"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
|Contact: Eric Mankin|
University of Southern California