Physical activity enhances insulin action in obese/overweight individuals. However, the exercise prescription required for the optimal enhancement is not known. The purpose of this study was to test the hypothesis that exercise training consisting of vigorous-intensity activity would enhance insulin sensitivity more substantially than moderate-intensity activity. Sedentary, overweight/obese subjects (n = 154) were randomly assigned to either control or an exercise group for 6 mo: (1) low-volume/moderate-intensity group (12 miles walking/wk at 40 to 55% peak O2 consumption (O2 peak)), (2) low-volume/high-intensity group (12 miles jogging/wk at 65 to 80% O2 peak), and (3) high-volume/high-intensity group (20 miles jogging/wk at 65 to 80% O2 peak). Training volume (miles/wk) was achieved by exercising 115 min/wk (low-volume/high-intensity group) or 170 min/wk (low-volume/moderate-intensity and high-volume/high-intensity groups). Insulin action was measured with an insulin sensitivity index (SI) from an intravenous glucose tolerance test. In the control group, there was a decrement (p < 0.05) in SI. In contrast, all the exercise groups significantly (p < 0.05) increased SI; the relative increment in the low-volume/moderate-intensity and high-volume/high-intensity groups (85%) were greater than in the low-volume/high-intensity group (40%). In conclusion, physical activity encompassing a wide range of intensity and volume minimizes the insulin resistance that develops with a sedentary lifestyle. However, an exercise prescription that incorporated 170 min of exercise/wk improved insulin sensitivity more substantially than a program utilizing 115 min of exercise/wk, regardless of exercise intensity and volume. Total exercise duration should thus be considered when designing training programs with the intent of improving insulin action.

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