The purpose of this study is: i) to determine if maximal oxygen uptake in downhill running is significantly different versus flat or uphill running, and ii) to compare cardiorespiratory and muscle fatigue responses to uphill vs flat and downhill running at similar running velocity and similar metabolic power. The investigators hypothesized that maximal oxygen uptake will be lower in downhill running. The investigators also anticipate attenuated cardiorespiratory and muscle fatigue responses to downhill running when compared to flat or uphill running performed at similar velocity but not when the comparison is done at similar metabolic power.

Inclusion Criteria:

• to be a well-trained runner with a VO2max > à 55 ml/kg/min
• nonsmoker or stopped smoking for 5 years
• BMI < 25
• affiliation to the social health insurance scheme
• signing an informed consent form.

Exclusion criteria:

• impossibility to give the subject enlightened information (subject in emergency situation, difficulties in understanding the study, ...);
• subject under the protection of justice
• subject under guardianship or curatorship
• contraindication to the practice of physical and sport activities
• lower limbs musculotendinous or articular problems
• respiratory, cardiovascular or metabolic pathology
• drug treatment in progress and impossibility to stop it within 7 days before the beginning of the study