Repetitive loading, in vivo, of the tibiae and femora of rats: effects of repeated bouts of treadmill-running.

Twenty-four male rats, aged 12 weeks, were subjected to approximately 20,000 loading cycles per day of treadmill running (2 h/day, 26.8 m/min and 10% gradient) for 5 (group A) and 10 days (group B); with corresponding controls (C5) and (C10). Rats in groups B and C10 were given weekly doses of tetracycline from 4 days prior to training. Following training, right tibiae and femora were tested to failure in torsion at 180 degrees/s. Sections were cut from the distal, mid and proximal diaphysis of left bones, bulk-stained in basic fuchsin and two transverse sections (50 microns) were cut and examined for the presence of microdamage and fluorescence. Results for mechanical testing showed a significant reduction in stiffness of tibiae (P less than 0.01) for groups A and B and a significant increase in twist angle (P less than 0.01) for group A when compared with controls. No evidence of microdamage was observed from histological analysis. But, labelling demonstrated reduced appositional growth of the periosteal and endosteal surface at the mid-diaphysis of exercised tibiae (P less than 0.01). These tibiae also showed fewer regions of, measurable, appositional growth than controls (P less than 0.05). Exercised femora showed increased appositional growth at the endocortical surface of the mid-diaphysis (P less than 0.05). Reductions in stiffness of exercised tibiae were significantly correlated with cross-sectional area.