I had the same problem when I was using a foot pedal, except if I tried to run, or step more quickly,(like crossing the street quickly at a crosswalk) my calf muscle would seize up and pretty much bring me to my knees. Just for reference, this started happening when I was still in my 30's and I considered myself in decent shape.
Since moving to my new machine with the start/stop button, I no longer have this issue. However, I went to do a quick testdrive of a HQ Sweet Sixteen, and within 5 minutes of using the foot pedal, I had irritated that calf muscle again. Took several days for it to settle back down.