All of the following apply to chronaxie EXCEPT

• A. It is represented by a point on the SD curve
• B. It is an index to muscle excitability
• C. It is measured in milliseconds
• D. It is represented by intensity

Answer: (D)

The important idea here is the strength-duration relationship and what chronaxie tells us about how easily a tissue can be excited. Chronaxie is the pulse duration needed to elicit a response when the stimulus is twice the rheobase current. In other words, it’s a time value on the strength-duration curve that marks where the current equals two times the minimum sustained current required for activation. Because it is a duration, chronaxie is measured in milliseconds, not in current strength.

This makes it an index of excitability: tissues with a shorter chronaxie respond to shorter pulses at a given relative strength, indicating higher excitability. On the strength-duration curve, chronaxie is depicted as a specific point along the time axis corresponding to twice the rheobase, not as a value on the intensity axis. Therefore, describing chronaxie as being represented by intensity is not correct, since its defining measure is time, not current.