Muscle strain is modulated more with running slope than speed in wild turkey knee and hip extensors

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Abstract
SUMMARY: We examined the length changes and electromyographic (EMG) activity of two hindlimb muscles in wild turkeys, to determine how these muscles modulate mechanical function with changes in running speed and slope. The muscles studied were the iliotibialis lateralis pars postacetabularis (ILPO), a biarticular knee and hip extensor, and the femorotibialis lateralis (FT), a knee extensor. Muscle length changes were recorded using sonomicrometry, and EMG activity was recorded from indwelling bipolar electrodes as the animals walked and ran at a range of speeds (1–3.5 m s–1). Treadmill slope was also varied, from a 12° uphill slope to a downhill slope of –12°. To test the hypothesis that the strain pattern in active muscles reflects the demand for mechanical work, we compared strain in the ILPO and FT across the range of slopes. Both muscles underwent active lengthen–shorten cycles during stance. We analyzed the lengthening and shortening part of the strain pattern separately to determine the response of muscle strain to surface slope. In both muscles stance phase shortening strain increased over the range of slopes studied, from 7.8±3.5% (ILPO) and 1.9±2.2% (FT) during downhill running at –12°, to 30.3±3.9% (ILPO) and 8.2±5.6% (FT) during uphill running at 12°. Stance-phase lengthening strain was also modulated with slope, from–15.6±3.2% (ILPO) and –22.1±9.6% (FT) during downhill running at –12°, to –4.2±2.5% (ILPO) and–9.0±5.6% (FT) during uphill running at 12°. The results suggest that for the ILPO and FT a change in net mechanical work output with running slope is likely mediated by a change in both the lengthening, energy absorbing portion of the contraction and the shortening, energy producing part of the contraction. We also found changes in the timing of EMG activity, and the relative portion of the stance period spent lengthening, which were consistent with a shift in muscle function from energy absorption during downhill running, to net energy production during uphill running. Generally, muscle strain was less affected by speed than by slope. Shortening strains were not significantly correlated with running speed. Only FT lengthening strain changed significantly with speed, ranging from–6.8±4.3% at 1 m s–1 to –15.3±4.7%at 3.5 m s–1. The consistent patterns of strain changes with running slope are evidence that strain pattern is modulated to meet the changes in demand for net mechanical work. The relatively poor relationship between strain and running speed may reflect the fact that changes in running speed during level running are not associated with a change in demand for net mechanical work. Taken together, the speed and slope results suggest that the demand for mechanical work is an important determinant of muscle length patterns in running and walking.