Acute Effects of High-Speed Repetitive Loading on Achilles Tendon Viscoelasticity and Ankle Agonist–Antagonist Co-Contraction: A Combined MyotonPRO and Surface Electromyography Analysis in Male Collegiate American Football Players

Background: Achilles tendon viscoelasticity and ankle neuromuscular co-contraction are functionally linked, yet how they jointly adapt during brief high-speed repetitive loading remains unclear. We simultaneously quantified Achilles tendon viscoelasticity (MyotonPRO) and ankle agonist–antagonist co-contraction (surface electromyography, sEMG) under two stance conditions and three repetition counts. Methods: Twenty-seven active male collegiate American football players (23.19 ± 2.63 years) performed rapid heel-raise/landing cycles in a bilateral ground stance (BGS) and a unilateral box stance (UBS) at 1, 5, and 10 repetitions. Tendon decrement (D), stiffness (S), and tone (F), the co-activation index (CoAct), and the co-contraction index (CCI) were analyzed using two-way (2 × 3) repeated-measures ANOVA. Results: D decreased significantly as repetitions increased (pooled across stance conditions: 1 rep 0.973 → 10 reps 0.937; F (2, 52) = 4.68, p = 0.014, ηp2 = 0.153), indicating improved elastic-recovery efficiency, whereas S and F remained stable. CoAct was maintained constantly at ≈72% regardless of stance or repetition (all p > 0.05), and the medial gastrocnemius was consistently activated first (agonist-led feedforward priming). Tissue-level elastic adaptation thus occurred without a concurrent change in neuromuscular co-contraction—a tissue–neural dissociation confirmed in both change scores and correlation structure. Conclusions: Acute enhancement of Achilles tendon elasticity during high-speed repetition appears to be a peripheral, tissue-level mechanical phenomenon expressed independently of neuromuscular co-contraction. Combined myotonometric–sEMG assessment reveals functional information not captured by imaging alone.