SD Curve and Chronaxie/Rheobase
Strength-Duration curves and Chronaxie determinations were historically performed from the 1930s-1960s to assess nerve injuries prior to the common recent use of EMG/NCV testing. These graphs were a means of evaluating the severity and subsequent recovery of a nerve injury.
Rheobase is determined as the current strength value (measured in mA) when a stimulus with a square wave DC pulse* of current of infinite duration (300 ms in practice) is applied to a muscle to obtain a twitch contraction. Rheobase is defined as the minimal current strength below which no response can be elicited.
Chronaxie is the minimal duration of the DC pulse that is required to excite the cell at twice the rheobase strength.**
*DC pulse of Current
The chronaxie of nerve is short, usually less then 1 ms; denervated muscle is a larger capacitor requiring more energy to discharge, usually with a chronaxie of greater than 1 ms. ***
Plotting a SD curve requires stimulating a muscle at its motor point with fixed pulse duration, ranging from .01 to 100 ms., recording the current strength in mA required to elicit a threshold twitch contraction.*** The mA values obtained vs. the pulse duration utilized is plotted on the X and Y axis respectively. ****
The curve of a denervated muscle will be shifted to the right compared to normal innervated muscle; subsequently shifting to the left during re-innervation.***
**Chronaxie and Rehobase values
***Chronaxie/rheobase of varying types of nerve/denervated muscle.
*** Graph of data obtained with SD testing
****SD curve of denervated abductor digiti minimi vs. recovery and normal