Thermo-Pad Comparative Studies


The therapeutic application of heat as a treatment modality requires maintaining an effective temperature range throughout the time of treatment. This temperature range is limited by the patients maximum temperature tolerance at the high end of the range and a minimum level above skin temperature required to get effective heat transfer to the skin. The patients tolerance of the treatment is decreased when the temperature is excessive. The therapist normally uses towels to buffer the patient from high temperature heat treatment pads. The subjective judgement of the number of towels required frequently reduces the surface temperature to an ineffective level. When the externally applied temperature is too low, the therapeutic effect is minimal. Presently used modes of heat application are subject to either wide ranges of temperature or elicit ineffective changes in skin temperature.


The purpose of this study is to determine heat loss characteristics of two types of heat treatment pads under laboratory conditions. The first type is the Hydrocollator pad (Chattanooga Co.) and the second is the Thermo-Pad (Hood Thermo-Pad Canada Ltd).


Five Hydrocollator pads and five Thermo-Pads were studied to determine heat loss characteristics over a two hour period. Temperature measurements were taken at five minute intervals throughout the period. All heat pads were placed on a Formica table top covered with a terry cloth towel. The Hydrocollator pads were inserted into a cover specifically designed to minimize heat loss. Thermo-Pads were inserted into a terry cloth sleeve consisting of one layer of towelling.

Yellow Springs Instrument (YSI) surface electrodes (YSI #409B) were placed between the covered pads and the terry cloth covered table top. Ten surface electrodes were connected to a YSI Telethermometer Model #2100 through a YSI switch box Model #4002. Data was collected with an IBM Personal Computer using a YSI Computer Interface Model #2120D.


The Hydrocollator pads are characterized by having a high initial temperature which rapidly and continually declines. As shown in Figure 1, the initial temperature is about 150 degrees F. when removed from the hot water bath. In contrast the Thermo-Pad have a temperature which rises rapidly from room temperature at their time of activation to about 105 degrees F. in the first 5 minutes. Thereafter their temperature slowly rises to a peak of about 111 degrees F. after 30 minutes. Later their temperature slowly declines. These characteristic curves shown in Figure 1 are relatively consistent over the population of pads used. The most important difference between these two types of heat treatment pad is the fact that the Thermo-Pad maintains a very uniform temperature over a 5 to 75 minutes from the time of activation (range 105 – 111 degrees F.). For the same period the Hydrocollator pad has a range of 150 – 107 degrees F.




Application of heat is a prime treatment modality for athletic injuries and within the field of physical therapy. While heat treatment is in widespread use, there are still unresolved questions related to its therapeutic effect. Heat is typically applied for a twenty minute duration. However, quantitative studies of initial skin temperature and sustained temperature throughout the treatment period are not available in the literature. In practice, the physical therapist or athletic trainer responds to the patient’s report of subjective sensation and buffers a heat treatment pad with additional towels if it is too hot. After a twenty minute period it is generally accepted that heat loss is so great that continued use is ineffective.

Therefore, the present study was undertaken to investigate the surface temperature at selected sites resulting from the application of external heat. Two commercial heat treatment modalities and a newly designed product referred to as Thermo-Pad were studied.


The purpose of this study was to determine surface skin temperature generated during three modes of heat application. Skin temperature was monitored for a twenty minute period and subjects were also asked to subjectively rate the sensation of temperature for comparison purposes.


Three heat modalities were used in this study. Initial surface skin temperature and retention of heat for twenty minutes were recorded. Hydrocollator pads, Cramergesic and Thermo-Pads were selected as treatment modalities.

Hydrocollator pads are a commercial product manufactured by Chattanooga Corporation. Cramergesic is a counterirritant manufactured by Creamer Company, and Thermo-Pads are manufactured by Hood Thermo-Pad Canada Ltd.. The application of each mode of treatment was consistent throughout the study. A Hydrocollator cover specifically designed for use with the Hydrocollator was used to minimize heat loss and to protect the skin from contact with an excessive temperature. A terry cloth towel was used as a cover for the Thermo-Pads. A similar terry cloth towel was used over the counterirritant. One quarter of a teaspoon of Cramergesic was applied by rubbing it into a 3″ x 3″ area of the skin for two minutes.


