Sensitivity of Structural and Mechanical Markers of Muscle and Tendon Damage Responses After Eccentric Plantar Flexor Exercise.

Vasenina, E, Fukuda, DH, Livingston, JJ, Antonio, BB, Herda, AA, Mangum, LC, and Stout, JR. Sensitivity of structural and mechanical markers of muscle and tendon damage responses following eccentric plantar flexor exercise. J Strength Cond Res 40(3): 293-303, 2026-This study aimed to investigate the acute structural and functional responses of the calf musculature and Achilles tendon following an eccentric exercise-induced muscle damage (EIMD) protocol and to evaluate the sensitivity of diverse assessment tools. Twenty young, recreationally active men performed a unilateral EIMD protocol consisting of 4 × 50 heel drops plus a final set to failure wearing a weighted vest equal to 40% of body mass, while the contralateral leg served as a control. Assessments included gastrocnemius and Achilles tendon thickness and length (ultrasound), calf circumference, pressure pain threshold, isometric strength, myotonography, and global/local pain ratings. Measurements were collected at baseline and 0, 24, 48, and 72 h postexercise. Significant visit × leg interactions ( p < 0.05) were observed across most markers. Gastrocnemius thickness increased by 22.4% immediately after and remained 12.9% elevated at 72 h. Achilles tendon thickness decreased by 21.7% postexercise, while tendon length increased by 12%. Isometric strength declined 15-25% at 24-48 h. Calf creep and relaxation times also decreased >15% across time points. Numerical Pain Rating Scale peaked at 48 h (mean increase of +37 points). Pressure pain threshold was significantly lower in the intervention leg at all time points ( p < 0.01). Eccentric plantar flexor exercise induced significant changes in muscle-tendon structure and function. Gastrocnemius thickness, tendon morphology, strength, myotonographic properties, and pain ratings were among the most sensitive indicators. These findings highlight the value of using a multimodal battery, including ultrasound, myotonography, strength testing, and pain ratings, to comprehensively monitor recovery after eccentric exercise.