Validity and Reliability of a Novel 3D Ultrasound for Gastrocnemius Medialis and Achilles Tendon Lengths

Overview

This study developed and validated a novel 3D ultrasound technique combining 2D imaging, 3D motion capture, and vector algebra to measure static lengths of the gastrocnemius medialis muscle–tendon unit (MTU), muscle belly, and Achilles tendon in vivo. The approach demonstrated adequate accuracy compared to MRI and showed high intra- and inter-rater reliability, supporting its use for assessing muscle and tendon morphology.

Background

Muscle–tendon units (MTUs) are fundamental components of the musculoskeletal system and exhibit plasticity in response to growth, loading, and pathology. Accurate assessment of individual muscle belly and tendon lengths is critical for diagnosing contractures and monitoring structural adaptations or treatment effects. Conventional 2D ultrasound has limitations in measuring whole MTU lengths, prompting the development of advanced ultrasound methods. However, many existing approaches lack validation against gold standards like MRI and have practical constraints such as requiring immobility and simplifying tissue properties to 2D.

Data Highlights

Key Findings

• The novel 3D ultrasound approach accurately measured static lengths of the GM MTU, muscle belly, and Achilles tendon compared to MRI.
• Intra-rater reliability was high, indicating consistent measurements by the same investigator across sessions.
• Inter-rater reliability was also high, demonstrating reproducibility between different investigators.
• The method combined 2D ultrasound imaging with 3D motion capture and vector algebra to overcome limitations of traditional 2D ultrasound.
• Use of anatomical landmarks and reflective markers ensured precise localization of muscle–tendon junctions and insertion points.

Clinical Implications

This validated 3D ultrasound technique offers a practical, non-invasive tool for clinicians to assess muscle and tendon lengths with high accuracy and reliability. It facilitates monitoring of structural adaptations and treatment effects in conditions affecting the gastrocnemius medialis and Achilles tendon. The method's ease of application and avoidance of MRI constraints may improve patient evaluation and therapy planning.

Conclusion

The study confirms that the novel 3D ultrasound approach is a valid and reliable method for in vivo measurement of gastrocnemius medialis muscle and Achilles tendon lengths. It holds promise for enhancing clinical assessment and research of muscle–tendon morphology.

References

1. Study Authors/University of Graz/2024 -- Assessment of a Novel 3D Ultrasound Technique for Evaluating Static Lengths and Lengthening Characteristics of the Gastrocnemius Medialis Muscle and Achilles Tendon In Vivo