Heat may provide quick and easy pain relief and can increase circulation to speed healing after muscle injury. It's used for many types of pain including joints, muscles, and soft tissue damage.
However, there's typically much debate over how to apply heat: moist or dry. Let's see what the research says...
Applying topical heat to a sore or injured area, obviously, makes your skin's temperature rise. But how is this helpful exactly?
It creates vasodolation: the widening of blood vessels that results from relaxation of the muscular walls of the vessels. More blood can then flow in and out of your tissues.
Having that extra blood flow means muscles receive more oxygen and nutrients. Carbon dioxide and metabolic wastes may also be taken away quicker.
Your cell's metabolism also doubles for every two degree centigrade increase in tissue temperature. So applying heat is not only like opening up an extra traffic lane, it increases the speed limit. This increase can be helpful to quicker healing and repair of damaged tissue.
Another effect of heat is the mediation of pain. Acute pain can be mediated by thermal heat due to sensitive calcium channels.
These channels react to the application of heat by increasing intracellular calcium. As this 2004 study concluded, "variation of intracellular calcium contents at a supraspinal level is involved in the modulation of acute thermal nociception." In other words, it can make ouchies feel a little less ouchy.
DRY OR MOIST?
When it comes to applying heat topically, whether to use dry heat moist heat is the first big fork in the road.
Moist heat can be applied through applications like hydrotherapy (immersing muscles in water aka bath time), moist gel packs, or hot water bottles.
It is normally applied for short periods of time (five to twenty minutes) and generally provides mild pain relief quickly.
Moist heat has been clinically demonstrated to transfer heat faster and deeper than dry heat. However, the shorter application time typically results in poor transfer to deep tissues/injuries.
Dry heat can be applied through applications like electric heating pads, dry heat packs, or infrared saunas.
It is normally applied for long periods of time (multiple hours), and thus pain relief can be delayed. However, temperature regulation is more consistent as it warms tissues gradually, so is typically more effective with deep tissues/injuries.
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A detailed study published in 2013 involving 100 subjects compared the beneficial effect of dry heat versus moist heat on exercise-induced delayed onset muscle soreness (DOMS). It was conduced to determine if moist heat, due to faster heat penetration, would work as well as dry heat. The study concluded:
...immediate application of heat, either dry (8 hours application) or moist (2 hours application), had a similar preservation of quadriceps muscle strength and muscle activity. ...the greatest pain reduction was shown after immediate application of moist heat ... immediate application of dry heat had a similar effect, but to a lesser extent ... It should be noted that moist heat had not only similar benefits of dry heat but in some cases enhanced benefits, and with only 25% of the time of application of the dry heat. Muscle strength, a measure of muscle damage, was preserved after exercise equally by dry heat or moist heat immediately after exercise. But if either modality was used 24 hours after exercise, they had little effect on recovery of strength. Another measure of damage is the force to flex the knee and the difference in force between flexion and extension ... related to changes in tissue stiffness ... moist heat and dry heat after exercise and 24 hours later both helped reduce damage ... showing that even though strength was not protected, if dry heat and moist heat were used 24 hours after exercise, structural damage to the elastic components in muscle or tendon appeared to be eliminated. This offers an interesting dichotomy... Since strength was reduced when heat was applied at 24 hours, there was damage to the muscle fibers unresolved by heat. But since elasticity was maintained if heat was applied at 24 hours, then a logical conclusion is that heat after 24 hours allowed the healing of tendon and connective tissue so that elasticity was retained but did not help the muscle fibers themselves. This is also shown in the skin resistance data ... moist heat immediately after exercise was just as effective as dry heat applied immediately and 1 day post exercise. But for pain, moist heat immediately after exercise was more effective than dry heat just after exercise. If either modality was applied 24 hours post exercise, the effects were minimal on pain. ... in spite of the fact that moist heat was only applied for ¼ the duration of dry heat, it was just as effective if not more effective in reducing pain and muscle damage after exercise.
I found an established body of research that may suggest the use of heat for pain and tissue recovery. Given the very low risk and very limited side effects, it would seem worth a try for individuals seeking a complementary and/or natural remedy for pain relief. Although moist heat could be argued as the "better" choice due to shorter application time, dry heat options are noted as generally being easier to apply in a typical day-to-day basis. If that dry heat sounds good and you need a heating pad, the one I use myself is big enough for low back, but also wraps-around for shoulders and neck.