Existing research on shortwave diathermy has been conducted using the pulsed shortwave diathermy induction drum, and limited research has been conducted using the ReBound continuous shortwave diathermy unit. Developed by ReGear Life Sciences, Inc. (Pittsburgh, PA), the ReBound is a U.S. Food and Drug Administration-approved modality that consists of various sized anatomically designed garments (sleeves) that contain induction heating coils for low-powered continuous shortwave diathermy. The ReBound has an interesting history. It was originally developed by the U.S. Navy to warm the SEAL team members and deep sea divers.13 Unlike traditional diathermy units, ReBound diathermy is able to treat an entire joint because of the encompassing sleeve worn by the patient (traditional diathermy units are only able to heat the area directly below with the drum). Because of the low power the ReBound unit operates on, it can safely deliver continuous shortwave diathermy.13
Since its emergence into the health care market in 2008, only one study has been conducted on the deep heating of the ReBound diathermy unit.7 This study found the ReBound was able to increase muscle temperature at a depth of 3 cm by only 2.3°C, which is well below the range desired for vigorous heating. However, at the end of the 20-minute treatment time tissue temperature was still increasing with the ReBound, whereas it had plateaued after 20 minutes with the pulsed shortwave diathermy induction drum. The purpose of the current study was therefore to extend the treatment time and evaluate the ReBound continuous shortwave diathermy unit compared to pulsed shortwave diathermy with regard to overall intramuscular temperature increases and cooling rates in the triceps surae muscle at a depth of 3 cm.
A repeated-measures counterbalanced (cross-over) design was used where each participant received both treatment conditions: MegaPulse II pulsed shortwave diathermy (EMS Physio Ltd., Oxfordshire, England) and ReBound continuous shortwave diathermy. The participants were randomized for treatment order based on preset randomized orders as they came in for treatment. The two independent variables were treatment condition and time. The two dependent variables were intramuscular tissue (3 cm depth) temperature increase and temperature decay. The treatment had two levels: MegaPulse II drum and ReBound unit. Dependent variables were recorded every 5 minutes during the entire study period (warming and cooling). Each participant had at least 4 days between sessions to ensure there was no carry over from session to session, and a maximum of 10 days between sessions.
Eighteen healthy adult participants (mean age: 22.56 ± 2.89 years; height: 171.73 ± 6.53 cm, weight: 65.77 ± 6.47 kg, subcutaneous fat: 5.17 ± 1.68 mm) were included in this study (male: n = 8; female: n = 10). Inclusion criteria for participants were age between 18 and 26 years and no previous lower leg injury within the past 6 months. Exclusion criteria included pregnancy, fever, peripheral vascular disease, infection/open wound, thrombophlebitis, blood-borne disease, edema, rash, history of ecchymosis, compromised circulation, compromised sensation to the area being treated, subcutaneous fat thickness of the calf greater than 15 mm, needle phobia, presence of a pacemaker, or allergy to the chemicals or metals being used. All participants were instructed to refrain from exercise for 2 to 3 hours prior to each treatment session. Institutional Review Board approval was obtained prior to the recruitment of participants. Participants were screened and provided written and verbal informed consent prior to beginning the study.
The triceps surae muscle on the non-dominant leg was used as the treatment leg. The largest girth of the triceps surae was measured for the location of the insertion point of the microprobe. The amount of subcutaneous fat was measured via diagnostic ultrasound (GE LOGIQ e; GE Healthcare, Wauwatosa, WI) and was recorded on the participant's data collection sheet. A mark was made on the medial aspect of the calf at a depth of 3 cm from the posterior surface of the calf to indicate where the microprobe would be inserted (Figure 1). The area of microprobe insertion was sanitized using 10% povidone-iodine followed by a 70% isopropyl alcohol swab. One of the four trained investigators inserted the intramuscular thermocouples for all trials to the previously described depth and insertion point. The investigator wore sterile gloves and had been trained in proper and safe insertion technique.
Measurement for thermocouple insertion.
