Originally Posted by Meridio
Not true; it's perfectly possible to bowl with a '45 degree' action and not get back problems. The key is to make sure the top half of your body is in the same alignment as the bottom half. If you have a side-on action, with your back foot parallel to the crease, then you should be looking over your front arm in your action. If you have a front-on action, you should be looking past your front arm; while if your back foot lands at 45 degrees then you should be looking through your front arm, so to speak.
Kyle Mills is one bowler I can think of with a 45 degree action; it's generally recognised as being a safe way to bowl now. The problems come when bowlers have their feet pointing one way and their torso the other; 45 degree actions are often mistakenly identified as side-on which can lead to the bowler being coached to bring their front arm across their body, causing back problems, which could be why they have been labelled as dangerous.
Yes I agree, having these 'mixed' actions as far as I'm aware is the biggest threat to back injuries in young kids growing up. As a coach we are always told to look out for these cases and try and eliminate it before the young bowler suffers any serious back injuries.
As you say, the hips and shoulders must be in sync in the back foot or else you have this 'mixed' action that puts real stress on the back.
When I bowl my back leg collapses, not sure it's necessarily a huge burden to my back, but being 6'6" it doesn't help me make the best use of my height when delivering the ball.