Skeletal muscles whose primary function is extension of a joint include the

The answer is triceps muscles. These muscles run along the back of the upper arm while the biceps run along the front. The triceps muscles allow the arm to extend at the joint of the elbow, therefore an antagonist of the biceps that bend the forearm.

the answer is lactic acid from anaerobic respiration. the acid in lactic acid is what creates sore and burning muscles. if you think about how acid burns that will give you a clue for questions like this.