The aim of this study was to determine the effect of mechanical properties of the pen on quality of handwriting in children. A total of 22 school-aged children, aged 8 to 14 years, wrote in cursive using a pen attached to a robot. The robot was programmed to increase the effective weight (inertia) and viscosity of the pen. Speed, frequency, variability, and quality of the 2 handwriting samples were compared. Increased inertia and viscosity improved handwriting quality in 85% of children (P < or = .05). Handwriting quality did not correlate with changes in speed, suggesting that improvement was not due to reduced speed. Measures of movement variability remained unchanged, suggesting improvement was not due to mechanical smoothing of pen movement by the robot. Because improvement was not explained by reduced speed or mechanical smoothing, we conclude that children alter handwriting movements in response to pen mechanics. Altered movement could be caused by changes in sensory feedback.