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First published online July 23, 2003

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Muscle force-length dynamics during level versus incline locomotion: a comparison of in vivo performance of two guinea fowl ankle extensors

Monica A. Daley^* and Andrew A. Biewener

Concord Field Station, Department of Organismic and Evolutionary Biology, Harvard University, Bedford, MA 01730, USA

View larger version (31K): [in a new window]   Fig. 1. Anatomy of the lateral gastrocnemius (LG) and digital flexor-IV (DF-IV) muscles in the guinea fowl hindlimb, illustrating the sites of transducer implantation. Forces were measured from the common gastrocnemius (G) tendon and the tendon to DF-IV (inset) via tendon buckle force transducers. Note that the guinea fowl does not possess independent free tendons to the lateral and medial heads of the gastrocnemius. Muscle fiber length changes and muscle activity were measured in the LG and DF-IV (inset) of the left hindlimb using sonomicrometry and electromyography, respectively.

View larger version (37K): [in a new window]   Fig. 2. Representative recordings of muscle force, length and electromyographic (EMG) activity for lateral gastrocnemius (LG, above) and digital flexor-4 (DF-IV, below) during level running at 1.3 m s-1, from one individual (bird 5). Shaded bars indicate the support phase of the locomotor cycle. Top, silhouettes of the bird show the phases of the stride for one cycle, with the recorded limb in black and the contralateral limb in gray.

View larger version (34K): [in a new window]   Fig. 3. Expanded and averaged recordings of muscle length, force and electromyographic (EMG) activity from the lateral gastrocnemius (LG, above) and digital flexor-4 (DF-IV, below), normalized to a single stride. The support phase of the stride is shaded in gray. Results are shown for bird 4, running at 1.3 m s-1 on the level (left), and at 16° incline (right). The traces of force and length are means ± S.D. obtained from 10 strides, and a single EMG trace is shown for reference. The error bars below the EMG traces indicate the mean ± S.D. for the timing of EMG activity (onset, offset).

View larger version (26K): [in a new window]   Fig. 4. Inter-individual (top) and intra-individual (bottom) variation in cumulative energy production by lateral gastrocnemius (LG, left) and digital flexor-4 (DG-IV, right) over the locomotor cycle, shown for level running at 1.3 m s-1. The final value is the net work generated per stride. The traces of average cumulative work output shown across individuals (birds 4, 5 and 6, top) encompass the range of inter-individual variation for both muscles. The mean ± S.D. of energy production over the normalized stride from one individual (bird 4, bottom) illustrates typical levels of stride-to-stride variation for each muscle.

View larger version (18K): [in a new window]   Fig. 5. Muscle stress plotted against fascicle strain, forming in vivo mass-specific work loops, shown for three level speeds (0.7, 1.3 and 2.0 m s-1, left to right) and two 16° incline speeds (0.7 and 1.3 m s-1, broken lines) for the lateral gastrocnemius (LG, top) and the digital flexor-4 (DF-IV, bottom) from one individual that was nearest the means for all birds (bird 4). The area enclosed in the loop is the net work performed per unit muscle mass per stride. Arrows indicate the direction of the work loop. Counter-clockwise loops indicate net positive energy, and clockwise loops indicate net energy absorption. Color indicates timing of electromyographic (EMG) activity relative to force and length change.

View larger version (24K): [in a new window]   Fig. 6. Summary histograms of peak muscle stress, active fascicle strain, net work per stride and tendon elastic energy recovery per stride (means ± S.E.M.) for the lateral gastrocnemius (LG, left) and digital flexor-4 (DF-IV, right) across speed for level (open bars) and incline (filled bars) locomotion. Shortening muscle strains are negative. See Table 3 for relevant sample sizes.

View larger version (23K): [in a new window]   Fig. 7. Mean muscle force and net work per stride against relative electromyographic (EMG) intensity for lateral gastrocnemius (LG, left) and digital flexor-4 (DF-IV, right), during locomotion on the level (open circles) and incline (filled squares). For clarity, results are shown for one representative individual (bird 3); however, the pattern of change from level to incline was similar across individuals, as indicated by the mean slopes and r2 values given in the text.

View larger version (29K): [in a new window]   Fig. 8. Net mass-specific muscle work against (A) net strain, (B) mean force, and (C) phase relationship between force and strain for lateral gastrocnemius (LG, i) and digital flexor-4 (DF-IV, ii). Symbols indicate different individuals. Statistical results for the effect of each variable on work from a general linear model are shown in Table 5.

View larger version (14K): [in a new window]   Fig. 9. Contribution of gastrocnemius (black portion) and digital flexors (white portion) to total center of mass (COM) work during incline running at 0.7 (middle) and 1.3 m s-1 (right), compared to the contribution to be expected if all muscles in the limb contributed equally to mass-specific work for incline running (H; left).

View larger version (18K): [in a new window]   Fig. 10. Schematic illustration of the effect of phase (between length and force) and strain pattern on digital flexor-4 (DF-IV) muscle work. The center panel shows the relationship between phase and work for the DF-IV (Fig. 8Cii). If the muscle undergoes a stretch-shorten cycle (large velocity) and muscle force and length are symmetrical, no net work results (A). However, if peak force precedes peak length, the muscle absorbs energy (B), and if peak force lags behind peak length, it produces energy (C). However, if the muscle contracts with constant velocity, whether positive, negative or isometric (as shown in D), phase has no impact on work. This explains why the interaction term, phase xvelocity, is a larger factor underlying DF-IV work than phase alone (Table 5); a change in velocity during force production (velocity) is required for phase to be an important factor in work. Vertical lines in A-D indicate peak force.