Both shooting and long throws are as much down to good timing as anything. You need everything to come together at once so that when you hit the ball your foot is travelling at max speed, bit like a golf swing in my opinion. Some of the top pros have the ability to hit a shot hard with hardly any backlift because their timing is that good.
However, I would say that
leg exercises like squats, leg extensions(do one leg at a time) and calf raises should improve the muscles required for a good shot.
Also, my secret to a long throw-in, work on your triceps, say
weighted dips for example, then when you are performing a throw bend your arms slightly and straighten them when you release the ball. The rules state that your arms must be straight but the chances of the ref noticing are none existent.
GP (long throw specialist)
