Effect of age and speed on the step-to-step transition strategies in children.

The development and acquisition of mature walking in children is multifactorial, depending among others on foot interaction with the ground, body dynamics and the knowledge of the 'rules' stemming from the gravity field. Indeed, each step the velocity of the centre of mass must be redirected upwards. This redirection may be initiated by the trailing leg, propulsing forward and upward the body before foot contact, or later by the loading limb after the contact with the ground. While it has been suggested that mature walking develops slowly from first independent steps to about 7 years of age, it is still unknown how children acquire the appropriate loading and propulsion forces during the step-to-step transition. To answer that question, twenty-four children (from 3 to 12 years old) and twelve young adults (from 20 to 27 years old) walked on force platforms at different walking speed. The ground reaction forces under each foot were recorded and the vertical velocity of the centre of mass of the body was computed. With decreasing age and increasing velocity (or Froude number), the occurrence of unanticipated transition is higher, related to a different ratio between the vertical support of the front and back leg. The different transition strategy observed in children indicates that body weight transfer from one limb to the other is not fully mature at 12 years old.