Loss of cardiorespiratory fitness and its recovery following two weeks of head-down bed rest and the protective effects of exercise in 55-to 65-yr-old adults

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- to 65-yr-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 yr) completed 2 wk of strict 6 degrees head-down bed rest (HDBR). Half of the participants performed approximately 1 h of daily exercises, including high-intensity interval cycling, aerobic cycling, and upper- and lower-body resistance training, whereas control participants were inactive. Stepincremental cycling tests to exhaustion were conducted pre-HDBR and at three times during the recovery phase (day 1 or 2, day 6, and 4 wk) to assess peak oxygen uptake (V_ O2). Peak V_ O2 was reduced in the control group throughout the first 6 days of recovery, but did return to pre-HDBR levels by the 4-wk recovery time point (interaction: P = 0.002). In the exercise group, peak V_ O2 was not different at any time point during recovery from pre-HDBR. Ventilatory threshold V_ O2 (interaction: P = 0.002) and heart rate at 15 W (interaction: P = 0.055) mirrored the changes in peak V_ O2 in each respective group. Overall, this study showed that approximately 1 h of daily exercise effectively protected 55- to 65-yr-old adults' cardiorespiratory fitness during 2 wk of HDBR. HDBR without exercise countermeasures caused substantial reductions in cardiorespiratory fitness, but fitness recovered within 4 wk of resuming daily activities. These findings highlight the importance of physical activity in late middle-age adults. NEW & NOTEWORTHY We report the complete time-course of cardiorespiratory fitness recovery back to baseline levels following 2 wk of head-down bed rest in 55- to 65-yr-old adults and found that multimodal training, consisting of high-intensity interval, aerobic and resistive exercises, performed throughout the 2 wk of head-down bed rest prevented reductions in cardiorespiratory fitness.

Automated summary

Cardiorespiratory fitness declines with age and can be accelerated by inactivity and bed rest. The recovery timeline for adults aged 55-65 is unknown, and the effectiveness of exercise during bed rest is unexplored in this age group. A study found that approximately 1 hour of daily exercise protected the cardiorespiratory fitness of adults aged 55-65 during 2 weeks of bed rest.