Muscles are usually electrically silent at rest. When weakly activated, the smallest weakest motor units, associated with the smallest motor neurones are first activated. This can be recorded as a pattern of motor unit potentials (MUPs) re[resenting the summed action potentials of all the simultaneously discharging fibres of the motor unit. The size and shape of the MUP depends upon the size of the motor unit (number and size of the fibres in the unit) and the position of the electrode. The electrode may be placed either superficially - over the muscle - or into the muscle (needle or fine-wire electrodes).
As the force of contraction of the muscle is increased - by increasing voluntary effort - more and increasingly large motor units (associated with progressively larger motor neurones) are recruited. The sizes and the number of overlapping MUPs also increase, until a complex interference pattern is produced. These records of the electrical activity of a muscle are called electromyograms (EMGs) and may be used to diagnose abnormalities of muscle function.
Myopathic disorders which specifically affect muscle fibres and their functioning will result in weakness of muscle contraction.
Note that the pattern of degenerating fibres will be random with respect to which motor units to which they belong.
Due to breakdown of the muscle fibres, the concentration of muscle enzymes (creatine kinase, lactate dehydrogenase, myoglobin) in the blood rises.
The degenerating muscle fibres will become sensitive to extraneous stimuli and individual muscle fibres may occasionally give off spontaneous bursts of action potentials. The fibres will contract but the twitches in these isolated fibres will be too weak to be detected at the whole muscle level. The discharges can be detected in the resting muscle as fibrillation potentials (or simply fibrillations) by an electrode inserted into the muscle.
The motor unit potentials will become small, sharp and oddly shaped (polyphasic) due to delayed and weakened action potentials in damaged fibres.
Even with maximal contraction the interference pattern of the EMG will be relatively simple and reduced in size.
Examples of myopathic disorders are polymyositis (disseminated, inflammatory disorder of the muscles); muscular dystrophy (an inherited fragility of the muscle fibres); glycogen storage disorders; familial periodic paralysis; myasthenia gravis; and many others.