Neuroscience Letters 384 (2005) 127–132 Trunk muscle proprioceptive input assists steering of locomotion Micaela Schmid a,b , Alessandro Marco De Nunzio a,b , Marco Schieppati a,b,∗ a Department of Experimental Medicine, Section of Human Physiology, University of Pavia, Pavia, Italy b Human Movement Laboratory, Centro Studi Attivit` a Motorie (CSAM), Fondazione Salvatore Maugeri (IRCCS), Scientific Institute of Pavia, Via Ferrata 8, I-27100 Pavia, Italy Received 16 February 2005; received in revised form 19 April 2005; accepted 22 April 2005 Abstract During locomotion, human subjects navigate in their environment and choose the direction by means of the internal representation of space that is continuously updated by sensory input. Aim of this study was to assess whether trunk proprioceptive information plays a role in the definition of the reference frame for orientation. Unilateral trunk muscle vibration was applied during locomotion along a straight path in seven subjects. Vibration was administered either from the onset or in the middle of a seven-step task, under eyes-open (EO) or blindfolded condition. The deviation of the walking trajectory was quantified by the distance of the seventh from the first foot print along the medio-lateral axis. Foot angles and stride lengths were computed for all foot-falls. Vibration produced a clear-cut deviation from the straight-ahead direction when delivered in the middle of blindfolded locomotion. With EO the deviation was much smaller. A mild deviation was obtained in blindfolded condition when vibration started at the onset of locomotion. All deviations from the straight-ahead were accompanied by coherent changes in foot orientation on the ground. Trunk proprioception plays a major role in the definition of locomotor trajectory. Trunk input seems to be weighted against vision and whole-body kinematic information. © 2005 Elsevier Ireland Ltd. All rights reserved. Keywords: Trunk muscle; Vibration; Locomotion; Straight-ahead; Reference frame When human subjects navigate in their environment, they need to identify the body position in space and the distribu- tion of the external objects with respect to them. The ability to orient in the space requires the definition of an internal body schema, which contains knowledge about representation of the space, body geometry and verticality [4,29]. The defini- tion of this schema is largely based on sensory information and experience. The CNS integrates many different sensory inputs, visual, vestibular, cutaneous, gravito-inertial, proprioceptive, in or- der to compute egocentric co-ordinates [19,23] both under static conditions and during navigation [2]. The weight as- signed to each of these inputs in the definition of the refer- ence frames is different, depending on the environment and the constraints in which the movement is performed [13]. ∗ Corresponding author. Tel.: +39 0382 592008; mobile: 39 335 8000 431; fax: +39 0382 592081. E-mail address: mschieppati@fsm.it (M. Schieppati). Vision seems to dominate in determining subjective body orientation [14] and may be considered at the top of a sen- sory hierarchic scale contributing to the reference frame def- inition [31]. However, in its absence, the information from the labyrinth and proprioceptors appears to be important [3,28]. The vestibular system provides both the allocentric reference and direct information as to the head movement in space. Muscle-spindle input forms a continuous proprio- ceptive chain from the feet to the eyes and applying tendon vibration at any level in this chain alters the internal repre- sentation of body posture [14,16,27]. Vibration almost selectively activates the primary termi- nations of the muscle spindle, very much as a tendon tap does [25]. Continuous muscle vibration has powerful effects on upright stance that are partly connected to the subject’s reactions to the illusions of movement produced by vibra- tion [14,17,22,7]. During stepping-in-place, hamstring mus- cles vibration produces involuntary forward stepping, and during treadmill locomotion involuntary step-like increase 0304-3940/$ – see front matter © 2005 Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.neulet.2005.04.059