Objective
To quantify the effect of botulinum toxin A injection, by the compound muscle action potential (CMAP) measure from the gastrocnemius muscles (GCM) and to compare them with the clinical data.

Methods
Seventeen legs of 10 cerebral palsy (CP) children were studied with botulinum toxin A injection on the motor points of their GCM. Each GCM was injected up to 6 units of the botulium toxin A per kilogram of the body weight. The CMAP were measured at the motor points of GCM with the surface electrodes on the post-injection day 1, day 3, day 7, 2 weeks and at 1 month then monthly thereafter for 6 months. Physician rating scale (PRS) and the angle of passive ankle dorsiflexion were evaluated at the same time.

Results
The amplitude and the area of the CMAP decreased from post-injection day 1 to 3 months. The most pronounced decrement was seen at 2 weeks post-injection (p＜0.05). The most pronounced increase of the dorsiflexion angle and PRS were seen at 1 and 2 months post-injection, respectively (p＜0.05).

Conclusion
The compound muscle action potential measure can be used for the neurophysiological quantification of the effect of botulinum toxin A, especially for the superficial muscles of extremities.