IMPACT OF PRE-EXERCISE REHYDRATION ON CENTRAL FATIGUE UNDER THE CONDITIONS OF HYPERTHERMIA
Kazys Vadopalas – Lithuanian Sports University, Kaunas
Relevance of the research. Human exercise performance depends on the body temperature. The results of numerous experiments have proved that work output decreases when the core temperature increases up to a critical point (in the case of persons of average physical activity – 38.7 ± 0.2o C and in highperformance athletes – 39.2 ± 0.1o C), especially when the intensive activation of the thermoregulatory and cardiovascular systems takes place. Hyperthermia can have a direct effect on voluntary activation of skeletal muscles as the temperature affects motor unit (MU) firing rate that is necessary for contraction summation in tetanic contraction. Dehydration might be factor acting in addition to overheating to impair exercise performance during hyperthermia. The maximum volume of liquid that is possible to be assimilated by physically active individuals is about 0.8–1.2 l / h. In sports training sessions and competitions we often come across the phenomena of hyperthermia and dehydration which interfere in achieving high sports results. It is not clear yet what is the effect of pre-exercise rehydration on central fatigue under conditions of hyperthermia. Research aim was to investigate effects of hyperthermia with and without dehydration on central fatigue during continuous high intensity exercise.
Research methods and organization. Ten males (medium distance runners) performed a maximal voluntary isometric contraction (MVC) of the knee extensors for 2 min (MVC – 2 min) under control conditions, after passive body heating (HT) and after (HT) plus rehydration (RH). During MVC – 2 min, at the 3 rd, 14th, 29th, 44th, 59th, 74th, 89th, 104th and 119th seconds superimposed electrical stimulation (TT-100 Hz) was performed. In HT and HT + RH experiments subjects sat immersed up to the waist in hot water (44 ± 1o C) bath for 45 min (air t 22o C, rh 40 %). During HT + RH experiment subjects slowly drank 1000 ml (100 ml every 6 min) the saline solution of 37o C in the course of 60 min (15 min before and during passive body heating). Rectal (Tre), skin and inner muscle temperatures were measured before and after both experiments. Heart rate (HR) was recorded every 5 s during passive body heating. Physiological heat stress index (PSI) was calculated as follows: PSI = 5(Tret – Tre0) × (39.5 – Tre0) -1 + (HRt –HR0) × (180 – HR0).
Results and discussion. Applying the modified methods of passive body heating, we evoked hyperthermia coupled with 1 % dehydration (rectal temperature was higher than 39o C, the subjects lost 1– 1.5 % of their body weight). We found that hyperthermia increased MVC fatigue and reduced voluntary activation of skeletal muscles compared to the control conditions. Rehydration did not influence the decrease of MVC during 2 min exercise, but after performing pre-exercise rehydration under the conditions of hyperthermia central fatigue decreased.
Conclusions. Our results suggest that pre-exercise rehydration might have an immediate positive effect in reducing thermal stress and thus reducing central fatigue even when exercise is performed during hyperthermia induced by passive warming of the body. The decrease of central fatigue is influenced by the changes in rectal temperature and cardiovascular system.