Validity of the oxygen uptake efficiency slope to assess aerobic fitness following bed rest

Achieving true peak oxygen uptake (VO 2 peak) is often difficult after the deconditioning that results from musculoskeletal unloading via bed rest or spaceflight. Consequently, submaximal measures predicting VO 2 peak are crucial for the effective assessment of cardiorespiratory functional reserve. One proposed index, oxygen uptake efficiency slope (OUES), is effort independent; however, its sensitivity to change in aerobic performance because of disuse and exercise training is unknown. Twenty‐five subjects performed a maximal exercise test before and after 14 days of 6° head‐down tilt bed rest. During bed rest, 9 subjects completed a high‐intensity exercise program (EX) while 16 were sedentary (CONT). In addition to VO 2 peak and ventilatory threshold (VT), OUES at 75% and 100% of total exercise time were determined. VO 2 peak (−13%), VT (−17%), OUES100 (−12%), and OUES75 (−10%) decreased in the CONT group; while VO 2 peak (+10%), VT (+8%), OUES100 (+6%), and OUES75 (+10%) increased in the EX group. The change in both OUES100 and OUES75 was significantly related to the change in VO 2 peak (OUES100: r = 0.85; OUES75: r = 0.75; P < 0.05). Bland‐Altman plots showed high agreement between OUES and VO 2 peak. These findings indicate that OUES is an objective submaximal index that can detect changes in cardiopulmonary reserve following deconditioning and exercise training. Support: NASA Johnson Space Center Human Research Program