1-finger exercise reveals unexpected limits to dexterity

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1-finger exercise reveals unexpected limits to dexterity

Even seemingly simple movements seem to push the hand's neuromuscular control

system to its limits, with implications for both human rehabilitation and robot

hands

http://www.eurekalert.org/pub_releases/2009-07/uosc-ofe070709.php

" Push your finger as hard as you can against the surface. Now as hard as you can

but move it slowly - follow the ticking clock. Now faster. Now faster. "

These were the commands for volunteers in a simple experiment that casts doubt

on old ideas about mechanisms to control hand muscles. Complete understanding of

the result may help explain why manual dexterity is so vulnerable to aging and

disease, and even help design more versatile robotic graspers.

A research team led by Francisco Valero-Cuevas of the University of Southern

California reports the paradoxical result in the Journal of Neuroscience.

" We expected to find, " says the report, " that maximal voluntary downward force

would scale with movement speed…. Surprisingly, maximal force was independent of

movement speed. "

The observation challenges theories that date back nearly seventy years about

how the properties of muscles influence their everyday function, and how

" redundant " our bodies are.

According to Valero-Cuevas, who holds a joint appointment in the USC Viterbi

School of Engineering's department of biomedical engineering and the USC

Division of Biokinesiology and Physical Therapy, in many tasks muscle force is

affected by physiological " force-velocity " properties that weaken muscles as

they move faster.

" That is why your bicycle has gears, and why as a child you could not speed up

much on level ground, " he explains.

Valero-Cuevas and his collaborators set up a simple experiment to characterize

how finger velocity made a difference in the force produced during the common

manipulation task similar to rubbing a surface, using a computer track pad or

iphone. Adult volunteers wearing a closefitting Teflon cover on their

forefingers applied fingertip pressure on a slippery Teflon surface linked to a

force-measuring sensor.

First, the volunteers simply pressed as hard as they could without moving. Then,

still pressing as hard as they could they moved their fingers against the

surface to the beat of a metronome.

" As expected, maximal downward force diminished when motion was added to the

task, " the researchers wrote. " But remarkably, there were no significant

differences … between slow, and fast movement speeds… even though the movement

speeds varied 36-fold. "

The paper, " Maximal Voluntary Fingertip Force Production is Not Limited by

Movement Speed in Combined Motion and Force Tasks, " goes on to discuss and rule

out several possible explanations for the result, including differing levels of

dexterity by the subjects, non-linear responses by muscles, and finger-muscle

asymmetries.

The explanation proposed by Valero-Cuevas and collaborators (and former

students) G. Keenan of the University of Wisconsin/Milwaukee, J.

Santos of Arizona State University, and Madhusudhan Venkadesan of Harvard

University, is that the universe of possible commands sent by the brain to the

muscles is severely limited by the mechanical nature of the task, even for

ordinary manipulation tasks.

That is, say the scientists, that even for seemingly-simple real-world tasks

where we must control both force and motion, the neuromuscular system can be

pushed to its limits of performance.

This complements other recent work by Valero-Cuevas showing how other ordinary

tasks like tapping a surface are extremely challenging to the nervous system.

Together these results begin to identify the mechanical pressures that could

have driven the evolutionary specializations of our brains and bodies that make

our hands so dexterous.

" These apparently esoteric results have tremendous implications for both humans

and robots, " Valero-Cuevas says. " For one, they bring together basic research

and clinical reality by helping explain the high vulnerability of dexterous

everyday function to disease and injury in spite of the many muscles and joints

we have.

" In addition, they suggest to engineers that adding redundant motors to robots

may actually be the key to making them dexterous. "

The detailed interactions among muscles and body mechanics are complex and defy

easy mathematical modeling at this time, he adds, but further study may offer

clues.

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The research was supported in part by grants from the NSF and NIH.