The power output of fast-glycolytic (FG) muscle fibres isolated from the iliofibularis (IF) muscle of desert iguanas (Dipsosaurus dorsalis) was measured at 35 sC using the oscillatory work-loop technique. To simulate cyclical muscle length changes during running, isolated fibre bundles were subjected to sinusoidal length changes and phasic stimulation during the strain cycle. At constant strain (12 %), the duration and timing (phase) of stimulation were adjusted to maximise power output. Using both hatchlings (4–8 g) and adults of varying sizes (15–70 g), the intraspecific allometries of IF length and contractile properties were described by regression analysis. The muscle length at which isometric force was maximum (L0, mm) increased geometrically with body mass (M, g) (L0=5.7M0.33). Maximum power output and the force produced during shortening showed no significant relationship to body size; work output per cycle (Wopt, J kg-1) under conditions required to maximise power did increase with body size (Wopt=3.7M0.24). Twitch duration (Td, ms), measured from the onset of force generation to 50 % relaxation, increased allometrically with body mass (Td=12.4M0.18). Limb cycling frequency during burst running (f, reported in the literature) and the frequency required to maximise power output in vitro (fopt) decreased with body size, both being proportional to body mass raised to the power 0.24. These findings suggest that limb cycling frequency may be limited by twitch contraction kinetics. However, despite corresponding proportionality to body size, limb cycling frequencies during burst running are about 20 % lower than the cycling frequencies required to maximise power output. Differences in the contractile performance of the IF in vitro and in vivo are discussed in relation to constraints imposed by gravitational forces and the design of muscular, nervous and skeletal systems.

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