Motor Unit Action Potentials
- 1Measure MUAP amplitude, duration, phases, and turns and relate each to fibre architecture
- 2Recognise instability, satellite potentials, and the signatures of remodelling
- 3Apply electrode-specific normative reasoning
A motor unit action potential (MUAP) is the spatial and temporal sum of the single-fibre action potentials belonging to one motor neuron, sampled through the spatial filter of a needle electrode. Because that sum depends on how many fibres fire, how tightly their discharges are synchronized, how densely they are packed, and how far each sits from the recording surface, the MUAP encodes the architecture of the motor unit with quantitative fidelity. Reading amplitude, duration, phases, and stability is therefore reading motor-unit anatomy and the security of its neuromuscular transmission — the central inference of needle EMG.
Neuropathy: Giant, long-duration MUAPs (>3 mV) from collateral reinnervation; reduced recruitment with rapid firing.
62 y/o male · Tibialis anterior · concentric needle · Chronic neuropathy from a right L5 radiculopathy. Real recording · 4 kHz.
Amplitude: a measure dominated by the nearest fibres
MUAP amplitude — the peak-to-peak excursion of the main spike — is generated almost entirely by the few muscle fibres lying within ~0.5 mm of the electrode tip. The recorded potential falls steeply with radial distance, so amplitude is exquisitely position-dependent: minute repositioning of the needle can double or halve it, which is precisely why amplitude alone is an unreliable index of overall unit size and why one must always optimize the needle to a crisp, short-rise-time potential before measuring. Held constant in position, amplitude rises with fibre density — most dramatically after collateral reinnervationcrowds new fibres around a surviving axon, generating the giant (>3–5 mV) units of chronic neurogenic disease seen in the figure. A short rise time (<500 µs) certifies that the spike arises from nearby fibres and that the amplitude reading is trustworthy.
Duration: the truest index of motor-unit size
MUAP duration — measured from the first deflection from baseline to its final return — is the single best electrical proxy for the total number of fibres in the unit. Unlike amplitude, duration is a whole-unit property: distant fibres contribute little to the spike but, because their action potentials arrive slightly desynchronized by differences in conduction distance and endplate position, they broaden the onset and terminal portions of the waveform. Duration therefore samples the entire territory rather than just the nearest fibres, and is comparatively insensitive to small needle movements. It lengthens when reinnervation adds fibres with widely scattered, immature endplates (increased temporal dispersion) and shortens in myopathy as fibre loss and atrophy strip fibres from each unit. Duration is consequently the measurement on which formal quantitative EMG and the neurogenic / myopathic distinction principally rest.
Phases and turns: the signature of synchrony
A phase is a segment of the waveform between two baseline crossings; a turn is any directional reversal that need not cross baseline. Both quantify the synchronywith which the unit's fibres discharge. A normal MUAP is two-to-four phased because its fibres, sharing similar conduction velocities and endplate distances, fire near-simultaneously. More than four phases defines a polyphasic potential; up to ~10–15% of units may be polyphasic in normal muscle (more with monopolar needles), so the finding is graded, not binary. Increased polyphasia and excess turns reflect desynchronized firing from variable conduction in immature reinnervated sprouts or from the scatter of surviving fibres in myopathy — making polyphasia a sensitive but non-specific marker that is abnormal in both neurogenic and myopathic processes.
A healthy MUAP is stable— it superimposes almost perfectly on consecutive discharges because every fibre fires reliably on every impulse. When neuromuscular transmission is insecure, as at the immature endplates of fresh reinnervating sprouts or in primary NMJ disease, individual fibres intermittently fail to fire, so the waveform's shape varies (jiggles) from sweep to sweep and may transiently block. An unstable MUAP is thus a marker of junctional security and of active, ongoing reinnervation; quantified at the single-fibre level this instability is measured as increased jitter and blocking.
Satellite and linked potentials
A satellite (linked) potential is a small, time-locked component trailing the main MUAP across an isoelectric gap, firing in lockstep with the parent unit. It represents a newly reinnervated fibre whose immature, thinly myelinated terminal sprout conducts slowly, so its action potential arrives late. Satellites are a specific marker of recent collateral reinnervation; over months, as the sprout matures and speeds up, the late component migrates earlier and is absorbed into the main waveform, converting a satellite into one more phase of an increasingly complex unit. Their presence dates the process — recent rather than remote.
Electrode-specific normative values
MUAP parameters are meaningful only against the right reference range, and reference ranges are electrode-specific. A concentric needle records between a central core and the surrounding cannula, yielding a restricted, directional pickup with relatively lower amplitudes and shorter durations and a sharp dependence on the side from which fibres are approached. A monopolar needle records against a distant surface reference, sampling a larger, less directional volume and producing higher amplitudes, longer durations, and more phases. Applying concentric norms to monopolar recordings will systematically overcall both unit size and polyphasia. Quantitative EMG further demands age-stratified, muscle-specific tables, since both duration and amplitude rise with patient age and differ by muscle.
- Amplitude is dominated by the nearest few fibres and is highly position-dependent; optimize rise time (<500 µs) before trusting it. It rises with fibre density and reinnervation.
- Duration is a whole-unit property and the best index of motor-unit size — lengthens with reinnervation, shortens in myopathy; it anchors quantitative EMG.
- Phases and turns index firing synchrony; >4 phases = polyphasic, a sensitive but non-specific marker abnormal in both neurogenic and myopathic disease.
- MUAP instability (jiggle/blocking) reflects insecure junctions — active reinnervation or NMJ disease; quantified as jitter on SFEMG.
- Satellite/linked potentials mark recent reinnervation; normative values are electrode-specific (concentric < monopolar for amplitude, duration, and phases).
- 1.Preston DC, Shapiro BE. Electromyography and Neuromuscular Disorders. 4th ed. Elsevier; 2021: Ch. 15.
- 2.Daube JR, Rubin DI. Needle electromyography. Muscle Nerve. 2009;39:244–270.
- 3.Stålberg E, et al. Standards for quantification of EMG and neurography. Clin Neurophysiol. 2019;130:1688–1729.
- 4.AANEM. Glossary of terms in neuromuscular electrodiagnostic medicine. Muscle Nerve Suppl. 2015.
- 5.Real signal: PhysioNet emgdb v1.0.0 (chronic L5 radiculopathy 62M, tibialis anterior, concentric needle).