Associations between Internal Load, Respiratory Metabolism, and Neuromuscular Activation during 1v1 Small-Sided Soccer Games

Small-sided soccer games (SSGs) are widely used in football training to simultaneously improve players' technical skills and physical fitness. However, the mechanistic relationship among internal load, respiratory metabolism, and neuromuscular activation during SSGs remains unclear. To examine the relationship between internal load and lower-limb neuromuscular activation during 1v1 SSGs, and to determine whether respiratory-metabolism variables mediate this association. A repeated-measures design was adopted. 60 physical education students (age: 18.75 ± 0.45 years; training experience: 3.27 ± 3.3 years; body mass: 69.35 ± 8.17 kg) completed a standardized 1v1 SSG protocol (8 × 1 min bouts with 1 min passive recovery). Internal load was quantified using the Firstbeat system - training impulse per minute (TRIMP·min^-1), total energy expenditure (EE_total), and heart rate (HR; average and peak) - while oxygen uptake (VO₂ mL·kg^-1·min^-1), minute ventilation (VE, L·min^-1), and respiratory rate (RespR, breaths·min^-1) were continuously estimated. Surface electromyography (sEMG) was recorded bilaterally from the rectus femoris (RF) and biceps femoris (BF), normalized to maximal voluntary contraction (MVC), and analyzed for root-mean-square amplitude (RMS), averaged EMG amplitude (aEMG), integrated EMG (iEMG), and median frequency (MF). Mediation analyses were performed to assess whether respiratory metabolism parameters mediated the effect of internal load on neuromuscular activation. Internal load indices showed consistent positive correlations with respiratory-metabolic variables, ranging from r = 0.436 between EE Total and Average VO₂ to r = 0.944 between Peak HR and Average VE (all p < 0.001). For BF, average VO₂ VE, and RespR correlated strongly with HR (r = 0.864-0.938); for RF, TRIMP correlated significantly with Peak VO₂ (r = 0.864, p < 0.001). BF metabolic indices were moderately correlated with MF (r = 0.268-0.340, p < 0.05), and RF average VO₂ correlated with aEMG (r = 0.28, p < 0.05). Mediation analysis revealed that:(1) BF models: The direct effects of HR (average/peak) on BF_MF were nonsignificant. In contrast, indirect effects via RespR, VE, and VO₂ were significant (p < 0.05), indicating full mediation by respiratory metabolism. (2) RF models: TRIMP·min^-1 had a significant positive direct effect on RF EMG indices (RMS, aEMG, iEMG; p < 0.05) and a significant negative indirect effect via Peak VO₂ (95% CI excluding 0), showing a "positive direct + negative indirect" dual-path mechanism. (3) Energy model: The effect of EE Total on RF_aEMG was fully mediated by Average VO₂ with no direct effect. Internal load and respiratory-metabolic responses showed consistent positive coupling during SSGs, forming a physiological basis for exercise performance. The BF muscle appeared to rely largely on respiratory-metabolic mediation for EMG frequency modulation, reflecting sensitivity to metabolic state. In contrast, RF activation was influenced by both direct internal load and indirect metabolic pathways. Findings suggest that BF activity may depend more on metabolic status, whereas RF activation may reflect combined influences of load intensity and metabolic mediation, providing insight for more precise assessment of load and muscle function in soccer contexts.