Participants were prone on the treatment table while the thermocouples were inserted and for the duration of the treatment. The 21-gauge, 1.5-in catheter thermocouple (IT-21; Physitemp Instruments, Inc., Clifton, NJ) placement at 3 cm depth was verified using a diagnostic ultrasound unit (GE LOGIQ e diagnostic ultrasound). Participants remained prone on the treatment table to allow the intramuscular temperatures to reach a plateau (no change in temperature more than 0.1° for three consecutive readings) using an electrothermometer (Iso-Thermex; Columbus Instruments, Columbus, OH) before beginning treatment. Following the baseline period, one of the two heat conditions (MegaPulse II or ReBound) was placed on the triceps surae for the predefined treatment period. Treatment parameters were calculated to induce a 4°C increase (vigorous heating) within the muscle. MegaPulse II pulsed shortwave diathermy parameters were 400 pps, 800 µs, and average power 48 W (max power 150 W), for 30 minutes. The concave drum unit was placed approximately 1 cm away from the participant's convex calf, creating a close proximity between the two without the drum ever actually touching the participant's skin (Figure 2). Treatment area was the size of the drum unit (approximately 200 cm2). ReBound continuous shortwave diathermy parameters were set at 100% intensity (35 W) for 45 minutes. Treatment area was the size of the treatment garment (18-inch circumference, 13 inches in length; Figure 3).
MegaPulse II shortwave diathermy (EMS Physio Ltd., Oxfordshire, England) drum set-up.
ReBound continuous shortwave diathermy (ReGear Life Sciences, Inc., Pittsburgh, PA) set-up.
After the treatment period, the modality was removed and the patient continued to be monitored for a 30-minute decay period. During the entire process, the thermocouples remained attached to the participant to record accurate temperatures. Due to the interference from the diathermy units' electromagnetic waves with the Iso-Thermex electrothermometer, the MegaPulse II drum and the ReBound unit were paused for approximately 5 seconds every 5 minutes during the heating period to obtain accurate temperature readings.
At the conclusion of the cooling period, the thermocouple was removed and the area was sanitized with a 70% isopropyl alcohol swab. The site of thermocouple insertion was covered with triple antibiotic ointment and a self-adhesive bandage. The intramuscular thermocouple was cleansed with a mild protein-dissolving detergent (Enzol; Johnson & Johnson, Irvine, CA), followed by high-level disinfection by placing it in 1.5% glutaraldehyde (MetriCide; Metrex Research, Romulus, MI) for a minimum of 12 hours.
A mixed-model analysis of variance with repeated measures was calculated for each of the dependent variables (intramuscular heating and cooling). We conducted a 2 × 7 mixed-model analysis of variance with repeated measures for intramuscular heating and 2 × 7 for intramuscular cooling. The grouping variable was condition: ReBound and MegaPulse II. Correlations between intramuscular temperature increase and amount of subcutaneous fat were also calculated. All statistics were two-tailed with the α level set a priori at 0.05 (SPSS version 20; SPSS Inc., Chicago, IL).
The purpose of our study was to compare the deep heating of two modalities: Megapulse II pulsed shortwave diathermy and ReBound continuous shortwave diathermy. We found that the MegaPulse II produced the greatest temperature increase. In terms of intramuscular temperature decay, there was no difference between the MegaPulse II and the ReBound.
MegaPulse II increased intramuscular temperature by an average of 3.48°C, with 12 of 18 (67%) participants reaching a 3°C increase or greater. Of those, 6 participants (33%) reached a 4°C increase or greater. In comparison, the ReBound increased an average of 3.08°C, with 8 of 18 (44%) participants reaching a 3°C increase or greater, and 5 of those participants (28%) reached a 4°C increase or greater.
