BUTCHER, MT*; CHASE, PB; HERMANSON, JW; CLARK, AN; BERTRAM, JEA; Florida State University; Florida State University; Cornell University; Florida State University; University of Calgary: A tale of three muscles: physiological comparisons of the equine deep and superficial digital flexors and soleus muscle fibers.

A thorough investigation into the physiology of equine skeletal muscle fibers was conducted as part of research to study the function of the digital flexors in equine locomotion. Physiological investigation involved mechanical measurements of force, unloaded shortening velocity (V_US) and kinetics of tension redevelopment (k_TR) on permeabilized muscle fibers from the deep digital flexor (DDF), superficial digital flexor (SDF) and soleus (slow muscle control). Individual fibers were attached to a force transducer at one end and a motor at the other via foil clips. Fiber mechanics were studied over of range of Ca²⁺ concentrations (pCa; pCa = -log [Ca²⁺]) from relaxing conditions (pCa 9) to maximum Ca²⁺ activation (pCa 5) at both 10°C and 30°C. Average maximal isometric force occurred at pCa 5 at each temperature and increased with temperature for all three muscles. At 30°C force at pCa 5 was greatest for DDF (114.6 � 9.6 mN/mm²) compared with SDF (92.7 � 8.1 mN/mm²) and soleus (91.6 � 6.5 mN/mm²) muscle fibers. V_US at pCa 5 also increased with temperature and was fastest at 30°C for DDF (2.7 � 0.5 FLs^-1) followed by SDF (1.8 � 0.8 FLs^-1) and the soleus (1.1 � 0.2 FLs^-1). Q₁₀ was 1.7-1.8 for V_US in all three muscles. Finally, the activation dependence of k_TR was determined for all three muscles at both 10°C and 30°C. There was a similar Ca²⁺-dependent increase in k_TR at each temperature for DDF and SDF compared with little to no Ca²⁺-dependent increase in k_TR in soleus fibers. This finding may indicate a more tightly coupled mechanism of Ca²⁺-regulation dynamics for the two relatively faster digital flexor muscles.