Cardiovascular and Thermal Strain during Manual Work in Cold Weather

In some occupations it is hard to protect the hands against the severe weather conditions in wintertime due to the requirement to maintain manual dexterity. Decrease in temperature of the hands increases risk of cold injury and deteriorates muscle function and manual dexterity, which in turn decreases productivity. A series of field and lab studies were performed to investigate cold stress and cold acclimation on the neuromuscular function of the hand. To quantify the cardiovascular and thermal strain during a working day in cold weather (–17.6±3.1oC (mean±sd)), five line workers (4 male, 1 female, 36.4±4.4 y) from the maintenance crew of New Brunswick Power were equipped with skin thermistors and heart rate monitors and followed for a day (0900h – 1600h). Typical tasks were videotaped and a time log of activities and hand protection levels were recorded. Hand and finger temperature dropped as low as 17.0–24.4oC and 7.9–12.6oC, respectively. Average temperatures over the working day period varied from a hand temperature of 24.8–31.4oC to 21.0–29.8oC for the index finger. The lowest mean body skin temperature ranged from 21.0–29.4oC while on average, this was 25.6–34.1oC. Typical tasks included climbing poles, cutting wires, and shovelling snow. Peak heart rate (HR) was 148–181 b·min. Although the measurements took place during a day of relatively mild weather and light activity, skin temperature of fingers and hands decreased to a level that has been previously demonstrated to impair manual dexterity. The intensity level of certain tasks was as high as 82–97% of the age-predicted HRmax. It was concluded that alternating high intensity tasks with low intensity tasks demanding manual dexterity, will decrease cardiovascular strain and may improve manual performance by warming the body and hands. This field study was followed up by laboratory experiments that were conducted to investigate the effects of physical activity and a resulting increase in core temperature (±0.5oC) on cold acclimation and neuromuscular function of the hand. Neuromuscular function of the hand was tested before and after hand cooling in 8oC water for 30 min while either bicycling at a submaximal level or sitting at rest, before and after two weeks of local cold acclimation (30 min/day, 5 days/week). Neuromuscular tests consisted of tactile sensitivity, hand grip strength, manual dexterity and an evoked twitch force in a custom made myograph. Temperature of core, index finger and hand were recorded daily as well as subjective thermal ratings. Thermal ratings and index finger temperature increased significantly during acclimation days from 1.2±0.7 (‘very cold’) to 2.1±1.3 (‘cold’) (P<0.01) and 8.7±0.7oC to 10.1±1.3oC (P=0.04), respectively. Neuromuscular function was impaired with cooling and neither acclimation nor an elevation in core temperature had a significant effect on manual performance. Subjective thermal ratings were the first to acclimate even when no improvements in actual temperatures were seen. We conclude that the discrepancy between subjective and actual temperature may pose an additional risk of cold injury on people exposed to repeated cold stress by changing their behavioural thermal regulation response. Geurts, C.L.M.; Cheung, S.S. (2005) Cardiovascular and Thermal Strain during Manual Work in Cold Weather. In Prevention of Cold Injuries (pp. 3-1 – 3-8). Meeting Proceedings RTO-MP-HFM-126, Paper 3. Neuilly-sur-Seine, France: RTO. Available from: http://www.rto.nato.int/abstracts.asp. Cardiovascular and Thermal Strain during Manual Work in Cold Weather 3 2 RTO-MP-HFM-126 1.0 INTRODUCTION In outdoor occupations that require manual work it is often hard to protect the hands against severe cold weather conditions. Although heavy insulative clothing may keep the body warm, hands are often exposed because gloves and mittens may impair manual dexterity [9], or the type of task requires people to wear gloves that do not protect the hands properly against the cold. For example, Finnish researchers have reported skin temperatures as low as 8°C in cheeks and fingers of outdoor workers during winter in Finland [2]. Similarly, people who work in the frozen food industry are regularly exposed to this kind of cold stress [4, 17, 20]. Exposure of the hands to cold may result in a decrease in muscle temperature causing a decrement in neuromuscular function [6-8,19], impairment of manual dexterity and performance [3,11], and may cause cold injury or frostbite, reducing work safety and increasing accident rates [2]. However, people can adapt or acclimate to their environment. Some research on people living in cold environments and people being exposed to repeated cold stress has found an enhancement of blood flow to the extremities [1,13,14,16]. It was suggested that the warm blood from the core may warm up the muscles and theoretically improve manual dexterity and tactile sensitivity during work in the cold [5], but this remains largely untested. Most local cold acclimation studies conducted in laboratories were performed on resting subjects. Occupations that require manual work, often also involve physical activity. Therefore the first objective was to investigate the thermal and cardiovascular strain in a typical outdoor occupation. Secondly, a follow up laboratory study was conducted to investigate the effects of physical activity and a concomitant elevated core temperature on cold acclimation and neuromuscular function of the hand. It was predicted that during manual work outdoors, the cardiovascular strain is fairly high resulting an increase in core temperature and this elevation of core temperature would improve neuromuscular function but decrease local cold acclimation.