High prevalence of hypohydration in occupations with heat stress-Perspectives for performance in combined cognitive and motor tasks

Abstract

Purpose: To evaluate the prevalence of dehydration in occupational settings and contextualize findings to effects on performance in cognitively dominated tasks, simple and complex motor tasks during moderate and high heat stress.

Methods: The study included an occupational part with hydration assessed in five industries across Europe with urine samples collected from 139 workers and analyzed for urine specific gravity. In addition, laboratory experiments included eight male participants completing mild-intensity exercise once with full fluid replacement to maintain euhydration, and once with restricted water intake until the dehydration level corresponded to 2% bodyweight deficit. Following familiarization, euhydration and dehydration sessions were completed on separate days in random order (cross-over design) with assessment of simple motor (target pinch), complex motor (visuo-motor tracking), cognitive (math addition) and combined motor-cognitive (math and pinch) performance at baseline, at 1°C (MOD) and 2°C (HYPER) delta increase in body core temperature.

Results: The field studies revealed that 70% of all workers had urine specific gravity values ≥1.020 corresponding to the urine specific gravity (1.020±0.001) at the end of the laboratory dehydration session. At this hydration level, HYPER was associated with reductions in simple motor task performance by 4±1%, math task by 4±1%, math and pinch by 9±3% and visuo-motor tracking by 16±4% (all P<0.05 compared to baseline), whereas no significant changes were observed when the heat stress was MOD (P>0.05). In the euhydration session, HYPER reduced complex (tracking) motor performance by 10±3% and simple pinch by 3±1% (both P<0.05, compared to baseline), while performance in the two cognitively dominated tasks were unaffected when dehydration was prevented (P>0.05).

Conclusion: Dehydration at levels commonly observed across a range of occupational settings with environmental heat stress aggravates the impact of hyperthermia on performance in tasks relying on combinations of cognitive function and motor response accuracy.

Fig 1. Illustration of the motor and cognitive tests employed in the laboratory study.

First task illustrates the visuo-motor tracking task (VMT), where the 5 boxes/targets appears on the screen 1 s before the 12 s tracking begins. The cursor (green line) moves from left to right over the screen (fixed velocity) and the subject is instructed to adjust force on the strain gauge transducer to keep the cursor within the designated box at the given time. VMT alternates with the MATH_TYPE or MATH_PINCH and TARGET_PINCH tasks (listed in the order of how the tasks appear). During VMT, MATH_PINCH and TARGET_PINCH, subjects used the strain gauge, whereas in the MATH_TYPE the number pad was used (bottom pic, right). Figure adopted from [26] with permission.

Fig 2. USG values across five different industries in four different European countries.

Symbols connected with lines represents participants at the ONSET and END of a work shift. Black diamonds [♦]: Police officers, both inside the office and outdoors, uniform is required. Black dot [●]: Agriculture workers, working outside with no protective clothing requirements. Black crucifix [+]: Tourism workers, inside the office and outdoors. Black square [■]: Construction workers, required to wear protective clothing. Black cross [X]: Aluminum extrusion workers, required to wear protective clothing.

Fig 3. TARGET_PINCH performance score.

Individual and mean changes in TARGET_PINCH performance with white bars representing euhydration (EUH: MOD and HYPER) and gray bars the dehydration session (DEH: MOD and HYPER). Values are expressed in percentage change from BASELINE (BASELINE expressed as 0) and individual participant changes in each condition and session. * Significantly different from BASELINE, # Significantly different from MOD (P<0.05).

Fig 4. MATH_TYPE performance score.

Individual and mean changes in MATH_TYPE performance during euhydration (EUH: MOD and HYPER [White bars]) and dehydration (DEH: MOD and HYPER [Grey bars]). Values are expressed in percentage change from BASELINE (BASELINE expressed as 0) and individual participant changes in each condition and session. * Significantly different from BASELINE and # significantly different from MOD (P<0.05).

Fig 5. MATH_PINCH performance score.

Individual and mean changes in MATH_PINCH performance during euhydration (EUH: MOD and HYPER [White bars]) and dehydration sessions (DEH: MOD and HYPER [Grey bars]). Values are expressed in percentage change from BASELINE and individual participant changes in each condition and session. * Significantly different from BASELINE and # significantly different from MOD (P<0.05).

Fig 6. VMT performance score.

Performance scores in the visuo-motor tracking (VMT) during euhydration (EUH: MOD and HYPER [White bars]) and dehydration (DEH: MOD and HYPER [Grey bars]). Values are expressed in percentage change from BASELINE (BASELINE expressed as 0) and individual participant changes in each condition and session. * Significantly different from BASELINE and # significantly different from MOD, (P<0.05).