Inter-individual variability in energetic cost of running may be partly explained by lower limbs mechanical characteristics. The transposition of muscle mechanical model with two components (contractile and elastic) to the whole lower limb mechanical behavior analysis during running. The main aim of present investigation is to study the effects of lower limb mechanical characteristic (force, velocity, stiffness) on the energetic cost of running using a lower limb mechanical approach including both contractile and elastic components.Twenty healthy men will run on a treadmill at 12km.h-1 in 8 different 5 min-conditions. Through conditions, stride frequency, sole stiffness and subject's body weight will be changed to allow artificial variations in lower limb mechanical characteristics. Energetic and non-invasive mechanical measurements analysis will be performed to obtain energetic cost of running and to compute force, velocity and stiffness of lower limbs for each condition.

Main aim: Identify the effect of mechanical characteristic induced on lower limbs on the energetic cost of running.

Secondary aim : Minimize the energetic cost of running by lower limb mechanical characteristics optimization.

Inclusion Criteria:

• healthy men being between 20 and 35 years old and doing regularly physical activities
• subjects must be able to run at 12 km/h during 15 minutes in aerobic condition
• not be sedentary and not do running competitions
• be affiliated to a social security system
• Sign a written inform consent

Exclusion Criteria:

• presenting a clinic sign of intolerance to exercise, and particularly to running
• overweight subjects
• having a medical treatment which may interfere with the running quality or with one of the physiological parameters measured
• taking part at the same moment in another medicinal research