Electromyography

What is the Link between Electricity and Muscle Activation?

Electromyography is the study of the electrical signal generated by contracting skeletal muscles. The early work on the relationship between muscle contraction and electricity was conducted in the 1600-1700s.

Before the 17th century, it was believed that movements were produced by moving spirits that travelled from the brain down hollow nerves to the muscles and that caused the muscles to increase in volume during a contraction. In contrast to this belief, in 1664 Jan Swammerdam (1637-1680) demonstrated that stroking the innervating nerve of the frog s gastrocnemius muscle generated a contraction even on an isolated muscle-nerve preparation when the connection to the brain was cut. The first intuition of the relationship between muscles and electricity is documented in the work of Francesco Redi (1626-1697) in 1666. Redi showed that the shock of the electric ray fish was muscular in origin, highlighting the rela-tionship between voluntary contractions and the pro-duction of an electrical signal. A decisive breakthrough came in 1791, when Luigi Galvani (1737-1798), who is credited as the father of neurophysiology, showed that electrical stimulation with metal rods of muscular tissue in frog's legs produced a contraction which generated a force. He reasoned that the factor respon-sible for movement was probably of electrical origin. Carlo Matteucci (1811-1868) definitively proved that electrical currents originated in muscles by showing, that injured excitable biological tissues generated direct electrical currents, and that they could be summed up by connecting elements in series, as in an electric pile.

In 1849, DuBois-Reymond (1818-1896) was the first to report the detection of voluntarily elicited electrical signals from human muscles. He constructed an apparatus that consisted of a surface electrode (a wire attached to a blotting paper immersed in a jar of saline solution) that caused a small deflection on the galvanometer when the fingers immersed in the saline solution were contracted. He understood that the impedance of the skin reduced the current that drove the galvanometer, and that a greater deflection could be produced by induced blisters so that the open wounds were in direct contact with the saline solution. However, methods to detect the electromyographic signal from a human improved slowly in the 19th century, when electricity started to be used for clinical applications. For instance, Guillaume Duchenne (1806-1875) applied electric stimulation to intact skeletal muscles to investigate muscular function in humans. Duchenne was one of the first to investigate electricity for therapeutic purposes, and is considered to be the father of electrotherapy. In 1922, Herbert Gasser (1888-1963) and Joseph Erlanger (1874-1965) used a cathode ray oscilloscope in place of the galvanometer to investigate conduction along the nerve axons that allowed them to observe action potentials. Great impact in the field was made by Edgard Adrian (1889-1977) and Detlev Bronk (1897-1975) in 1929, when they introduced the first needle electrode to record the electrical signal generated during a muscle contraction. It was a groundbreaking innovation with immediate impact on the clinical community. Since then, a series of technological advances have fostered the investigation of muscle function through the analysis of the electrical activity associated with muscle contraction. So how are muscle contraction and electricity related?