I think that is one of the biggest advantages on the trainer. Forces you to pedal in a more rounded motion but definitely due to not having the momentum/inertia you do on road and highlights your weaknesses. I don't enjoy the trainer but I endure it knowing that it is only going to do good.
My feeling is that we can adapt to anything. I don't know that adapting to a low-inertia trainer makes sense. What might be thought of as a weakness in terms of power output might be a strength in terms of resting for the proper ratio on each pedal stroke.
Riding with proper cycling shoes clipped in certainly feels superior, but riding, especially sprinting in vans or a similar shoe on platform pedals can do a lot for your pedal stroke. Pedaling in squares or rather ellipses if we're going by sensation and making it look like a smooth circle to others is what I find to put out the best sustainable output. That upstroke has an elastic feel to it when things are really clicking, there's no power generation involved it's just popping back up. Very similar to the leg swinging back forward when running; we direct it's path with a sort of internal visualization of the path the body should take, but the movement should feel like a reflex of sorts.
I believe that elasticity is lost on a low-inertia trainer.
Anyway, leg muscles used in pedal stroke? All of them in some way or another. I think we get in trouble when we try to isolate the specific muscles. The body is one gigantic muscle and you should be using all of it as you ride. Better to think of the proper sensation to experience. In that regard, my current understanding is this:
- we have to have spinal rotation to generate good power
- there should be a sensation that the body is being pulled up at all the joints, like a marionette doll
- the tension should be stabilized from the core such that you feel so smooth it could make you think the same amount of power is being generated at all parts of the pedal stroke if you didn't know better
- all of the muscles should have some sort of cyclical tension/relax phase to them
- on the upstroke it feels as though energy is moving up the lateral portion of the leg starting from the outer area of the ball of the foot up the calf crossing the knee to the quad then crossing to the glute up the back to right behind the ear
- on the downstroke the energy returns down the spine from the ear out to the center of the quad around 12 o'clock
- as the pedal goes down the energy spirals towards the inside of the leg, with even the adductor magnus adding a slight twist and more down/back force between 3 and 5ish-o'clock
- eventually the energy leaves towards the inside ball of the foot
- the shoulders and arms will mirror the legs to a large extent in that the line of energy or tension spirals up and down, with a good example that you can see being alberto contador climbing out of the saddle. the sensation is similar while seated, but much more subtle so it's difficult to see in others if you haven't already felt it yourself
- probably the key checkpoint is to feel that everything is lifting in a way to make it feel as if you're floating, because when that happens the body is generally moving the best it can.
- the sensation of the energy coming up to and then returning from the area near the ear is one worth chasing if you haven't felt it before. it feels rather like your foot is almost stepping on an imaginary ground at the ear level in the sense there is no compression of the body between the ear and the foot. not sure how else to describe that right now.