Heat acclimation improves exercise performance

ncbi.nlm.nih.gov - Heat acclimation improves exercise performance: "Heat acclimation increased power output at lactate threshold by 5%".

frontiersin.org - Post-exercise Hot Water Immersion Elicits Heat Acclimation Adaptations in Endurance Trained and Recreationally Active Individuals: "Endurance athletes experience frequent, sustained elevations in body temperature during training and competition; as a consequence, endurance athletes are considered to be partially heat acclimatized."

ncbi.nlm.nih.gov - Strategies and factors associated with preparing for competing in the heat: a cohort study at the 2015 IAAF World Athletics Championships.

ncbi.nlm.nih.gov - Effect of two-weeks endurance training wearing additional clothing in a temperate outdoor environment on performance and physiology in the heat.
Training in additional clothing for two-weeks in a temperate environment was not an effective heat acclimation strategy for triathletes.

ncbi.nlm.nih.gov - Adaptation to hot environmental conditions: an exploration of the performance basis, procedures and future directions to optimise opportunities for elite athletes.
ncbi.nlm.nih.gov - Effects of endurance training and heat acclimation on psychological strain in exercising men wearing protective clothing.

See also:
commons.nmu.edu - The effect of winter versus summer running on lower extremity MUSCULOSKELETAL INJU RY RATE IN RECREATIONAL RUNNERS
karger.com - Thermoregulation and Human Performance
journals.plos.org - Impact of Environmental Parameters on Marathon Running Performance
trailrunnermag.com - The Importance of Not Getting Too Cold During Winter Runs