Repetitive loading, in vivo, of the tibia and femora of rats: effects of a single bout of treadmill running.

The presence of microdamage in the tibiae and femora of rats following repetitive loading in vivo was investigated by subjecting 48 male rats, aged 12 weeks, to treadmill running (26.8 m.min-1 on 10% grade) for 0.56 hours (5,000 cycles, E1), 1.13 hours, (10,000 cycles, E2), 2.27 hours (20,000 cycles, E3), and 3.4 hours (30,000 cycles, E4) with Group C as control. Following exercise, tibiae and femora were excised and the right limbs were tested in torsion at 180 degrees.sec-1. Transverse sections were cut from the proximal, mid- and distal diaphysis of left tibiae and femora, bulk stained in basic fuchsin, cut to 50 microns thick, and examined for the presence of microdamage. Following these periods of loading, tibiae and femora showed no evidence of microdamage initiation, as evidenced by light microscopy, or corresponding alterations in mechanical properties. It was concluded that the magnitude of loading produced by single bouts of intensive exercise, which encompassed up to 30,000 loading cycles, was insufficient to initiate fatigue microdamage in tibiae or femora of rats.