Hi Algabon,
I took a brief look at the circuit and I found it not worth building. A muscle stimulator must be able to produce different programs as individuals require individual treatment.
The frequency a muscle stimulator uses is fixed (15.384KHz (~65ms), used in hospitals), but there are different programs varying in times between pulse bursts and of course the intensity of the bursts must be variable too.
More than 40V - even in the audio frequency range might be fatal if the electrodes are not placed properly. At 40V the device causes discomfort already.
If you want to use for medical treatment I'll help you out and send you my design including the software. (Device name LIMS = Low Impact Muscle Stimulator) The output has a safety circuit not to exceed 39V.
It is also not recommendable to use a normal (inverted) mains transformer since its capability to transform square wave AF signals is not the best. Better use a self-made iron powder or ferrite core transformer with an air gap to increase battery life (less DC transfer, hence less current flow).
Under no circumstances use power - even gained by a power supply from mains voltage. With the smallest malfunction you will electrocute patients. Using an on board battery charger it should be safe enough to disconnect the entire device during the charging process.
Regards
Hans
P.S. The device of that link produces only half waves only. It must produce AC! (Preferrably a sine wave). Otherwise it's totally worthless for medical applications.