As anticipated, based on previous research,7 the MegaPulse II had greater intramuscular temperature increases than the ReBound, but the absolute increases were not as high as we had anticipated. Neither the MegaPulse II nor the ReBound was able to consistently reach the vigorous heating level of a 4°C increase. Previous studies have shown the MegaPulse II is capable of vigorously heating intramuscular tissues by 4°C at a depth of 3 cm.6–8 However, only 33% of the participants in this study experienced vigorous heating of greater than 4°C in the triceps surae muscle group. Although the exact cause of this difference is unknown, some factors may have influenced this result. First, in pilot testing this study, we discovered there is a large difference in quality of heating with the Mega-Pulse II depending on the placement of the drum. If the drum was touching the skin overlying the triceps surae muscle group, then the participant reported an intense heat shortly after the start of the MegaPulse II treatment. Conversely, if the drum was placed too far away (1 inch or more), the participant did not report feeling any sensation of warming and the intramuscular temperature likewise did not drastically increase. The MegaPulse II drum is concave, so it needed to be placed directly centered over the convex calf muscle, as close as possible without touching the skin. There may have been some variability with respect to modality set-up due to differences in participants' triceps surae muscle size and length. Additionally, our heating may have been slightly less than other studies where the drum was in direct contact with the skin.1,3–9,14
Second, the greatest amount of tissue temperature increase while using the MegaPulse II is centered directly underneath the drum. One research study examined tissue temperature at a depth of 3 cm at different areas beneath the drum and found that the exact center increased approximately 1.5°C more than the periphery of the drum (center: 4.58 ± 0.87°C; edges: 3.02 ± 1.02°C and 3.28 ± 1.63°C).8 Our study focused on consistently inserting the needle 3 cm deep into the belly of the triceps surae muscle group, but due to varying sizes of our participants' calf muscles, it cannot be assumed that the thermocouple was directly centered underneath the drum with every participant. Based on the aforementioned study, there is a great amount of variability in the drum's heating pattern, and this variability may have contributed to the variability seen in our heating results.
Finally, the MegaPulse II uses high-frequency electromagnetic waves to produce heat, and as a side effect also interferes with technological equipment in the area. The Iso-Thermex electrothermometer used to capture the intramuscular temperature readings became inactivated whenever the MegaPulse II was turned on. For this reason, we were forced to pause the MegaPulse II treatment every 5 minutes to allow the Iso-Thermex to obtain an accurate reading. This continual interruption of the MegaPulse II treatment may have affected its overall efficacy and ability to vigorously heat the muscle tissues.
Despite the slightly lower than anticipated temperatures, MegaPulse II was still able to consistently produce vigorous heating of greater than 3°C within the triceps surae muscle at a depth of 3 cm. Vigorous heating is indicated to enhance treatments such as cross-friction massage, stretching, and joint mobilizations.2–4,11 Although this study only looked at heating the triceps surae muscle, other studies have shown that using the MegaPulse II with vigorous heating parameters and stretching at the ankle,4,5,12,14 hamstring,2,3,11,15 and shoulder1,16 have been effective in increasing extensibility and range of motion within muscles and joints.
ReBound continuous shortwave diathermy has potential advantages over the MegaPulse II in the clinical setting due to its smaller size and accessibility, but once again it was shown not to be able to produce vigorous heat within a deep muscle. Draper et al.7 reported that after a 30-minute treatment with the ReBound, the intramuscular temperature only increased 2.31°C in the triceps surae muscle at a depth of 3 cm. The results showed a steady increase all the way up to the 30-minute mark with no signs of leveling off, from which we inferred that there was potential for the ReBound to continue heating to the vigorous heating level if treatment time was extended. Therefore, in designing our research study, we extended the ReBound treatment to 45 minutes to see if the extra time would allow the ReBound unit to heat the muscle to vigorous heating levels. The results of our study replicated Draper et al.'s findings over the first 30 minutes, but although the ReBound unit heat continued to increase over the last 15 minutes in our study, it was not enough to reach the vigorous heating level. At 30 minutes, the muscle had increased an average of 2.39°C (compared to 2.31°C reported in Draper et al.'s study), and at 45 minutes the muscle had increased an average of 3.08°C.
The rate of heating in muscle tissues at a depth of 3 cm seems to be consistent between Draper et al.'s study and our study, with temperatures increasing at a rate of 1°C every 15 minutes when applied at 100% intensity. This is a much slower rate of heating than is reported in the FAQ document on the ReBound web site, which claims to induce a 1°C temperature increase every 2 to 3 minutes when applied at 100% intensity, achieving vigorous heating within 8 to 10 minutes.13 Given that research has demonstrated that the ReBound unit is effective at achieving only moderate levels of deep heating (2° to 3°C), this evidence-based rate of increase finding (1°C every 15 minutes) is important for clinicians in determining appropriate treatment time to achieve the desired therapeutic effect.
