Effect of threshold loads on voluntary control of slow and rapid inspiratory movements.

The purpose of this study was to examine the hypothesis that voluntarily produced inspiratory movements are preplanned. Subjects performed both rapid (0.5 s) and slow (2.0 s) voluntary inspirations to a target volume of 50% of their inspiratory capacity under two conditions: 1) normally unloaded with random loading; and 2) normally loaded with random unloading. The load was a 10 cmH2O threshold load. When the load was unexpectedly applied, the subjects undershot the target volume by 397 ml (fast) and 284 ml (slow). When the load was unexpectedly removed, the subjects overshot the target volume by 303 ml (fast) and 224 ml (slow). The duration of inspiration did not change significantly. These observations are consistent with an "impulse-timing" model of preplanned voluntary movement, which incorporates reflex modification of the movement to compensate for loads which may be added or removed.