Figure 4. Total sympathetic nerve activity responses to static handgrip.

Data for individual astronauts (1-5) are depicted as the same line type in each panel as in Fig. 3. In-flight mean values (•) represent the mean of data from three subjects. The data from astronaut 3 were excluded in this figure because his baseline nerve activity tracing was shifted upward during static handgrip in space; although we could count the burst frequency as well as the burst incidence, we could not obtain the accurate total sympathetic nerve activity from his tracings. BL, baseline; HG₁, first minute of static handgrip; HG_F, static handgrip at fatigue; a and b, the first and second halves of the intermediate period after the first minute of static handgrip and before fatigue; C₁ and C₂, first and second minutes of post-handgrip circulatory arrest; R₁ and R₂, first and second minutes of recovery. * P < 0.05 and ** P < 0.01, compared with baseline.