Slowing of dexterous manipulation in old age: force and kinematic findings from the 'nut-and-rod' task.

The task of sliding a nut from a rod has been used to study manual slowing in old age (Smith et al. in Neurology 53:1458-1461, 1999; Neurobiol Aging 26:883-890, 2005). In this experiment, we sought to determine if the age-related slowing in this task occurs with losses of motor precision, as indicated by the forces exerted on the rod. The forces exerted by the nut on the rod were monitored along with the kinematics of the hand in old and young adults while they attempted to lift a nut from three vertically oriented rods of different shape (straight, single curve, double curve). Old adults performed the task 64% slower than young adults for the straight rod, 100% slower for the single-curve rod, and 80% slower for the double-curve rod. Old adults did not differ from the young adults in the amount of force exerted against the rods in the horizontal plane, or in the steadiness of these forces, but exerted greater force impulses in the vertical direction over the course of a trial (359% straight, 236% single curve, 214% double curve) and much more force in the vertical direction (255% straight, 267% single curve, 159% double curve). Old adults also performed the task with 35% greater average roll of the hand into pronation. We suspect that old adults tilted the nut, even for the straight rod, dragging it against the rod to create the elevated vertical forces. These observations support previous speculation that old adults do not control the external moments applied to grasped objects as well as young adults.