Effect of body mass on physiological and time-trial performance responses to mixed-method cooling applied between...

This study investigated the effect of body mass (BM) on the efficacy of mixed-method cooling (ice slurry ingestion + cooling vest) applied between exercise bouts. Sixteen males completed two experimental trials in the heat (35°C, 50% relative humidity) in a randomized and counterbalanced order. In each trial, participants completed two 30-min cycling bouts (25-min cycling at 55% of maximal oxygen uptake followed by a 5-min time-trial) separated by a 15-min break. During the break, participants received mixed-method cooling (COOL: 5.0 g·kg^-1 of ice slurry ingestion [-1.4 ± 0.2 °C] with a cooling vest) or no cooling (CON: 5.0 g·kg^-1 of fluid ingestion [28.5 ± 2.7 °C]). Participants were categorized according to their BM values as Heavier (72.2 ± 6.9 kg, n=8) or Lighter (59.3 ± 4.1 kg, n=8). Compared with CON, COOL reduced rectal temperature during the break in the Heavier group (-0.32 °C, p=0.013, d=1.14) and at the end of the second bout in both groups (Heavier: -0.31 °C, p=0.005, d=0.90; Lighter: -0.26 °C, p=0.016, d=0.46). The difference in rectal temperature at the end of the break between trials correlated with BM (r=-0.506, p=0.046, n=16). In an additional linear mixed-model analysis of rectal temperature across all time points, BM was not a significant covariate (BM: p=0.650, BM×trial: p=0.120, BM×trial×time: p=0.275). Heart rate was lower in COOL during the break and second bout in the Heavier (p<0.05, d=0.46-0.88) and Lighter groups (p<0.05, d=0.25-0.72). COOL mitigated the decline in mean power output in the second time-trial compared to the first time-trial (p=0.027, d=0.47). However, there was no significant difference in the ergogenic effect between the groups. Mixed-method cooling during the 15-min break attenuated thermal and perceptual strain during subsequent 25-min exercise and improved cycling 5-min time-trial performance. Given that BM was not a significant covariate for rectal temperature, BM-related differences in the efficacy of mixed-method cooling should be interpreted cautiously.