Physiological responses to nine different exercise:rest protocols.Med. Sci. Sports Exerc, Vol. 21, No. 1, pp. 90-95, 1989. This study determined the metabolic responses to different exercise: rest protocols during circuit exercise using hydraulic resistance. In experiment 1, nine subjects underwent nine different 27 min exercise circuits. There were three variations of three exercise:rest protocols (2:1, 1:1, 1:2). The VO2for the nine circuits averaged (mean ± SEM) 1.94 ± 0.03 1·min-1(43% of treadmill VO2max), with the largest difference between the protocols being 13%. Heart rate averaged 152.2 ± 3.1 beats·min-1, with the largest difference between the protocols being 8%. Increasing the exercise duration per minute or the number of exercise bouts per minute had minimal effects on the mean VO2and heart rate response to hydraulic resistive exercise. In experiment 2, nine subjects underwent three different 9 min exercise circuits using exercise:rest protocols of 2:1, 1:1, and 1:2 while work and VO2were simultaneously measured. Surprisingly, increases in work were not necessarily accompanied by corresponding increases in VO2.