Scientists have found that activities combining movement and force tax the brain to capacity, which is contrary to previous belief that muscles themselves tire out. This finding may help understand why minor damage to the neuromuscular system can at times profoundly affect one's ability to complete everyday tasks.
"Our results show how much the mechanics of the body, and a given task, affect what the brain can or can't do," said Francisco Valero-Cuevas of the Brain-Body Dynamics Lab at the University of Southern California, who led the research.
"The so-called 'problem' of muscle redundancy-having too many muscles and joints to control-may not be the only challenge the brain faces when controlling our bodies. Rather, we seem to have about as many muscles as we need, and not too many, as others have proposed in the past.
"The scientific world and the clinical world have long been arriving at conflicting conclusions, and this work begins to resolve the paradox.
"While neuroscience and biomechanics studies have suggested that muscles and joints are, in theory, redundant and provide numerous alternative solutions to simple tasks, clinicians routinely see people seeking treatment for hand disability resulting from relatively minor conditions such as aging," added Valero-Cuevas.
The study followed previous experiments that suggested the brain and complex musculature can barely keep up with requirements posed by our anatomy and the mechanics of even ordinary, real-world, finger tasks like rubbing a surface.
The conclusions begin to explain why even minor damage to the neuromuscular system seems to produce real deficits in manipulation.
The research focused on simultaneous force and motion-specifically from fingers either pushing or rubbing a surface-with volunteers conducting the experiment at defined, yet varying, speeds.
Knowing the force-producing properties of muscle, the researchers expected the rubbing motion would show reduced downward force as the speed of motion increased.
Surprisingly, whether rubbing slowly or at a pace 36-times faster, speed had little affect on the downward force the volunteers could produce.
The researchers interpret the results to mean the brain is sufficiently occupied by the physical demands of combining motions and forces, so the muscle properties are not the limiting factors for how much force the fingers can create.
"This begins to explain the clinical reality that when something in the system is damaged, either in the brain or body, we can see losses of function. We are not as 'redundant' as we thought," said Valero-Cuevas.
The research team is conducting additional research to determine what exact neural and anatomical mechanisms are producing these results.
The current study has been published in the Journal of Neuroscience.