Heat transfer by three types of hot pack and its implication on the flexibility of the lower back : a randomized , controlled trial

201 Mailing address: Paulina Fuentes-León – Universidad Católica del Maule – Av. San Miguel, 3605 – Talca, Chile – Phone: +56 71 2203463 – E-mail: gmarzuca@gmail.com – Financing source: Nothing to declare – Conflict of interest: Nothing to declare – Presentation: Jun. 2015 – Accepted for publication: Jul. 2016 – Number of registration in clinical trials: NCT02391376. DOI: 10.1590/1809-2950/15088923022016

Se registraban cada cinco minutos la temperatura de la compresa y de la región lumbar y la existencia de eritema. Se empleó la prueba de Schober realizada antes de la primera y después de la última sesión. Tras quince minutos de tratamiento, la compresa de semillas presentó una temperatura final mayor.

INTRODUCTION
The use of various agents in physiotherapy aids in the recovery process of corporal tissues. Thermotherapy uses the benefits of heat through different application method, which can be deep or superficial. One of the most common superficial thermotherapies is the use of hot pack. Among its benefits are an initial increase in blood flow 1,2 , muscle relaxation 3 , antispasmodic effect 3,4 , decreased fatigue and excitability 2 , increased muscle flexibility 2,3,5 , decreased joint stiffness [6][7][8] , and increased elastic properties 5,7 . In recent years there was an increase in the belief and use of alternative medicine methods, such as the therapeutic use of seed pack to relieve pain and decrease muscle contractures, mainly contributing in solving musculoskeletal problems 9,10 .
Despite the fact that hot pack are commonly recommended by some health professionals due to their economical and easy use, there is a lack of information about the real capability of different types of pack to maintain and transfer heat 11 , such as moist heat, seeds and gel pack. Studies about superficial heat effects, through the application of hot pack, are usually associated with other therapeutic techniques (e.g., elongation). Therefore, the specific effect of the hot pack therapy cannot be clearly identified. Surely, this lack of information is something to worry about in physiotherapy, considering the everyday outpatient administrations which have used hot pack. It is therefore necessary to establish which of the various types of existing pack present better thermal properties for therapeutic practice.
In order to determine the efficiency of different types of pack, this study aims at comparing the effects generated through superficial heat transfer by the application of three types of hot pack on the flexibility of the lower back of healthy, male students between the ages of 18 and 25 from the Catholic University of Maule (UCM), Talca, Chile.

METHODOLOGY
This randomized double-blind controlled longitudinal study was approved by the Bioethics Committee of UCM, Chile. It is also in agreement with the Human and Animal Rights requirements of the International Committee of Medical Journal Editors' Uniform Requirements for Manuscripts Submitted to Biomedical Journals. The Consolidated Standards of Reporting Trials (CONSORT) Statement was used as a guideline for the study (Figure 1).

Participants
Three groups were randomly assigned using the online tool from http://www.graphpad.com/quickcalcs/ index.cfm 12 , each one consisting of 15 healthy students from UCM between 18 and 25 years old with normal body mass indexes (18.5 to 25 kg/m 2 ). A total of 45 participants were evaluated based on a sample calculation using an interval of confidence of 95% and a standard error of 5% 13 . In the first session an interview was performed in order to record medical history and health status, explain the study and obtain the informed consent signature. The exclusion criteria included recent scars or wounds on the lower back region, history of peripheral vascular diseases, impaired sensitivity, "not sedentary" (sports practice equal to or more than 3 times per week), body mass index above 25 kg/m 2 and a fat index above 14% according to Faulkner's formula.

