Improving your ability to produce a more powerful pedal stroke does not come from doing exercises like leg extensions and leg curls. A powerful downstroke comes from using your hips. Exercises like the barbell hip thrust and deadlift stiff leg will effectively train your legs to drive from the hips and not your knees, which will boost your power on the bike. These five exercise will focus on all the main muscles groups that cycling demands.
To improve your pedal stroke you will always want to use multi-joint exercises that engage multiple muscle groups. If you are new to these exercises, focus on mastering a couple of these exercises before you start expanding your training routine.