Effects of age on intramuscular fascicle kinematics of the medial gastrocnemius muscle of track and field Masters athletes

Aging results in a decline in muscle mass and intrinsic mechanical properties, yet its impact on intramuscular force transmission and fascicle kinematics remains poorly understood. We investigated whether age, sex, and habitual high-intensity athletic training modulate medial gastrocnemius (GM) muscle mechanics in 172 competitive masters athletes (n=108 male, n=64 female; aged 35–90 years). Participants performed ramp plantar flexion to maximum voluntary contraction (MVC) while GM architecture was assessed via B-mode ultrasound. Fascicle length, pennation angle, fascicle rotation, and strain were quantified at rest and during contraction. Resting pennation angles did not vary with age ( p≥0.30) or sex ( p≥0.39). During contraction, superficial, but not deep fascicle rotation (change in pennation angle) decreased by 6.6±51.3% per decade ( p=0.003), whereas deep fascicle rotation remained stable ( p=0.27). Furthermore, resting fascicle length showed a significant age-related decline of 1.8±13.7% per decade ( p=0.002). Finally, Achilles tendon force decreased with age (-4.3±16.3% per decade, p<0.001), independent of sex ( p=0.12). Exploratory subgroup analyses showed that while athletic specialization (endurance vs. power) did not significantly influence fascicle kinematics, high-performing athletes (>85% age grade) maintained significantly greater superficial fascicle rotation than lower-performing peers (p=0.027). These findings indicate that while age-related alterations in superficial fascicle kinematics occur even in well-trained individuals, preserved architectural gearing remains a distinct marker of elite athletic performance. In conclusion, our results suggest that aging alters intramuscular force transmission pathways independently of lifelong training status, highlighting the limits of exercise in fully mitigating structural neuromuscular aging.