The role of head position and prior contraction in manual aiming

• Arm positioning undershot the horizontal position with the head turned away from the arm.
• Arm positioning overshot a target with the head turned towards the arm.
• A pre-contraction caused undershooting of the reference position regardless of head position.
• Head position and a pre-contraction have independent effects on positioning accuracy.

We sought to determine if the asymmetrical tonic neck reflex influences the accuracy of self-selected arm positioning without vision and to ascertain if such accuracy is influenced by a pre-contraction of the prime movers. Participants reproduced an arm position using their abductors with the head in midline, rotated towards and away from the arm. Arm movements were made with and without a pre-contraction of the abductors. Twenty participants performed eight trials in each of the six different conditions. Compared to the midline position, participants undershot the reference position with the head turned away and overshot the position with the head rotated towards the arm. A pre-contraction caused undershooting regardless of head position. Results suggest that head position and pre-contraction may have significant and independent effects on arm positioning.