In a strength-duration curve, which factors are considered variable?

• A. Rheobase
• B. Waveform
• C. Both A and B
• D. Neither A nor B

Answer: (B)

The strength-duration relationship shows how much current you need to evoke a response at different pulse lengths for a given stimulus shape. Rheobase is the minimum current required for an infinitely long pulse and chronaxie is the pulse duration at twice that current; these describe the tissue’s excitability under fixed conditions. When you plot the curve, you largely hold the waveform constant and vary the pulse duration to see how the threshold current changes, so the tissue’s basic excitability parameters stay the same on that curve. What you can change to alter or compare curves is the waveform itself—the shape and polarity of the pulse—which shifts the threshold needed at any given duration. Because the waveform is the controllable factor that affects how easily fibers respond, it is the variable in this context, whereas rheobase is a property of the tissue under the chosen conditions rather than a variable along the curve.