The hips and shoulders aren't so much springs as they are pivot points for the springs to bend and turn when necessary IMO.That was my ponder. This leads to the segmented movement argument. I can make a finite amount of power if I use only my arm or leg. With my back pressed to a wall for example. They are a spring in the functional sense but with potential energy only. When I add the powerful spring in my mid-section the strike is exponentially stronger. So if the arms and legs are springs, I would add that the waist is also a spring. And you could argue that the shoulders and hips are also springs.
Anatomy supports this (also supports the other discussion about hands=feet elbow=knee hip=shoulder)
The wrist and ankle joints are gliding joints. Great for making single direction movement and small circles.
The knees and elbows are hinge joints; great for.. well.. hinging lol. Larger amount of movement but stuck to one plane.
The shoulder and hips are the most free moving joints in the body, a ball and socket joint. This gives you movement at every angle but requires a lot of muscle to stabilize.
So now if we start thinking about the types of joints our body has and apply the theory of the arms and legs being springs attached to the spine you can very clearly see how the arms and legs can compress (much like a spring) through the hinging joint. Once the joint moves in too close to the body it becomes weak and requires muscles to hold the structure (instead of bone). To get around this, let your ball and socket joint relax and pass the tension to the larger spring of your spine. Think about how a very strong heavy spring bends under pressure that isnt applied directly to the center. Then you snap back to center and hopefully hit your target. This is all assuming contact has already been made and you cant simply just punch the guy......