Adaptation strategies of Icelandic horses with induced transient hindlimb lameness at walk, trot and tölt.

This study tested how Icelandic horses change their movement when a mild, temporary hindlimb hoof pain is induced at walk, trot, and tölt, to see which motion signals best reveal lameness. It found pelvic minimum asymmetry (PDmin) and hip-hike are the most informative, especially at trot, while tölt and walk under saddle show only subtle changes.

Why this matters

• Icelandic horses have extra gaits (notably tölt), which can make lameness detection more difficult using traditional visual cues tailored to walk and trot.
• Objective lameness metrics validated in trot (e.g., pelvic minimum/maximum asymmetries) are not well established for tölt or walk in this breed.
• Identifying robust, gait-appropriate parameters helps clinicians choose the best gait to examine and improves sensitivity to mild hindlimb lameness.

Study design and methods

• In vivo experiment with 11 Icelandic horses assessed before and after inducing a mild, transient hindlimb lameness using sole pressure (hoof-associated discomfort).
• Gaits and contexts tested:
  • In-hand: walk, trot (straight-line handling).
  • Ridden: walk, sitting trot, tölt.
• Instrumentation: inertial measurement unit (IMU) system to record upper-body motion:
  • Head vertical movement asymmetries: HDmin (difference in head minima), HDmax (difference in head maxima).
  • Pelvic vertical movement asymmetries: PDmin (difference in pelvic minima; “impact”/early-stance related), PDmax (difference in pelvic maxima; “push-off”/late-stance related).
  • Hip-hike: side-to-side difference in upward pelvic excursion at the tubera coxae (associated with unloading the lame limb).
• Outcomes also included temporal stride parameters (not detailed in the abstract’s results).
• Analysis: linear mixed models compared sound vs induced-lame conditions within each gait; differences reported as estimated marginal mean changes (mm); statistical significance as specified in the study.

Key findings

• PDmin (pelvic minimum asymmetry) increased with induced lameness in all tested situations except in-hand walk:
  • Walk ridden: PDmin +5.07 mm.
  • Trot in-hand: PDmin +10.72 mm; trot ridden: PDmin +9.85 mm.
  • Tölt ridden: PDmin +4.88 mm.
  • Walk in-hand: no significant PDmin change reported.
• PDmax (pelvic maximum asymmetry; push-off related) increased only for trot in-hand:
  • Trot in-hand: PDmax +4.80 mm.
  • No significant PDmax change was reported for ridden trot, ridden walk, or ridden tölt.
• Hip-hike increased on the lame hindlimb side:
  • Trot in-hand: +20.90 mm; trot ridden: +10.81 mm.
  • Tölt ridden: +4.28 mm.
• Overall pattern: trot (especially in-hand) produced the clearest, largest asymmetries; changes at tölt and ridden walk were smaller.

How to interpret PDmin, PDmax, and hip-hike

• PDmin (difference in pelvic minima):
  • Reflects asymmetry around the lowest pelvic point that occurs near hoof impact/early stance.
  • Increase indicates the horse is protecting the lame limb at loading, letting the pelvis drop less on that side.
  • Showed consistent sensitivity to induced hoof pain across gaits, strongest at trot.
• PDmax (difference in pelvic maxima):
  • Reflects asymmetry around the highest pelvic point near late stance/push-off.
  • Increase suggests reduced propulsion from the lame limb.
  • Only clearly increased at trot in-hand, implying push-off deficits were less consistently detectable in ridden gaits or tölt for this mild, hoof-associated pain.
• Hip-hike:
  • Represents greater upward excursion of the pelvis over the lame side, a classic strategy to unload the painful limb during stance.
  • Large increases at trot (in-hand and ridden) and smaller but detectable increases at tölt support its utility alongside PDmin.
• Gait mechanics context:
  • At trot (a running gait with higher limb loading), impact-related asymmetries (PDmin) are prominent and easier to detect.
  • At walk and tölt under saddle, overall limb loading is lower and the timing of the pelvic minimum differs relative to stance phases, blunting PDmin changes.
  • These biomechanical differences explain why trot is more diagnostically sensitive for mild hindlimb lameness.

