Muscular Activity and Surface EMG

Abstract

In human movement, the skeletal muscles act as the active force elements. The force exerted by a muscle as well as the coordination of several muscles involved e. g. in a complex movement are controlled by the neural control of the muscles. A specific innervation pattern or program is transmitted from the CNS to the effectors via the axons and motor endplates causing there an excitation of the muscular cell membranes. The excitation consists of depolarisation and repolarisation of the membrane resting potential. Hereby, the excited cells can be considered as elementary electrical sources and sinks which form an electrical field within the body as a volume conductor resulting in the electromyographic signal (EMG). The active state of the membrane is linked to the bioenergetic metabolisms within the cell resulting finally in the activation of the contractile structures; the skeletal muscle cells can be considered as elementary force generators which contribute in their population to the resulting muscle force. Characteristic for the active state is the electrophysiological excitation and the mechanical contraction. These basic phenomena are commonly known and have been reported in literature extendedly. Also the knowledge of the impacts by the organisation in motor units, the influence of sensory feedback and reflexes, the kinesiological properties of force exertion and movements is very advanced. The electromyogram can be utilized to monitor the “active” muscular inputs to the biomechanical system. Because of the electro-mechanical coupling, there exists a close relationship between EMG activation and force exertion. Therefore, the EMG patterns can be analysed and can contribute to a better understanding of a movement rather than consideration of the mechanical aspects isolatedly.