An Indian Institute of Technology, Madras, alumnus has discovered that humans' ability to use their fingertips for performing various tasks is the result of a complex neuro-motor-mechanical process orchestrated with precision timing by the brain, nervous system and muscles of the hand.
Madhusudhan Venkadesan of Cornell University's Department of Mathematics made this finding as part of a study to understand the biological, neurological and mechanical features of the human hand that enable dexterous manipulation.
'When you look at the hand, you think, five fingers, what could be more straightforward?' said University of Southern California biomedical engineer Francisco Valero-Cuevas, who co-authored the study with Venkadesan.
'But really we don't understand well what a hand is bio-mechanically, how it is controlled neurologically, how disease impairs it, and how treatment can best restore its function. It is difficult to know how each of its 30-plus muscles contributes to everyday functions like using a cell phone or performing the many finger tasks it takes to dress yourself,' the researcher added.
During the study published in The Journal of Neuroscience, the researchers asked volunteers to tap and push against a surface with their forefinger, and recorded the fingertip force and electrical activity in all of the muscles of their hands.
The research team recorded 3D fingertip force as well as the complete muscle coordination pattern simultaneously using intramuscular electromyograms from all seven muscles of the index finger.
The subjects were asked to produce a downward tapping motion, followed by a well-directed vertical fingertip force against a rigid surface.
It was found that the muscle coordination pattern clearly switched from that for motion to that for force before contact. Venkadesan's mathematical modelling and analysis revealed that the underlying neural control also switched between mutually incompatible strategies in a time-critical manner.
'We think that the human nervous system employs a surprisingly time-critical and neurally demanding strategy for this common and seemingly trivial task of tapping and then pushing accurately, which is a necessary component of dexterous manipulation,' said Valero-Cuevas.
'Our data suggest that specialized neural circuitry may have evolved for the hand because of the time-critical neural control that is necessary for executing the abrupt transition from motion (tap) to static force (push),' he added.
Valero-Cuevas says that the new findings may help explain why it takes young children years to develop fine finger muscle coordination and skills such as precision pinching or manipulation, and why fine finger manipulation is so vulnerable to neurological diseases and aging.
'In the tap-push exercise, we found that the brain must be switching from the tap command to the push command while the fingertip is still in motion. Neurophysiological limitations prevent an instantaneous or perfect switch, so we speculate that there must be specialized circuits and strategies that allow people to do so effectively,' he said.
'If the transition between motor commands is not well timed and executed, your initial forces will be misdirected and you simply won't be able to pick up an egg, a wine glass or a small bead quickly,' he added.
Even more importantly, he said, the findings may provide a functional explanation for an important evolutionary feature of the human brain -- its disproportionately large sensory and motor centres associated with hand function.
Valero-Cuevas believes that the understanding of the neuromuscular principles behind dexterous manipulation may particularly help people who have lost the use of their hands by guiding rehabilitation and helping to develop the next generation of prosthetics.
