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Haptics Theories Upturned: Pressure Independent of Velocity

It has been a staple of our understanding of the processes behind human dexterity for decades, that the faster you push at something, the more force you exert. Now, it seems that is not actually accurate.

A research team led from the University of Southern California has reported strange results in the Journal of Neuroscience.

The researchers set up a simple experiment to characterise 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.

"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."

"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 report goes on to strip away potential reasons for skewed data, one at a time, including differing levels of dexterity by the subjects, non-linear responses by muscles, and finger-muscle asymmetries. With all ruled out, the only explanation left, is that physiological "force-velocity" properties 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," explained Valero-Cuevas, one of the authors of the study and 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.

The implications for haptic control systems, and artificial replication of force projection are quite profound, as it appears there are finite limits to force exerted by the muscles of the human body, regardless of the speed at which they are moving.

References

Maximal Voluntary Fingertip Force Production is Not Limited by Movement Speed in Combined Motion and Force Tasks

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