I find that sitting in a tense position while driving can cause some of the same issues you mentioned. I've learned to change my position frequently and if I notice myself tensing up, to consciously relax. And whenever I stop for a break I stretch as many ways as I can.

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.

Perhaps a physical therapist will offer stretching exercises to help with this and all quilting pains.
sandy

Try a different chair....the seat height can affect your legs...I have short legs and if the seat is too far off the ground (so that my feet dangle) it will make my legs go to sleep....stretching to use a foot pedal in a similar position would cause me grief

Good suggestions, I think I will try the book under the other foot idea. Yes...my shoulders and back are in severe pain after FMQ'ing. I am trying to not tense up so much, but my concentration is focused on the quilt!

It is important to keep moving and vary your activities to help avoid pain. Break up your activities and been careful about repetitive motion for too long at a time. When my husband built my sewing center he made the top drawer in the drawer section to the left of my machine to be an ironing center complete with a specially hinged ironing board that pops up. It was such a nice thought but I rarely use it because I need to get up and walk to the ironing board to press during the construction process. Sitting too long is not good for our bodies.

18 months ago I broke both bones in my right leg and dislocated my ankle so I sympathize. One thing I have learned is to make sure you are sitting directly in front of the needle not off to the side. I also put my foot pedal on a book and a non slip pad under it so it doesn't move. I also bought a neat but inexpensive chair (99.00) that I had tried at my LQS and is comfortable to sit in for long periods of time. Make sure to have something to rest your left foot on so it is even with your right,

I found this to be a problem as well...so I looked and felt how my foot was pressing the pedal. You push your toes down which puts constant stress on your ankle and tendon in your lower leg. I turned my pedal around where the higher end of the pedal was at the front of my foot. Yeah!, I had a lot less strain on my ankle and lower leg...barely had to touch the pedal with now the top of the pedal at the front of my foot. Try this and note how your foot is now not having to press so far down and your lower leg is not being used as much either. Different feel, but works for me.

I have also experienced such pains. My DD made the brilliant suggestion of doing an online search for Yoga for Quilters. I got some great simple exercises that have made a huge difference. Hope you find something that works for you.

I do not do a lot of FMQ, guess I need to get over my fear of it not being perfect the first time out, although I know that practice makes perfect. My pain is mostly in my upper arms and shoulders from my posture while I'm quilting/sewing. Someone recommended that I use the rubber door stops you can get almost anywhere and put them under the back of your sewing machine, so you're not leaning too far forward while sewing. Also, try using a phone book, or anything else that would bring your left foot to the same level as the right foot. My biggest problem is having my right leg go to sleep if I've quilted for too long, although it doesn't happen as much when I'm piecing. I try to take breaks after a certain amount of time and that seems to help. Please try some alternatives as recommended by QB members, and you will probably find the right equation for less pain and more quilting. Good luck.