Everyone has trouble staying fit and motivated during the colder months of the year. Not only is it harder to stay motivated to train in cold conditions, but the lower temperatures generally means an increased incidence of colds/flu and other similar illness's.
Motivation to train is extremely hard in cooler conditions, not only is it harder to wake up early but the cold can actually decrease performance and immune system function. In order to stay motivated and fit it is important to stay warm. This goal can be achieved by ensuring appropriate clothing is worn when exercising; covering the body to keep warmth in and decrease the effects of cool air and wind chill is the aim. Clothing is the most important factor when trying to combat cold stress. Exposed skin is at a much higher risk of being affected by cold then skin substantially covered. It is also advisable to keep dry, therefore having athletic clothing which will keep your core warm but allow sweating to perform normally is the most beneficial in trying to decrease the effects of cold. If clothing becomes wet eg: rain, it should immediately be changed to decrease the effects of cold stress. Another helpful tip is to maintain a high rate of heat production to ensure the body is handling the effects of cold, by increasing the heat produced by the body it offsets the effects of cold.
The last issue about exercising in the cold is the increased incidence of illness especially the common cold or flu. By wearing the appropriate clothing and following the above steps it will stop the detrimental effects of cold.
ACSM's Resource Manual for Guidelines for exercise testing and prescription.