Quantifying the impact of heat on human physical work capacity; part IV: interactions between work duration and heat stress severity

Abstract

High workplace temperatures negatively impact physical work capacity (PWC). Although PWC loss models with heat based on 1-h exposures are available, it is unclear if further adjustments are required to accommodate repeated work/rest cycles over the course of a full work shift. Therefore, we examined the impact of heat stress exposure on human PWC during a simulated work shift consisting of six 1-h work-rest cycles. Nine healthy males completed six 50-min work bouts, separated by 10-min rest intervals and an extended lunch break, on four separate occasions: once in a cool environment (15 °C/50% RH) and in three different air temperature and relative humidity combinations (moderate, 35 °C/50% RH; hot, 40 °C/50% RH; and very hot, 40 °C/70%). To mimic moderate to heavy workload, work was performed on a treadmill at a fixed heart rate of 130 beats·min−1. During each work bout, PWC was quantified as the kilojoules expended above resting levels. Over the shift, work output per cycle decreased, even in the cool climate, with the biggest decrement after the lunch break and meal consumption. Expressing PWC relative to that achieved in the cool environment for the same work duration, there was an additional 5(± 4)%, 7(± 6)%, and 16(± 7)% decrease in PWC when work was performed across a full work shift for the moderate, hot, and very hot condition respectively, compared with 1-h projections. Empirical models to predict PWC based on the level of heat stress (Wet-Bulb Globe Temperature, Universal Thermal Climate Index, Psychrometric Wet-Bulb Temperature, Humidex, and Heat Index) and the number of work cycles performed are presented.

Work–rest regimens for work in hot environments: A scoping review

“…Fifty‐six sources were retained for full‐text screening, to which 28 sources were added from cross‐referencing. In the end, 49 sources met the inclusion criteria, of which 35 (71%) were original studies 16,26–28,35–37,39–66 and 14 other sources: five technical reports, 14,67–70 three reviews, 71–73 two guidelines, 21,22 two book chapters, 74,75 one symposium paper, 76 and one abstract 77 . No Cochrane review, systematic review, nor meta‐analysis was found.…”

“…No laboratory study focused on the combination of physical and mental tasks performed concurrently. Of the 33 experimental studies that investigated endurance tasks or a combination of endurance and mental tasks, 25 (76%) used a single‐task such as walking, 35,37,40,43,45,47,49,50,52,59–61,63–65 cycling, 26–28,39,42,51,62,66 or block stepping 44,48 . The remaining eight studies (24%) incorporated specific occupational tasks related to agriculture, construction, and tourism, 16 welding, 36 repetitive box lifting, 54 walking with load carrying, 49,55,56 walking with arm curls, 58 or walking with shoveling 53 .…”

“…Of the 33 experimental studies that investigated endurance tasks or a combination of endurance and mental tasks, three (9%) used a self‐paced strategy (one reported the associated metabolic rate measured using second‐by‐second time‐motion or real‐time analysis 16 while the other two did not report intensity data 36,54 ); the remaining 30 (91%) used a fixed‐pace strategy (Figure 5B). The latter used different techniques to set work intensity (Figure 5C), either by fixing exercise intensity at a given metabolic rate ( k = 20, reporting intensity in W, 35,37,40,47,48,54,58–60 in % of VO 2max , 42,43,49,55,64,65 in kcal/h, 44,52,53 metabolic equivalent of tasks 50 or not reported 56 ), a fixed rate of metabolic heat production, 26–28,39,45,51,62,66 or fixing it at a given level of heart rate 61 or perceived effort 63 . Of the 30 experimental studies that used a fixed‐intensity strategy, five used various intensity patterns during the simulated work 40,47,49,50,53 .…”

“…Of the 35 experimental studies included, 33 (94%) reported heart rate (Figure 6B) and 22 (63%) reported skin temperature (Figure 6C). Eleven studies (31%) measured effort perception using the 6–20 Borg Scale and one using the CR10 scale 35 (Figure 6D), and 10 (29%) measured thermal perception using category scales (7‐point ASHRAE scale 26,27,42,59,60,65 and another scale 28,58 ), visual analog scales, 63 or did not provide details 61 (Figure 6E). Two experimental studies (6%) reported measures of fatigue perception using the Multidimensional Fatigue Inventory 63 or a category‐ratio 0–10 scale 35 (Figure 6F).…”

“…For physical performance, Mulholland et al 58 measured maximum oxygen consumption using a graded exercise test at the end of the work‐rest protocol. Sawka et al 37 and Uchiyama et al 63 reported distance completed; Kraning and Gonzalez 50 reported endurance time; Maresh et al 54 reported the total number of boxes lifted; and Smallcombe et al 61 reported physical work capacity. In the study of Beshir et al, 41 the participants performed a sustained one‐dimensional vertical compensatory tracking task where they had to maintain alignment between a vertical line and a fixed target using a hand controller.…”