ROBERTS, T.J.; HIGGINSON, B.K.; NELSON, F.E.; GABALDON, A.M.; Oregon State University; Oregon State University; Oregon State University; Oregon State University: Versatile function of muscle stretch-shorten cycles during running.

The muscles that power running must be remarkably flexible in their mechanical function. For example, the muscular system performs almost zero net mechanical work during level running, but it must perform net positive work (producing mechanical energy) during uphill running and net negative work (absorbing mechanical energy) during downhill running. We used sonomicrometry and EMG to test the hypothesis that the strain pattern in active muscles reflects the demand for mechanical work. Wild turkeys ran at 2 m/s on a treadmill angled at 0, +12, or -12 degrees. Muscle length and activity were measured in the femorotibialis muscle, a knee extensor, and the caudal head of iliotibialis lateralis (ITL), a knee and hip extensor. Both muscles underwent cyclic stretch-shorten cycles during stance and EMG activity for all running inclines, but shortening strain increased with increasing incline, while lengthening decreased. For example, during downhill running the ITL lengthened by 14.4% and shortened by 7.2% for a net lengthening strain of 7.2% during stance, while during uphill running the muscle lengthened by only 3.9% and shortened by 28.0% for a net shortening strain of 24.1%. In addition to altering strain magnitudes, both muscles changed the relative timing of the stretch-shorten cycle, spending more time lengthening for downhill running and more time shortening for uphill running. Shifts in relatively timing of EMG activity were also consistent with a shift from active lengthening contractions for downhill running to active shortening contractions for uphill running. These results suggest that stretch-shorten cycles of force production may provide a number of mechanisms for altering mechanical function with changes in demand for mechanical work. Supported by NIH AR46499.