Why do we move the way we do?

There are several advancing theories as to how the brain calculates motor commands in humans. With these theories, there are also various mathematical models to describe how the brain optimizes certain movements.

One way to optimize movement is to use kinematic models, which deal with position, velocity, and any other derivative of position. Kinematic models optimize movements in regard to precision, speed, or smoothness. Another way to optimize movements is with dynamic models, which deal with forces generated by muscles and the resulting torques placed on joints. Dynamic models can optimize energy used by muscles or stresses placed on joints.

It is possible to examine the dynamic effects on movements by looking at the effects, if any, that gravity has on arm movements. Because gravity is a one directional force, movements in different directions should be affected distinctly. On the other hand, if there was no differential effect, dynamics would be said to play no role in movement.

To test this, I used an augmented reality setup with high speed cameras known as an OptoTrak. In this setup, subjects would look through a semi-transparent mirror and see nine virtual images in the sagittal plane in front of them-one being the center of the circle formed by the other eight. The subjects held a ping pong paddle with an infrared marker on it that the cameras could monitor. The subjects would start with the marker in the center object, and then move to the random object to which they were instructed. Two subjects performed eighty movements with an unweighted paddle, and then eighty movements with a half pound weight attached to the paddle.

The results of the experiment were inconclusive. There was no apparent trend between weighted and unweighted, which might suggest the dynamic model plays no role in movement. However, there were too few subjects with too few trials to make such a generalization. Further experimentation is needed.