Collision activity during training increases total energy expenditure measured via doubly labelled water

Collision sports are characterised by frequent high-intensity collisions that induce substantial muscle damage, potentially increasing the energetic cost of recovery. Therefore, this study investigated the energetic cost of collision-based activity for the first time across any sport. Using a randomised crossover design, six professional young male rugby league players completed two different 5-day pre-season training microcycles. Players completed either a collision (COLL; 20 competitive one-on-one collisions) or non-collision (nCOLL; matched for kinematic demands, excluding collisions) training session on the first day of each microcycle, exactly 7 days apart. All remaining training sessions were matched and did not involve any collision-based activity. Total energy expenditure was measured using doubly labelled water, the literature gold standard. Collisions resulted in a very likely higher (4.96 +/- 0.97 MJ; ES = 0.30 +/- 0.07; p = 0.0021) total energy expenditure across the 5-day COLL training microcycle (95.07 +/- 16.66 MJ) compared with the nCOLL training microcycle (90.34 +/- 16.97 MJ). The COLL training session also resulted in a very likely higher (200 +/- 102 AU; ES = 1.43 +/- 0.74; p = 0.007) session rating of perceived exertion and a very likely greater (- 14.6 +/- 3.3%; ES = - 1.60 +/- 0.51; p = 0.002) decrease in wellbeing 24 h later. A single collision training session considerably increased total energy expenditure. This may explain the large energy expenditures of collision-sport athletes, which appear to exceed kinematic training and match demands. These findings suggest fuelling professional collision-sport athletes appropriately for the muscle damage caused alongside the kinematic work required.