Relationship between performance at different exercise intensities and 1 skeletal muscle characteristics

27 28 The hypothesis investigated whether exercise performance over a broad range of intensities is 29 determined by specific skeletal muscle characteristics. Seven subjects performed 8-10 30 exhaustive cycle trials at different work loads ranging from 150 to 700 W (150 min 20 s). 31 No relationships between the performance times at high and low work loads were observed. A 32 relationship (P<0.05) was observed between the percentage of FTx fibers and the exercise 33 time at 579±21 W (~30 s; r=0.88). Capillary-to-fibre-ratio (r: 0.58-0.85) was related 34 (P<0.05) to exercise time at work intensities ranging from 395 to 270 W (2.5-21 min). 35 Capillary density was correlated (r=0.68; P<0.05) with the net rate of plasma K 36 accumulation during a ~3-min bout, and was estimated to explain 50 to 80% (P<0.05) of the 37 total variance observed in exercise performances lasting approximately 30 s -3 min. The 38 Na,K pump β1 subunit expression was found to account for 13-34% (P<0.05) during 39 exhaustive exercise of approximately 1-4 min. In conclusion, exercise performance at 40 different intensities is related to specific physiological variables. A large distribution of FTx 41 fibres may play a role during very intense efforts, i.e. ~30 s. Muscle capillaries and the 42 Na,K pump β1 subunit seem to be important determinants for performance during 43 exhaustive high-intensity exercises lasting between 30 s to 4 min. 44 45