The main findings of the study indicated that a 4-week combined electromyostimulation and plyometric training program significantly increased jumping height and sprint run in physically active men.
-Dr. Mikel Izquierdo
I believe that getting EMS strength phases done a few times a year is enough to make an impact with speed, not just isometric knee extension. My experience is that doing 4 six periods a year is enough to get a statistical impact. How much more will doing it with higher periods I don’t know. What I do know is that EMS works and important for athletes that have limits with training time, such as professionals that spend a lot of time traveling. Some of the interesting things I have seen first hand is the changes in pain and function within a few treatments. If you are interested in EMS and sport, and want to see some new approaches I suggest investing into chapter 6 of the Kinetics Manual. There you will see some new Polish approaches.