Effects of exercise on the diameter of collagen fibrils in the central core and periphery of the superficial digital flexor tendon in foals

Objective—To determine the effects of exercise on collagen fibril diameter distribution in the superficial digital flexor tendon (SDFT) of foals. Animals—43 Dutch Warmblood foals. Procedure—From 1 week until 5 months of age, group-1 foals (n = 14) were housed in stalls and not exercised, group-2 foals (14) were housed in stalls but were exercised, and group-3 foals (15) were maintained at pasture. Biopsy specimens were collected from the SDFT at 2 months, and 8 foals in each group were euthanatized at 5 months. Remaining foals were housed together in a loose stall and paddock until euthanatized at 11 months. After euthanasia, specimens were collected from the SDFT; all specimens were analyzed by use of electron microscopy. Collagen fibrillar index (CFI), mass average diameter (MAvD), and area dependent diameter (ADD) were compared among groups. Results—Exercise-related differences in fibril distribution were not detected among groups at 2 months. At 5 months, ADD in peripheral specimens was significantly greater in group 1 than group 3. At 11 months, MAvD in core specimens was significantly less in group 3, compared with the other groups. However, in peripheral specimens, MAvD was significantly less in groups 2 and 3. Conclusion and Clinical Relevance—Collagen fibril restructuring in the SDFT of foals is in part an exercise- driven process. Withholding exercise may cause a delay in fibril development that can be partially overcome by increasing exercise at a later age. Exercise type may also affect remodeling of the SDFT in foals. (Am J Vet Res 2001;62:1563–1570).