1. If you move well you most likely won't have pain or injury, and if you don't have pain or injury then you'll be more likely to move well.
2. One thing I really like to watch is how 2 and 3 year olds walk; very minimal force impact at heel strike (they basically just lay the foot flat on the ground each step), smooth rolling of the hips, completely relaxed arm swing, etc.
3. In many athletic movements (key: well performed athletic movements), the hamstrings and rectus femoris perform more as isometric muscles, allowing the tendons of these muscles to create and transfer force.
4. This is a huge problem and it affects EVERYONE.
5. Movement is improved through motor learning. The key is focus and specific control. I can't learn something without focus (motor LEARNING), and I can't use something if I can't control it (motor CONTROL). Motor learning takes focus and practice. If the correct movement is performed, the muscles will do their job.