I was watching a lecture on the Research Channel recently about painful lower back disorders (such as Ankylosing Spondylitis and Osteoporosis) and their resulting changes in the normal lordotic curve (the slight inward curve) of the lumbar region of the spine. It went on to say that certain disorders of the spine can cause a loss of this normal lordotic curve forcing the person to have to walk with a slightly stooped or hunched posture with their knees slightly bent.... This type of posture when walking is very similar not only to what has been reported in dozens (if not hundreds) of BF sightings but also to the way the creature in the Patterson film walked. I'm not saying that Patty (or any other BF) has a spinal disorder, I'm saying it is very possible that a BF's spinal anatomy appears to lack the normal lordotic curve that is found in normal human spinal anatomy. Also, IF the Patterson film is not authentic then the person wearing the suit greatly appears to be suffering from one of these (very painful) disorders, which would make them very unlikely to be walking that quickly and effortlessly in mountainous terrain.

It can not be discounted that nature/evolution has provided the spinal strength and possible lack of curve that you speak of to allow this creature to live within an habitat that causes it to traverse terrain that is less than hospitable for us. Nature creates a balance and that is not a pun, the slight hunch seen may be natures way of creating a balance mechanism for a creature that ascends and descends mountainous terrain.

I am very familiar with AS, the family of diseases it belongs to (Spondyloarthropathies) runs in my family. If you are interested in learning more about AS, PM me. I will state that it's very rare in the human/mammal population.