Equine upper limb kinematics: Comparison between biplanar high-speed fluoroscopic kinematography and optoelectronic motion capture in one Mini Shetland Pony.

Equine proximal limb skeletal movements were analyzed in a Mini Shetland Pony using two methods—biplanar high-speed fluoroscopy (FluoKin) and optoelectronic motion capture (OMC)—revealing that FluoKin detected greater ranges of motion than OMC, highlighting limitations in skin marker-based methods.

Background and Motivation

• There is a lack of studies describing the kinematics of the proximal limb skeleton (shoulder, hip region) in living horses and related equids.
• In horses, the scapula does not rotate around a fixed pivot, but rather an instantaneous center of rotation—a complex movement pattern seen in all therian mammals.
• This complex scapular movement challenges traditional kinematic analysis techniques that rely on skin markers placed on the surface, as skin movement relative to bone can cause inaccuracies.
• The study aims to improve accuracy in measuring bone movement by overcoming limitations related to skin marker displacement errors through advanced imaging techniques.

Research Objective

• To describe in detail the skeletal movement of the proximal limbs in a living equid (Mini Shetland Pony) during locomotion.
• To compare two measurement methods: optoelectronic motion capture (OMC) and biplanar high-speed fluoroscopic kinematography (FluoKin).
• To evaluate the range of motion (ROM) differences detected by these two methods.

Methodology

• Simultaneous measurement of limb movement using:
  • Optoelectronic motion capture (OMC) — a non-invasive method using skin markers and cameras to track motion.
  • Biplanar high-speed fluoroscopic kinematography (FluoKin) — an X-ray based imaging technique that visualizes bones directly with two synchronized fluoroscopic cameras, allowing precise tracking of skeletal movement.
• The subject was a single Mini Shetland Pony walking and trotting on a treadmill to ensure controlled and repeatable gaits.
• Data analysis was conducted using scientific rotoscoping, a technique aligning fluoroscopic images with 3D bone models for precise motion quantification.

Key Findings

• Range of Motion (ROM) measured using FluoKin:
  • Scapula ROM during walking: 27.6° (± 0.9°)
  • Scapula ROM during trotting: 23.3° (± 3.7°)
  • Hip joint ROM during walking: 29.5° (± 2.3°)
  • Hip joint ROM during trotting: 33.2° (± 3.8°)
• Comparing FluoKin with OMC:
  • FluoKin consistently measured higher ROM values than OMC.
  • The greatest discrepancies were found in the elbow and stifle joints, with ROM differences up to 33.0° and 28.1°, respectively.
  • This suggests that OMC underestimates actual bone movement likely due to skin marker errors caused by skin displacement relative to underlying bones.

Conclusions and Implications

• The study provides the first detailed insights into the proximal limb skeletal kinematics of a live pony during walking and trotting.
• FluoKin proves to be a superior method for accurately capturing the complex rotations and movements of proximal limb bones compared to traditional skin marker-based OMC.
• Findings highlight the limitations of using skin markers in animals where the scapula and other limb bones do not move around fixed pivot points, emphasizing the necessity of bone-based imaging for precise biomechanical studies.
• The improved accuracy in measuring equine limb kinematics could inform veterinary diagnostics, rehabilitation, and the design of assistive devices or treatment strategies in horses and related species.