The three modes of heat application were applied to the right and left shoulders and the right and left hamstring. Hydrocollator pads and Thermo-Pads were also located on the lower back. The specific sites for the application of heat treatment and the modality applied are summarized in the following table.


Right shoulder…………………..Hydrocollator…………………..Cramergesic

Left shoulder…………………….Thermo-Pad……………………Thermo-Pad

Lower back……………………..Hydrocollator…………………..Thermo-Pad

Right hamstring…………………Hydrocollator…………………..Cramergesic

Left hamstring…………………..Thermo-Pad……………………Thermo-Pad

The sites of the application were defined as follows:

1. Lower back; three inches above the superior iliac cres on the right side of the lower back.

2. Right and left hamstring; six inches superior to the popliteal space in the back of the knee.

3. Right and left shoulder; four inches below the acromioclavicular joint (mid portion of the deltoid)


Twenty-eight college aged students were used as subjects. Subjects reported for two separate sessions. In session 1, Hydrocollator pads and Thermo-pads were alternately applied to the shoulders and thighs. A Hydrocollator pad was also applied to the lower back. During session 2, Thermo-Pad and Cramergesic were alternately applied to the shoulders and thighs. In addition a Thermo-Pad was applied to the lower back.

The subjects were placed in a prone position on a training table and surface electrodes were positioned and held in place by a 2″ x 2″ micropore non allergenic tape.


Skin temperature was measured at each site using a Yellow Springs Instrument (YSI) surface electrode Model #409B attached to a YSI telethermometer, Model #2100, and a compatible switch box (YSI Model #4002). The telethermometer was interfaced with an IBM Personal Computer using the YSI computer interface Model #2120D. Temperatures were recorded from each site periodically throughout the twenty minute period. A data entry program on the IBB PC prompted the operator for switch box changes as required. The operator was also prompted to enter the subjects perceived temperature responses.

The subject’s Perceived Temperature response for each site and modality was recorded at 2 minutes, 5, 10, 16, 18, and 20 minutes, throughout the study. These responses were based on a subjective scale defined as:

1. NONE complete inactivity, no sensation felt.

2. MILD limited sensation of heat felt.

3. MODERATE very comfortable.

4. HOT too uncomfortable, as much as I can stand.

5. TOO HOT must add temperature buffer towels.

This scale was discussed with the subject prior to the session and was posted within the subject’s view during the session.


Under therapeutic conditions, the skin temperature resulting from the use of the Hydrocollator was much lower than that observed under laboratory conditions as shown in the “Comparison Study of Heat Loss From Two Types of Heat Treatment Pads”. The temperature buffering resulting from Hydrocollator covers and additional terry cloth towels masks the pronounced differences in thermal characteristics of Hydrocollator and Thermo-Pads.

Figure 2 shows the subjects perceived temperature. The subjects perceived the temperature of the Hydrocollator to be higher than that of the Thermo-Pad. The Hydrocollator data shows an initial average of 3.2 to 3.3 (beyond being comfortable) and a range of 1 – 5 while the perceived temperature of the Thermo-Pad showed an average of 2.1 to 2.2 (mild to moderate sensation) with a range of 1 – 3 in most instances. The subjects perceived the temperature of the Thermo-Pad to be higher than the Cramergesic.

Figure 3 shows the actual temperatures recorded. There is remarkable consistency in the Thermo-Pads as shown in graph lines Thermo-Pad 1 and Thermo-Pad 2. The skin temperature generated by Hydrocollator pads was higher at all time periods throughout the twenty minute study and presented greater variability despite the technicians constant addition of towels. The moderate increase in skin temperature resulting from the Thermo-Pads and their ability to generate constant heat, relieves the technician from responding to the subjects need for compensatory measures. The subjects are protected from high temperatures that would potentially burn the skin. This advantage is intrinsically related to the fact that the Thermo-Pad is producing heat based on the ongoing process of crystallization of the contained food grade sodium acetate. In contrast the Hydrocollator pads, while having a substantial heat capacity, are charged with heat in a hot water bath and continually lose heat following removal from that medium.