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.

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).

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.

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.

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.

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.

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.

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).