Muscle fibers are torn and disrupted. The human body is only designed to run about 20 miles. Thus our preoccupation with running marathons can be damaging to the body. Muscle cells and fibers are damaged due to overstressing of the muscles involved. Ending a workout very quickly stops the pumping action of the muscles that assists the drainage of body fluids diffused throughout the muscle tissue.
Blood pressure drops and heart rate slows, and drainage of waste products slows also.
This also contributes towards stiffness and pain in muscles. The muscles must be conditioned to accept the battering we dish out to them. Distance training, conditioning the muscles to function efficiently while running over long distances, should be the first priority in training. Secondly, training down hill is important. Thirdly, shoes must be worn that effectively cushion the increased landing shocks while running down hill.