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The Journal of Experimental Biology 204, 3507-3522 (2001)
© 2001 The Company of Biologists Limited

The effects of speed on the in vivo activity and length of a limb muscle during the locomotion of the iguanian lizard Dipsosaurus dorsalis

Frank E. Nelson^* and Bruce C. Jayne

Department of Biological Sciences, PO Box 210006, University of Cincinnati, Cincinnati, OH 45221-0006, USA

*Present address: Oregon State University, Zoology Department, 3029 Cordley Hall, Corvallis, OR 97331-2914, USA (e-mail: nelsonfr{at}ava.bcc.orst.edu)

Accepted July 18, 2001

The caudofemoralis muscle is the largest muscle that inserts onto the hindlimb of most ectothermic tetrapods, and previous studies hypothesize that it causes several movements that characterize the locomotion of vertebrates with a sprawling limb posture. Predicting caudofemoralis function is complicated because the muscle spans multiple joints with movements that vary with speed. Furthermore, depending on when any muscle is active relative to its change in length, its function can change from actively generating mechanical work to absorbing externally applied forces. We used synchronized electromyography, sonomicrometry and three-dimensional kinematics to determine in vivo caudofemoralis function in the desert iguana Dipsosaurus dorsalis for a wide range of speeds of locomotion from a walk to nearly maximal sprinting (50–350 cm s^–1). Strain of the caudofemoralis increased with increasing tail elevation and long-axis rotation and protraction of the femur. However, knee extension only increased caudofemoralis strain when the femur was protracted. The maximum and minimum length of the caudofemoralis muscle and its average shortening velocity increased from the slowest speed up to the walk–run transition, but changed little with further increases in speed. The times of muscle shortening and lengthening were often not equal at higher locomotor speeds. Some (20–25 ms) activity occurred during lengthening of the caudofemoralis muscle before footfall. However, most caudofemoralis activity was consistent with performing positive mechanical work to flex the knee shortly after foot contact and to retract and rotate the femur throughout the propulsive phase.

Key words: terrestrial, locomotion, muscle, electromyography, sonomicrometry, physiology, kinematics, lizard, Dipsosaurus dorsalis.

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