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First published online May 24, 2004
Journal of Experimental Biology 207, 2277-2288 (2004)
Published by The Company of Biologists 2004
doi: 10.1242/jeb.01006

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Mechanical function of two ankle extensors in wild turkeys: shifts from energy production to energy absorption during incline versus decline running

Annette M. Gabaldón^*, Frank E. Nelson and Thomas J. Roberts

Department of Zoology, Oregon State University, 3029 Cordley Hall, Corvallis, OR 97331, USA

View larger version (29K): [in a new window]   Fig. 1. In situ calibration of tendon strain to muscle force for the lateral gastrocnemius, LG, in one turkey. The sciatic nerve was electrically stimulated to generate a muscle contraction while tendon strain and muscle force were measured simultaneously. (A) Strain values for the deep (str1) and superficial (str2) aspects of the bony tendon were averaged (strLG) to account for bending. Muscle force was measured by attaching the free distal end of the tendon to a servomotor. (B) Tendon strain and muscle force during stimulation. The region where the muscle was developing force (indicated by the arrows) was the segment used for calibration. (C) Relationship of muscle force and tendon strain. The slope of the line was determined using linear regression analysis. (D) Pure bending of the bony tendon in the region where the strain gauges were glued. The deep (str1) and superficial (str2) tendon strains are averaged (strLG) to cancel most of the strain due to pure bending.

View larger version (44K): [in a new window]   Fig. 2. Muscle force, fascicle length and power output for the lateral gastrocnemius, LG, and peroneus longus, PL, muscles in a turkey running at a steady speed of 2 m s–1 on level ground. Shaded areas are stance, when the foot is in contact with the ground; unshaded areas are swing, when the foot is off the ground and being repositioned for the next step. L, fascicle length; L0, resting segment length.

View larger version (29K): [in a new window]   Fig. 5. Mean±S.E.M. values of net work, average force, net fascicle strain, and average velocity for the lateral gastrocnemius (LG) and peroneus longus (PL) muscles in all turkeys running at a steady speed of 2 m s–1 on a level, incline, and decline. Filled symbols for the LG and PL are averages over the period of stance; open symbols for the LG are averages over the period of force production in stance. For clarity, S.E.M. values for open symbols are not shown; hidden open symbols for LG net work are the same as the stance values. Net work output for the LG and PL changed significantly with running slope (P<0.05; r2=0.99 for LG and r2=0.97 for PL). On steeper inclines, LG and PL did increasingly greater amounts of net positive work; on steeper declines, both muscles did increasingly greater amounts of net negative work. Average muscle force in stance remained constant across all slopes for both the LG and PL (P>0.05), whereas net fascicle strain, expressed as percentage length change relative to resting fascicle length, and average muscle velocity changed significantly with running slope, in parallel with the changes in net work output. For net fascicle strain, P<0.01; r2=0.99, LG and r2=0.98, PL; for average muscle velocity, P<0.01; r2=0.99, LG and r2=0.98, PL. L, fascicle length; L0, resting segment length.

View larger version (39K): [in a new window]   Fig. 3. Muscle force and fascicle length for the lateral gastrocnemius (LG) and peroneus longus (PL) muscles in three turkeys running at a steady speed of 2 m s–1 on level ground. Shaded areas are stance; unshaded areas are swing. Force–length relationships for the LG and PL over one complete stride (swing and stance) are also shown for each bird. Arrows in a counterclockwise direction indicate net positive work in the work loop. Arrows for PL at the start of stance indicate a small fraction of active lengthening performed in the stretch–shorten cycle. L, fascicle length; L0, resting segment length.

View larger version (17K): [in a new window]   Fig. 4. Stance net mechanical work output for the lateral gastrocnemius (LG) and peroneus longus (PL) muscles during steady speed running at 1–3 m s–1 on the level (open circles), 12° incline (filled triangles) and 12° decline (filled squares). Values are means ± S.E.M.; N=5 birds for the LG and N=4 for the PL; numbers above a symbol indicate a different number of birds at that speed. Across a range of running speeds, both the LG and PL performed greater net positive work in stance during incline versus level running, and they developed net negative work in stance during decline running.

View larger version (30K): [in a new window]   Fig. 7. (A) Muscle force and fascicle length for the lateral gastrocnemius (LG) and peroneus longus (PL) muscles during one full stride from a turkey running at a steady speed of 2 m s–1 on a declined (–12°), level and inclined (+12°) treadmill. Shaded areas are stance; unshaded areas are swing. Dashed vertical lines indicate the time to peak muscle force in stance. During decline running, the PL shifted the time to peak force production from late stance into early stance to correlate with muscle lengthening. The LG did not shift the time to peak force production in response to decline running. (B) A summary graph of mean ± S.E.M. values for all birds showing the time to peak force for LG (filled triangles) and PL (open circles) muscles in relation to running slope. L, fascicle length.

View larger version (49K): [in a new window]   Fig. 6. Muscle force, electromyographic (EMG) activity, fascicle length, and power output for the lateral gastrocnemius, LG, and peroneus longus, PL, muscles in two different turkeys running at a steady speed of 2 m s–1 on a declined (–12°), level, and inclined (+12°) treadmill. Shaded areas are stance; unshaded areas are swing. Mechanical energy production (positive power) in stance increased during incline versus level running for both the LG and PL, as muscle shortening increased over their periods of force production. During decline running, both the LG and PL shifted mechanical function and instead absorbed mechanical energy in stance by forcefully lengthening. L, fascicle length; L0, resting segment length.

View larger version (21K): [in a new window]   Fig. 8. Average muscle force, net fascicle strain and net work output for the peroneus longus (PL) analyzed over individual segments of the stretch–shorten cycle during steady speed running at 2 m s–1 on a level, inclined and declined treadmill. (A) The lengthening region of the stretch–shorten cycle and (B) the shortening region of the stretch–shorten cycle (analyzed as the region in stance following lengthening). L0, resting segment length.

View larger version (19K): [in a new window]   Fig. 9. Effects of running slope on (A) ankle joint angle changes during a single stride at a running speed of 2 m s–1 and (B) ankle net excursion in stance, calculated as the difference in ankle angle between toe-off, the end of stance, and toe-on, the beginning of stance. Values are means ± S.E.M. for N=6 birds.