Where do most joint injuries tend to occur? why?

The ankle is where the joint injury tends to occur because Calcium is released from the sarcoplasmic reticulum. Aside from calcium - assuming that we already have the motor end-plate signal to initiate contraction - then acetylcholine is also necessary, but you would need ATP to bind the myosin as well. Credits to Med School Physiology. 

Fossil found in strata reveal that unused body part decrease in size