There are two things that govern how we move: our brains and the muscles themselves. Every time we pick up a cup or wave to a friend, neurological stimuli must make their way from the brain to the muscles involved to generate a contraction or relaxation in those muscles and a subsequent movement. But local properties of the muscles also play a role.
How much force a muscle produces in response to neurological stimuli depends on its length, velocity, and stiffness, in addition to its strength. The same signal from the brain could generate very different movement depending on these properties. Northeastern assistant professor of physical therapy, movement and rehabilitation sciences Christopher Hasson is interested in the interplay between those two phenomenon and wants to know whether the brain is aware of—and compensates for—variations in those properties.
The way to test this is to perturb the muscle properties and see how the brain responds – but this is rather difficult to do in living humans – at least without doing some radical surgery.
To decouple neural activity from muscle activity he created a computer game that asks subjects to hold their arm in a stationary position and simply flex their triceps or biceps to control the virtual arm they see on screen. Sensors stuck to the person’s actual muscle read its activity and that is sent to the computer to activate the virtual muscles. It’s an accuracy game and success is measured by how close they can position the virtual arm near a fixed target.
But here’s the cool part: the on-screen arm contains virtual muscles that may or may not function like the properties of the actual arm. Hasson first gives subject time to perform the task and adapt to the virtual arm. Once the subject becomes proficient, he can instantly alter the muscle properties by programming in things like reduced strength or increased stiffness. He hypothesized that if the brain “knows” what a particular muscle’s properties are, then it will more readily learn how to maneuver the virtual arm if it’s muscles are programmed to behave like real human muscles, rather than in a more artificial way.
He’s done some initial tests on his hypothesis and the work is under review for publication. You’ll have to stay tuned to find out the results!