Procedures
A different type of hot pack was applied in the lower back of each subject: moist heat pack (MHP) (Enraf Nonius BV® Enno Moist 25x30 cm, Model 3448167, Netherlands), seed pack (SP) (craft 25×30 cm, Talca, Chile) or gel pack (GP) (Flexipack® 25×30 cm, Model Nav11, Chattanooga Group, Hixson, TN, USA) following the manufacturer's guidelines and in accordance with clinical practice for 15 minutes for 3 consecutive days 14 . On day one, prior to the application of the pack, the initial evaluation of flexibility of the lumbar spine of each subject was recorded using the Schober test. On day 3, after the application of each pack, the final evaluation of the flexibility of the lower back was performed using the same test. During the sessions, the temperature of the lumbar area, the temperature of the pack and the pigmentation change of the lumbar area were assessed every 5 minutes through a photographic record.
In order to carry out this study, three evaluators were needed. Evaluator number 1 performed skinfold measurements (tricipital, subscapular, supra-iliac and abdominal) and initial and the final Schober tests for each subject. Evaluator number 2 selected the pack to be assessed and performed the temperature measurements of the lumbar area and pack. Evaluator number 3 performed the task of taking pictures before and after each session, applied the Bedford scale for thermal comfort sensation and recorded the data. Evaluators number 1 and 3 did not know to what type of pack each subject was exposed. The application of each type of pack was made for a group of 15 subjects, separately for each group, during 3 consecutive days in the same week, without having knowledge of the type of pack designated to each subject. The study was carried out in a bright room at a temperature of 21°C, each subject was comfortably placed on a stretcher in prone position, with a cushion in the abdominal area and forefoot zone.
The method used to assess the flexibility was the Schober Test. In order to perform the evaluation, the patient was placed in a standing position with his bare feet slightly apart; two points were marked on the skin, one above the spinous apophysis S1 and another point  10 cm above that mark. The patient performed a maximal trunk flexion without flexing the knees and the increase of the distance between the two marks was recorded. After this evaluation, the pack was applied to the lower back and wrapped in two layers of towels in order to avoid skin injuries from the heat. The temperature of the lower back and of each type of pack was recorded at minutes 0 (baseline measurement), 5, 10 and 15. Temperature was measured by using a digital pyrometer (Etekcity® Lasergrip 774 Non-Contact Infrared Thermometer, Anaheim, CA, USA) at a set distance of 50 cm and it was held perpendicular to the patient's skin and the pack. The photographic record of the lower back of each subject, before and after each session of superficial thermotherapy, was recorded using a digital camera (Sony® DSC-W610, SONY Corporation, Japan). The camera was placed perpendicular to the lower back, at a distance of 50 cm. The brightness of the photograph was taken into account so as not to affect the analysis of the erythema later. The erythema analysis of the lower back was performed with the freeware Picture Color Analyzer (Shizuoka, Japan; http://www. isao.com). The software measures the red color intensity of each pixel (attributing a value between 1 and 255) within the chosen area. The average red color value obtained in arbitrary units is the result of the following formula: Average red color = total red color value / (255 × number of pixels in the area).

Statistical analysis
The data was analyzed through the statistical program GraphPad Prism® (version 5.01; Inc., El Camino Real, CA USA, 2007). Data is presented as mean ± standard deviation. The intragroup analysis was performed using the paired t-test. The intergroup comparison was made using one-way analysis of variance (one-way ANOVA) with Tukey's post-hoc test. To evaluate the behavior of packs and the lumbar region temperatures over the time, repeated measures two-way ANOVA with Bonferroni post-hoc test was used. The significance value adopted was P<0.05.

Pack temperature
The initial temperature of the pack was 56.2±1.0°C, 57.8±3.4°C and 49.7±2.9°C for the MHP, SP and GP groups, respectively. The pack that suffered greater reduction in temperature by the end of the session was MHP, with a 14.2 °C decrease, finishing the process with 42.0±1.1 °C. The GP presented a decrease of 10.2 °C, ending the session at 39.5±1.0°C. The SP group achieved the lowest variation between the start and end of the application, a reduction of 8.2°C, completing the process of heat transfer at 49.6±2.7°C. There were significant differences in pack heat maintenance at 0, 5, 10 and 15 minutes (repeated measures two-way ANOVA; P<0.001) (Figure 2). The MHP and the SP started the protocol with similar temperatures, but the SP sustained a higher temperature during the entire session when compared to MHP (Bonferroni post-hoc test; P<0.001). When compared to the GP group, the SP sustained higher temperatures from the beginning of the protocol until the end of the application (Bonferroni post-hoc test; P<0.001). Also, GP initial temperature was lower than MHP (Bonferroni posthoc test; P<0.001), remained lower during the 5 th and 10 th minutes (Bonferroni post-hoc test; P<0.01 and P<0.001, respectively), but there was no difference at minute 15.

