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The effects of a 10-wk training regimen on the mechanical properties of the femur and humerus were evaluated in 2.5- and 25-mo-old Fischer 344 female rats. The rats trained on a rodent treadmill 5 days/wk for 10 wk. Duration, grade, and speed increased until the rats maintained 1 h/day at 15% grade and either 15 m/min (old rats) or 36 m/min (young rats). Excised bones were mechanically tested with a 3-point flexure test for mechanical properties of force, stress, and strain. Fat-free dry weight (FFW) and moment of inertia were also obtained. With aging, similar increases were observed in both the femur and humerus for FFW, moment of inertia, and force. Ultimate stress was reduced in the senescent femur while strain was elevated; a similar but nonsignificant trend was observed in the humerus. Irrespective of age, training increased FFW in the femur and, to a lesser degree, in the humerus. Breaking force was elevated for both bones after training. In young and old bones, the training-induced differences in bone mass and force were similar, despite differences in training intensity. In the old trained rats, femur ultimate stress was greater than that in control rat femurs and similar to that in young rat femurs. The results of the present study indicate that training effects were not limited by age.

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To quantify cortical bone response to weight-bearing exercise, bone size, mineral content, and formation were measured at the femoral midshaft in swine. Bone formation was measured histomorphometrically on the periosteal, endosteal, and osteonal surfaces. Sedentary adult crossbred sows (3 years, 229 kg) were randomly assigned to basal (B, n = 6), control (C, n = 7), or trained (T, n = 7) groups. The basal and control groups did not exercise and were killed initially (B) or after 20 weeks (C). The trained group walked on a treadmill 20 minutes/day at 5 km/h and 5% grade, 5 days/week for 20 weeks. Bone length, area, or fat-free dry weight was not different with time (B versus C) or with training (C versus T). Periosteal modeling was stimulated by walking. Periosteal formation surface and mineral apposition rate (MAR) were greater in trained than control femora. No effects of walking were measured on the endosteal surface. Intracortical remodeling was not affected by walking. The number of labeled osteons (22.4 cm-2) was not different among groups, but osteonal MAR was greater in trained (1.18 microns/day) than control (0.96 mu/day) femora. Walking for 20 weeks in the previously sedentary sows was not a sufficient stimulus to create differences in gross measures of bone size or mineral content but did increase periosteal and intracortical MAR. The primary effect of increased exercise appeared to be osteoblast activation.