Electromyography

Skeletal muscles are composed of thousands of muscle fibres that contract when they are activated by nerve pulses. These pulses are generated in motoneurons, whose cell bodies are located in the anterior horn of the spinal cord. Nerve pulses travel down the nerve axons of the motoneuron and come in contact with the muscle at the neuromuscular junction. Each motoneuron innervates a number of muscle fibres, which contract almost simultaneously when they are excited by a nerve pulse. Thus, a single motoneuron and all the fibres innervated by this motoneuron constitute the functional unit of muscles, called motor unit (MU). The muscle fibre membrane has a resting potential of approximately —70 mV. A nerve pulse at the innervation point (i.e. at the neuromuscular junction), temporarily alters this resting potential to +40 mV. The change in membrane potential, referred to as depolarization, propagates in both directions along the muscle fibres from the innervation point. The depolarization voltage travelling down the muscle fibres of a motor unit can be detected with an electrode placed near the muscle fibres or on the skin above the contracting fibres. The characteristic shape of the depolarization voltage is called motor unit action potential (MUAP). In order to sustain a muscle contraction, motor units must be repeatedly activated. The resulting sequences of MUAPs are called motor unit action potential trains (MUAPTs). The EMG signal is the electrical signal associated with a muscle contraction that can be detected with a sensor positioned inside a muscle or on the skin on top of a muscle. It represents the summation of the electrical activity from a number of MUAPTs that are simultaneously activated during a muscle contraction. The EMG signal ranges between a few mikroV to several mV depending on the level of muscle activity. The activation of a motor unit transforms the nerve signals into force and movement so that the electrical manifestation of a MUAP is accompanied by a contraction of the innervated muscle fibres. The force generated by the contraction of a single motor unit is called force twitch. For sustained contractions, force twitches superimpose if they occur in close enough succession, producing a tetanic, or fused, force. The force produced by a contracting muscle is composed of the individual forces generated by all active motor imitc within the muscle.