Minimizing command timing variability is a key factor in skilled actions

Variability between movements prevents the best athletes from making a perfect shot every time. While fluctuations in the amplitude of neural sensory inputs and motor outputs are thought to be primarily responsible, increasing evidence suggests that they account for only a fraction of the observed variability. Here, we propose that a significant portion of the variability is due to imprecisely timed motor commands. This theory best explained the position and force variability observed during discrete and periodic movements. Furthermore, we show how the timing variability in the non-dominant arm's muscles is larger than in the dominant arm, suggesting a relationship between handedness and the command timing variability of the left- and right-hands. Our theory provides a unifying computational perspective of movement variability that is essential for understanding motor control.