The effect of full-limb flexion tests on static and dynamic muscle activity and locomotion asymmetry in owner-sound horses.

Overview

• This study investigates how full-limb flexion tests impact muscle activity and movement symmetry in horses that show no lameness.
• The research aims to understand whether these commonly used veterinary tests affect muscle function or gait asymmetry during trotting.

Introduction and Purpose

• Context: Flexion tests are commonly used in equine veterinary examinations to detect potential locomotor problems by flexing a horse’s limb and then observing its gait.
• Problem: The neuromuscular effects of these tests are not well understood, resulting in variability in responses and questions about their reliability for diagnosing lameness or evaluating horses pre-purchase.
• Objective: To study how full-limb flexion affects:
  • Static muscle activity — muscle activation during the limb being held in a flexed position, and
  • Dynamic muscle activity — muscle activation during the subsequent trot.
• Additionally, to assess if the flexion test impacts locomotion asymmetry during trotting.

Methods

• Subjects: Sixteen warmblood horses, all owner-sound (no lameness), randomly assigned to four groups (4 horses per group).
• Data collection: Surface electromyography (EMG) electrodes monitored muscles in the limb regions during tests:
  • Forelimbs: Splenius, Triceps brachii caput longus, Longissimus dorsi, Extensor digitorum communis, Ulnaris lateralis muscles.
  • Hindlimbs: Gluteus medius, Biceps femoris, Semitendinosus, Extensor digitorum longus, Flexor digitorum profundus muscles.
• Procedures:
  • Each horse underwent a 60-second full-limb flexion test, holding the limb in a flexed position.
  • Immediately following, horses were trotted in a straight line to assess dynamic responses.
• Measurements:
  • EMG data were collected and normalized side-to-side to allow comparison.
  • Inertial measurement units (IMUs) collected kinematic data assessing asymmetry in head, withers, and pelvis movement.
• Statistical analysis:
  • One-way repeated measures ANOVA compared muscle activity during the static flexion phase versus baseline.
  • Two-way repeated measures ANOVA with statistical parametric mapping analyzed dynamic muscle activity during trot-up compared to baseline.
  • Linear mixed-effect models assessed differences in kinematic asymmetry measures.

Results

• Muscle activity: No significant changes were found in the normalized muscle activation levels in either forelimb or hindlimb muscles during:
  • The static flexion phase.
  • The dynamic trotting phase following flexion.
• Locomotion asymmetry:
  • Pelvic movement during the suspension phase (when the limb is lifted) increased in vertical displacement on the same side as the flexed limb.
  • Specifically, maximum vertical movement differences after left and right hindlimb flexion tests were approximately 8.4 mm and 8.1 mm respectively, which were statistically significant.
  • No significant asymmetry changes were noted in head or withers movement.

Conclusions and Limitations

• Conclusions:
  • Full-limb flexion tests do not significantly alter muscle activation patterns in the muscles studied, neither statically during flexion nor dynamically during trotting after flexion.
  • The significant pelvic displacement observed suggests that flexion can influence gait mechanics slightly, though not via altered muscle activation in the measured muscles.
  • This suggests the neuromuscular function is largely unaffected by these flexion tests in clinically sound horses.
• Limitations:
  • Only full-limb flexion tests were analyzed; other more localized tests (distal limb, or specific joint—carpal or tarsal) were not included.
  • The force applied during flexion was not standardized, which may influence reproducibility of results.
  • The study focused on a specific set of muscles and may not reflect the response of all muscles involved in limb movement.

Implications

• The results call into question the reliability of flexion tests to induce neuromuscular changes observable by EMG in unaffected horses, which may impact interpretations in lameness or purchase evaluations.
• While gait asymmetry was detected at the pelvis, the functional significance and clinical relevance require further investigation.
• Veterinarians should consider these findings when using full-limb flexion tests to assess equine locomotion issues.