The most likely reason the ReBound unit was not capable of producing a 4°C temperature increase may be the difference in power output. ReBound continuous shortwave diathermy operates at a constant power output of 35 W.13 To achieve vigorous heating using the MegaPulse II, it has been recommended that an average power output of 48 W be used during treatment.6,8 There is a greater than 10-W difference between the two power outputs, which appears to be the difference between moderate and vigorous heating. To achieve moderate heating using the MegaPulse II, a power output of only 24 W is recommended.1 There is no practical medium between moderate heating (2°C increase at 24 W) and vigorous heating (4°C increase at 48 W), but it may reasonably be hypothesized that if a 3°C increase was desired using the MegaPulse II, a power output of 36 W could be used, which is a similar power output to the ReBound (35 W) and the resulting temperature increase would presumably be similar (3°C increase). Thus, it appears the power output of the ReBound may simply be too low to achieve the desired vigorous heating levels. In theory, because the ReBound is a continuous heating modality (always on) and the MegaPulse II is a pulsed heating modality (continually switches on and off throughout the treatment), one should be able to use a lower power output with a continuous modality, but it appears the ReBound's power output is too low to produce the same effects. The manufacturers of the ReBound set the maximum power output of the modality at 35 W,13 so future research in manufacturing a newer model of the device may look into the effects of increasing the maximum power output.
The secondary purpose of our study was to examine the decay rates of the three modalities after completion of their respective treatments. There were no differences in decay rates between the MegaPulse II and ReBound. MegaPulse II decreased an average of 2.90 ± 0.44°C over the 30-minute cooling period, whereas ReBound only decreased an average of 1.99 ± 0.67°C. Although it appears the ReBound is able to retain its heat much longer, the starting points of the two treatments were not equal, with MegaPulse II heating up to 40.04 ± 0.96°C and ReBound only heating up to 39.32 ± 0.89°C after their respective treatment protocols. Both modalities resulted in triceps surae temperature decay to approximately 37°C (37.14 ± 0.44°C for MegaPulse II and 37.33 ± 0.56°C for ReBound) after 30 minutes.
Similar to previous studies,6,7 MegaPulse II resulted in triceps surae temperature decay of 0.89 ± 0.64°C after 5 minutes, 1.54 ± 0.49°C after 10 minutes, and 2.01 ± 0.43°C after 15 minutes (Table 3). Vigorous heating of muscle tissue provides more potential to elongate the tissue, and thus provide a more effective treatment. A slower decay rate signifies that the muscles are able to retain their heat longer, allowing a clinician to work on the heated muscles or joints via stretching or joint mobilizations.1 Based on the values obtained in our study, the triceps surae muscle was able to retain a 2°C increase from baseline values for approximately 10 minutes on average for both the MegaPulse II and ReBound (Table 3), despite the fact that the ReBound increased intramuscular temperature 0.39°C less than the MegaPulse II. Termed the “stretching window,”1,10,17 this time period signifies the amount of time a clinician has to work with a heated muscle or joint before the effects of the heating modality begin to wear off. Although previous studies have found the stretching window to be 15 to 20 minutes using the MegaPulse II,3,4,7,11,12 our research indicates that the stretching window may be open for as long with the ReBound as with the MegaPulse II, despite not reaching the desired 4°C vigorous heating level.
Ultrasound has a stretching window of approximately 7 minutes before the muscle temperature drops 2°C.10 Previous research7 and this study show that MegaPulse II's stretching window is approximately 15 minutes before the muscle temperature drops 2°C. The variation in decay rates can be attributed to the size of the modalities. MegaPulse II has a much larger surface area compared with ultrasound, and thus is able to heat a greater amount of muscle and surrounding tissues in the area. This finding is clinically significant for health care practitioners when determining which modality to use for a given patient. The treatment area of the MegaPulse II drum is 200 cm2, whereas the treatment area of a standard 5 cm effective radiating area ultrasound head is only 20 cm2. Given that the recommended ultrasound treatment area is two times the effective radiating area, which in this case is approximately 40 cm2, the treatment area of the Mega-Pulse II drum is approximately five times larger than that of ultrasound. If using the MegaPulse II can treat the area desired to be heated, it is advantageous to use the larger modality so the tissues and musculature can retain that heat for a longer period of time.
