Asynchronous recovery of medial and lateral gastrocnemius mechanical properties and jump performance following localized plantar flexor fatigue.

OBJECTIVE

To investigate the acute and short-term (24 h) effects of localized plantar flexion fatigue on muscle mechanical properties, countermovement jump (CMJ) kinetics, and dynamic balance, focusing on recovery profiles of the gastrocnemius medialis (MG) and lateralis (LG).

DESIGN

Prospective, repeated-measures experimental study.

METHODS

Twenty-eight healthy volunteers (n = 28) performed a standardized unilateral isometric plantar flexion fatigue protocol until task failure (Borg CR-10 ≥ 8). Assessments were conducted at baseline, immediately post-fatigue, and 24 h post-fatigue (24 h). Outcomes included myotonometric properties (tone, stiffness, elasticity, relaxation time, and creep) of the MG and LG via MyotonPRO, CMJ kinetics via force plate, and dynamic balance via the Modified Star Excursion Balance Test (mSEBT).

RESULTS

Significant Time × Region interactions were found for stiffness (P = 0.038, ηp² = 0.276) and relaxation time (P = 0.024, ηp² = 0.171). Both muscle heads showed an acute decrease in stiffness (P < 0.001); however, LG stiffness recovered by 24 h (P = 0.330), while MG stiffness remained reduced (P < 0.05). Although CMJ height recovered at 24 h (P = 0.842), eccentric and braking phase durations remained prolonged (P < 0.001; ηp² = 0.357 and ηp² = 0.264), indicating altered neuromuscular strategies. For dynamic balance, a delayed impairment was observed in anterior reach distance, which declined only at 24 h compared to baseline (P = 0.021, ηp² = 0.133).

CONCLUSION

Localized plantar flexion fatigue induces region-specific mechanical alterations and asynchronous recovery. Restoration of jump height at 24 h masks persistent deficits in eccentric phase durations and MG stiffness. These findings highlight a "neuromuscular vulnerability window" at 24 h post-fatigue, where delayed impairments in MG mechanics and anterior stability may increase potential susceptibility to injury despite apparent recovery. Practically, monitoring head-specific muscle mechanics, rather than global performance outcomes, is critical during this 24 h window to optimize training modification and guide objective return-to-play decisions.

TRIAL REGISTRATION: NCT07457398 (Registration date: February 25, 2026).