Breathless but buoyed: a female athlete's record-breaking ultraendurance swim 1,900 m above sea level.

We describe the physiological adaptations evoked during a 3-mo training block in an elite female swimmer building toward a 34.3-km ultraendurance solo swim across Lake Tahoe (ambient temperature ∼13.9°C, water temperature ∼17.5°C, and elevation 1,900 m). The athlete underwent physiological assessments including measures of resting metabolic rate, peak oxygen uptake (V̇o2peak), cycling exercise economy, simple reaction time, and grip strength. In addition to training history and measures taken before the record attempt, we captured the physiological responses immediately following completion of the 34.3 km distance. The athlete completed the ultraswim 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 1,382.9 kcal·day-1 to 1,647.6 kcal·day-1. Exercise economy and V̇o2peak remained relatively constant throughout the training. Following the 10.5-h swim, body mass changed by <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.NEW & NOTEWORTHY Ultraendurance exercise requires cardiometabolic fitness, intentionality toward nutrient replenishment, psychological resilience, and some degree of musculoskeletal durability. When ultraendurance exercise requires sustained swimming, add to these the burdens of phase-locked breathing, immersion in cold water, and the need to stay afloat. Here, we record the record-breaking performance of an elite female ultraswimmer, demonstrating that although body mass remains stable following completion of the event, psychological processing speed is strongly compromised.