Thank you all for your educative help.
A few follow-up questions:
From a lay persons point of view I've noticed that increasing the pulse width seems to have the most noticeable effect on the apparent strength of the contraction. Is there any reason (not to use the maximum that my machine will allow - 400us?
My machine will, support different parameters on each channel. Is there any merit it setting separate parameters for each channel (e.g. one type of stimulation for one vastus medialis and another for vastus lateralis, concurrently? Would this help avoid the training effect?
Finally, I'd appreciate some help identifyng the motor points for optimum electrode placement (mine are small, 2 per channel). Prof Ronzio mentions vastus medialis and rectus [femoris] - rectus and vastus lateralis, but I've seen many different photos/diagrams on the internet, that don'r mention the rectus femoris.
Does it make any difference where the positive and negative electrodes are placed