This study investigated the thermal effect of therapeutic moist hot packs on the underlying soft tissue of the anterior thigh in 6 healthy volunteers (23 to 31 years of age). Temperatures at 4 depths (surface, 1 cm, 2 cm, and 3 cm) were recorded via a needle thermistor and a surface thermosensor for 5 minutes before, 20 minutes during, and for 75 minutes after, hot packs application.

Repeated measures ANOVA showed significant increases from baseline to maximum temperature at all depths during the 20 minutes of hot pack application. Surface temperature increased approximately 10° c in 11 minutes while deeper layers increased considerably less and in more time. Upon removal of the hot pack, temperatures began to decline, except at the 3 cm depth which continued to rise another 0.4° c for the next 12 minutes. This continued rise after hot pack removal accounted for almost one-third the overall increase at this depth.

The results show that: a) statistically significant increases in temperature occur to a depth of 3 cm during and after a 20 minute hot pack application; b) the greatest rise in temperature occurs at the surface and in the shortest amount of time; c) the 3 cm depth demonstrates the smallest rise, reaching its maximum at an average of 12 minutes after the hot pack is removed, and d) subcutaneous tissue temperature does not reach the so-called "therapeutic range" with a standard hot pack application. Possible explanations for the continued rise in temperature at the 3 cm depth are given.


One of the most commonly utilized superficial heating modalities is the moist hot pack.1, 2 Clinically, these packs are used to relieve pain, reduce muscle spasm, increase joint range of motion, and promote healing. 1, 2 Physiologically, moist hot packs a) increase blood flow secondary to vasodilation; b) increase local metabolism; c) increase nerve conduction velocity; d) alter viscoelastic properties of connective tissue, and e) produce muscle relaxation.1, 3, 4

To achieve these benefits, it is believed that the temperature of the target has to be elevated into the "therapeutic heat" range.2, 5 This means that the "resting" temperature of the part, for example 36-38 degrees c, has to be brought to between 40-43 degrees c, for benefits to occur. However, if temperature goes above 45 degrees c, pain and tissue damage ensue. Therefore, the presumed therapeutic window is relatively narrow.

Hot packs heat by the mechanism of conductive heating, defined as, "an exchange of thermal energy in which there is physical contact between two surfaces".1 How hot a target tissue becomes, and how quickly this rise occurs, depends upon a number of factors, namely a) the temperature gradient (which takes into account both the difference in temperature between adjacent regions and the distance between them); b) the specific heat of the tissue (specific heat is the number of calories required to raise one gram of tissue one degree centigrade); c) the insulating nature of the tissue separating adjacent regions; d) the duration of the treatment, and e) hemodynamics, i.e. the ability of the blood flowing through the tissue to remove heat from the region.

Although a number of investigators have studied the temperature changes at various depths caused by moist hot packs, 5-9 none have specifically addressed the tissue of whether the " therapeutic range" is actually achieved in muscle, nor have they investigated the time-course of these changes after the hot pack is removed. Furthermore, some of these studies utilized techniques that are not practiced clinically, such as replacing hot packs every ten minutes,9 or complicating the treatment by occluding blood flow to the limb some time after the hot packs were applied.9

Therefore, the purposes of this study were: 1) to determine the minute-by-minute temperature achieved at four depths (surface, 1 cm, 2 cm, and 3 cm deep) during a typical 20 minute application of a hydrocollator pack to the anterior surface of the human thigh; 2) to determine the temperature every minute at the same four depths for 45 minutes following removal of the hydrocollator pack; 3) to compare the values achieved to determine if the "therapeutic rang" is ever actually reached; 4) to discuss the findings in