The effects of intensity on the energetics of brief locomotor activity.
The energetic costs associated with locomotion are often estimated only from the energy expended during activity and do not include the costs incurred during recovery. For some types of locomotion, this method overlooks important aspects of the metabolic costs incurred as a result of the activity. These estimates for energetic cost have also been predicted from long-duration, low-intensity activities that do not necessarily reflect all the behavior patterns utilized by animals in nature. We have investigated the effects of different activity intensities on the metabolic expenditure (per unit distance traveled) associated with brief exercise, and offer a more inclusive analysis of how the energetics of short-duration activities might be analyzed to estimate the costs to the animal. Mice ran on a treadmill for 15 or 60 s at 25 %, 50 % or 100 % of maximum aerobic speed (MAS) while enclosed in an open-flow respirometry system. Following the run, each mouse was allowed to recover while remaining enclosed in the respirometry chamber. Excess exercise oxygen consumption (EEOC), the excess volume of oxygen consumed during the exercise period, increased with the duration and increased linearly with the intensity of exercise. In contrast, the volume of oxygen consumed during the recovery period, or excess post-exercise oxygen consumption (EPOC), was independent of exercise intensity and duration and accounted for more than 90 % of the total metabolic cost. The net cost of activity (C(act)), calculated by summing EEOC and EPOC and then dividing by the distance run, increased as both activity duration and intensity decreased. The values for C(act) ranged from 553 ml O(2 )g(-)(1 )km(-)(1) for a 15 s run at 25 % MAS to 43 ml O(2 )g(-)(1 )km(-)(1) for a 60 s run at 100 % MAS. Combining these data with data from a companion paper, we conclude (1) that EPOC is independent of both the duration and intensity of activity when exercise duration is brief in mice, (2) that EPOC accounts for a majority of the oxygen consumed as a result of the activity when exercise durations are short, and (3) that animals can minimize their energy expenditure per unit distance by running faster for a longer period.