Linking in vivo muscle dynamics to in situ force-length and force-velocity reveals that guinea fowl lateral gastrocnemius operates at shorter than optimal lengths

Figures

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  Figure 1.. Measuring the force-length relationship. The force-length (F-L) relationship was determined through a series of 200 ms tetanic contractions at varying isometric fascicle length conditions (see methods). For each contraction (example in A), we measured the total force, and the active force (dark dot) was estimated based on the muscle fascicle shortening and fitted passive force-length relationship (light dot). B) After a series of contractions, separate curves were fit to the active and passive F-L data (solid and dashed lines, respectively. See methods for details. An individual contraction cycle from A is overlaid onto the F-L curve in B for illustration. C) We normalized data, with force as a fraction of Fmax, and length divided by L0 to calculate fractional length. Different colored lines correspond to the normalized F-L relationships across birds. Shaded area indicates the force plateau, defined as the region between ±5% of Fmax.

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  Figure 2.. Measuring the force-velocity relationship. The force-velocity relationship (F-V) was obtained from a series of isotonic contractions at varying levels of fractional force. Two examples are shown here, for 0.3 (light) and 0.9 Fmax contractions (A-B). The F-V relationship was fitted to data from each individual bird, using a hyperbolic curve fitting (representative shown in panel C).

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  Figure 3.. F-V relationships across individuals. The F-V relationships across individuals (n = 11) in normalized units. Power curves are shown in dashed lines in Watts per kilogram muscle. The grey vertical line indicates the mean Vopt (2.9 L0s−1) and the grey shading indicates the mean ± 5% Vopt.

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  Figure 4.. Representative in vivo data recording. Muscle fascicle length (top, left axis), muscle force (dashed line, top, right-hand y-axis), EMG intensity (see methods) (bottom, left axis), and raw EMG signals (bottom, right hand y-axis) for four consecutive strides for a single bird during walking. Grey shaded blocks indicate times of foot-ground contact.

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  Figure 5.. Representative work loops during walking (purple, A and C) and running (green, B and D) of two representative individuals (ind2 and ind7). All work loops of the trial are plotted in gray. Averages are represented by the colored lines. Shaded areas around these mean curves indicate the ± 95% confidence interval. Time of EMG activity is represented by the solid lines, whereas dashed lines indicate absence of EMG activity. Closed circles indicate the time of foot contact (Ton) and open circles indicate the time of foot off (Toff). The individuals selected span the range from the most (A) and least (B) isometric contracting LG in walking and running. Values indicate the mass specific work output (Jkg−1, mean ± SD).

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  Figure 6.. In vivo muscle-tendon force and fascicle strain operating ranges relative to the force-length plateau. (A) To summarize in vivo data, we show mean stress and strain at EMGon (square), Ton (down pointing triangle), Frise (filled circle), Fpk (upward triangle), and Ffall (open circle). Error bars indicate 95% CI. The grey shaded rectangle indicates the force plateau (±5% Fmax) of the isometric F-L relationship. The muscle is activated at lengths slightly longer than the optimum length, and rapidly shortens at low force across the plateau. No significant force rise for stance occurs until foot contact (Ton). B) The effect of activation on the F-L relationship. Active F-L relationships with tetanic 200ms stimulation (dark, top), tetanic 50ms stimulation (medium, middle), and twitch stimulation (light, bottom). The corresponding passive curves are also shown but overlap. Open circles indicate individual data points from a single isometric contraction. For comparison, we also show the in vivo force and fractional length at 50% force rise and decay for walking (purple) and running (green). With decreasing stimulation and isometric force magnitude, the F-L plateau shifts rightward to…

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  Figure 7.. Relating in vivo operating range to the force-velocity relationship. Mean velocity and normalized force at Ton (downward triangles) and Fpk (upward triangle) are shown along with the mean F-V and P-V relationships across individuals (light blue and dark blue, respectively). Shaded regions indicate 95% CI. The grey box and vertical dashed line indicate mean ±5% Vopt. Muscles shorten at low velocity at Fpk, and higher velocity at Ton, when the muscle produces shortening work. Fractional forces are higher in running (green) compared to walking (purple).

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  Figure 8.. LG work loops in obstacle strides (color) compared to level strides (grey) in walking (A) and running (B). Data from published studies on walking (A, Ind 3) and running (B, Ind 6) over obstacles (Gordon et al., 2020; Schwaner et al., 2023). Force is normalized relative to peak force during steady level gait at the same speed because Fmax is unknown. Fascicle length has been normalized such that peak force occurs at 0.8, as found in the present study. However, L0 was not directly measured for these individuals. Shading represents 95% CIs. Obstacles elicit earlier and steeper force rise, at longer fractional lengths compared to level. Peak forces occur on estimated F-L plateau in obstacle strides. This suggests that the low forces in late swing in steady strides create a safety buffer, with potential for the muscle to rapidly generate higher forces in response to unexpected perturbations, without a change in muscle activation.