Aged bone displays an increased responsiveness to low-intensity resistanceexercise

The ability of bone to respond to increased loading as a function of age wa s tested by use of three-point bending and histomorphometry. The hindlimbs of male Fischer 344 rats of three age groups (young = 4 mo, adult = 12 mo, and old = 22 mo; n = 10 per age group) were progressively overloaded by tra ining the rats to depress a lever high on the side of a cage while wearing a weighted backpack. This squatlike movement required full extension of the hindlimbs. Exercised (Exer) rats performed 50 repetitions three times per week for 9 wk. Pack weight was gradually increased to 65% of body weight. C ontrols (n = 10 per age group) performed the same exercise without addition al weight. Neither the mechanical properties of the femur nor histomorphome try in the proximal tibia was significantly affected in young or adult rats . However, old Exer rats were found to have significantly smaller medullary areas and a decreased trabecular spacing than their age-matched controls. These results suggest a greater sensitivity to increased loading in aged ra ts.