Seasonal variation in work performance and heart rate response to exercise. A study of 1,835 middle-aged men

A near maximal bicycle exercise test in 1,835 presumably healthy Norwegian men indicated a seasonal variation in physical fitness. Thus, the total amount of work performed was significantly higher, and the work pulse on equivalent loads significantly lower during the summer than during the autumn. Although the differences were small, they may invalidate comparisons made between exercise tests in populations tested at different seasons of the year. In particular, there was a considerable and sudden change in the above mentioned parameters from June--August to September--October.