so it's okay to bend at hip but not a waist? your head tilts down same anyways in both ways?
so top boxers does not bend at the waist but at the hips? It does look like they do bend at the waist and the hip from time to time
You
can/should bend where your hips are (some, there is an intelligent limit), which is different than using the lower back to bend. Mostly it should be the knees, though.
Tyson and the other guys taught by Cus DID bend at the back a lot for the peek-a-boo style. Also their backs (including Tyson's) were notoriously way out of wack and were constantly needing readjustments (between rounds, even) and trying all sorts of things to help their constant back pain. It worked, but at a price.
There is a science to all boxing movement. When ducking/weaving--
1. Mostly use your knees (level change) so you don't have to pitch forward and get out of position/off-balance.
2. Somewhat use your hips if you want/need. Shouldn't be pitched forward approaching 45º down or more.
3. Your back should be bent only slightly forward (still relatively straight) or not at all. That's the range. No one's back is designed to do spring back up quickly from leaning forward, esp. during combat, regardless of the strength of the muscles built around it.
This is not only for health, but for best technique/results in the ring.
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As for bending sideways, some limited range of that is useful and expected with proper movement along with punching. Ducking (which means forward in this case) with the lower back is a really bad idea. Also you shouldn't be leaning forward too much when you do. It's not even a long term thing, bad problems will manifest very quickly. You can't spring up, return to basic fighting stance/posture as fast to punch, either, so you miss opportunities and can get bullied or even injured during that time