Comparison of collagen fibril populations in the superficial digital flexor tendons of exercised and nonexercised thoroughbreds.

This study was undertaken to test the hypothesis that collagen fibrils, the submicroscopic units of strength in tendon, would hypertrophy in response to a specific defined training programme. Fibril diameters were measured in central and peripheral regions of superficial digital flexor tendon (SDFT) samples from five 18-month-old horses which underwent a subsequent 18 month training programme and 6 age- and sex-matched controls. Central region fibrils from the trained horses had a mass-average diameter (MAD) of 105.3 nm, which was significantly lower (P < 0.01) than that of 131.7 nm for the same region in the control horses. This reduction in fibril diameter in the region of tendon which is predisposed to injury was interpreted as evidence of microtrauma, as it implies the region is weakened by the training regimen. Repeated episodes of microtrauma may accumulate and eventually result in degenerative lesions and clinical tendonitis.