threshold tracking allows for the strength of a test stimulus to be adjusted by a computer in order to activate a defined fraction of the maximal nerve or muscle potential. a threshold tracking experiment consists of a 1-ms stimulus being applied to a nerve in regular intervals. the action potential is recorded downstream from the triggering impulse. the stimulus is automatically decreased in steps of a set percentage until the response falls below the target (generation of an action potential). thereafter, the stimulus is stepped up or down depending on whether the previous response was lesser or greater than the target response until a resting (or control) threshold has been established. nerve excitability can then be changed by altering the nerve environment or applying additional currents. since the value of a single threshold current provides little valuable information because it varies within and between subjects, pairs of threshold measurements, comparing the control threshold to thresholds produced by refractoriness, supernormality, strength-duration time constant or "threshold electrotonus" are more useful in scientific and clinical study.