REPETITIVE LOADING, INVIVO, 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-mu-m) were cut and examined for the presence of microdamage and fluorescence. Results for mechanical testing showed a significant reduction in stiffness of tibiae (P < 0.01) for groups A and B and a significant increase in twist angle (P < 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 < 0.01). These tibiae also showed fewer regions of, measurable, appositional growth than controls (P < 0.05). Exercised femora showed increased appositional growth at the endocortical surface of the mid-diaphysis (P < 0.05). Reductions in stiffness of exercised tibiae were significantly correlated with cross-sectional area.