KS, Assuming amplitude remains constant, increasing the phase (pulse) duration increases the phase (pulse) charge or amount of electrical energy delivered. Thus a more robust effect as you have noticed. We use 400usec phase duration with 100usec interphase interval in otherwise healthy post-operative and post-injury patients with excellent elicitation of quads. We use NMES until the pt is able to perform a volitional contraction at a level of force we deem acceptable for that specific pt and that time in their rehab. Motor point location is well described in a recent paper by Gobbo et al where the concept of functional motor points are discussed versus anatomical motor points. Functional motor points are described as the location where the most robust muscular response is found for the least amount of electrical energy. The Gobbo paper references Botter et al who mapped functional motor points of the quads. The two papers I mention both show images of these motor points. Thus, there are clusters of motor points in all the individual quad muscles. Keep in mind though that NMES is not directly depolarizing the skeletal muscle but rather the motor nerve to the muscles. You will see a large variety of recommendations for electrode placement but there remains a lack of universal agreement. I believe this is due to individual pt variability with functional motor point locations. Thus, one electrode placement site may be more or less effective for one pt versus another. Specific to TKA, read the papers of Lapsley (Jennifer, I believe) for more details specific to TKA.