Physiological determinants of heart rate responses and bout-to-bout stability during 2v2 and 4v4 small-sided games in football players

Purpose Injury prevention in sport is increasingly recognized as a health priority, and monitoring internal load during routine training offers a pragmatic pathway to reduce overuse risk and support long-term athlete health. This study examined the extent to which three practical physiological markers (resting heart rate (HRrest), submaximal warm-up heart rate after 60-s recovery (HRR), and Yo-Yo IR1 HRmax) explain mean heart rate (HRmean) and bout-to-bout variability (CV%) during two commonly used small-sided game (SSG) formats (2v2 and 4v4) in football players. Methods Forty male amateur players completed HRrest, HRR, and Yo-Yo IR1 testing, followed by randomized, counterbalanced 2v2 and 4v4 SSG sessions (3 × 3-min bouts). HRmean and CV% were computed for each format. Associations were assessed using Pearson correlations and multiple linear regression. Results HRmean did not differ significantly between 2v2 and 4v4 (166.9 ± 5.1 vs. 165.3 ± 6.0 bpm; t₃₉ = 1.35, p = 0.19), whereas CV% was lower in 2v2 (2.05 ± 1.16%) than 4v4 (2.85 ± 1.43%; t₃₉ = −2.52, p = 0.016). In 2v2, no physiological marker correlated with HRmean (all p > 0.30). In 4v4, HRR showed a moderate significant correlation with HRmean ( r = 0.36, p = 0.023). Regression models explained 3.1% (2v2) and 16.3% (4v4) of HRmean variance, with HRR emerging as the only significant predictor in 4v4 ( β = 0.20, p = 0.047). In a repeated-measures linear mixed-effects model (random intercept for player; fixed effect for format), the resting HR × format interaction for CV% was significant ( p = 0.016). Conclusion HRR is the sole physiological marker meaningfully associated with internal load during SSGs (and only in 4v4) while neither HRmean nor CV% in 2v2 is explained by the tested markers. More constrained formats (2v2) produce more stable cardiovascular responses. By identifying simple heart-rate–based indicators and game formats that yield more stable cardiovascular responses, these findings can inform scalable load-management strategies that help mitigate maladaptive overreaching and contribute to broader injury-prevention efforts in athletic populations, although future studies are necessary to confirm such hypothesis.