How do humans turn? Head and body movements for the steering of locomotion Halim Hicheur and Alain Berthoz

Turning in humans represents a greater challenge than straight ahead walking in terms of postural control. Indeed, when steering along a curved path humans have to adapt their locomotor pattern by taking into account centrifugal acceleration, which tends to keep their bodies along a straight trajectory. Many observations have been reported with respect to changes in the locomotor pattern between straight and curved walking, at the level of the lower limbs (legs) coordination. In this study, we investigated how head and body movements are coordinated for the steering of the whole body along a curved path or a straight path (control condition). We manipulated two factors - walking speed and path curvature - in order to analyze how centrifugal acceleration affects locomotor control at the level of upper limbs movement. Head and body orientation relative to walking direction were found to be determined both by biomechanical factors (induced by stepping activity) and geometrical features of the path. However, with increasing centrifugal acceleration, only geometrical constraint remains and head and body were rigidly oriented in advance of the future walking direction, independently of the stepping activity. Walking speed condition did significantly affect maximal, minimal as well as amplitude of head and body deviation during straight walking, but not during turning. Our observations are discussed within the general framework of biological motion control and some suggestions are proposed for the implementation of these experimental observations for the design of humanoids robots