MOON, B.R.*; URQUHART, M.R.; LINDSTEDT, S.L.; CONLEY, K.E.: Long tendons reduce muscle shortening in rattlesnake tails

The tailshaker muscles of western diamondback rattlesnakes (Crotalus atrox) sustain high frequency contractions that involve minimal shortening. Muscle segments in the proximal part of the tail have long tendons that insert onto the distal tail vertebrae; segments in the distal part of the tail lack tendons and insert directly onto the rattle shaker element. Studies of other animals have indicated that muscles with long tendons shorten less and do less work than muscles with short or no tendons. These results lead to the hypothesis that tailshaker muscle segments with and without tendons shorten differently. We used sonomicrometry to measure muscle shortening in vivo during rattling, and to test whether the muscle segments with tendons shorten less than do the segments that lack tendons. Rattling frequencies of 20-100 Hz involve muscle shortening of only 1-6%. The proximal muscle segments with long tendons contract at the same frequency as, but shorten significantly less than, the distal segments that lack tendons. This result indicates that the distal muscle segments do most or all of work to shake the rattle whereas the proximal segments do less work and probably stabilize the vertebrae from which the distal segments originate.