Effects of reduced muscle temperature on the oxygen uptake kinetics at the start of exercise T. SHIOJIRI, 1 M.SHIBASAKI, 2 K. AOKI, 3 N. KONDO 4 and S. KOGA 5 1 Laboratory of Exercise and Sports Science, Yokohama City University, Japan 2 Division of Intelligence Science, Graduate School and Technology, Kobe University, Japan 3 Division of Education, Graduate School, Kobe University, Japan 4 Faculty of Human Development, Kobe University, Japan 5 Applied Physiology Laboratory, Kobe Design University, Japan SHIOJIRI, T., SHIBASAKI, M., AOKI, K., KONDO, N. & KOGA, S. 1997. Effects of reduced muscle temperature on the oxygen uptake kinetics at the start of exercise. Acta Physiol Scand 159, 327±333. Received 25 September 1995, accepted 5 November 1996. ISSN 0001±6772. Laboratory of Exercise and Sports Science, Yokohama City University; Division of Intelligence Science, Graduate School and Technology, Division of Education, Graduate School, Faculty of Human Development, Kobe University; and Applied Physiology Laboratory, Kobe Design University, Japan. The purpose of this study was to examine the effects of reduced muscle temperature (T m ) on gas exchange kinetics and haemodynamics at the start of exercise. Six male subjects performed moderate cycle exercise under reduced (C) and normal (N) T m conditions. T m and rectal temperature were signi®cantly reduced by immersion in cold water (by 6.6 °C and 1.8 °C, respectively). The increases in oxygen uptake ( _ VO 2 ) and oxygen pulse ( _ VO 2 /HR) during phase 1 (abrupt increase after the start of exercise) were signi®cantly lower under C than under N. The time constant for O 2 under C (36.0  7.7 (SD) s) was signi®cantly greater than under N (27.5  4.4 s); however, the time constants of cardiac output under C (38.3  16.6 s) and N (33.7  18.5 s) were similar. These results suggest that the slower _ VO 2 on-response under reduced T m conditions is caused by decreased O 2 extraction in working muscle and/or by impairment of oxidative reactions by reduced muscle temperature. Keywords cardiac output, exercise transition, oxygen uptake kinetics, reduced muscle temperature. Cold water immersion reduces body temperature and muscle temperature ( T m ), and decreases heart rate (HR) (McArdle etal. 1976, Bergh & Ekblom 1979) and muscle blood £ow (Ishii et al. 1992) during exercise. A leftward shift in the oxygen dissociation curve of haemoglobin also occurs when T m decreases. Consequently, oxygen transport from the lungs to working muscles is proba- bly impaired during exercise in the presence of reduced T m . On the other hand, a reduction inT m decreases the rate of adjustment of aerobic resynthesis of ATP be- cause of its positive thermal dependence, and thus the decreased rate of adjustment of ATP resynthesis at re- duced T m must be compensated for by a greater fraction of available active muscle mass than at normal T m to meet the O 2 demand of exercise (Ferretti 1992). Beelen & Sargeant (1991) have suggested that recruit- ment of muscle ¢bres, including faster ¢bre types, is greater during exercise when T m is reduced than at normal T m , and thus a relatively slower adjustment of aerobic resynthesis of ATP and a higher contribution of anaerobic resynthesis at the start of exercise are required at reduced T m . We therefore hypothesized that _ VO 2 kinetics at the start of exercise are slower at reduced T m because of impaired oxygen transport to working muscles and/or restricted oxygen utilization in the muscles. However, some investigators (Beelen & Sargeant 1991, Ishii et al. 1992) have reported that _ VO 2 kinetics at the start of exercise at reduced T m and nor- mal T m are similar. We questioned their conclusions concerning kinetics that have not been quantitatively Correspondence: Tomoyuki Shiojiri, Laboratory of Exercise and Sports Science, Faculty of Science,Yokohama City University, 22^2 Seto, Kanazawa-ku,Yokohama 236, Japan. Acta Physiol Scand 1997, 159 , 327^333 Ó 1997 Scandinavian Physiological Society 327