Background: Exercise-induced fatigue (EIF) and exercise-induced muscle damage (EIMD) impair neuromuscular performance and delay post-exercise recovery. However, human evidence regarding the structural and functional recovery kinetics following Lactiplantibacillus plantarum GKK1 supplementation remains limited. Objective: This randomized, double-blind, placebo-controlled trial investigated whether 4 weeks of L. plantarum GKK1 supplementation improves functional recovery and modulates biochemical responses after an EIMD protocol in healthy men. Methods: Forty-eight healthy men with no regular exercise habits were randomly assigned to receive either placebo (n = 24) or L. plantarum GKK1 (two capsules daily, totaling 1.0 × 10¹¹ CFU; n = 24) for 28 consecutive days. The trial was registered at ClinicalTrials.gov (NCT06893549). After supplementation, participants completed 100 maximal plyometric jumps. Countermovement jump (CMJ), isometric mid-thigh pull (IMTP), and Wingate anaerobic performance were assessed before EIMD and at 3, 24, and 48 h post-EIMD. Blood biomarkers of muscle damage, inflammation, oxidative stress, endocrine response, and sympathoadrenal activity were analyzed, and urinary 3-methylhistidine and creatinine were measured. Results: Compared with placebo, GKK1 supplementation was associated with smaller post-exercise decrements in the CMJ rate of force development, relative peak force, jump height, IMTP relative peak force and peak RFD, and Wingate anaerobic performance (p < 0.05). GKK1 also attenuated post-exercise increases in CK, myoglobin, hs-CRP, and TBARS, and was associated with more favorable testosterone, HGH, catecholamine, and dopamine responses. At 24 h post-EIMD, urinary 3-methylhistidine and the 3-methylhistidine/urinary creatinine ratio were lower in the GKK1 group than in the placebo group. No adverse changes were observed in clinical safety biomarkers. Conclusion: These findings suggest that GKK1 may be a safe nutritional strategy to facilitate functional recovery, attenuate secondary inflammatory responses and modulate urinary markers associated with muscle breakdown following muscle-damaging exercise in healthy young men.

While high-intensity interval training (HIIT) enhances physical conditioning, its specific impact on the TMG-derived mechanical properties of rowing-related muscles and sport-specific performance remains underexplored. This study investigated the effects of a 4-week rowing ergometer-based HIIT intervention on 2000-m rowing performance and muscle mechanical properties, evaluated via tensiomyography (TMG). Seventeen rowers were randomized into the HIIT group (n = 9) or the control group (CON, n = 8). Over four weeks, the HIIT group completed two weekly rowing ergometer HIIT sessions (ten 60-s all-out sprints, 30-s recovery), while the CON group continued regular training without additional HIIT. Pre- and post-intervention, participants underwent a 2000-m ergometer trial and TMG assessments across five rowing-related muscles. The HIIT group demonstrated a 3.1% reduction in 2000-m completion time, together with increases in mean power output (7.7%) and stroke rate (6.5%) (group × time interaction η²p = 0.53, 0.39, and 0.27, respectively; p < 0.05). TMG analysis showed shortened delay time (Td) and half-relaxation time (Tr) across all targeted muscles after false discovery rate correction. Contraction time (Tc) was shortened in the tibialis anterior and vastus medialis, while the erector spinae showed a nominal uncorrected change that should be interpreted cautiously. Maximal displacement (Dm) and sustain time (Ts) remained unchanged. These findings suggest that short-term rowing-specific HIIT is associated with improved 2000-m performance and TMG-derived peripheral temporal properties; however, unchanged Dm should be interpreted only as no detectable change in radial displacement under the present TMG conditions rather than definitive evidence of unchanged muscle stiffness. Because of the small sample size, unbalanced total training load between groups, and the absence of direct physiological or biomechanical measurements such as VO₂max, lactate, electromyography, and stroke biomechanics, these findings should be interpreted as preliminary.