Along with intramuscular temperature increases and decay rates, we also examined whether there was any correlation between subcutaneous fat and temperature changes. We found no significant correlations between the two using the MegaPulse II (P = .442) or ReBound (P = .770). Through their respective mechanisms of heating the structures from the inside out (starting deep within the muscle and working out toward the skin), these results were as we expected because the superficial skin and fat layers should not contribute to muscle heating using these modalities. No other known studies at this time have examined the correlation between subcutaneous fat and temperature changes, but it is a relationship worth researching. The participants in our study were limited to a subcutaneous fat level of less than 15 mm in the calf, and our participants only had an average of 5.2 ± 1.7 mm of subcutaneous fat. More research needs to be conducted examining participants with varying levels of subcutaneous fat to obtain a more thorough analysis of whether this affects temperature changes.
Although it was not a primary purpose of this study, after data was collected we calculated total Joules delivered during each treatment (Joules = Watts × time in second) and plotted Joules against temperature increases over the course of treatment (Figure 6). One way to standardize energy delivered during different treatment types or with different treatment parameters is to calculate the total Joules delivered. Current recommendations for low-level laser therapy specify that dosage be provided in Joules (total energy), rather than the previously recommended Joules/cm2, because this allows for standardization of therapy results with different laser types.18 Similarly, a recent systematic review on therapeutic ultrasound reported that studies demonstrating an effect of treatment with ultrasound typically applied greater ultrasound energy per session (average of 4,228 J) compared to studies that showed no benefit (average of 2,019 J).19 We found that, although ReBound delivered more Joules during the treatment time than MegaPulse II (94,500 vs 86,400 J), MegaPulse II was able to achieve a significantly greater temperature increase than the ReBound (Figure 6).
Intramuscular heating based on Joules delivered. The MegaPulse II pulsed shortwave diathermy (Drum) unit is manufactured by EMS Physio Ltd., Oxfordshire, England, and the ReBound continuous shortwave diathermy unit is manufactured by ReGear Life Sciences, Inc., Pittsburgh, PA.
To our knowledge, this is the first study that has evaluated temperature increase as a function of total energy (Joules) delivered. In terms of shortwave diathermy, it would appear that therapeutic effect may be more a function of the rate of energy delivery, as opposed to the amount of energy delivered. We make this suggestion based on the fact that MegaPulse II delivered its 86,400 J of energy over 30 minutes, whereas the 94,500 J delivered by the ReBound was over 45 minutes. As stated previously, the limited power output of the ReBound unit may make it difficult or impossible to achieve vigorous therapeutic heating levels in deep tissue.
There were limitations to our study. We only looked at healthy, college-aged, predominately white participants with no current or previous lower-leg injury within the past 6 months. Heating structures specifically in need of vigorous heat, such as scar tissue or joint adhesions, may affect heating patterns in ways we were not able to examine. As previously mentioned, we also only used participants whose calf subcutaneous fat level was below 15 mm. Incorporating a more diverse group of participants with regard to physical size, health, age, and ethnicity may be beneficial in future studies. Our study also only examined the triceps surae muscle at a depth of 3 cm. Additionally, we did not evaluate clinical outcomes (eg, pain or range of motion) following deep heating. Previous studies using shortwave diathermy have measured heating6–10,12 or clinical outcomes,2–5,14–16,20 but no studies have evaluated both magnitude and effectiveness of heating simultaneously.
Future studies should examine different muscles at varying depths to see how the deep heating modalities affect different areas of the body. The majority of research studies focus on the triceps surae muscle, but most clinical treatments and practice of MegaPulse II use diathermy for the shoulder, elbow, and hamstrings. Future studies may focus on these areas of the body when researching the deep heating effects of diathermy.