Background: It is questionable whether postures that are regarded as sedentary behavior in able-bodied persons evoke comparable physiological responses in adults with stroke or cerebral palsy (CP). This study aimed to compare metabolic demand and muscle activity in healthy controls, adults with stroke, and adults with CP during sedentary behavior and light physical activities. Methods: Seventy-one adults (45.6 [18.9] y, range 18–86) participated in this study, of which there were 18 controls, 31 with stroke, and 22 with CP. The metabolic equivalent of task (MET) and level of muscle activation were assessed for different sedentary positions (sitting supported and unsupported) and light physical activities (standing and walking). Results: During sitting supported and unsupported, people with mild to moderate stroke and CP show comparable MET and electromyographic values as controls. While sitting unsupported, people with severe stroke show higher METs and electromyographic values (P < .001), and people with severe CP only show higher METs compared with controls (P < .05) but all below 1.5 METs. Standing increased electromyographic values in people with severe stroke or CP (P < .001) and reached values above 1.5 METs. Conclusions: Physiologic responses during sedentary behavior are comparable for controls and adults with mild to moderate stroke and CP, whereas higher metabolic demands and muscle activity (stroke only) were observed in severely affected individuals.