The effects of intensity on the energetics of brief locomotor activity

The energetic costs associated with locomotion are often estimated only fro m the energy expended during activity and do not include the costs incurred during recovery. For some types of locomotion, this method overlooks impor tant aspects of the metabolic costs incurred as a result of the activity. T hese estimates for energetic cost have also been predicted from long-durati on, low-intensity activities that do not necessarily reflect all the behavi or patterns utilized by animals in nature. We have investigated the effects of different activity intensities on the metabolic expenditure (per unit d istance traveled) associated with brief exercise, and offer a more inclusiv e analysis of holy the energetics of short-duration activities might be ana lyzed to estimate the costs to the animal. Mice ran on a treadmill for 15 o r 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 all owed to recover while remaining enclosed in the respirometry chamber. Exces s exercise oxygen consumption (EEOC), the excess volume of oxygen consumed during the exercise period, increased with the duration and increased linea rly with the intensity of exercise, In contrast, the volume of oxygen consu med during the recovery period, or excess post-exercise oxygen consumption (EPOC), was independent of exercise intensity and duration and accounted fo r more than 90 % of the total metabolic cost. The net cost of activity (Cac t), calculated by summing EEOC and EPOC and then dividing by the distance r un, increased as both activity duration and intensity decreased. The values for Cact 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 wi th data from a companion paper, we conclude (1) that EPOC is independent of both the duration and intensity of activity when exercise duration is brie f 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 an imals can minimize their energy expenditure per unit distance by running fa ster for a longer period.