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. 

No because the eaeht is on a 23.5 degree axis tilt and so when it is winter the northern hemisphere is pointing away from the sun and the soutubern hemisphere is pointing toward the sun