Background: In 1966, five 20-year-old men underwent a comprehensive physiological evaluation of the capacity for adaptation of the cardiovascular system in response to 3 weeks of bed rest and 8 weeks of heavy endurance training; these same participants were reevaluated before and after training at the age of 50. The aim of the present study was to reexamine these same men 40 years following the original assessments.
Methods and results: In all three studies, minute ventilation and expired gases were analyzed during exercise testing with Douglas bag collection. Cardiac output (CO) was determined using the acetylene rebreathing technique. Compared with the original 30-year interval, the decline in maximal oxygen uptake (VO(2max)) (-11% vs -25%), maximal CO (+6% vs -11%), and maximal stroke volume (+10% vs -10%) were greater between 50 and 60 years of age. The annualized decline in VO(2max) (55 mL/min/y) between ages 50 and 60 was approximately fourfold higher than the decline between 20 and 50 years (12 mL/min/y).
Conclusions: In the original five participants of the Dallas Bed Rest and Training Study, VO(2max) declined after 40 years of living due to a balanced decrease in central and peripheral determinants of oxygen uptake. The rate of decline in VO(2max) and its components accelerated after the age of 50 years secondary to age and clinical comorbidities. The net proportional decline in VO(2max) for a period of 40 years of life was comparable with that experienced after 3 weeks of strict bed rest at the age of 20 (27% vs 26%, respectively).