The independent and interactive effects of thermal stress and mental fatigue on manual dexterity.

Many occupations and sports require high levels of manual dexterity under thermal stress and mental fatigue. Yet, multi-stressor studies remain scarce. We quantified the interactive effects of thermal stress and mental fatigue on manual dexterity. Seven males (21.1±1.3y) underwent 6 separate 60-min trials characterised by a combination of 3 air temperatures (HOT: 37°C; NEUTRAL: 21°C; COLD: 7°C) and 2 mental fatigue states (MF: mental fatigue induced by a 35-min cognitive battery; No-MF: no mental fatigue). Participants performed complex (O'Connor test) and simple (Hand-Tool test) manual tasks pre- and post-trials to determine stressors-induced performance changes. We monitored participants' rectal temperature and hand skin temperature (Thand) continuously and assessed reaction time (Hand-Click test) and subjective mental fatigue (5-point scale). Thermal stress (p<0.0001), but not mental fatigue (p=0.290), modulated Thand (HEAT: +3.3°C [95%CI +0.2, +6.5]; COLD: -7.5°C [-10.7, -4.4]). Mental fatigue (p=0.021), but not thermal stress (p=0.646), slowed reaction time (~10%) and increased subjective fatigue. Thermal stress and mental fatigue had an interactive effect on the complex manual task (p=0.040), with COLD-No-MF decreasing performance by -22% [-39, -5], while NEUTRAL-MF, COLD-MF, and HEAT-MF by -36% [-53, -19], -34% [-52, -17], and -36% [-53, -19], respectively. Only mental fatigue decreased performance in the simple manual task (-30% [-43, -16] across all thermal conditions; p=0.002). Cold stress-induced impairments in complex manipulation increase with mental fatigue; yet combined stressors' effects are no greater than that of mental fatigue alone, which also impairs simple manipulation. Mental fatigue poses a greater challenge to manual dexterity than thermal stress.