Rheobase is defined as:

• A. The minimum stimulus amplitude, of infinite duration, that produces a minimum visible contraction (twitch)
• B. The maximum amplitude before fatigue occurs
• C. The rate of stimulus delivery necessary for contraction
• D. The duration of the pulse at which contraction begins

Answer: (A)

In excitation of nerves and muscles, the strength–duration relationship matters: a response depends on both how strong the stimulus is and how long it lasts. Rheobase is the smallest current that will elicit a response if the pulse length were effectively unlimited. In other words, it’s the minimum stimulus amplitude needed to produce a twitch when duration is not a limiting factor. This is why the statement describing the minimum stimulus amplitude, of infinite duration, that produces a minimal visible contraction (twitch) fits rheobase. In practice we approximate rheobase with long pulses, and as you lengthen the duration, the required amplitude to reach threshold approaches this value. Chronaxie—a related concept—is the pulse duration at twice the rheobase that evokes a response. The other descriptions refer to fatigue thresholds, rate or pattern of delivery, or threshold duration, which are not rheobase.