Objectives: We examined: (1) relationships among sub-maximal heart rate (sub-maxHR) following 4 minutes of the Yo-Yo intermittent recovery test level 1 (IR1) and maximal-effort performance (distance covered at volitional exhaustion), (2) test re-test reliability of sub-maxHR, and (3) whether changes in maximal-effort Yo-Yo performance are related with changes in sub-maxHR in professional rugby league players. Methods: Yo-Yo IR1 performances of 32 players were assessed before and after 28.5 ± 6.8 days of pre-season training. Relationships between sub-maxHR and maximal-effort Yo-Yo performance, and changes in maximal-effort performance and sub-maxHR were compared. Results: Maximal-effort Yo-Yo IR1 performance and sub-maxHR correlated negatively (r = -0.73, CI -0.85 to -0.55, very large). Coefficient of variation (CV) of sub-maxHR was 2.4% (CI 2.0-3.3%), worthwhile changes of small (0.7 [CI 0.5-1.0%]), moderate (2.1 [CI 1.6-3.0%]) and large (4.3 [CI 3.3-6.1%]) were identified. Training-induced changes in sub-maxHR and Yo-Yo IR1 performance displayed a negative correlation (r = -0.57, CI -0.74 to -0.33, large). Conclusions: sub-maxHR has a very large negative relationship with maximal-effort Yo-Yo IR1 performance. Training-induced changes in sub-maxHR demonstrated a large negative relationship with changes in Yo-Yo IR1 performance. The CV and worthwhile changes can determine meaningful variations in sub-maxHR in professional rugby league players.
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