Environmental Effect

THE EFFECT OF A COLD ENVIRONMENT ON HUMAN PERFORMANCE

Juha Oksa Ph.D. Finnish Institute of Occupational Health, Oulu, Centre of Expertise for Health and Work Ability, Physical Work Capacity team

Introduction

When humans are exposed to cold ambient temperatures cooling may occur, and even while exercising, when body heat production is increased, this may result in subnormal body temperatures.

It is well verified that subnormal body and especially muscle temperature has an adverse effect on neuromuscular and physical performance capacity (e.g. 2).

The following mini review will focus on the effects of cooling on dynamic exercise, functional properties of the muscles and some neural aspects of muscle function.

Dynamic Exercise

In general, the ability to perform dynamic exercises is more readily disturbed by cooling than isometric exercise. The decrement in performance is usually expressed as absolute decrease (%) or related to decrease in muscle temperature (% • °C-1 decrease in muscle temperature).

Regardless of the exercise type, duration or component of performance (endurance, velocity, force etc.) decrease in dynamic performance after cooling is in the order of 2 - 10 % • °C-1 decrease in muscle temperature (4, 19). However, even bigger values have been reported. Bergh and Ekblom (3) found that cooling produced a 55 % decrement in maximal working time while muscle temperature was decreased by 3.4 °C, corresponding to 16 % • °C-1 decrease in muscle temperature.

Also, Oksa et al. (15) found that during drop jump exercise the highest decrease in performance was 17 % • °C-1. The latter implies that exercise type, which is very fast and efficiently utilises the elastic properties of the working muscles is espe¬cially susceptible for cooling.

Another important factor which needs to be taken into account is that maximal performance capacity (e.g. force and endurance, 12, 13) decreases due to cooling but the energy cost and motor unit recruitment during submaximal exercise is higher (13, 14).
This results in higher relative strain of a given work in cold environment as compared with thermoneutral environment.

Dose – response relationship

A dose – response relationship between muscle temperature and performance can be found as exemplified in Figure 1.

Whether the muscle is passively cooled or actively rewarmed after cooling is of no importance; the predominant factor in determining the outcome of physical performance is muscle temperature (15). Figure 1.

The relationship between muscle temperature and performance. Transversally within the muscle there is a temperature gradient the steepness of which is dependent on severity of cold exposure (18).

For example, exposing the bare forearm to 5 °C still air for 120 minutes induces a gradient in m.

flexor carpi radialis where muscle temperature from the warmer core decreases 0.5 °C for each 0.5 cm towards the shell of the muscle (14).

Contributing factors

There are individual differences in physical characteristics, which may modify the thermal responses to cooling and protect against loss of performance. First, subcutaneous fat acts as a thermal insulator slowing the rate of cooling (9).

Second, with increasing body size the surface area - body mass ratio decreases thus decreasing the area for heat loss and along with the increase in body size the body heat content also increases. Due to these factors increased body size slows the rate of cooling (6).

Third, a fit person is able to produce more heat than unfit, therefore maintaining thermal balance more effectively (10). All these factors may help to maintain performance capacity in cold environment.

On the other hand, with subjects matched for fitness level and amount of subcutaneous fat but differing with the amount of muscle mass (normal vs. muscular subjects) it was observed that substantial amount of muscle mass after passive cooling does not seem to protect against loss of performance (12).

The functional properties of skeletal muscle

In addition to decreased physical performance cooling has also a profound effect on func¬tional properties of skeletal muscle (8). It has been well verified that the rate of tension development in the beginning of muscle contraction i.e. the time to maximum force level (twitch or tetanic tension) is temperature dependent (e.g. 17).

The temperature sensitivity (Q10) of the rate of tension development in humans has been shown to be approximately 1.5 (17). A similar temperature dependence has been found also for the rate of relaxation at the end of muscle contraction (20).

The Q10 of the rate of relaxation in humans has been reported to be approximately between 1.7 - 2.3 (20) The velocity of muscle contraction itself, shortening and lengthening, is also slower in a given time when muscle tissue is cooled (8). Therefore, the power production of the muscle during shortening is less and power absorption during lengthening is more thus leading to a less powerful contraction of a muscle.

Cooling also slows nerve and muscle conduction velocity, which may result in a slower and weaker muscle contraction (5, 7). The decrease in conduction velocity has been reported to have a Q10 of approximately 1.4 (11) and absolute decrease in nerve conduction velocity has been reported to vary between 1.1 - 2.4 m • s-1/°C (7).

Reflex activity

Force production is regulated peripherally and/or centrally. Peripheral regulation is mainly conducted through reflex pathways, the stretch reflex (T-reflex) playing a major role. Many studies concerning the effects of cooling on stretch reflex have shown that cooling suppresses stretch reflex amplitude (e.g. 7).

There is evidence that the suppressed T-reflex ampli¬tude is due to decreased activity of the muscle spindles and thus decreased gammamotoneuron excitability (1) and these changes may lead to a decreased force production of a muscle (16). On the other hand, it also has been shown that during low-intensity repetitive work in cold. (when forearm muscle strain is higher in relation to same work in thermoneutral condition), stretch reflex responses are being enhanced in relation to thermoneutral responses.

This probably indicates that the increased strain of the working muscles were met by increas¬ing the reflex activity, therefore, in cold recruiting more muscle fibres in order to maintain the given work level (14).

Conclusions

It is evident that cooling deteriorates physical performance, its components, functional pro¬perties of the muscle and neural functioning. The amount of deterioration is dependent on the amount of cooling i.e. how much muscle temperature is lowered. There seems to be no "threshold" in muscle temperature after which performance starts to decrease, after the decrement starts immediately when muscle temperature decreases.

While maximal working capacity is lowered and submaximal work requires increased fibre recruitment the relative strain for a given athletic performance in cold weather can be higher. Since already peripheral and very superficial cooling is sufficient to decrease performance capacity this needs to be taken into account (e.g. to increase heat loss during exercise: do not expose working muscles to cold air) while designing exercise regimes in cold and especially when preparing for athletic competitions.

References

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Wiles CM, Edwards RHT. The effect of temperature, ischaemia and contractile activity on the relaxation rate of human muscle.Clinical Physiology 1982; 2: 485-497. rights reserved, Qatar Orthopaedic and Sports Medicine Hospital