Can energetic expenditure be minimized by performing activity intermittently?

Previous research has shown that the energetic expense per unit distance tr aveled for one bout of short-duration activity is much greater than the ene rgetic expense associated with long-duration activity. However, animals are often seen moving intermittently, with these behaviors characterized by br ief bouts of activity interspersed with brief pauses. We hypothesized that, when multiple bouts of brief activity are performed intermittently, the en ergetic cost per unit distance is less than when only one short bout is per formed. Mice were run 1, 2, 3, 5, 9 or 13 times for 15 s at their maximal s peed within a 375 s period while enclosed in an open-how respirometry syste m on a treadmill. The mice were also run continuously for 375 s. Following the last sprint and the continuous run, the mice remained in the respiromet ry chamber until their (V) over dot O-2 reached resting levels. Excess exer cise oxygen consumption (EEOC), the excess volume of oxygen consumed during the exercise period, increased from 0.03+/-0.01 to 0.40+/-0.02 ml O-2 g(-1 ) (mean +/- S.E.M., N=9) with activity frequency. However, the excess post- exercise oxygen consumption (EPOC), or volume of oxygen consumed during the recovery period, was independent of activity frequency (range 0.91-1.16ml O-2 g(-1)) and accounted for more than 80 % of the total metabolic cost whe n activity was performed intermittently. Lactate concentration was measured at rest, immediately after running and immediately after recovering from r unning 1, 5 and 13 times within the 375 s period. After running, [lactate] was significantly higher than resting values, but following recovery, [lact ate] had reached resting values. The net cost of activity, C-act, calculate d by summing EEOC and EPOC and then dividing by the distance run, decreased significantly from 132+/-38 to 6+/-1 mi O-2 gl km(-1) as activity frequenc y increased. When these values for Cact were compared with the cost of runn ing continuously for the same amount of time, the values were identical. Th erefore, we conclude that animals can minimize energetic expenditure by per forming brief behaviors more frequently, just as they can minimize these co sts if they increase the duration of continuous behaviors.