Adaptation of rat tibiae in response to repetitive loading

Abstract

ADAPTATION OF RAT TIBIAE IN RESPONSR TO REPETITIVE LOADING. M.R. Forwood and A.W. Parker Anatomy Department, The University of Queensland, Australia 4072 Forty eight male rats, aged 12 weeks, were subjected to approximately 20,ooO loading cycles per day of treadmill running (2 hrs/day, 26.8 m.min-* and 10% gradient) for 5 (Group A) and 10 days (Group B); with corresponding controls (C5) and (CIO). Rats in groups B and Cl0 were given weekly doses of tetracycline 4 days prior to training. Following training, right tibiae were tested to failure in torsion at 180 deg.s-l. Following bulk staining in basic fuchsin, six transverse sections (50 pm) were cut from the diaphysis of left bones and examined formicrodamageand fluonxcence.Resultsformechanicaltesting showedasignificantreduction in stiffness (P < 0.01) for groups A and B and a significant increase in twist angle (P < 0.01) for group A 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 labelling than controls (P < 0.05). Changes observed in mechanical properties and the periosteal and endocortical surface of exercised tibiae are consistent with long term effects observed in growing bones following intensive running training (Keller et al. O.R.S., 1983, Matsudaet al., J. Appl. Physiol. 60: 2028, 1986; Salem et al. I.C.B. XII: 353, 1989) but are inconsistent with Frost’s mechanostat theory which would predict s imulated modelling inresponse to increased mechanical usage.