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

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.