I read about a study that used EMG recording of muscles to find good exercises for the shoulder. How do they know for sure the device isn’t picking up signals from nearby muscles?

This is a very good question and a problem that researchers have to deal with everyday. Electromyography (EMG) is the tool used to measure electrical activity in muscles. EMG tells researchers a lot about how muscles work.

There are two ways to hear and record the electrical signals of muscles. Small patches or electrodes can be placed on the skin over the muscle. The electrodes are placed over the middle section of each muscle. This method gets the best signal without picking up signals from other muscles.

A second method uses fine-wire electrodes. Fine needles are inserted in to the belly of the muscle. The wires are taped down to keep them from moving when the muscle contracts. Fewer muscle fibers are recorded with wires compared to electrodes.

Scientists know that cross-talk is a problem with EMG studies. They take this into account when looking at their findings.