Which of the following is NOT a normal chronaxie value for skeletal muscle?

• A. 1/1000 second
• B. 1 sigma
• C. 1 millisecond
• D. 1000 milliseconds

Answer: (D)

Chronaxie is the time needed for a stimulus at twice the threshold (rheobase) to trigger a response, and in skeletal muscle this time is in the millisecond range. That means values around one millisecond are normal. So both 1/1000 second and 1 millisecond fit the expected range. A duration of 1000 milliseconds equals one second, which is far outside the typical chronaxie for skeletal muscle, so it isn’t a normal value. The label “1 sigma” isn’t a time unit at all, so it doesn’t represent a chronaxie duration, but the clearly extreme duration here is the one-second value.