DOI: 10.32098/mltj_2026_6512

Reliability and relationships between plantar flexor musculotendinous unit assessments: Ultrasonography, myotonography, and isometric strength

Authors

  • Ecaterina Vasenina Human Performance Lab, Department of Health Sciences and Human Performance, The University of Tampa, Tampa, USA
  • David H. Fukuda Physiology of Work and Exercise Response (POWER) Laboratory, Institute of Exercise Physiology and Rehabilitation Science, University of Central Florida, Orlando, USA
  • Jack J. Livingston Physiology of Work and Exercise Response (POWER) Laboratory, Institute of Exercise Physiology and Rehabilitation Science, University of Central Florida, Orlando, USA
  • Ashley A. Herda Department of Health, Sport, and Exercise Sciences, University of Kansas Edwards Campus, Overland Park, USA; Department of Orthopedic Surgery and Sports Medicine, University of Kansas Medical Center, Kansas City, USA
  • Colby L. Mangum REhabilitation, Athletic Assessment, & DYnamic Imaging (READY) Laboratory, Institute of Exercise Physiology and Rehabilitation Science, University of Central Florida, Orlando, USA
  • Jeffrey R. Stout Physiology of Work and Exercise Response (POWER) Laboratory, Institute of Exercise Physiology and Rehabilitation Science, University of Central Florida, Orlando, USA

evasenina@ut.edu

Keywords

Plantar flexor, muscle thickness, Achilles tendon, reliability, isometric force

Abstract

Objectives: The objective of this study was to determine the test-retest reliability of gastrocnemius muscle thickness and stiffness measurements, maximal isometric force using seated and standing plantar flexion, and Achilles tendon length, thickness, and stiffness. Additionally, we aimed to assess the correlations between musculotendinous unit properties and unilateral isometric force assessments of the plantar flexor muscles.

Methods: Twenty physically active males (23.79 ± 3.64 years, 175.56 ± 9.63 cm, and 83.41 ± 16.09 kg) completed the study. Participants underwent anthropometric assessments, ultrasound imaging for gastrocnemius muscle thickness and Achilles tendon properties, and myotonography for mechanical tissue characteristics. Unilateral standing and seated isometric plantar flexion tests were conducted using a force plate. Reliability was determined using intraclass correlation coefficients (ICC3,k) and coefficients of variation (CV). Stepwise regression assessed relationships between musculotendinous properties and isometric peak force.

Results: Achilles tendon length, calf muscle thickness, and calf circumference exhibited excellent reliability (ICC >0.9), while Achilles tendon thickness (ICC = 0.89), calf echo intensity (ICC = 0.83), and seated isometric peak force (ICC = 0.93) demonstrated good reliability. Standing isometric peak force (ICC = 0.83) also showed good reliability, while calf stiffness (ICC = 0.96), calf frequency (ICC = 0.97), and Achilles tendon stiffness (ICC = 0.78) exhibited good to excellent reliability. Calf decrement correlated with standing peak force (R2 = 0.23, p = 0.018), while Achilles decrement was associated with gastrocnemius muscle thickness (R2 = 0.374, p = 0.004).

Conclusion: Achilles tendon length, gastrocnemius muscle thickness, and stiffness are reliable for plantar flexor assessment. Relationships between muscle-tendon properties and force production highlight their structural influence. Standardized protocols are recommended for improved consistency.

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Published

2026-03-31

Issue

Vol. 16 No. 1 (2026)

How to Cite

1.
Vasenina E, Fukuda DH, Livingston JJ, Herda AA, Mangum CL, Stout JR. Reliability and relationships between plantar flexor musculotendinous unit assessments: Ultrasonography, myotonography, and isometric strength. MLTJ [Internet]. 2026 Mar. 31 [cited 2026 Nov. 1];16(1). Available from: https://mltj.online/index.php/mltj/article/view/6512