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Objective—To determine the effects of exercise on biomechanical properties of the superficial digital flexor tendon (SDFT) in foals. Animals—43 Dutch Warmblood foals. Procedure—From 1 week until 5 months of age, 14 foals were housed in stalls and not exercised, 14 foals were housed in stalls and exercised daily, and 15 foals were maintained at pasture. Eight foals in each group were euthanatized at 5 months, and remaining foals were housed together in a stall and paddock until euthanatized at 11 months. After euthanasia, SDFT were isolated and fit in a material testing system. Mean cross-sectional area (CSA) was measured and traction forces recorded. Normalized force at rupture (forcerup), normalized force at 4% strain, strain at rupture, stress at 4% strain (stress4%strain), and stress at rupture were compared among and within groups. Results—At 5 months, mean CSA and normalized forcerup were significantly greater and stress4%strain significantly less in the pastured group, compared with the other groups. At 11 months, CSA and normalized forcerup were not significantly different among groups, because forcerup increased significantly from 5 to 11 months in the nonexercised group and decreased significantly in the pastured group. Conclusions and Clinical Relevance—Exercise significantly affected the biomechanical properties of the SDFT in foals. Evenly distributed moderate- and low-intensity exercise at a young age may be more effective for development of strong, flexible tendons in horses than single episodes of high-intensity exercise superimposed on stall rest. This effect may impact later susceptibility to SDFT injury. (Am J Vet Res 2001;62:1859–1864)

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).