Cold climate and conditions associated with cold weather have been known to have an effect on performance of athletes. Although not as profound as negative effects that accompany heat stress, there are many cold related injuries and illnesses that can affect athletes and their performance. Thus, it is important to identify risk factors that predispose individuals to cold related injuries, and methods that can be used to avoid these injuries all together.
Testing on humans in the cold does meet some ethical barriers, making it difficult to test the limit of human capacity in the cold, but a large amount of research has been done on lab rats. We can use this information to draw conclusions on how humans might reach our limit in similar situations, and help guide future studies to identify key physiological processes that will help us better understand our adaptations to cold climate. First, it is important to define key terms and concepts in the human physiological response to cold and how it affects our performance. Secondly, focus will be brought to injuries that are of importance from chronic exposure to varying degrees of cold. Finally, observe various insights from studies that have shed new light on cold tolerance research and reflect on how it will aid future advancements in the field of question.
Review of Relevant Literature Initially, it is important to get a good understanding of how the human body responds being introduced to a cold environment. The two main mechanisms of focus will be peripheral vasoconstriction and metabolic heat production. Peripheral Vasoconstriction Vital organs located in the body’s “core” can only function optimally when a certain body temperature is maintained (approximately 37.8°C).
When core temperature drops to a certain point, the body will divert blood flow from the “shell,” or the skin and the peripheral regions of the body, in order to provide more heat delivery to vital organs of the core. The blood is diverted by a bodily function known as vasoconstriction, where blood vessels contract in order to increase blood pressure and decrease flow in these tightened and narrowed areas. This is important because a significant source of body heat is transferred via convection, a method of heat transfer through blood, in this case.
Vasoconstriction of peripheral blood vessels reduces heat loss from the core to the shell, and furthermore allowing vital organs to remain at optimally functioning temperature (Dougherty et al., 1993). Only problem with this phenomenon is that with the reduction of blood flow to ears, nose, fingers, toes, etc., these areas become vulnerable to cold injury (which will be looked at in more specific detail later) and loss in manual dexterity (Brajkovic et al., 2003). The body also has an effective counter measure in the event that there isn’t enough blood in the periphery, which is called cold-induced vasodilation (CIVD), which reduces the effect of peripheral vasoconstriction and allows for some blood return and prevent the occurrence of cold-related injury (Iida, 1949).
Metabolic Heat Production Another way the body combats a drop in core temperature is by producing heat energy through skeletal muscle contraction via catabolism, breaking down nutrients to fuel the body. Skeletal muscle can voluntarily contract by our every movement and produce heat as a by-product, but in the event where when voluntary movement does not produce enough heat, the body will begin to systemically contract and relax in an involuntary manner, causing us to shiver.
Shivering will typically start in the muscles of the torso and eventually move to the limbs if more heat is generated in attempt to offset heat loss (Bell et al,. 1992). To put the heat energy generated by shivering into perspective, consider the fact that typical shivering raises body oxygen uptake values to approximately 600-700ml/min in comparison to approximate resting values of 210-300ml/min (Toner et al., 1996). In a fairly recent study by Eyolfson et al., 2001, maximal shivering levels were reported to produce an oxygen uptake of 2.2L/min during cold water immersion, where heat loss occurs at a much faster rate (Figures 1 and 2).
Adapting to the Cold Now that we have a basic understanding of how our body can make changes in order to adapt to acute exposures to the cold, it is imperative that we take a closer look to longer term adaptations and how it can have an effect on performance. There are three main methods that individuals can acclimatize themselves to cold climate which are through habituation, metabolic or insulative acclimatization.
Habituation Castellani et al., 2006, describes habituation as a decrease in response pronunciation, relative to unacclimatized individuals. Habituation can also be defined as “a reduction in behavioural perception of a repeated stimulus.” Thus, a decrease in shivering and cold induced vasoconstriction will take place as a result (Young, 1996). Habituation can occur from long term exposure to moderate levels of cold temperature over long periods of time to produce warmer average core temperature and less discomfort while in the cold and furthermore greater drop in core temperature after prolonged cold exposure (Savourey et al., 1992).
Metabolic Acclimatization The next approach to adaptation is metabolic acclimatization. This allows for an amplified shiver response resulting in an increase in heat production (Young, 1996). This could also mean there is increased oxygen consumption at maximal shivering and it occurs at an earlier instance in time. This is important because it means that it further reduces the risk of cold related injuries to the periphery and ensures that core temperature is regulated.
Insulative Acclimatization And the final approach is insulative acclimatization where, according to Castellani et al., 2006, the body adopts improved heat conservation methods. This includes an increased decline in initial body temperature, resulting in an earlier vasoconstriction response of the body, possibly attributed by a stimulated sympathetic nervous system response from exposure to the cold (Young, 1996).
Effects on Performance After looking at these basic concepts of the body’s physiological responses and adaptations, the next step would be to analyze their direct impacts on aerobic fitness and physical performance. Studies done comparing sedentary to aerobically fit individuals (Bittel, 1988), lead us to believe that fitness does not have a large effect on overall capability to perform, but that these differences arise from individual differences from person to person such as body composition, age, and sex (to a certain extent). A study conducted by Lounsbury et al., in 2005 looked at novice and expert swimmers and their ability to swim in severe cold water condition.
Results had suggested that the expert swimmers could swim further distances, but not longer durations (Lounsbury et al., 2005). This leads us to the conclusion that limb fatigue could have played a role in temperature maintenance and had caused the expert swimmers to cool down at a faster rate that the novice swimmers because they had expended larger amounts of energy over the course of their swim, consequentially meaning muscle cooling had caused expert swimmers to retire early. What is known with certainty is that increased metabolic activity from exercise can help warm the periphery and protect from frostbite (freezing of tissue) and hypothermia, where core drops to a temperature of 35°C or lower (Figure 3) (Blatteis et al., 1976).