Is the human arm made of tunable springs?

A study was conducted to determine in what physiological conditions, if any, the human arm deviated from 'spring-like' behavior during movement. Simple, short force pulses applied in mid-motion were found to consistently shift the desired endpoint of a movement across all four subjects. Far from being spring-like, the human arm during simple point-to-point movement did not behave in a conservative manner and could not even be described as a stable passive dynamic system around a desired trajectory. In fact, muscle properties and automatic reflex response can result in significant deviations from the desired trajectory, with over-compensation in the form of an active response, without changes in 'central command'.