Clinical implications for gait selection and measurement

• Prioritize trot for mild hindlimb lameness detection in Icelandic horses:
  • In-hand trot is most informative for both PDmin and PDmax and shows the largest hip-hike changes.
  • Ridden trot is also useful, though some effects are slightly smaller.
• Use PDmin and hip-hike as primary objective parameters:
  • Expect clear increases with hoof-associated pain at trot; PDmax may additionally help in-hand.
• Be cautious when relying on tölt or walk under saddle:
  • Only subtle PDmin and hip-hike changes may be visible; a normal-looking tölt does not exclude mild hindlimb pain.
  • If tölt is assessed, pair it with trot findings before drawing conclusions.
• Practical examination tips:
  • Evaluate on a straight line in-hand at trot when possible; add ridden trot if relevant to the horse’s use.
  • If available, use IMU-based systems to quantify PDmin/PDmax and hip-hike; if not, visually assess for increased hip-hike on the suspected lame side.
  • Repeat across sessions/surfaces to confirm consistency, as mild lameness signals can be subtle outside trot.

Strengths and limitations

• Strengths:
  • Within-horse comparison of sound vs induced-lame states improves sensitivity to change.
  • Objective IMU measurements minimize observer bias.
  • Both in-hand and ridden conditions across three gaits reflect common clinical scenarios for Icelandic horses.
• Limitations:
  • Lameness was transient and hoof-associated (sole pressure); findings may differ with other pathologies (e.g., proximal suspensory, stifle, hock).
  • Sample size was modest (n = 11); effect sizes and thresholds need replication.
  • Authors note the need to verify results in clinically lame Icelandic horses with varied diagnoses and severities.
  • Factors such as rider influence, speed control, and individual tölt style may affect asymmetry but were not detailed in the abstract.

What this means for future research

• Validate PDmin and hip-hike in clinically lame Icelandic horses across a spectrum of hindlimb conditions and severities.
• Determine diagnostic thresholds and accuracy (sensitivity/specificity) for PDmin, PDmax, and hip-hike in trot and tölt.
• Examine how speed, rider posture, saddle fit, and surface modify asymmetry metrics in tölt vs trot.
• Integrate IMU data with kinetic measures (force plates/instrumented shoes) to map pelvic minima/maxima to stance sub-phases in tölt and walk.
• Develop automated algorithms tailored to Icelandic gaits for real-time lameness screening.

Terminology and metric definitions

• Tölt: a four-beat, lateral, ambling gait characteristic of Icelandic horses, typically smooth with no aerial phase and lower peak limb loading than trot.
• In-hand vs ridden: handled from the ground (in-hand) versus under a rider (ridden), which can influence loading and symmetry.
• PDmin/PDmax: differences in pelvic vertical minima/maxima between left and right halves of the stride; PDmin relates to impact/early stance, PDmax to push-off/late stance.
• HDmin/HDmax: analogous head movement asymmetries, more commonly used for forelimb lameness; recorded here but not emphasized in the reported results.
• Hip-hike: side-to-side difference in upward pelvic movement over each hindlimb; larger on the lame side indicates unloading behavior.

Take-home points

• Trot is the preferred gait for detecting mild hindlimb lameness in Icelandic horses, especially in-hand.
• PDmin and hip-hike are the most reliable objective indicators; PDmax adds value mainly at in-hand trot.
• Ridden walk and tölt show only small asymmetry changes with mild hoof pain, so normal findings in these gaits do not rule out lameness.
• Objective IMU measurements can enhance detection and monitoring across gaits and contexts.
• Further validation in clinically lame horses with diverse pathologies is warranted before broad clinical standardization.