In an electromyogram, muscle activity is measured and modelled, for example using a diagram. Commercially available EMG devices for home use also employ a numerical scale in conjunction with a coloured background.
The measured muscle activity and the derived thresholds provide the baseline data for biofeedback therapy. The measurement itself can be carried out in different ways, and ultimately depends on the objective of the therapy: muscle tone can be detected using either electrodes on the skin, anal or vaginal probes, or via fine-wire electrodes placed directly in the muscle under the skin. Regardless of the placement of the electrode, the measurement is always based on the same electrical and biochemical processes.2
Electromyography exploits the electrical properties of the semi-permeable muscle fibre membrane and records the ion balance between the intracellular and extracellular spaces of the muscle cell. In the relaxed, i.e. non-contracted state, the cell interior is negatively charged, while the exterior of the cell is positively charged. This charge difference results from the differing distribution of negatively charged potassium and positively charged sodium ions and is referred to as the resting potential, which usually amounts to between -80 and -90 mV.3
When the first motor neuron (supraordinate motor neuron) issues a movement command to the motor nerve, transmitter substances are emitted, which ultimately affect the diffusion properties of the muscle fibre membrane. Positive sodium ions flow into the cell interior, so that a positive charge briefly arises here. This process is known as depolarisation. This causes an action potential to be induced, rising from -80mV to + 30mV. However, shortly after this a direct ion backflow restores the negative charge – and repolarisation occurs.
The action potential is transmitted along the muscle fibres in the form of an electrical excitation, resulting in a release of calcium ions. This initiates chemical processes that cause the contractile cell elements to contract. In this way, a muscle contraction is induced. The electromyogram measures the action potential of the muscle fibre membrane on the basis of the depolarisation-repolarisation process, and can then model the muscle activity in a diagram.
Electromyography and the biofeedback therapy based on it are now being used more and more for diagnostics and therapy. They have also become firmly established in rehabilitation, as a way of monitoring and improving movement patterns and muscle activities during various training activities. Ergonomically designed devices also allow the biofeedback therapy to be continued in a home environment.4
4 EMG-Fibel – Eine praktische Einführung in die kinesiologische Elektromyographie, Peter Konrad, Version 1.0 September 2005