Randomization determines the mode of muscular strengthening : either exclusively eccentric or exclusively concentric.The principal objective is to demonstrate the superiority of the improvement in the quadriceps isokinetic torque after isokinetic muscular strengthening by the eccentric mode compared to the concentric mode.The following parameters are also evaluated : the variations in the level of pain , the parameters of walking ( maximum speed over 10 and 200 meters , analysis on a computerized Gaitrite treadmill ) , static equilibrium ( on a FUSYO force platform ) , and the functional status of the patient using the Western Ontario and MacMaster Universities osteoarthritis index ( WOMAC ) questionnaire after the strengthening period and at 6 months .