of Organismic and Evolutionary Biology, Harvard University, Cambridge, MA 02138, USA
² Department of Environmental Biology, University of Adelaide, Adelaide, SA 5003, Australia

View larger version (16K): [in this window] [in a new window]   Fig. 1. A schematic drawing of the lateral view of a wallaby hindlimb showing the two muscles examined in this study: biceps femoris (BF) and vastus lateralis (VL). One pair of sonomicrometry crystals (SONO; represented as black circles) was implanted in the BF and two pairs were implanted in the VL, proximally and distally. EMG electrodes (not shown) were implanted in each muscle adjacent to the pairs of SONO crystals.

View larger version (5K): [in this window] [in a new window]   Fig. 2. Stride parameters did not differ significantly between level (open symbols) and incline (filled symbols) hopping at either speed examined. Time of contact (tc; squares) decreased significantly between 3.3 m s–1 and 4.2 m s–1 on both grades (P<0.01), while swing phase duration (circles) and stride time (triangles) did not differ significantly between speeds (P>0.05). Symbols are offset on the x-axis for clarity. Error bars represent ± 1 s.e.m.

View larger version (24K): [in this window] [in a new window]   Fig. 3. Representative joint angle patterns normalized to percentage of stride for the (A) hip and (C) knee during level (open) and incline (filled) hopping at 4.2 m s–1. The shaded region denotes the stance phase. (B) A significant increase in hip extension (P=0.007) produced significantly greater net hip extension (P=0.013) during incline hopping. (D) The knee exhibited significantly less net flexion (P=0.006) during incline hopping due to a significant decrease in initial joint flexion (P=0.021). Initial hip flexion and knee re-extension were not significantly different between level and incline hopping (P=0.108 and P=0.289, respectively). There were no significant differences in average joint angle changes between speeds, and data for 3.3 and 4.2 m s–1 were pooled. Error bars represent ± 1 s.e.m.

View larger version (32K): [in this window] [in a new window]   Fig. 4. Representative muscle fascicle strain and activation patterns from wallaby #3 during level and incline hopping at (A,B) 3.3 and (C,D) 4.2 m s–1. Shaded regions indicate stance phase. Muscle fascicle strain was recorded from one sight in the biceps femoris (BF) and two sights in the vastus lateralis, proximally (VLprox) and distally (VLdist). In general, both muscles exhibited decreased initial stretch followed by increased shortening during incline as compared with level hopping. There were no significant differences in fascicle strain due to speed. Note: the second EMG electrode in the VL (bottom trace) did not provide reliable data in these trials.

View larger version (17K): [in this window] [in a new window]   Fig. 5. Histograms showing mean stretch, shortening and net strain between level and incline hopping in the (A) biceps femoris (BF) and (B) vastus lateralis (VL) (averaged for 3.3 and 4.2 m s–1). Insets indicate regions for which strains were measured during stance. The BF exhibited significantly greater shortening during inclined hopping (P<0.001), which produced significantly greater net shortening during stance (P<0.001). Due to high levels of variation, there were no significant differences in strain between level and incline hopping or between sites in the VL. Error bars represent ± 1 s.e.m.

View larger version (11K): [in this window] [in a new window]   Fig. 6. (A) A schematic showing the timing of electromyogram (EMG) activity in the biceps femoris (BF) (white) and vastus lateralis (VL) (dark grey) relative to the stride cycle. EMG onset occurred at the same time in both muscles and tended to occur later in the cycle during incline hopping, which led to the muscle tending to be active for a greater portion of stance phase. (B) EMG intensity, measured as normalized mean spike amplitude (MSA), did not differ significantly between level and incline hopping in either muscle. Error bars represent ± 1 s.e.m.

View larger version (18K): [in this window] [in a new window]   Fig. 7. Mean net joint moments (A,C) and muscle stresses (B,D) from a wallaby (body mass, 6.76 kg) hopping on a level runway and a 14° incline (three trials each). Hip joint moment (A) and resulting biceps femoris (BF) muscle stress (B) increased significantly between level and incline hopping. Joint moment at the knee (C) decreased significantly during incline hopping; however, due to the action of biarticular muscles, vastus lateralis (VL) stress (D) remained similar between level and incline trials. Broken lines represent ± 1 s.e.m.

View larger version (9K): [in this window] [in a new window]   Fig. 8. Work loops predicted for the (A) biceps femoris (BF) and (B) vastus lateralis (VL) during level and incline hopping calculated from muscle stress (Fig. 7) and mean fascicle strain. The BF likely does little work during level hopping but produces a substantial amount of positive work during incline hopping (counter-clockwise work loop). The VL likely shifts from absorbing a large amount of energy (negative work) during level hopping to producing a small amount of positive work on an incline. Estimated work values are in J kg–1 muscle. Circles on the ends of the work loops indicate the beginning of stance. The scale bar along the x-axis represents 10% fascicle strain in this direction.