Nowadays, eccentric exercise can be realized in a dynamic way with an ergocycle wich permits eccentric (ECC) lower limbs contractions at a defined power output. With this type of ergometer, O2 consumption (VO2) in ECC is three fold lower than in concentric (CON) for the same power output. Consequently, ECC program could be used in chronic disorders with limited capacities, such as obesity.

The investigators hypothesis that an eccentric training program, as it will induce higher mechanical constraints, would improve body composition, especially by reducing fat mass. This effect could be explained by an increased rest energy expenditure and a better improvement in biological parameters (particularly lipid profile and insulin-resistance) after ECC training (versus CON training).

This study aims to compare the modification of fat mass after an ECC program versus a CON program at the same VO2. The secondary goal is to assess the physiological mechanisms involved in the modification of body composition

Thirty adults will be randomized into 2 groups: CON or EXC. The patients of ECC group will follow habituation sessions (2weeks) to avoid the secondary muscular effects of high intensity eccentric exercise. The training program will last 12 weeks for the two groups. Before and after, each subjects will participate to the following measurements: total and segmental body composition (DXA), aerobic capacities (maximal incremental exercise test, VO2max), muscular lower limbs strength (isometric end isokinetic dynamometer), rest energy expenditure (indirect calorimetry), biological parameters (lipid profile, insulin-resistance), physical activity (actimetry), food intakes and quality of life (questionnaires).

Inclusion Criteria:

• - Adults
• Obese (BMI>35)
• Inactive (<10 METS/h/week)
• No other chronic disease (cardio pulmonary, osteo articulary or muscular diseases), no surgery during the last 3 months, no contraindication to physical activity

Exclusion Criteria:

• (BMI<35)