LAUSR

Cardiorespiratory fitness declines with age and this decline can be accelerated by inactivity and bed rest. Recovery of fitness is possible, but the timeline in 55-65-year-old adults is unknown. Furthermore, the effectiveness of exercise to prevent deconditioning during bed rest is unexplored in this age group. Twenty-two adults (11 women, 59 ± 3 years) completed two weeks of strict 6° head-down bed rest (HDBR). Half of the participants performed approximately 1 hour of daily exercises, including high-intensity interval cycling, aerobic cycling, and upper- and lower-body resistance training, while control participants were inactive. Step-incremental cycling tests to exhaustion were conducted pre-HDBR and at 3 times during the recovery phase (day-1 or 2, day-6, and 4 weeks) to assess peak oxygen uptake (V̇O2). Peak V̇O2 was reduced in the control group throughout the first 6 days of recovery, but did return to pre-HDBR levels by the 4-week recovery time point (interaction: p=0.002). In the exercise group, peak V̇O2 was not different at any time point during recovery from pre-HDBR. Ventilatory threshold V̇O2 (interaction: p=0.002) and heart rate at 15 W (interaction: p=0.055) mirrored the changes in peak V̇O2 in each respective group. Overall, this study showed that approximately 1 hour of daily exercise effectively protected 55-65-year-old adults' cardiorespiratory fitness during two weeks of HDBR. HDBR without exercise countermeasures caused substantial reductions in cardiorespiratory fitness, but fitness recovered within 4 weeks of resuming daily activities. These findings highlight the importance of physical activity in late middle-age adults.