Between-Day Reliability of Markerless Motion Capture for Single-Leg Squat and Landing Tasks

Summary

Can markerless motion capture be trusted for repeated athlete monitoring? A 2025 study published in the International Journal of Sports Physical Therapy evaluated the between-day reliability of markerless motion capture during single-leg squats and single-leg landing tasks. Using Theia3D, researchers found low measurement error and good-to-moderate reliability, with performance comparable to traditional marker-based motion capture, supporting its use for repeated athlete monitoring and biomechanical reliability research.

Editor's Note: The following summary details independent academic research conducted in clinical research settings. Theia3D is an offline software solution engineered exclusively for research and human performance analysis.

Why This Matters

Markerless motion capture makes it possible to collect biomechanical data more easily and more frequently during tasks associated with injury risk, such as single-leg squats and landings. However, for this data to be useful in practice, measurements must be reliable across testing sessions.
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Between-day reliability is especially important for athlete monitoring and recovery research, where practitioners need to distinguish true biomechanical change from normal measurement error. Without reliable measurements, it becomes difficult to interpret whether changes in movement patterns reflect adaptation, recovery, or simply noise in the data.

This study directly addresses a critical question for sports scientists and practitioners: Can markerless motion capture reliably quantify trunk and lower-limb kinematics during single-leg tasks across multiple days?
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Study Overview

This study assessed the between-day reliability of trunk and lower-limb kinematics during single-leg squat and single-leg landing tasks using markerless motion capture with Theia3D (v2022.1.0.2309).

Markerless Motion Capture: 19 recreational athletes, two sessions one week apart, 8-camera Qualisys Miqus setup, 100 Hz.

Marker-Based Motion Capture: 10 different recreational athletes, identical tasks, 10-camera Qualisys Oqus, 100 Hz.

Tasks: Single-leg squat, forward single-leg landing, medial single-leg landing.

Key Findings
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Low Measurement Error Across Most Variables

Theia3D demonstrated acceptable absolute reliability, with RMSD and SEM values below 5° for most joints and planes. Hip flexion showed higher between-day error (~5–7°), consistent across both markerless and marker-based systems.

Moderate to Good Relative Reliability
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Single-leg squat: mean ICC = 0.77; Forward landing: mean ICC = 0.83; Medial landing: mean ICC = 0.80. Most kinematic variables met accepted thresholds for sports science applications.

Comparable Performance to Marker-Based
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57 out of 80 discrete variables showed lower SEM and MDC values for markerless vs. marker-based, suggesting Theia3D can achieve reliability on par with or better than traditional lab-based systems.

What This Means for Sports Scientists and Practitioners

This study demonstrates that Theia3D can reliably capture trunk and lower-limb kinematics during single-leg tasks commonly used for biomechanical monitoring, recovery research, and return to training monitoring.

Read the full study here.
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Interested in Reliable Markerless Athlete Monitoring?
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If you’re evaluating motion capture tools for biomechanical monitoring, recovery research tracking, or performance monitoring, Theia3D provides validated, repeatable biomechanics data without markers or wearables.

Contact us today to learn how markerless motion capture can fit into your workflow.

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