Age-related directional bias of fingertip force.

We studied the direction of the three-dimensional fingertip force vector in young and old adults during a simple pressing task with the index finger. Ten young and ten old subjects pressed against a force plate with their index finger and maintained target forces that ranged from 2.5 to 15 N. Subjects viewed a display of the force normal to the force plate; forces tangential to the force plate were not displayed. Young adults produced fingertip forces that were nearly perpendicular to the plate at all target forces while old adults produced fingertip forces that deviated in the proximal direction of the horizontal plane, and ulnar direction of the vertical plane. The fingertip force deviated from the onset of pressing in both groups, but in young subjects the force aligned to perpendicular within 200 ms. Additional study is required to determine if these biased force vector directions contribute to clumsiness and slowing that are characteristic of fine manipulation in old age.