WHEELER, D. L., J. E. GRAVES, G. J. MILLER, R. E. VANDER GRIEND, T. J. WRONSKI, S. K. POWERS, and H. M. PARK. Effects of running on the torsional strength, morphometry, and bone mass of the rat skeleton.Med. Sci. Sports Exerc., Vol. 27, No. 4, pp. 520–529, 1995. Intensity and duration effects of weight-bearing exercise on the rat skeleton were investigated. Eighty-four 3-month-old female Spraguc-Dawley rats were assigned to control and nine exercise groups. Exercised rats were run on a treadmill for either 30, 60, or 90 min·d-1at low (VO2max± 55%), medium (VO2max± 65%), or high (VO2max± 75%) intensity 4 d·wk-1for 10 wk. Rat femurs, tibias, and vertebrae were harvested for torsional mechanical tests, bone density assessment, and morphometry. Our results indicate exercise has a significant effect (P< 0.05) on the femoral mechanical response (energy absorbed and twist angle), tibial morphometry (cortical bone area and thickness), and tibial and vertebral bone density measurements but had no effect (P> 0.05) on bone strength when compared with control values. Higher intensity exercise decreased tibial and femoral torque (P< 0.05). Long duration exercise increased tibial and femoral stiffness and decreased twist angle and energy absorbed (P< 0.05). These results indicate bone adapts to its loading environment by increasing bone mineral density, increasing cortical bone area, increasing stiffness, decreasing energy absorbed, and decreasing twist angle. High-intensity exercise decreased the maximum force the bone could withstand, whereas long duration exercise made the bone more brittle.