The “super” in superficial digital flexor tendon (SDFT) relates to the fact this energy-packed tendon lies just under the skin on the back of horses’ legs, where we can easily feel it with our hands. In other words, it’s “superficial,” as opposed to deep.

Arguably, the horse’s SDFT is “super” in more ways than one; each tendon packs muscular energy into its fibrous strands and then releases it when horses need it most, as they gallop, jump, or propel themselves at high speeds in any direction, says Claire O’Brien, MRes, PgDip, BSc (Hons), FHEA, director of the Science Learning Centre at the University of Limerick, in Ireland.

Though this super tendon equips the horse to accomplish stunning athletic feats, it is by no means perfect, O’Brien explains. The SDFT is prone to traumatic and especially overuse injuries with a very high re-injury rate after healing.

In this article we’ll take a closer look at the latest scientific knowledge on this strange, marvelous, yet vulnerable equine tendon.

Hierarchically Aligned Elastic Bands

At a microscopic level within the SDFT, molecules of stretchy type I collagen group together to create strands of 100-atom-wide collagen microfibrils. Like individual threads within yarn, these microfibrils line up to form thicker strands called collagen fibrils, which serve as the “fundamental load-bearing material,” O’Brien says. Wrapped in cross-linked bands, each fibril aligns with other banded fibrils to form collagen fibers. Multiple collagen fibers then combine to form fascicles, and the fascicles group together to form fascicle bundles.

These fascicle bundles all fit into a fairly elastic and lubricated structure called the interfascicular matrix (IFM). The IFM “plays a pivotal role” in efficient SDFT functioning, primarily because it allows for smooth gliding of the fascicles, she says. The entire tendon has a width of about ¾ inch.

“The tendon is really like this big group of lots of elastic bands,” explains Christopher Elliott, BVSc (hons), MANZCVS, Dipl. ACVSMR, of Palm Beach Equine Clinic, in Wellington, Florida.

Stretchy yet solid, thanks to this unique material composition, the SDFT can extend up to 15% during a gallop and possibly more when jumping, O’Brien says