Analysis of a natural motor event called “mirror activity” has revealed that the long contraction of muscles in one hand can cause frequent involuntary reactions from the muscles in the opposite hand.
These findings provide detailed evidence on how brain-body connectivity works and regulates muscle behavior, which may help identify some of the biological events responsible for motor disorders such as Parkinson’s disease.
The study, “Inverse relationship between amplitude and latency of physiological mirror activity during repetitive isometric contractions,” was published in the journal Neuroscience.
Uncontrolled muscular activity is a hallmark symptom of Parkinson’s disease, often manifesting as involuntary tremors or muscle contraction. Many patients also experience what is called “mirror movements,” which is a disease-related form of the natural muscular event of mirror activity.
Mirror activity is a phenomenon characterized by involuntary muscular activity on one side of the body that is triggered by voluntary, controlled movements of the other limb. In neurologically healthy people, mirror activity does not directly lead to visible movements. Still, this muscular response can be detected by surface electromyography (EMG), which is a method used to register the electrical activity of muscles.
This mirror muscle activity is believed to happen as nerve cells that are on one side of the brain — and control the electrical signals of only one side of the body — are so highly active that their signals start to “leak” to surrounding brain areas, triggering nerve cells on the opposite side of the brain.
German and Russian researchers have now conducted a study, which enrolled 24 healthy men, to gain more detailed insight into how mirror activity works.
Participants were asked to press a sensor at a constant force (80% of maximum voluntary contraction) and at certain intervals by simultaneously contracting the thumb and index finger of one hand while the other hand was resting. EMG sensors were placed on the back of both hands to record all muscle activity that occurred during the task.
Participants were also asked to focus only on the active hand and to avoid intentional movements with the non-active hand.
Data revealed that after several repetitions of voluntary contractions, the strength (or amplitude) of the electrical impulses detected on the resting hand due to involuntary contractions increased. In addition, the time delay (latency) between the voluntary muscle contraction and the start of involuntary muscular activity decreased. This demonstrates an inverse relation between latency and amplitude.
“Our results extend the existing knowledge of [mirror activity] occurring due to high-effort unilateral [on one side only] contractions with constant force requirements,” the researchers wrote.
This study showed “for the first time” that mirror activity is not “time-locked to the onset of contracting muscle bursts”; rather, it “starts after varying latencies” during contraction of the muscles on the opposite side. “These latencies show a dynamic behavior as a function of repeated muscular effort,” they said.