Limits in motor control bandwidth during stick balancing

Abstract

Why can we balance a yardstick but not a pencil on the tip of our finger? As with other physical systems, human motor control has constraints, referred to as bandwidth, which restricts the range of frequency over which the system can operate within some tolerated level of error. To investigate control bandwidth, the natural frequency of a stick used during a stick-balancing task was modified by adjusting the height of a mass attached to the stick. The ability to successfully balance the stick with the mass positioned at four different heights was determined. In addition, electromyographic signals from forearm and trunk muscles were recorded during the trials. We hypothesized that 1) the probability of successfully balancing would decrease as mass height decreased; and 2) the level of muscle activation in both agonist and antagonist would increase as the natural frequency of the stick increased. Results showed that as the mass height decreased the probability of successfully balancing the stick decreased. Changes in the probability of success with respect to mass height showed a threshold effect, suggesting that limits in human control bandwidth were approached at the lowest mass height. Also, the level of muscle activation in both the agonist and antagonist of the forearm and trunk increased linearly as the natural frequency of the stick increased. These changes in muscle activation suggest that the central nervous system adapts muscle activation to task dynamics, possibly to improve control bandwidth.

Trunk muscle coactivation is tuned to changes in task dynamics to improve responsiveness in a seated balance task

“…Quantitatively speaking, the responsiveness of control can be expressed in terms of the bandwidth, which reflects the attenuation and/or phase shift from the input (pencil starts to fall over) to the output (move fingertip under the pencil). Using a stick balancing task, the responsiveness of human motor control was studied by lowering a mass affixed to the stick (Reeves et al, 2013). By lowering the mass, the natural frequency of stick movement increased and the task became more challenging.…”

Less precise motor control leads to increased agonist-antagonist muscle activation during stick balancing

“…To assess the effect of focal height on the probability of success, the number of successful and failed trials was used to determine the probability of successful balancing for each subject at each focal height, based on Reeves et al (2013). Briefly, the probability of success can be approximated by the following:…”

“…This step yielded 3 differences (100-75 cm, 100-50 cm, and 100-25 cm) for each of the variables (angular velocity, forearm agonist and antagonist, and trunk agonist and antagonist). Next, 4 univariate regression models with angular velocity used as an (Reeves, Pathak, Popovich, & Vijayanagar, 2013). First, the time periods where peaks in agonist muscle activation levels during the stick balancing trials were determined (round end of arrow).…”

“…In a recent stick balancing study, it was shown that the forearm and trunk muscle activation changed with task dynamics (Reeves, Pathak, Popovich, & Vijayanagar, 2013). Task dynamics were modified by adjusting the height of a mass affixed to the stick.…”

Understanding robust control theory via stick balancing

“…This paper aims to begin changing that by presenting the most essential concepts in robust control using human stick balancing. This simple case study is popular in the sensorimotor control literature [1], [11], [12]. Moreover, we model it as an inverted pendulum on a moving cart that is extremely familiar to engineers and scientists [13], [14].…”