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First published online December 10, 2003
Journal of Experimental Biology 207, 211-223 (2004)
Published by The Company of Biologists 2004
doi: 10.1242/jeb.00764

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Dynamics of leg muscle function in tammar wallabies (M. eugenii) during level versus incline hopping

A. A. Biewener^1,*, C. McGowan¹, G. M. Card^1, and R. V. Baudinette²

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 (18K): [in a new window]   Fig. 1. Diagram showing the muscle-tendon anatomy of the tammar wallaby hindlimb and the positions of attachment of the tendon force buckles and implantation of the sonomicrometry (SONO) and electromyographic (EMG) electrodes in the lateral gastrocnemius (LG) and plantaris (PL) muscle bellies.

View larger version (23K): [in a new window]   Fig. 2. Representative recordings of muscle-tendon force, fascicle strain and EMG in the plantaris and gastrocnemius muscles of wallaby #2 during (A) level versus (B) incline hopping at 4.2 m s-1. Whereas gastrocnemius force measured in the Achilles tendon represents the medial gastrocnemius (MG) and lateral gastrocnemius (LG) combined, muscle strain and EMG were recorded from the LG muscle head. Little or no significant differences in force, fascicle strain or neural activation were observed when animals shifted from level to incline hopping at a given speed. The locomotor cycles showing stance and swing phases are those shown in Fig. 4 to describe the in vivo force-length behavior of these muscles.

View larger version (19K): [in a new window]   Fig. 8. Summary histogram showing the mean timing of force and EMG of the wallaby lateral gastrocnemius (LG) and plantaris (PL) muscles during level versus incline hopping. Because no significant timing differences were observed at 3.3 m s-1 versus 4.2 m s-1, trials were pooled for these two speeds and averaged among the four animals. Error bars represent ±1 S.E.M.

View larger version (32K): [in a new window]   Fig. 3. Representative changes in (A) knee, (B) ankle and (C) metatarsophalangeal (MP) joint angles versus time for three strides of wallaby #1 while hopping at 4.2 m s-1 on the level and on an incline. Because of small shifts in relative stance time for level versus incline hopping at the same speed, the graphs have been normalized to the same equivalent time period (time base shown is the average for the two conditions).

View larger version (20K): [in a new window]   Fig. 4. Representative in vivo work loops (force versus length) performed by (A) the lateral (LG) and medial gastrocnemius (MG) and (B) the plantaris (PL) muscles of wallabies #2 and #3 during level versus incline hopping. The strain and activation patterns for gastrocnemius are based on measurements made of the LG head. The period of muscle EMG is shown in gray. The path of force relative to length change is shown by the arrows. The net work performed (area inside the loop) is also shown for each muscle cycle. Net work is positive when the loop is counterclockwise in nature (e.g. wallaby #2 LG) and negative when the loop moves in a clockwise direction (e.g. wallaby #3 LG). Although different patterns were observed among individual animals, work loop patterns were always consistent across level and incline hopping conditions within an individual animal. The patterns shown here for wallaby #2 versus wallaby #3 differed the most among the four animals. Gastrocnemius work by the two animals not shown was generally more similar to that of wallaby #3. Differences in force-length behavior were generally greater for the gastrocnemius than for the plantaris. In general, the PL did less net work (negative or positive) than the LG.

View larger version (29K): [in a new window]   Fig. 5. (A) Histogram showing the mean (± S.E.M.) lengthening, shortening and net strains recorded from the four wallabies during level versus incline hopping (averaged for trials at 3.3 m s-1 and 4.2 m s-1). (B) Stance phase strains were measured over two intervals (based on level hopping recordings): from foot down to peak force and from peak force to foot off. Although the magnitude of lengthening and shortening strain was greater in the plantaris (PL) compared with the lateral gastrocnemius (LG), the net fascicle strains of the two muscles measured over the course of limb support were quite similar.

View larger version (10K): [in a new window]   Fig. 6. Net mass-specific muscle work (J kg-1 muscle) performed by the gastrocnemius (LG + MG) and the plantaris (PL) during level versus incline hopping at 3.3 m s-1 and 4.2 m s-1. Values are means ± S.E.M. (N=4).

View larger version (14K): [in a new window]   Fig. 7. Peak stresses (means ± S.E.M., N=4) developed in the gastrocnemius and plantaris tendons during level versus incline hopping at 3.3 m s-1 and 4.2 m s-1.