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1 Sport and Health Sciences, Exeter University, EXETER, United Kingdom
2 School of Sport and Health Sciences, University of Exeter, Exeter, Devon, United Kingdom
* To whom correspondence should be addressed. E-mail: a.m.jones{at}exeter.ac.uk.
We hypothesized that a period of endurance training would result in a speeding of muscle phosphocreatine concentration ([PCr]) kinetics over the fundamental phase of the response and a reduction in the amplitude of the [PCr] slow component during high-intensity exercise. Six male subjects (age 26±5 years) completed 5 weeks of single-legged knee-extension exercise training with the alternate leg serving as a control. Before and after the intervention period, the subjects completed incremental and high-intensity step exercise tests of 6-min duration with both legs separately inside the bore of a whole-body magnetic resonance spectrometer. The time-to-exhaustion during incremental exercise was not changed in the control leg (PRE: 19.4±2.3 vs. POST: 19.4±1.9 min) but was significantly increased in the trained leg (PRE: 19.6±1.6 vs. POST: 22.0±2.2 min; P<0.05). During step exercise, there were no significant changes in the control leg but end-exercise pH and [PCr] were higher after, compared to before, training. The time constant for the [PCr] kinetics over the fundamental exponential region of the response was not significantly altered in either the control leg (PRE: 40±13 vs. POST: 43±10 s) or the trained leg (PRE:±8 vs. POST: 40±12 s). However, the amplitude of the [PCr] slow component was significantly reduced in the trained leg (PRE: 15±7 vs. POST: 7±7 % 
[PCr]; P<0.05) with there being no change in the control leg (PRE: 13±8 vs. POST: 12±10 % [PCr]). The attenuation of the [PCr] slow component might be mechanistically linked with enhanced exercise tolerance following endurance training.
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