Decrease in work rate in order to keep a constant heart rate: biomarker of exercise intolerance following a 10-day bed rest

During a 15-min exercise carried out at a heart rate (HR) slightly above that corresponding to the gas exchange threshold, to keep HR constant, work rate significantly decreased; the decrease was more pronounced after a 10-day horizontal bed rest. The work rate decrease at a fixed HR can be considered a systemic biomarker of exercise intolerance during microgravity/inactivity and could also be easily and reliably determined during spaceflights or in patients.