A recent study demonstrated significant differences in the biochemical characteristics of the tendons of Friesian horses when compared to other breeds. Over long periods of time, horses have been selectively bred for different purposes, and it is quite possible that these practices have had an impact on the locomotor tissues of Friesian horses. In the 18th and 19th centuries, Friesian horses became well known for their high knee action and elegant performance and were used in short-distance trotting races. While much has changed, the current KFPS breeding goal for movement still requires a great deal of flexion and power in the legs.
In a 2020 study conducted by Utrecht University in the Netherlands, a group of researchers hypothesized that tendons in racing breeds (Thoroughbreds) have biochemical properties that increase strength, whereas those in sporting breeds, such as Warmbloods and Friesians, would have tendons with more elastic properties. The team compared the common digital extensor tendon and superficial digital flexor tendon of each breed, and their results were interesting. They found significantly higher water percentage, lower collagen concentrations, and higher crosslink concentrations in the superficial digital flexor tendons of Warmbloods and Friesians when compared to Thoroughbreds. DNA content was also significantly lower in Warmbloods and Friesians than in Thoroughbreds.
The basic function of any tendon is to transmit the muscle’s force to the bone to produce movement. Tendons are strong bands of soft connective tissue that attach skeletal muscles to bones. Tendons are comprised of many parallel strands of collagen, which are the basic units of a tendon. A thin sheath wraps each collagen fiber, and a number of fibers combine to make up a tendon. The mechanical strength of a tendon relies on the orientation of collagen molecules within the tendon fibrils and the stabilization provided by the chemical crosslinks between the collagen molecules. Recent studies investigating the Friesian horse have reported differences in connective tissue metabolism and homeostasis compared to other breeds.
Friesians have a relatively small gene pool and have been intensively selected for breeding based on a specific conformation type and gait pattern associated with a hyperextended, hyper-elastic, and hyper-flexing locomotive pattern. Tendon and ligament damage generally occurs commonly in horses, with the superficial digital tendon and the suspensory ligament being the most frequently affected.
The superficial digital tendon is a spring-like, energy-storing tendon that has a major role in supporting the fetlock joint and is highly susceptible to injury during activities such as galloping or jumping. The suspensory ligament supports the fetlock and protects it from hyper-extension (i.e., dropping too low) during exercise. Recent studies have reported that Friesian horses have significantly fewer superficial digital flexor tendon injuries than Standardbred horses but significantly more suspensory ligament and intersesamodean ligament injuries than Warmblood horses.
From a functional perspective, the racing Thoroughbred relies on stronger tendons, while sport horses such as Friesians and Warmbloods rely on more elastic tendons. These significant biochemical differences in tendon properties between breeds are likely a result of selective breeding practices over time, although this requires further biomechanical and genetic studies.
Reference: M.E. Verkade, E. Hazeleger, C.H.A. van de Lest, W. Back. Biochemical differences between distal limb extensor and flexor tendons among equine breeds selected for racing and sport. The Veterinary Journal, Volume 262, 2020.
