Delivered by Ingenta to : unknown IP : 66.186.132.170 Thu, 29 Dec 2005 16:33:04 RESEARCH ARTICLE Postural Sway During Single and Repeated Cold Exposures Tiina M. Ma ¨ kinen, Hannu Rintama ¨ ki, Juha T. Korpelainen, Ville Kampman, Tiina Pa ¨a ¨ kko ¨ nen, Juha Oksa, Lawrence A. Palinkas, Juhani Leppa ¨ luoto, and Juhani Hassi MA ¨ KINEN TM, RINTAMA ¨ KI H, KORPELAINEN JT, KAMPMAN V, PA ¨ A ¨ KKO ¨ NEN T, OKSA J, PALINKAS LA, LEPPA ¨ LUOTO J, HASSI J. Postural sway during single and repeated cold exposures. Aviat Space Environ Med 2005; 76:947–53. Introduction: Tissue cooling changes sensory and neuromuscular functions that are also involved in postural control. The purpose of the study was to determine how acute and repeated exposures to cold affect whole body postural control. Methods: Postural sway was measured from 10 subjects during standing with eyes open (EO) and closed (EC) using an inclinometer-based method. Sway was assessed at at 10°C on 10 consecutive days and at 25°C on days 1, 5, and 10. Sway path length, area, velocity, side-to-side and forward-backward movement were as- sessed. At the same time, rectal and skin temperatures, muscle tonus/ shivering, thermal sensations, and comfort were recorded. Results: Acute exposure to 10°C caused thermal discomfort, significantly low- ered (26.1–26.5°C) mean skin temperatures, slightly lowered rectal tem- perature (36.7°C) and increased (140 –260%) muscle tone, increased sway path length (67– 87%, p  0.05), velocity (63–71%, p  0.05), total sway area (42– 67%, p  0.05), and forward-backward movement (35–57%, p  0.05) compared with 25°C. Side-to-side movements were not altered in the cold. Postural sway increased with EC, and further when exposed to cold, but the effect of cold was smaller compared with EO. Repeated exposures over the 10-d period decreased sway 10 – 40% both at 25°C and at 10°C (p  0.05– 0.01), suggesting motor learning. The difference in sway between 25°C and 10°C remained the same throughout the 10-d period, suggesting that the observed cold habitua- tion responses do not affect sway. Conclusions: The results demonstrate that postural control is impaired in cold, which may affect physical performance in cold environmental conditions. Keywords: postural control, cold strain, cold acclimation, habituation, thermoregulation, human. P OSTURAL CONTROL is an essential element of human daily activities. Sufficient postural control is important in dynamic activities, such as physically de- manding occupations (21). An impaired balance may result in decreased performance and injuries resulting from slipping, tripping, or falling accidents. Control of human posture is a complex phenomenon. Maintaining postural stability in the field of gravity requires that the center of mass falls within the area of support. This area is relatively small, requiring constant fine-tuning of movements in the different joints to maintain posture. Sensory information of the body’s posture is gained through visual, somatosensory, and vestibular systems. The afferent information is integrated at the spinal cord, medulla, midbrain, and cerebral cortex. Finally, pos- tural control is obtained by preprogrammed anticipa- tory postural adjustments, muscle reflexes, peripheral elasticity of muscles and tendons, as well as prepro- grammed and voluntary corrections (13). Cold exposure may affect postural control through a variety of mechanisms. The cold environment itself, with icy surfaces and a reduced amount of light during the winter, can endanger postural stability (4). Different physiological responses related to cooling may also af- fect postural control. For example, shivering may affect postural control due to increased muscle tone. It is not known whether this increased tension in muscles has a beneficial or disadvantageous effect on sway. When cooling progresses, the muscle tone is changed into tremor and associated with visible shaking or shudder- ing. It is possible that shivering causes perturbations in fine motor control (17), requiring more tuning of move- ments compared with a warm environment. Cooling also affects the sensory systems involved in postural control. For example, the ankle mechanorecep- tors are important sensory components for maintaining balance. Previous studies examining the functional properties of the sole and ankle mechanoreceptors have demonstrated that local cooling of feet increases pos- tural sway (15,16,24). The proprioceptors located in the muscles, tendons, and joints can also be affected by cooling, resulting in changes in neuromotor functions. Cooling may, for example, decrease the activity of the muscle spindles, leading to suppression of tendon-re- flex amplitudes, consequently affecting neuromuscular control (19). The neural transmission of both afferent and efferent information may be slowed due to cooling From the Centre for Arctic Medicine, University of Oulu (T. M. Ma ¨kinen, J. Hassi), the Department of Physiology, University of Oulu (H. Rintama ¨ki, T. Pa ¨a ¨kko ¨nen, J. Leppa ¨luoto), the Finnish Institute of Occupational Health (H. Rintama ¨ki, J. Oksa), the Department of Neu- rology, University of Oulu (J. T. Korpelainen), the Microelectronics and Material Physics Laboratories and EMPART Research Group of Infotech Oulu, University of Oulu (V. Kampman), Oulu, Finland; and the Department of Family and Preventive Medicine, University of California, San Diego, CA (L. A. Palinkas). This manuscript was received for review in April 2005. It was accepted for publication in July 2005. Address reprint requests to: Tiina M. Ma ¨kinen, Centre for Arctic Medicine, Thule Institute, University of Oulu, P.O. Box 5000, FIN- 90014 University of Oulu, Oulu, Finland; tiina.makinen@oulu.fi. Reprint & Copyright © by Aerospace Medical Association, Alexan- dria, VA. 947 Aviation, Space, and Environmental Medicine • Vol. 76, No. 10 • October 2005