MUAPs

MUAP stands for motor unit action potential. It is the summed electrical activity produced by all skeletal muscle fibers innervated by a single motor neuron when that neuron fires. The motor unit fibers respond almost synchronously, and the resulting potentials can be detected by electromyography (EMG) electrodes. In clinical practice MUAPs are recorded with intramuscular needle electrodes or surface electrodes during voluntary contraction or minimal activation. The recorded waveform is described by amplitude, duration, number of phases, and area; shapes range from biphasic to triphasic or polyphasic, reflecting fiber recruitment and conduction properties. Individual MUAPs are typically followed by silent intervals, and separating single-unit activity can be aided by spike-triggered averaging or decomposition techniques.

Normal MUAPs vary by muscle but are generally a few hundred microvolts in amplitude and a few milliseconds in duration, often biphasic or triphasic, with occasional polyphasia.

Pathology affects MUAP morphology. In neurogenic disorders such as peripheral neuropathy or motor neuron disease, surviving motor units may reinnervate additional muscle fibers, producing larger, longer-duration MUAPs with increased polyphasia. In myopathic disorders, loss of muscle fibers within motor units tends to yield smaller amplitude, shorter-duration MUAPs, with altered recruitment patterns at lower force levels.

Clinical use and interpretation rely on MUAP analysis to distinguish neurogenic versus myopathic patterns, assess disease progression, and complement nerve conduction studies and clinical examination. Limitations include dependence on electrode type, subcutaneous fat, contraction level, and recording technique, necessitating expert interpretation within the broader clinical context.