Chronaxie is defined as the minimum stimulus duration, using twice the rheobase, that produces the same minimum visible contraction.

• A. The minimum stimulus duration, using twice the rheobase, that produces the same minimum visible contraction.
• B. The maximum stimulus duration that produces a visible contraction.
• C. The amplitude of current required to elicit a contraction.
• D. The rate at which the stimulus is delivered.

Answer: (A)

Chronaxie comes from the strength–duration relationship that describes tissue excitability. Rheobase is the smallest current needed to evoke a contraction when the pulse is very long. Chronaxie is the minimum pulse duration required to produce a visible contraction when the current is set to twice the rheobase. That exactly matches the statement: the shortest stimulus duration that yields the same minimum visible contraction at twice the rheobase. This shows how current and pulse width trade off to reach the threshold response. The other ideas aren’t definitions of chronaxie: the maximum duration is not the defining measure, rheobase concerns the current required with a long pulse, and rate of delivery refers to frequency, not duration at a fixed current.