‘Driving’ a new pair of arms: Neurology, recovery and rehabilitation

‘Driving’ a new pair of arms: Neurology, recovery and rehabilitation.

Brendan Marrocco, an Iraq War veteran who lost all four limbs in a roadside bomb attack, was recently released from a Baltimore hospital after receiving a double-arm transplant. Northeastern University news office asked Christopher Hasson, a sensorimotor control expert and a newly appointed assistant professor in the Department of Physical Therapy, to explain the brain’s role in the long recovery and rehabilitation process.

Marrocco received the double-arm transplant six weeks ago and has already reported movement in the elbow of his left arm. What is the brain’s role in learning how to control a novel object, which in this case is a new pair of arms? A human arm is marvelously complex—and presents a formidable control problem for the brain. The scale of this problem is best illustrated by comparison with driving a car. With an automatic transmission you have three things to control: The steering wheel turns the car left or right, the gas pedal speeds up the car, and the brake slows it down. Healthy adults learn the basics of driving relatively quickly, but fine-tuning takes much longer and can only be achieved through many hours of practice. During this fine-tuning process the brain refines its knowledge of how the car responds to control actions. In Marrocco’s case, he must learn to “drive” his new arms; however, the control problem explodes in complexity. For each arm he must learn to control motions at three joints with 12 muscles; if you include the hand that adds at least 14 more joints and more than 20 more muscles. Imagine trying to learn to drive a car with more than 30 different controls! Fortunately, Marrocco has a head start, as he has had prior experience controlling arms. This may explain why he learned to perform basic movements relatively quickly. However, fine-tuning his control will take much longer. You are the principal investigator of Northeastern’s Neuromotor Systems Laboratory, in which you study how movement control in older adults is affected by age-related changes in the neuromuscular system. How will Marrocco’s relative youth—he is only 26 years old—contribute to the recovery and rehabilitation process? Although it’s natural to think that a relatively young adult such as Marrocco would have a clear advantage over someone who is older, age may not be a critical factor in terms of the motor learning aspects of recovery. It was once widely thought that after you reach adulthood the structure of your brain stabilizes and becomes fixed, and therefore older adults would have difficulty learning new skills as their brains are more resistant to change.