Proximal hindlimb flexion in the horse: effect on movement symmetry and implications for defining soundness.
Authors: Starke S D, Willems E, Head M, May S A, Pfau T
Journal: Equine veterinary journal
Summary
# Editorial Summary: Proximal Hindlimb Flexion and Movement Symmetry Flexion tests remain a cornerstone of lameness diagnosis and prepurchase evaluation, yet their biomechanical effects on hindlimb movement have received limited scientific scrutiny. Starke and colleagues used inertial measurement units on the tuber coxae and sacrum to quantify vertical movement symmetry in 13 objectively sound horses before and after proximal hindlimb flexion, comparing kinematic data with subjective visual assessment. The flexion test reliably introduced asymmetry by decreasing sacral elevation on the flexed limb's mid-stance phase whilst increasing it on the contralateral side—a phenomenon manifesting as the characteristic hip hike—though the magnitude of this effect varied considerably between individual horses and diminished with successive strides. Critically, depending on each horse's baseline asymmetry, flexion could either exacerbate or paradoxically improve movement symmetry, suggesting the test's utility lies in amplifying pre-existing deficits rather than inducing them de novo. For practitioners, this means proximal hindlimb flexion can sensitize subtle lameness to clinical visibility, potentially lowering the detection threshold when soundness remains difficult to confirm objectively—though the highly individual response underscores why negative flexion tests require cautious interpretation.
Read the full abstract on PubMed
Practical Takeaways
- •Proximal hindlimb flexion can reveal subtle lameness in sound-appearing horses by exacerbating minor asymmetries above the visual detection threshold during prepurchase or clinical examinations
- •Individual responses to flexion are highly variable—some horses show increased asymmetry while others show decreased asymmetry—so flexion results must be interpreted with caution relative to baseline gait
- •The effect is transient and decreases over subsequent strides, so timing and standardization of flexion duration are important for consistent clinical interpretation
Key Findings
- •Proximal hindlimb flexion introduced additive changes to baseline movement symmetry with variable individual responses
- •Main systematic effect was decreased sacral upward movement following mid-stance of the flexed limb and increased upward movement of the non-flexed limb, manifesting as 'hip hike'
- •Flexion exacerbated subtle asymmetry in sound horses, potentially elevating barely-perceptible lameness above the visibility threshold
- •Effect of flexion decreased with increasing stride number, suggesting a transient rather than sustained change