Q&A: How do humans control their bodies?

UNIVERSITY PARK, Pa. — How humans move is an open question, according to Mark Latash, distinguished professor of kinesiology at Penn State. Investigations continue into how the nervous system generates meaningful movements, but his decades of research into motor control — the ways the body and nervous system interact with each other and the environment to produce movement — have led to insights into how people control their actions and how simple tests can predict future neurological problems.

Sayan Deep De, doctorate candidate in kinesiology, has collaborated with Latash on many studies, including a recent examination of how the nervous system controls force produced by the hand.

Latash and De recently discussed human movement and how measurements of motor control could help predict a person’s risk for Parkinson’s disease.

Q: How do humans control their bodies? 

Latash: That is a big question, and there is a lot that remains unknown, which is one reason why motor control exists as a field of study. 

People produce movements without necessarily thinking about their exact progression. As long as the movement proceeds in a satisfactory way, there is no reason to monitor and correct it in spite of frequent small changes in the external forces and body states. These small changes are dealt with "automatically" and ensure success within acceptable ranges of forces, coordinates, velocities, and other important mechanical variables. 

De: We all initiate our movements without a clear, 100% certainty of what they will cause. For example, when you are walking, your steps do not have to be of a specific length, and your brain does not have to recalculate the muscle forces at each step based on every pebble in the path. That would be too slow and take too many cognitive resources, but the central nervous system changes the parameters of muscles, joints and the whole body to successfully accomplish many tasks — in this example, walking. 

The central nervous system allows your body to be dynamically stable. It is constructed to automatically adjust to small disturbances from inside or outside the body — things like changes in your baseline activity of neural cells, hormones, the terrain or the wind. 

Q: How does the way we control our body change with a condition like Parkinson’s disease? 

Latash: Parkinson’s disease is associated with progressive death of cells in the brain that produce and release dopamine, a neurotransmitter. By the time people are diagnosed, the brain is already very compromised. As a result, the body loses dynamic stability of movements. 

De: When you complete a physical task, you seldom work with one muscle or system. Typically, these things work together in groups. 

Imagine a video game where you have to maintain a spaceship at a target height while the screen scrolls. You control the spaceship’s altitude by pressing two buttons with your index and middle fingers. You could press harder with your index finger and softer with your middle finger, or vice versa. As long as the combined force keeps the spaceship at the target height, you’re winning the game. 

A healthy person would use a broad range of options to keep their spaceship in the middle of the screen. In other words, if they applied 40% of the force with their index finger and 60% with their middle finger this time, they might shift to 70% with their index finger and 30% with their middle finger and then change again to a different balance the next time they play the game. If they were measured 10 times, they might have 10 different balances — not because they are consciously choosing a different balance but because that is how their body regulates the force. Their nervous system allows for a wide range of solutions. 

Prior research by Dr. Latash’s group has shown that — many years before they become symptomatic — people with Parkinson’s disease perform differently than healthy people in a game like this. They tend to find a single solution — as a random example, 35% in one finger and 65% and the other — and use that solution consistently. They lack the ability to use flexible solutions, which is a sign of dynamic stability. 

The spaceship would successfully stay in the same position for both people, but the way that they kept that spaceship flying would be more rigid in someone with Parkinson’s disease, and this would be true years before they displayed the symptoms that reduce quality of life or typically lead to diagnosis. 

Q: How could this difference be used to screen for Parkinson’s disease? 

Latash: The average age of diagnosis for Parkinson’s disease is 60 years old, but people typically show changes in brain structure for years before they are diagnosed. 

Previous research from my lab demonstrated differences in the range of solutions that people in the early stages of Parkinson’s disease use on force tasks, as in the video game scenario we discussed.  

Based on this research, we could connect four force sensors to someone’s hand and then give them tasks where they apply a specific amount of force. This would allow us to identify which people use a small range of solutions. Those results may serve as an early indicator of Parkinson’s disease. 

This force-sensor test could be run in 15 minutes at a routine doctor’s appointment, and the necessary equipment is very inexpensive. People who did not demonstrate a broad range of solutions in the force tasks could be referred to a neurological specialist who could begin to evaluate them for Parkinson’s or other neurodegenerative conditions. 

Q: What would be the value of early Parkinson’s diagnosis? 

Latash: Parkinson's treatment mostly tries to replace the neurotransmitter lost due to the neuronal death. The problem with this is that patients can lose 70% of relevant neurons before they are symptomatic. But existing neuroprotective drugs can greatly slow the progression of the disease. As with many conditions, early diagnosis is very helpful in disease management. 

While the test we propose cannot be used for diagnosis, it would potentially offer a useful way to screen and identify individuals who might benefit from an MRI or other more sophisticated and expensive tests for a definitive diagnosis. 

Q: What is next for this research? 

De: Older adults have many problems with motor control, due either to diseases like Parkinson’s or just the progression of aging. Understanding how we control our bodies is the critical first step towards helping people age well and supporting anyone with conditions that affect their mobility. 

Force testing on fingers is just one example of a broad range of fine and gross motor skills that we seek to understand. To develop treatments and diagnose problems, we need to understand how things work. 

Recently, we published a paper from my doctoral research that showed movement and force production are probably controlled using different neurological pathways. This is one more step towards a broader understanding of human motor control. And that understanding may someday lead us to treatments for people who have lost function in any part of their body. 

Latash: As Dr. De said, insights into how people move have huge implications for people with movement disabilities and for everyone as they age. Like other motor control researchers, we will continue to seek these discoveries.