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46

The exact mechanism is unclear. Here are some possible causes: rapid collapsing of cavities inside the joint [1]; rapid ligament stretching [1]; breaking of intra-articular adhesions [1]; escaping gases from synovial fluid [2]; movements of joints, tendons and ligaments [2]; mechanic interaction between rough surfaces [2], mostly in pathological situations ...


25

The joint you are thinking of is not a knee, nor is it an elbow, instead it is an ankle which is bending the same way as us humans. You can see from the below diagram that the knee - the joint between the femur and tibia - is just further up the leg normally hidden by feathers. Birds have a comparatively elongated metatarsus which gives the impression that ...


15

Is joint-cracking harmful? No. Donald Unger was told by his mother that he'd get arthritis if he cracked his knuckles so he cracked his left knuckles every day for 60 years but never his right knuckles. He had no arthritis or any other problems in either hand and got a publication (D. L. Unger, "Does knuckle cracking lead to arthritis of the fingers?", ...


8

A paper was just published that explains this phenomenon in PLOS One, found here. It looks like the sound is caused by the formation of a gas cavity in synovial fluid of the joints. They do mention that contrary to what is stated in the most upvoted answer here, the sound does not come from the collapse of the bubble, but rather its formation. And in ...


6

Nobody really knows where it comes from. The currently most popular theory is that pulling the joint apart leads the gases in the joint's cartilage to accumulate and form a bubble which then pops when you let it spring back. The only thing that has been researched is whether it has an effect on the joint, but people who do it regularly don't seem to have any ...


3

I couldn't find studies analysing clearly enough tendons size, but a couple of sources show that they are probably correlated: One study found correlation between dimensions of Achilles'tendon and other ankle tendons (see table 2). Another study found correlation between height and dimensions of some tendons (see table 2). Anyway, since correlation is not ...


2

As far as I remember the pathology course from medical school, chronic long-lasting inflammation often leads to proliferation of connective tussie and ultimately to fibrosis. The actual mechanism here is the lack of oxygen which is used-up by different immune system cells to produce peroxydes and superoxydes.


1

The additional degrees of freedom are provided by the entire cervical spine, rather than the atlantoaxial joint alone, which is the joint between C1 and C2. What you're describing as roll (when you point one ear to the ground), is provided by contraction of the sternocleidomastoid. The sternocleidomastoid muscle pulls the mastoid process (behind the ear) ...


1

The intermediate radio-ulnar joint is fibrous in arms. [Source: Slideshare.net] You can see the the interosseus membrane between radius and ulna which is composed of white collagen fibres. Furthermore, there is a cartilaginous joint between epiphysis and diaphysis of humerus which is made up of hyaline cartilage. But it is lost during development and is ...


1

The simple answer is: our ancestors did...as did our ancestors ancestors, and so on. We are not optimally designed, but rather, we take what our ancestors give us, and work with that. The horses for example. They have added another joint by essentially extending their toes, and walking on them, which gives them an advantage in running. It is possible to ...


1

Osteoarthritis results from the gradual degradation of articular cartilage. When the cartilage is damaged extensively, e.g., all the way through the cartilage and into the bone, an inflammatory response ensues in the bone. This often happens when bone is rubbing against bone. One of bone's responses to injury and inflammation is the production of new bone. ...


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