Force control and its relation to timing.

Previous work (Keele & Hawkins, 1982; Keele, Pokorny, Corcos, & Ivry, 1985) has suggested two general factors of coordination that differentiate people across a variety of motor movements, factors of timing and maximum rate of successive movements. This study provides comparable evidence for a third general factor of coordination, that of force control. Subjects who exhibit low variability in reproducing a target force with one effector, the finger, tend to show low variability with two other effectors, the foot and forearm. In addition, ability in force control cuts across different force ranges and across situations where force control is either the primary goal or the secondary goal. Force records obtained during a periodic tapping task show that, although force control is largely independent of timing, there are some interactions between the two factors. Force variation appears to distort timing a small amount in part because larger forces speed up implementation of movement, thereby shortening preceding intervals and lengthening following ones, and in part because force variation alters central-timing mechanisms.