Voluntary muscle contraction is the result of communication between the brain and individual muscle fibers of the musculoskeletal system. A thought is transformed into electrical impulses, which travel down motor neurons (in the spine and peripheral nerves) to the neuromuscular junctions that form a motor unit. The individual muscle fibers within each motor unit contract with an “all or none” response when stimulated, meaning the muscle fiber contracts to its maximum potential or not at all. The strength of contraction of a whole muscle depends on how many individual fibers are activated, and can be correlated with electrical activity measured over the muscle with an EMG sensor.

An electromyogram, or EMG, is a graphical recording of electrical activity within muscles. Activation of muscles by nerves results in changes in ion flow across cell membranes, which generates electrical activity. This can be measured using surface electrodes placed on the skin over the muscle of interest. Electrical activity correlates with strength of muscle contraction, and is dependent on the quantity of nerve impulses that are sent to the muscle. This is easily visible in large muscles such as the bicep muscle in the arm.

In this activity, you will graphically record an EMG using an EMG sensor as you squeeze a Hand Dynamometer. The measured grip strength will be correlated with the muscular electrical activity as measured in the EMG. You will see if electrical activity changes as a muscle fatigues during continuous maximal effort. Finally, you will investigate the brain’s response to “coaching,” or your ability to regain maximum grip strength given a visual stimulus.