INTRODUCTION: Aerobic deconditioning may occur during International Space Station (ISS) flights. This paper documents findings from exercise testing conducted before, during, and after ISS expeditions.METHODS: There were 30 male and 7 female astronauts on ISS missions (48 to 219 d, mean 163 d) who performed cycle exercise protocols consisting of 5-min stages eliciting 25%, 50%, and 75% peak oxygen uptake (Vo_2peak). Tests were conducted 30 to 90 d before missions, on flight day 15 and every 30 flight days thereafter, and on recovery (R) days +5 and +30. During pre- and postflight tests, heart rate (HR) and metabolic gas exchange were measured. During flight, extrapolation of the HR and Vo₂ relationship to preflight-measured peak HR provided an estimate of Vo_2peak, referred to as the aerobic capacity index (ACI).RESULTS: HR during each exercise stage was elevated (P < 0.05) and oxygen pulse was reduced (P < 0.05) on R+5 compared to preflight; however, no other metabolic gas analysis values significantly changed. Compared to preflight, the ACI declined (P < 0.001) on R+5, but recovered to levels greater than preflight by R+30 (P = 0.008). During flight, ACI decreased below preflight values, but increased with mission duration (P < 0.001).CONCLUSIONS: Aerobic deconditioning likely occurs initially during flight, but ACI recovers toward preflight levels as flight duration increases, presumably due to performance of exercise countermeasures. Elevated HR and lowered oxygen pulse on R+5 likely results from some combination of relative hypovolemia, lowered cardiac stroke volume, reduced cardiac distensibility, and anemia, but recovery occurs by R+30.Moore AD Jr, Lynn PA, Feiveson AH. The first 10 years of aerobic exercise responses to long-duration ISS flights. Aerosp Med Hum Perform. 2015; 86(12, Suppl.):A78–A86.