DISCUSSION
It is important to determine the effects of the application of hot packs with different compositions on the flexibility of the lower back in healthy subjects, through the direct use of superficial thermotherapy properties. Many postural and muscular problems in adult lesions are related to the lack of flexibility due to improper alignments of the spine and pelvis, causing problems in the joint's range of motion, muscle pain and injuries and altering posture and motor skills [15][16][17] .
For proper heat transfer, it is necessary to consider various factors, such as resistance imposed by the skin. The smaller the skinfold, the higher the change of intramuscular temperature 18 . In order to avoid discrepancies in the results associated to this issue, subjects with fat indexes above 14% (according to Faulkner's formula) and with a body mass index above 25 kg/m 2 were excluded from the study. The application of topical heat at temperatures of around 50°C may not be able to significantly increase muscle temperature 19 . However, under these conditions the epidermal-dermal layer can reach temperatures above 40°C 19 , similar to those observed in this study, which are capable of increasing blood flow in the hypodermic layer 20 . Nozaki et al. 21 stated that intramuscular temperature depends on the thickness of the dermal layer, making it necessary to consider the number of layers of insulation between the pack and the skin. Mclean 22 observed that the magnitude of the temperature transferred from hot packs did not correlate to the expected level of subjectperceived heat or the pack-skin distance, using a depth of 9 layers of towels without specific thickness and without consideration of the impact that the amount of exerted pack-skin compression has in the transmission of heat. This is why two layers of towels were used in this study for the different types of pack to be established, with the same thickness and trying not to generate external pack-skin pressure. It is important to highlight the possible influence of the "compression factor" and its relation to the thermal exchanging area during the application process. The compression can affect the method of heat conduction, because there are irregularities on the surface of the pack that make them not to fit perfectly. It causes a series of spaces occupied by air that produce greater resistance, since air is a poor heat conductor, unlike the high conductivity which occurs during direct pack-skin contact 23 .
From the 3 types of hot pack used, the group that reached the highest pack temperature was the SP group, showing significant differences when compared with the MHP and GP groups during the whole evaluation. The average initial temperature of the lower back recorded in this study do not have significant differences among the three groups, however, differences were found within the 5 minutes after heat exposure. The lower back temperatures of the MHP and SP groups were significantly greater when compared to the GP group, and the MHP group was better for transmitting heat when compared to the SP group. The pack that presented the highest heat transfer after 5 minutes of the procedure was the MHP group, despite the fact that the SP was the one that achieved the highest pack temperature. It is opposed to the assumption of Myrer et al. (1997) 18 , in which the higher the temperature gradient between the pack and the body, the higher the heat transferred. This discrepancy could be explained by the type of heat generated by the MHP, its moisture condition and the specific heat property of the hot pack material. Moist heat can penetrate more quickly than dry heat because water molecules conduct heat better than air molecules, so the moist heat can be used at lower temperatures and with shorter exposure periods than dry heat 24 .
When comparing the difference between initial and final pack temperatures, the MHP group was the one that achieved the greatest heat loss, obtaining significant differences from the SP and GP groups, indicating that it was the group that had the greatest ability to transfer heat. The second highest temperature reduction was observed in the GP group, but it does not perform an optimal heat transfer to the skin, which can be explained by the loss of the gel thermoreversible capacity that is altered by changing its freezing and boiling point according to the gel manufacturing catalog (Colony Processing Inc.). The pack that managed to maintain its temperature, showing significant differences throughout the evaluation when compared with the MHP and GP groups was the SP group, while this may be attributed to its physical properties. However, other studies, from which to make a comparison, were not found.
The theoretical temperatures mentioned for the MHP and GP groups were not achieved by the protocol established for clinical use, on which hot packs reach an initial temperature of 71-79°C 7 . None of the packs used reached that temperature, indicating that these types of pack, despite being kept in a pack device at a constant temperature 80°C 21 , fail to accomplish a complete transfer of heat. Gel packs reached a maximum theoretical temperature of 52°C 25 and the time suggested to heat these packs is 55 seconds (Orthomed Instrument Catalog, inc.), which was insufficient to reach this temperature.
The erythema analysis was carried out through the comparison of the lower back, red pixel count before and after the heat procedure. It showed significant increases for the three groups, even though the GP group showed less variation in temperature in the lower back. Due to the above, it is necessary to protect and verify the condition of the skin every 5 minutes when this type of therapy is applied 7 .
Most studies investigated some kind of thermotherapy application in conjunction with stretching protocols 9,26-28 ; so the heat effects are mostly indirectly evaluated 29 . It is therefore necessary to more objectively determine the amount of heat that each type of pack transfers to the corporal zone and how that affects flexibility. Maintaining flexibility preserved during life is an essential aspect of clinical practices, due to its functional consequences that entail limiting the patient's ability to perform household activities, professional activities or sporting tasks, affecting their quality of life 30 .
It is expected that a healthy person gets a variation of 5 cm in the Schober test 31 . When completing the process of applying superficial thermotherapy through different types of pack, the final Schober test evaluation confirmed that there was a change in the lengthening capacity of soft tissues, resulting in a clinically significant increase of lumbar spine flexibility.
The intragroup analysis showed significant improvement in the flexibility of the three groups after the application of heat through superficial thermotherapy. It may be due to the therapeutic effects of the heat in the tissues, including alterations of viscoelastic properties, increased muscle flexibility 2,3,5 and decreased articularstiffness [6][7][8] . These benefits can alter the properties of the soft tissue of the lower back, increasing tissue elongation (creep), which implies a further relaxation at the muscular level 10,32 .
The three hot packs tested presented different physical properties, but the intergroup analysis showed significant differences in the lumbar flexibility variation between the MHP and GP groups. The MHP provided a more pronounced increase in the lengthening of lower back tissues. This study reveals the importance of choosing an appropriate pack for clinical use to achieve the best desired therapeutic effects. Further research should be developed in this area.