Abstract
Chronaxie is used to describe the relative excitability of a given tissue, in this case, muscle. The strength-duration curve is a plot of the lowest current required for stimulation versus pulse duration. Chronaxie is the duration at which the current required for stimulation is twice the long-duration asymptote, the rheobase a. The current required for stimulation is equal to a+b/t, where b/a is the pulse duration, the chronaxie, which is a constant that depends partly on the type of excitable tissue. This is the Weiss-Lapique expression for the strength-duration curve, which is hyperbolic-like, with the required current increasing with decreasing pulse duration. Chronaxie has importance when subcutaneous tissues are stimulated with skin-surface electrodes. The Chronax^<[○!a]> (OG Giken, CX-2) now on the market is used for clinical examination of muscle function. It takes a fairly long time to determine the strength-duration curve. Moreover, muscle vibration must be confirmed by visual inspection. It is not practically and clinically accepted. As it is impossible to apply it to muscle fatigue examination, it should be improved. In the present study, we developed a new computer-based chronaximeter. It consists of a notebook computer, an electrical interface circuit, and a stimulation electrode. The new meter is characterized by rapid measurement for the strength-duration curve and by portability. Muscle vibration is detected with a piezoresistive accelerometer mounted on the electrodes, instead of by visual inspection. Strength-duration curves were obtained for biceps brachii, flexor carpi radialis, tibialis anterior, and rectus femoris by using a stimulation electrode situated at the motor point of the given muscle. From these curves, the chronaxie values for biceps brachii were calculated to be 0.129 ms; for flexor carpi radialis, 0.168; for tibialis anterior, 0.265; and for rectus femoris, 0.188. The chronaxie values obtained were shown to be sufficiently different.
