Sprint velocity of the lizard Sceloporus occidentalis was maximal at preferred body temperature (Tb, 35 degrees C). Mean running velocity (VR) and stride frequency (f) at this temperature were 3.23 +/− 0.7 (mean +/− S.E.M.) ms-1 and 15.6 +/− 0.3 s-1, respectively. VR and f did not change significantly when Tb was raised to 40 degrees C. At Tb values between 25 and 35 degrees C the thermal dependencies of VR (Q10 = 1.23) and f (Q10 = 1.12) were quite low. At Tb values below 25 degrees C the thermal dependence of these factors increased markedly. Stride length (LS) was independent of Tb from 15 to 40 degrees C. Lizards with a Tb of 10 degrees C were largely incapacitated, and VR, f and LS were all greatly reduced. Comparisons with measurements of the contractile properties of skeletal muscle of this species suggest that stride frequency is limited by the twitch contraction time at temperatures below 23 degrees C. At higher temperatures, sprint performance is nearly independent of the thermal effects on the muscles.

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