Redundancy in movement may boost motor learning

Ranjini Raghunath

The experimental setup authors used to measure redundancy in arm movements of the subjects
The experimental setup authors used to measure redundancy in arm movements of the subjects   (Photo: Singh et al)

Whether you toss a ball or play an arpeggio, your body takes an all-roads-lead-to-Rome approach when it comes to movement. You can move your muscles and joints in many different ways to do the same task. This “redundancy” in muscle movement has historically been considered “noise”, but growing evidence suggests that it may promote and reinforce motor skills. By calculating and comparing the level of redundancy with motor learning rate, a new study shows that people who are able to explore more ways to do the same task also appear to learn motor skills faster. The paper was published in the Proceedings of the National Academy of Sciences.

There are several unanswered questions when it comes to redundancy. Why are there so many more muscle configurations than are actually needed for a task? How does the brain pick one configuration over another every time it does the same task? Why do some people have a greater level of redundancy i.e. the ability to move their muscles in more ways than others?

To address some of these questions, the scientists asked human subjects to move a robotic arm from one point to another repeatedly, and then tracked how many different ways each subject manipulated their joints, in order to gauge their redundancy levels. Then, the team measured the subjects’ motor learning rates by “disturbing” them: by pushing against their arm while they did the task, and tracking how quickly they corrected their movements to compensate for the disturbance. The subjects were able to correct their movement paths gradually with practice over the course of 400 trials. In another experiment, some of the subjects were first asked to move a cursor along a straight line. The researchers then deliberately rotated the cursor angle by 45 degrees, forcing the subjects to adapt to this new path. A total of 60 subjects were recruited for the experiments.

The researchers found that subjects who are able to manipulate their joints in more ways also appear to adapt more quickly to disturbances and learn faster. Redundancy levels were also generally higher in the subject’s dominant hand compared to the non-dominant one.

Understanding what promotes motor learning has clinical applications. During rehabilitation, patients with Parkinson’s disease or stroke are made to do a task in the same way again and again to teach them lost or new skills. “Now, you can make them do the same task in different ways so that the person learns faster,” says first author Puneet Singh, PhD student at the Centre for BioSystems Science and Engineering, Indian Institute of Science (IISc).

Although the neural basis is unclear, such gleanings provide valuable insights into how the brain adapts to changes over time. “Biological systems learn typically through genes and evolution over a long time [generations], but the brain is special in that it can learn within a lifetime,” says senior author Aditya Murthy, Professor and Chairman, Centre for Neuroscience, IISc. “As we grow up, our muscles change, the length of the bone increases, or as we grow older, our muscles become weaker. Our brains are constantly accommodating these changes, constantly learning. ”

Singh is now curious to find out if the reverse is also true — when redundancy is suppressed, is learning reduced? He also hopes to work with patients to explore possible links between redundancy and impaired motor learning.