Which Muscles are Really Used During the Pedal Stroke?
One of the most persistent myths in the biking world surrounds the pedal stroke and goes something like this: " Without being attached to the pedals you can not use your hamstrings properly, which forces you to rely too much on the quads to power the pedal stroke. By not being able to curl the knee joint during the upstroke of the pedal stroke you create muscular imbalances and tire out the quads faster." That is what most of us have been told. However, this understanding of which muscles are used and how they are used during a pedal stroke is completely wrong and potentially dangerous over the long run.
According to this theoretical model of muscles used during the pedal
stroke, the hamstrings are used maximally from 8 o'clock to 10 o'clock
position, while the glutes and quads are responsible for the downstroke
part of the pedal stroke. This paints a completely false picture of the
situation and leads a lot riders to assume that the hamstrings are only
there to flex the knee joint on the upstroke, which would be impossible
to do if you weren't attached to the pedals. This, of course, would mean
that it would be impossible to optimally pedal without clipless pedals,
which is where the faulty logic that tells riders that it is impossible
to pedal optimally without them stems from.
What does this mean for you?
1 - You can (and should) be able to pedal your bike very effectively with flat pedals. This myth is one of the most common ones I hear from riders as to why they don’t want to try flat pedals when in fact, flat pedals will actually clean up and improve your pedal stroke.
2 - You should train your legs to produce a powerful downstroke using the hips as the primary power source, not the knee joint. This means that leg curls and leg extensions are bad exercise choices since they reinforce this "knee powered" pedal stroke. Exercises like single leg deadlifts and single leg squats are much more effective since they train the legs to drive from the hips, not the knees.
3 - When riding don’t worry about "spinning circles" or "keeping equal pressure on the pedals" or whatever else someone has told you that essentially means you need to curl the hamstring through the return portion of the pedal stroke. While a good, efficient pedal stroke may feel like you are spinning circles the reality of what your muscles are doing to produce that feeling are much different. Your body has one way to optimally produce lower body locomotion and you simply want to apply it to the pedal stroke.
More info here