GABALDON, A.M.; NELSON, F.E.; ROBERTS, T.J.: Gastrocnemius Muscle Mechanics in Turkeys During Uphill and Downhill Running

To produce different kinds of movements, animals either recruit different muscles or alter the mechanical function of active muscles. Studies indicate that the lateral gastrocnemius (LG) muscle in turkeys changes mechanical function with changes in running incline. On level ground, the LG produces force isometrically in stance, minimizing energy cost while performing little work. On uphill ground, the LG actively shortens to increase work output. Here, we investigated LG mechanics during downhill treadmill locomotion in wild turkeys (Meleagris gallopavo). We hypothesized that the LG changes from a power-producing motor in uphill running to a brake in downhill running (absorbing work to decelerate and lower the body). Sonomicrometer crystals were implanted along a proximal fascicle of the LG muscle in two turkeys. Bipolar EMG electrodes were implanted adjacent to the crystals to record muscle electrical activity and strain gauges were glued to the bony tendon of the LG to measure force. Birds ran on a horizontal, inclined (+12°), and declined (-12°) treadmill. As predicted, the LG actively shortened to produce work when the turkeys ran uphill and actively lengthened to absorb work when they ran downhill. The muscle was equally effective at absorbing mechanical energy as it was at producing it; average power in stance at 2 m/s was +54 W/kg muscle (uphill) and –60 W/kg muscle (downhill). Muscle forces during stance were similar for uphill vs. downhill running, but the cost of producing force was somewhat greater for uphill running, as indicated by a higher integrated, rectified EMG/force ratio. These findings support the idea of flexibility of mechanical function within an individual muscle. Supported by NIH AR46499.