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Control of wrist and arm movements of varying difficulties

  • 11/11/2011 10:43:00 AM
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Control of wrist and arm movements of varying difficulties

 Jason Boyle, Ph.D

Our muscles are controlled by “motor units”, which each consist of a neuron, and the muscle fiber(s) it activates or “innervates”. The muscle that responds is termed an “effector”. Brain mapping studies have shown that a disproportionate area of the motor cortex governs certain effectors of the body. For example, your fingers, lips, and tongue are highly innervated organs that can execute complex movement patterns, but your toes are not quite as skilled. So naturally this is an ideal system to operate under, since our daily activities require that we manipulate objects with our hands and speak with our mouth. Likewise, as technology advances, our daily interaction with electronic devices has become almost second nature. However, this interaction (movement pattern) can be altered by visual manipulations to our perceptual system.

Typically, movement with electronic devices involves grasping, moving or dragging a cursor from one defined target area to another. This target-to-target pattern has repeatedly shown an inverse linear relationship between speed and movement, with speed decreasing as difficulty of the task increases (size of or distance between targets). An illustration of this is the way fewer accuracy constraints tap back and forth between two pieces of poster board compared to two business cards. In a human-computer interaction task, when all perceptual information is held constant (target size and distance between targets), the wrist and arm show remarkably similar movement patterns (movement-initiation, time spent reversing in the target, movement-termination). However, when the visual gain (picture zoom) is doubled for the wrist and not the arm, based on movement-initiation and movement-termination markers, the motor system provides faster movement times for the wrist compared to the arm. This increase in speed is notsurprising because even though the task is still the same (dimensions of target size and distance), perceptually it has become easier. This increase in visual gain would be the same if you moved a computer mouse from icon to icon under 100% view and then under 200% view. The accuracy demands from icon to icon are viewed perceptually as easier. Interestingly though, when the arm is afforded this visual gain increase and not the wrist, no increase in speed of movement patterns is seen for the arm. These findings suggest that due to higher innervations of the motor units in the wrist compared to the arm, the system is more highly organized in controlling and exploiting the affordances provided to the perceptual system. This higher level of control in the wrist compared to the arm would prove especially important in a setting where the wrist and arms make manipulations to an amplified target on a screen.


Today’s technological advances in surgery have limited the amount of patient invasiveness through laparoscopic and robotic assisted surgery. However appealing these procedures may seem to a patient, these techniques require surgeons to make precise microscopic manipulations with the hands and arms while viewing their amplified task on a screen. Future investigations of this relationship between visual amplification and movement output could potentially provide a greater understanding of how the motor system receives and processes information, while simultaneously planning and executing movements in a safer manner.


For further readings related to this topic:
  1. Balakrishnan, I.L., & MacKenzie, I.S. (1997). Performance differences in the fingers, wrist, and forearm in computer input control. In Proceedings of the CHT ’97 Conference on Human Factors in Computing Systems. New York ACM, 303-310. 
  2. Kovacs, A. J., Buchanan, J. J., & Shea, C.H. (2008). Perceptual influences on Fitts’ law. Experimental Brain Research, 190, 99-103.



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