A slow limit: extensive motor training cannot overcome a limit on the production of slow and smooth motion.

Previous research from several paradigms indicated that people have difficulty in producing slow and smooth movements (SSM). It is not clear whether these difficulties are due to biomechanical constraints, planning constraints, or lack of experience with moving slowly. Here, we investigated the latter possibility: we empirically tested whether short-term practice and long-term expertise with moving slowly would result in an increased ability to perform SSM. In , novice participants completed 10 training sessions in which they moved a stylus on a digitizing tablet as they traced an ellipse that moved at different frequency/peak velocity combinations, with frequencies ranging from 0.25 Hz to 0.875 Hz. In , experts in slow movement (tai chi performers) and fast movement (karate performers) completed the same task in one session. The results indicated that all participants had difficulties in producing SSM, as evidenced by an increase in submovement rate with decreasing frequency. Participants in did show a marked improvement in their ability to produce fewer submovements while reducing mean squared jerk (MSJ). These short-term improvements were not evident in long-term slow motor expertise (i.e., tai chi). Taken together, our results suggest that SSM are likely difficult to produce due to a combination of several factors: lack of experience, planning, biomechanical, and neural constraints. People can improve their ability to produce smooth slow movements in the short term (10 training sessions) in a sinusoidal tracking task by reducing the rate of submovements, but they are not able to eliminate them entirely. This lack of an ability to move slowly and smoothly was confirmed in a second experiment, where there was a lack of difference observed between long-term slow movement practitioners (i.e., tai chi experts) and novices.