Moon, B.R.*; Urquhart, M.R.; Conley, K.E.: In vivo shortening in the superfast tailshaker muscle of rattlesnakes

Sound-producing muscles typically contract at high frequencies with minimal shortening. The tailshaker muscle of western diamondback rattlesnakes (Crotalus atrox) contracts at frequencies of 20-100 Hz and has a constant energetic cost per twitch, but its extent of shortening is unknown. Decreasing lateral displacement of the rattle with increasing twitch frequency suggests that the muscles shorten less at higher twitch frequencies. However, the constant cost per twitch suggests that muscle shortening is constant. We are using sonomicrometry to measure muscle shortening in vivo and to test three factors that may account for the apparent discrepancy between muscle energetics and mechanics: (1) A tradeoff between lateral and torsional rotation of the rattle with increasing twitch frequency may involve a tradeoff in shortening among the three groups of tailshaker muscles. (2) The extent of eccentric contraction may vary with twitch frequency, which could change the force and displacement per twitch with minimal or no change in cost. (3) Rattle motion may be ballistic, with the maximal displacement being limited by the timing of contralateral twitches rather than by the extent of active muscle shortening. Preliminary results indicate that all three factors are important. Quantitative analysis, now under way, may show how each factor contributes to changes in muscle and rattle mechanics without changing the muscle energetics.