Morphological and biochemical effects of strenuous exercise on immature long bones.

To determine the effects of intense exercise on the growth of long bones in immature animals, young male white leghorn chickens were run five days per week starting at four weeks of age on motor-driven treadmills. Work intensity was determined on the basis of maximal oxygen consumption (VO2 max) with the exercise intensity maintained at 70-80 percent VO2 max. Young animals ran continuously for 30 minutes, older animals 45 to 60 minutes each day. Runners and controls (10 animals per group) were sacrificed at 8, 12, 14 and 20 weeks of age. The lengths of the femurs and tibiotarsus were significantly stunted at 8-, 12- and 14 weeks in the runners but had nearly recovered at 20 weeks of age. Both bones also demonstrated significantly decreased total cross-sectional areas in 8-, 12- and 14 week-old runners as well as decreased cortical cross-sectional areas. The tibiotarsus also remained significantly smaller in the 20-week-old runners, but the femur had recovered in terms of radial growth. Intermolecular pyridinoline collagen crosslinks were identical in amount in the two groups with the femur collagen significantly less cross-linked than the tibiotarsus. The delayed growth of the exercised avian young bone is consistent with data obtained from children and young mammalian models. The osteogenic response to exercise that produces an increased bone mass in adult tissue appears either suppressed or overcome in young avian bone indicating that it may be erroneous to assume that data obtained from adult tissue are also applicable to young growing bone.