The use of Ice (Cryotherapy) is useful for calming down inflammation swelling and recent injuries. Swelling caused by inflammation is a completely healthy process but it can be painful and stubborn to control. Ice is a good, drugless way of decreasing pain and reducing swelling, for example, a freshly pulled muscle.
The use of Heat (Thermotherapy) is for muscle pain and takes the edge off symptoms like aches and stiffness. Chronic pain, particularly back pain involves tension, anxiety and sensitisation and heat can soothe and de-stress.
A combination of both heat therapy and cold therapy is called contrasting therapy. It can be helpful in some injury recovery, but the exact benefits are unknown.
Both heat and ice have the potential to cause harm when used incorrectly. Heat can make inflammation significantly worse and ice can aggravate symptoms of tightness and stiffness. Ice therapy is pointless if you already cold and shivering and heat therapy is useless if you're already hot and sweaty.
So, the obvious answer is how do you know when it's just muscle pain and needs heat and when it is an injury that needs ice? That is a tough call, but ice usually wins but only for the first 48 hours, at which point, the most effective method can be reviewed. Muscle injuries as a rule, normally occur during exercise or intense effort and causes pain almost immediately. If the muscle is truly torn, you would swap ice for heat when the swelling was reduced. As heat only aggravates swelling and inflammation.
In studies heat, ice and strong pain medication were seen equally as effective at treating pain. But ice and heat are cheaper and more readily available, not to mention much safer to use.
So to conclude:
Heat is for aches and pains.
Ice is for injuries, swelling and inflammation
