MAXIMUM SPEED AND MECHANICAL POWER OUTPUT IN LIZARDS

The goal of the present study was to test the hypothesis that maximum running speed is limited by how much mechanical power the muscular sys tem can produce, To test this hypothesis, two species of lizards, Cole onyx variegatus and Eumeces skiltonianus, sprinted on hills of differe nt slopes, According to the hypothesis, maximum speed should decrease on steeper uphill slopes but mechanical power output at maximum speed should be independent of slope, For level sprinting, the external mech anical power output was determined from force platform data. For uphil l sprinting, the mechanical power output was approximated as the power required to lift the center of mass vertically, When the slope increa sed from level to 40 degrees uphill, maximum speed decreased by 28 % i n C. variegatus and by 16 % in E. skiltonianus. At maximum speed on a 40 degrees uphill slope in both species, the mechanical power required to lift the body vertically was approximately 3.9 times greater than the external mechanical power output at maximum speed on the level. Be cause total limb mass is small in both species (6-16 % of body mass) a nd stride frequency is similar at maximum speed on all slopes, the int ernal mechanical power output is likely to be small and similar in mag nitude on all slopes, I conclude that the muscular system is capable o f producing substantially more power during locomotion than it actuall y produces during level sprinting, Thus, the capacity of the muscular system to produce power does not limit maximum running speed.