abnormal spontaneous activity
A healthy muscle at rest is electrically silent: when you are not asking it to do anything, it sits quietly, and the needle hears almost nothing. Abnormal spontaneous activity is the opposite, a muscle that crackles, pops, or hums on its own while the patient is fully relaxed. Hearing this unexpected chatter is one of the most important findings in the whole needle exam, because resting muscle should not be noisy.
Several distinct kinds matter. Fibrillation potentials and positive sharp waves are the firing of individual muscle fibres that have lost their nerve supply; cut off from their nerve, they twitch on their own, too small to see through the skin but loud and regular through the needle, sounding like rain on a roof or a marching rhythm. They are the hallmark of recent denervation and take two to three weeks to appear after a nerve injury. Fasciculations are larger, irregular pops from a whole motor unit firing on its own, the electrical version of the visible muscle twitches many people get, usually benign but sometimes a clue to motor neurone disease. Other patterns, like myotonic discharges that wax and wane like a revving engine, point toward specific muscle diseases.
Abnormal spontaneous activity is one of the strongest, most objective signs the needle exam offers, and a cornerstone of localizing a lesion and dating it. Finding fibrillations in a particular set of muscles, and not in their neighbours, lets the examiner trace which root or nerve has been damaged; finding them tells you the damage is active rather than old and stable; and finding none, weeks after symptoms began, argues against significant ongoing nerve injury. It is detective work done largely by ear.
In a foot drop, the examiner finds dense fibrillation potentials in the muscle that lifts the foot but a silent muscle just next to it on the other side of the leg. Because the noisy muscle and the quiet one are supplied by different nerves, the spread of spontaneous activity helps decide whether the damage is at the common peroneal nerve, the sciatic, or the L5 root.
Which muscles crackle and which stay silent traces the damaged nerve or root.
Fibrillations are not specific to nerve injury: they can also appear in muscle diseases such as inflammatory myopathy, so they must be read alongside the motor unit shapes and the nerve conduction findings, not in isolation.