Breathless but buoyed: A female athlete's record-breaking ultraendurance swim 1900 meters above sea level.

We describe the physiological adaptations evoked during a 3-month training block in an elite female swimmer building towards a 34.3 km ultraendurance solo swim across Lake Tahoe (ambient temperature ~13.9°C, water temperature ~17.5°C, elevation 1900 m). The athlete underwent physiological assessments including measures of resting metabolic rate, peak oxygen uptake (V˙O₂peak), cycling exercise economy, simple reaction time, and grip strengths. In addition to training history and measures taken prior to the record attempt, we captured the physiological responses immediately following completion of the 34.3 km distance. The athlete completed the ultra-swim across Lake Tahoe in a record-setting 10:34:48 (hh:mm:ss)-the fastest time ever recorded for a woman. From the beginning of training to the weeks preceding the Lake Tahoe swim, resting metabolic rate increased from 1382.9 kcal∙day^-1 to 1647.6 kcal∙day^-1. Exercise economy and V˙O₂peak remained relatively constant throughout training. Following the 10.5 h swim, body mass changed by less than 1 kg. Simple reaction time and grip strength values declined to substantial and mixed quantities, respectively. This study provides insights into the training regimen adopted by a female swimmer and the physiological ramifications of a record-breaking ultraendurance performance.