Here's a trick I found relative to flexshaft foot controls that
works well for me. Try it it may work for you.
I like to place the pedal so that the arch of my foot is resting on
the hinge part, and control the speed with my heel. What this does
is allow you to rest the weight of your foot (leg) on the solid back
of the pedal, and shift position slightly to press the pedal. It is
much less tiring because you don’t actually press on the pedal, you
just shift the weight of your foot.
This doesn’t work on the pedals that are a little square box with
the pedal sticking out of it.