Cooling and neuromuscular performance in man

Abstract

The quality and quantity of the problems arising in neuromuscular performance during work or leisure time activities in cold should be known better. Especially, the possible dose - dependent relationship between different levels of cooling and rewarming exercise in relation to changes in neuromuscular performance have remained obscure. This thesis focuses on the effects of a standard cold exposure and different levels of cooling as well as the effect of rewarming exercise on muscular performance and co-activation of agonist - antagonist muscle pairs (reflected by EMG) during stretch-shortening cycle. During the standard cold exposure 48 male subjects (age 25±2 yr, height 176±4 cm and weight 72±2 kg) sat at 10°C for 60 min wearing shorts and jogging shoes. The exposure of same duration to 27°C was considered as thermoneutral reference. Exercise type which prominently utilises the elastic properties of the muscles (very fast stretch-shortening cycle exercise), drop jump, was found to be most susceptible for cooling. To study the effects of different levels of cooling the subjects sat at 27°C, 20°C, 15°C and 10°C for 60 min on different occasions. After the exposure to 10°C the subjects performed rewarming walking exercise. The results showed that muscular performance and its variables decreased when ambient exposure temperature was lowered (decreasing muscle temperature) but recovered with rewarming exercise (increasing muscle temperature). An unexpectedly low level of cooling, exposure to 20°C, was found to be sufficient to significantly decrease muscular performance (17 % · °C⁻¹ in Tₘ). In relation to muscle temperature a dose - dependent change in the co-activation of the working agonist-antagonist muscle pairs was found. The activity of the agonist muscle during preactivity and stretch phases increased (ea. 34 % and 32 %, respectively), whereas during the shortening phase it decreased (ea. 21 %). During the shortening phase, on the contrary, the activity of the antagonist muscle increased (ea. 47 %). This ''braking effect" of agonist-antagonist muscle pairs during shortening phase is, in part, responsible for the decreased muscular performance. Alterations in the T- and H-reflexes could be related to the EMG-activity changes of the lower leg muscles observed during a stretch-shortening cycle after cooling.