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To better understand the functional role of early postural adjustments associated with gait initiation, we studied ground reaction forces before the first step in a group of 10 young and 18 elderly normal subjects as well as in a group of 11 patients with marche à petits pas, an age-related gait disorder in which difficulties in executing the first step are encountered. Kinematic data and ground reaction forces were recorded synchronously and processed by an ELITE motion analysis system. Start difficulties in the patients might be explained by impaired amplitudes and directions of anteroposterior forces as well as smaller values of vertical forces than in the controls walking with comparable step lengths. Indeed, the abnormalities of anteroposterior forces lead to defective propulsive forces, whereas the smaller upward reaction forces make the elevation of the starting leg more difficult. These perturbations are likely to reflect impairments of muscular synergies of the lower limbs manifested by disturbances in the agonist-antagonist coupling, and lack of coordination between both limbs.