Figure 1.

Interactions in human physiology and performance. (A, B and C) show the 3 types of interaction (additive, synergistic, antagonistic) that can occur when combining independent stressors. (D, E and F) show examples of 3 interaction sub-types (‘the most severe component takes precedence’, ‘relative addition’ and ‘impact nullification’). (G to K) shows the recent studies (refs. 3, 5, 4, 2 and ⁴ respectively) that have employed an individual and combined stressors approach to investigate human performance under thermal and hypoxic stress. Together these studies provide evidence that as the individual stressors impact increases, there is a progressive shift from additive to antagonistic (hypo-additive) interactions.