High-intensity 5 km running offers an ideal framework to analyze the organism's multidimensional responses. Since previous research primarily analyzed isolated aspects of fatigue, this study aimed to examine the integrated acute neuromuscular, metabolic, and perceptual responses to a 5 km run. Twenty-one recreational male runners participated. Pre- and post-race assessments included body composition, blood lactate, m. rectus femoris ultrasound thickness, quadriceps maximal voluntary isometric contraction (MVIC), heart rate, perceived exertion (Borg CR10), and 5 km finish time. Statistical analysis was performed in the Jamovi software,utilizing descriptive statistics, the Shapiro–Wilk test of normality, the Wilcoxon signed-rank test with effect size r calculation, and Spearman’s correlation coefficient, at a significance level of p < 0.05. Post-race measurements revealed a significant decrease in quadriceps MVIC (pre: 305.26 ± 98.83 N vs. post: 258.85 ± 88.47 N; p = 0.002) and an increase in blood lactate (pre: 0.81 ± 0.35 vs. post: 6.90 ± 1.44 mmol/L; p < 0.001), alongside high average heart rates (165 ± 16 bpm). However, ultrasound-assessed muscle architecture remained unchanged. The 5 km run induced pronounced neuromuscular and metabolic fatigue. Unchanged muscle architecture suggests that acute strength decline is primarily mediated by metabolic and neural mechanisms, rather than immediate structural-morphological factors.

High-intensity 5 km running offers an ideal framework to analyze the organism’s multidimensional responses. Since previous research primarily analyzed isolated aspects of fatigue, this study aimed to examine the integrated acute neuromuscular, metabolic, and perceptual responses to a 5 km run. Twenty-one recreational male runners participated. Pre- and post-race assessments included body composition, blood lactate, m. rectus femoris ultrasound thickness, quadriceps maximal voluntary isometric contraction (MVIC), heart rate, perceived exertion (Borg CR10), and 5 km finish time. Statistical analysis was performed in the Jamovi software, utilizing descriptive statistics, the Shapiro–Wilk test of normality, the Wilcoxon signed-rank test with effect size calculation, and Spearman’s correlation coefficient, at a significance level of p < 0.05. Post-race measurements revealed a significant decrease in quadriceps MVIC (pre: 305 ± 99 N vs. post: 259 ± 88 N; p = 0.002) and an increase in blood lactate (pre: 0.8 ± 0.4 vs. post: 6.9 ± 1.4 mmol/L; p < 0.001), alongside high average heart rates (165 ± 16 bpm). However, ultrasound-assessed muscle architecture remained unchanged. The 5 km run induced pronounced neuromuscular and metabolic fatigue. Unchanged muscle architecture suggests that acute strength decline is primarily mediated by metabolic and neural mechanisms, rather than immediate structural–morphological factors. These findings highlight the value of an integrated assessment approach for understanding acute fatigue responses following high-intensity 5 km running and may contribute to more precise training-load prescription and recovery monitoring in recreational runners.

Acute neuromuscular responses to running may vary according to exercise duration and physiological demands. This study compared changes in vastus medialis contractile properties, quadriceps maximal voluntary isometric contraction (MVIC), and interlimb asymmetry following 5 km and half-marathon running trials. Twenty-eight male recreational runners (36.96 ± 11.35 years) completed both trials 10 days apart. Maximal radial displacement (Dm), contraction time (Tc), and contraction velocity (Vc) were assessed using tensiomyography, while quadriceps MVIC was assessed using isometric dynamometry before and after each trial. Absolute interlimb asymmetry and its direction were also analysed. The unadjusted analysis showed a greater pre-to-post increase in Dm following the half-marathon; however, the effect of the running trial on post-running Dm was not significant after adjustment for baseline Dm. Tc and MVIC decreased after both trials, whereas Vc increased, with no significant running trial × time interactions for these outcomes. TMG- and MVIC-derived absolute interlimb asymmetry remained unchanged. Acute post-running responses differed according to the neuromuscular characteristic assessed rather than consistently according to running distance. Changes in contractile properties and maximal force-generating capacity were not accompanied by a systematic increase in the magnitude of between-leg differences.