Fig. 5

Schematic illustrating intensity-dependent pupil and physiological/biochemical responses. In contrast to other physiological/biochemical parameters, which exhibit pronounced increases at the LT/VT/CT points, the pupils dilate markedly not only around the LT/VT/CT points or at higher exercise intensities (additional dilation, i.e., PDT), but at very-light-intensity exercise (initial dilation point). This profile may indicate early activation of the ascending arousal system and explain the effects of exercise below the LT/VT/CT (area of the vertical green bar) on arousal states. Parameters that the current study did not measure (LAC, NA and A) were drawn based on previous studies and reviews [9, 10, 42, 43]. LAC, blood lactate; NA, blood noradrenaline; A, blood adrenaline; \({\dot{V}}_{\text{E}}\), minute ventilation; \({\dot{V}}_{{\text{O}}_{2}}\), oxygen intake; HR, heart rate; PDT, pupil dilation threshold; LT, lactate threshold; VT, ventilation threshold; CT, catecholamine threshold; LC, locus coeruleus