INTERMITTENT EXERCISE ALTERS ENDURANCE IN AN 8-LEGGED ECTOTHERM

Most animals move intermittently, yet many proposed performance limitations of terrestrial locomotion are based on steady-state measurements and assumptions. We examined the effect of work-rest transitions by exercising the ghost crab, Ocypode quadrata (28.1 +/- 8.1 g), intermittently on a treadmill at 0.30 m/s, a supramaximal speed [i.e., greater than the speed that elicits the maximal rate of oxygen consumption (VO2)]. Duration of the exercise and pause periods, ratio of exercise to pause, and speed during the exercise period were varied to determine the effect on performance. Crabs fatigued after 7.5 min of continuous running, a distance capacity (i.e., total distance traveled before fatigue) of 135 m. When the task was done intermittently with 2-min exercise and 2-min pause periods, the crabs fatigued after 87 min (a total distance of 787 m), representing an 5.8-fold increase in distance capacity compared with continuous exercise at the same absolute speed (0.30 m/s) and a 2.2-fold increase in distance capacity compared with continuous exercise at the same average speed (0.15 m/s). Pause periods < 30 s did not result in greater distance capacity compared with continuous exercise at the same average speed. Longer (3-5 min) and shorter exercise periods (less-than-or-equal-to 30 S) decreased distance capacity. Leg muscle lactate increased 10-fold to 15-mu-mol/g leg during intermittent exercise. However, significant amounts of lactate were cleared from the leg during the brief pause periods. The average VO2 during intermittent exercise was not statistically different from maximal VO2. The aerobic cost of intermittent exercise, both per time and per distance, was equal to or greater than the aerobic cost of continuous exercise at the same average speed. Further examination of dynamic physiological adjustments, such as in intermittent exercise, is likely to alter the performance limits established under a steady-state paradigm.