I don’t think your idea is very unique = =. . Forgive me. . Because there are students nearby who use power tools to make generators and then apply for patents. . The result is just fine looking at the picture = =. .
You can try to consider springs and rack and gear combinations. Pull the rack and the gear turns (the handle can be made into a gear). Then consider how to reset. . When it comes to fitness equipment, you should consider its safety