Superficial digital flexor tendon lesions in racehorses as a sequela to muscle fatigue: a preliminary study.
Authors: Butcher M T, Hermanson J W, Ducharme N G, Mitchell L M, Soderholm L V, Bertram J E A
Journal: Equine veterinary journal
Summary
# Editorial Summary Superficial digital flexor tendon (SDFT) injuries represent one of the most common career-ending conditions in racing Thoroughbreds, yet their biomechanical origins remain poorly understood. Rather than viewing SDFT lesions as isolated tendon failure, Butcher and colleagues investigated the entire suspensory system by implanting strain gauges in the forelimb musculature of three horses exercised at walk, trot and canter on a motorised treadmill, correlating real-time muscle and tendon loading data with architectural and fibre-type analysis. The research revealed that whilst the SDFT is highly fatigue-resistant and efficient at elastic energy storage—particularly during trotting—the synergistic deep digital flexor muscle possesses fast-twitch fibres that fatigue rapidly; as running speed increases and the deep digital flexor fatigues, the SDFT becomes progressively overloaded as it must compensate for loss of support and fetlock stabilisation. This preliminary work suggests that SDFT injuries may result not from intrinsic tendon weakness but from cumulative overload caused by the deep digital flexor's inability to sustain effort, pointing toward training protocols and diagnostic approaches that assess the entire distal limb system rather than the tendon in isolation—a paradigm shift with significant implications for injury prevention, conditioning programmes and clinical assessment in equine athletes.
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Practical Takeaways
- •Digital flexor tendon injuries may be preventable by conditioning programs that specifically build fatigue resistance in the deep digital flexor muscle, as its failure shifts excessive load to the superficial digital flexor.
- •Training intensity and duration should be gradually increased to allow adaptation of the fast-contracting deep digital flexor, which fatigues more readily than the superficial digital flexor.
- •Diagnostic and prevention strategies should evaluate the entire distal limb suspensory system rather than focusing on the tendon in isolation.
Key Findings
- •Superficial digital flexor muscle demonstrates fatigue-resistant and high force production properties, primarily storing and returning elastic energy at the trot.
- •Deep digital flexor muscle has fast-contracting properties that render it susceptible to fatigue, with increasing force contribution at higher running speeds.
- •Overloading of the superficial digital flexor tendon is proposed to result from fatigue of the synergistic deep digital flexor muscle, which normally stabilizes fetlock hyperextension.
- •Strain gauge measurements in 3 Thoroughbreds walking, trotting and cantering revealed differential load-sharing between digital flexor tendons across gaits.