Electromyogram

Introduction

Electromyography is one of the many electrodiagnostic tests conducted for the purposes of studying the electric functions of the human body. While there are many different types of sensors and protocols for EMG, it can be described as both a neurological and a musculoskeletal test that is targeted to the peripheral nervous system pathway.

Definition

Electromyography is a process in which the electrical signals of the muscles are captured via an electrode. EMG tests can provide data about the impulses from the nerves responsible for contraction and the reactions of the muscle fibers to the said impulses. Depending on the device used, the resulting raw data can be exported as a graph called electromyograph, therefore in some cases giving the name electromyography to the original test.

Nerve Conduction Studies (Electromyelogram)

EMG tests are generally followed by an electromyelogram otherwise called nerve conduction studies (NCS). Being a more active testing approach then EMG, NCSs include electrical inputs to observe the reaction of the nerves more specifically. Paremeters such as action potential, latency, amplitude and conduction velocity are observed with EMG after an artificial electrical signal is administered. Data gathered from a NCS can be used to determine the type and extend of a damage to a nerve.

Physiology

In an electromyograph, a value called "Motor Unit Potential" (MUP) is observed for the target muscle. These are the electrical potential created by the muscle for the purpose of executing a voluntary conraction. Inferences are then made from these MUPs depending on the frequency, amplitude and time taken to generate the contraction. EMG uses the electrophysiological properties of muscles to allow a differianciation of a myopathy from a neuropathy and can help determine the type and progression of these conditions.

Types of EMG

Needle EMG

The electrode is placed within the tip of a needle, which is inserted to the target muscle and can be relocated as necessary. Needle EMG is the preferred method for diagnostic purposes due to being more targeted and reliable then a surface electrode. Although the process is considired safe, the potential risks of pain, bleeding, infection, and pneumothorax remain as a result of the needle being used.

Surface EMG (sEMG)

Surface measurements of muscle activity is generally reserved for research purposes. Using an adhesive electrode on the skin over the targeted area enables an easier test. However, a singular superficial electrode measurement picks up signals from multiple muscle fibers and all the tissue in between, compromising signal integrity thus making it non-viable for diagnostic uses.

EMG in Rehabilitation

Electromyography has also found uses within certain fields of rehabilitiation, biofeedback therapy being one of them. Conditioning of a biological action is a proven concept. This approach has been succesfully used with visual and auditory feedback in the past, and converting the outputs of an electromyograph into a similar feedback had varying degrees of success in coordinating muscle movement for the muscles of the pelvic floor. The same principle also showed promise for patients with recent knee surgeries although to a lesser extend. Alternative uses for the surface varience of EMG has also been tested to find mixed results, one of them being the inspiratory muscles.

EMG in Research

Electrophysiological properties of the human body are still a subject of vigorous study due to the intricaties and the complexity of the nervous system as a whole. EMG has been proven to be an invaluable tool in collecting data and helped built some of the current consepts of the musculoskeletal system in the literature. As research progresses, combined use of EMG with other types of electrodiagnostic tools resulted in a vast array of studies to discover and evaluate new approaches for rehabilitation such as motor imagery and sensorial feedback. Studies aiming to implement EMG into more specific areas such as activities of daily living have also been prevelant, especially with the progress of technological adaptations of EMG.

Technological Research and Development

Thanks to the multidiciplinary research on the subject, EMG has not remained only to be clinical testing device. From gait analysis to wheelchairs using a human-machine interface with EMG sensors this technology proved to be an exciting prospect. This potential also paved the way for the sensor technology to become more accesible and less costly.