It's a complex procedure and it varies depending on how certain players shoot. It involves your calf muscles, the Gastrocnemus(calf muscle) must contract as you need to extend your toes to create a flat line from your knee to your toes increasing your accuracy, which increases the power of your shot. Both your Tibialis anterior(muscle on your shin) and Peronaus Longus(runs along the inside of your lower leg) are extended and they allow you to adjust the angle your foot makes as it makes contact with the ball. Your Quadriceps contract as you straighten your leg, this is very important to the power the shot. Your hamstrings will extend and fast, and if they aren't strong enough or you didn't warm up enough you may end up like Michael Owen and snapping them clean. The hip flexor is very important to the movement of your entire leg and like the Quadriceps is very important to powering your shot. The abs, back and gluteus maximus are important if you're rotating you're body to shoot. So like I said earlier, it all depends on how you decide to shoot. If you're volleying the ball, quadriceps, hip flexors, abs and back power the shot. If you're striking a dead ball, there are three common options, if you hit it like Giggs, with lateral spin most of the power comes from hip flexors, back and abs. If you hit it like ronaldo with top spin, then you need to power it with abs and quads. If you thump it like Gerrard or Lampard, it's mainly from your quadriceps. Hope that helps