BUTCHER, MT*; CHASE, PB; HERMANSON, JW; BERTRAM, JE; CLARK, AN; SYME, DA; Univ. of Calgary; Florida State Univ.; Cornell Univ.; Univ. of Calgary; Florida State Univ.; Univ. of Calgary: Work and power characteristics of skinned fibers from the deep and superficial digital flexor muscles in the forelimbs of horses

The objective involved making measurements of mechanical work and power on permeabilized fibers from the deep (DDF) and superficial (SDF) digital flexors as well as soleus (SOL) for a slow muscle control. Individual fibers underwent sinusoidal oscillations over a range of physiological frequencies (0.5-16Hz) and strain amplitudes (0.01-0.07 ML) under relaxing conditions (pCa 9), maximum activation (pCa 5) and rigor conditions at both 10°C and 30°C. Work and power were analyzed by both the workloop technique and a Nyquist technique of determining the elastic and viscous moduli of fibers to evaluate the real and imaginary components following correction for phase differences measured in rigor. Results from the workloop analysis on activated fibers (pCa 5) showed a general trend of increased mean work and power absorption with increasing strain and cycling frequency for all three muscle fiber types at each temperature. At 30°C only, work and power were actively generated (avg. 0.002-0.05 Wkg^-1) at 0.01 ML strain for all three fiber types although at differing cycling frequencies: DDF (4-7 Hz), SDF (4-5Hz) and SOL (1Hz). Workloop results were corroborated by Nyquist analysis (30°C only) on a subset of fibers which showed mean cross-bridge work being done against the apparatus by DDF, SDF and SOL fibers at 0.01 ML strain and further a marginal mean capacity for cross-bridge work to be done by DDF fibers at 0.02 ML strain between 4-6Hz. On average, work done by cross-bridges peaked at 1Hz for SOL, 4Hz for SDF and 6Hz for DDF fibers. Physiological findings contribute to an integrative model of digital flexor function in equine locomotion.