Notes on EMG
Lithu Muralee and Julian Wexer
-EMG stands for Electromyography
-Electromyography is the experimental technique concerning the development, recording, and analysis of myoelectric signals
-Myoelectric signals are signals formed by physiological variations in the state of muscle fiber membranes
-Neurological EMG vs Kinesiological EMG
-Neurological is an artificial muscle response from external electrical stimulation
-Kinesiological is activation of muscles within specific tasks
-Within postural tasks, functional movements, work conditions, and treatment/training regimes
-Kinesiological EMG is an evaluation tool for applied research, physiotherapy/rehabilitation, sports training, and interactions of the human body to industrial products and work conditions
-Benefits of EMG are that it allows you to look into the muscle, allows you to measure muscular performance, helps decision making before and after surgery, documents treatment and training regimes, helps patients tain their muscles, allows analysis to improve sports activities, and detects muscle response in ergonomic studies
-The motor unit is the smallest functional unit to describe the neural control of the muscular contraction process
-Motor unit defined as cell body and dendrites of a motor neuron, the multiple branches of its axon, and the muscle fibers that the neurons are applied to
-Brain can excite muscle fibers
-Resting potential of not contracted muscle fiber membranes is about -80 mV
-Neuron induces Na+ ions to flow into the muscle fiber membrane in depolarization, which is followed by repolarization, which restores the exchange of Na+ and K+ ions
-The Na+ influx causes the action potential in the muscle to go from -80 mV to +30 mV, depolarization occurs, and levels return to normal, but the excitation leads to a release of calcium ions, which causes the muscles to contract
-EMG measures the action potentials
-Electrodes are placed on the skin over a muscle and are spaced apart and depending on the space between them, the electrodes form a potential difference between them
-Motor unit action potentials or MUAPS are summed up to get a superposed signal
-Two most important mechanisms influencing the magnitude and density of the observed signal are the recruitment of MUAPs and their firing frequency, which are the main control strategies to adjust the contraction process and modulate the force output of the involved muscle
-A raw EMG signal is an unfiltered and unprocessed signal detecting the superposed MUAPs.
-When a muscles is relaxed there is a baseline in the graph of raw EMG, but when the muscle is contracted, noise can be seen, with the averaged baseline noise not being higher than 3-5 microvolts, and with the target being 1-2
-Raw EMG spikes are of random shape, and the raw recording burst cannot be precisely reproduced in exact shape
-Tissue characteristics can influence the EMG signal, as electrical conductivity varies with tissue type
-The EMG detected can be from other muscles, which can influence the EMG signal, but this cross talk typically doesn’t exceed 10%-15%
-If the change of distance between the electrodes changes, the EMG signal is affected
-Outside electrical noise can impact the EMG signal reading
