insertional activity

When the EMG needle first pokes into a muscle, the muscle fibres it nudges respond with a brief crackle of electrical noise, like the rustle you get from disturbing dry leaves. That short burst, which dies away within a fraction of a second once the needle stops moving, is the insertional activity. The examiner deliberately moves the needle in small steps and listens to this rustle at each spot.

Normal insertional activity is a brisk, short-lived burst that stops almost the instant the needle stops. Its length and character are clues in themselves. If the burst goes on too long after the needle settles, the muscle membranes have become irritable, which happens when a muscle has lost its nerve supply or is inflamed. If insertional activity is reduced or absent, the muscle tissue may have wasted away and been replaced by fat or scar, as in long-standing severe damage, leaving little live muscle for the needle to provoke.

Insertional activity is the first thing the examiner notes at each needle position, before asking the patient to contract. On its own it is a soft, subjective sign and rarely diagnostic, but increased insertional activity often heralds the more definite abnormal spontaneous activity that follows, and absent activity is a sobering hint that a muscle has been badly and chronically damaged. It sets the stage for everything the needle exam looks at next.