Longitudinal changes in Achilles tendon and triceps surae muscle architecture during a 156-km mountain ultramarathon

This study aimed to assess the longitudinal changes in triceps surae muscle-tendon architecture during a mountain ultramarathon. Experienced trail runners [n = 55, 78% men, age: 45.2 (13.5) yr] participated in a 156-km trail run (6,000 m climbing) consisting of six 26-km laps. The resting architectural properties of triceps surae muscle-tendon were measured using ultrasound imaging for Achilles tendon cross-sectional area (AT CSA), medial gastrocnemius muscle pennation angle, thickness, length, and fiber length. Measurements were performed the day before the race (baseline), at 52 km (T1), at 104 km (T2), at 156 km (T3), and 12 h after the race (H12). Among finishers (n = 41), there was a significant biphasic change in AT CSA during the race (P = 0.001). First, a significant decrease in AT CSA occurred between baseline and T1 (P = 0.006), with a greater decrease for participants averaging speed >8 km/h (P = 0.014). Second, there was a significant increase in AT CSA especially between T2 and T3 (P = 0.006) that was correlated with a decrease in average speed (P = 0.001) and alteration of spaciotemporal running parameters (P < 0.05). Changes in muscle-tendon architecture were not significantly different between finishers (n = 41) and nonfinishers (n = 14). In 47 participants (85.5%) who completed the follow-up, AT CSA at H12 was greater compared with baseline (P = 0.010). The main finding is the significant and biphasic modification of the AT CSA during a 156-km mountain ultramarathon with an initial decrease corresponding to mechanical stress followed by a secondary increase suggesting adaptive mechanotransduction persisting after 12 h.NEW & NOTEWORTHY Achilles tendon cross-sectional area (AT CSA) demonstrated significant adaptive modifications during a 156 km mountain ultramarathon in trained athletes. Initially, a decrease in AT CSA, especially at higher running speeds, is consecutive to the biomechanical stress on the plantar flexor muscle-tendon unit (MTU). Subsequently, there is a significant increase in AT CSA persisting up to 12 h after the race, which likely corresponds to an adaptive process to limit the compressive and tensile load on the tendon.