Abstract
In this article both movement errors and successful movements are considered to be the product of varying ratios of muscle force signals and the composite of neuromotor noise in which the force signal is embedded. Based on earlier work we derived four propositions which together form a theoretical framework for understanding the incidence of error in conditions of time pressure and mental load. These propositions are (1) motor behaviour is an inherently stochastic and therefore noisy process, (2) biophysical, biomechanical and psychological factors all contribute to the level of neuromotor noise in a movement signal, (3) endpoint variability of movement is related to the signal-to-noise ratio of the forces which drive the moving limb to the target, and (4) optimal signal-to-noise ratios in motor output can be arrived at by adjusting limb stiffness. In an experiment with a graphical aiming task in which subjects made pen movements to targets varying in width and distance, we tested the prediction that time pressure and dual task load would influence error rates and movement noisiness, together resulting in biomechanical adaptations of pen pressure. The latter is seen as a manifestation of a biomechanical filtering strategy to cope with increased neuromotor noise levels. The results confirmed that especially under time pressure error rates and movement noise were enhanced, while pen pressure was higher in both conditions of stress.
