J Rehabil Health Sci 2016; 14:9-13Ⓒ 2016 Osaka Prefecture University

Journal of Rehabilitation and Health Sciences
9Online ISSN 2185-8004

Original Article

Deep heating using microwave diathermy decreases muscle hardness:
A randomized, placebo-controlled trial
Koji Nonaka†１, Masataka Kataoka１, Shinya Ogaya１, Kenichi Ito２ and Junichi Akiyama３
１

Department of Physical Therapy, School of Comprehensive Rehabilitation, Osaka Prefecture University,
3-7-30, Habikino, Habikino-shi, Osaka 583-8555, Japan;
２Hirakata Kohsai Hospital, 1-2-1, Fujisakahigashimachi, Hirakata-shi, Osaka 573-0153, Japan;
３Health Welfare Laboratory, Kibi International University, 8, Iga-machi, Takahashi-shi, Okayama 716-8508, Japan.
Received 5 November 2016; accepted 8 December 2016

Backgroud: Microwave diathermy treatment is one of the deep heating modalities used in rehabilitation.
However, no study has reported on the change in the range of motion (ROM) and muscle hardness after microwave diathermy treatment.
Objective: We aimed to determine the effect of microwave diathermy on muscle hardness and passive
ROM.
Methods: Twenty healthy adults (12 men and 8 women) participated in this study. The subjects were randomly assigned to receive microwave diathermy treatment (n＝10; six men, four women) or placebo (n＝10;six men, four women). Microwave diathermy (100 W) and placebo treatments were administered to eachsubjectʼs right calf for 15 minutes in the microwave and placebo groups, respectively. The hardness of thesoleus muscle and passive ROM of ankle dorsiflexion were measured before and after the intervention.
Results: Soleus muscle hardness and passive ROM of ankle dorsiflexion did not change before and after
the placebo intervention. However, in the microwave group, there was a decrease in the soleus muscle hardness and an increase in ankle dorsiflexion ROM following the intervention.
Conclusion: Our findings indicate that microwave diathermy treatment can decrease muscle hardness andincrease passive ROM.
Key words : deep heating; muscle condition; joint range of motion

1 Introduction
Muscle hardness is defined as the resistance of themuscle against perpendicular pressure, while muscle stiffness is defined as the resistance that is built when themuscle is extended longitudinally.1-3 Thus, muscle hardness and muscle stiffness are different, but they are related.1 Muscle hardness can be used to objectively determine
a muscleʼs condition1, 2 because it can increase under several conditions, such as spasms and damage.1, 4 It is important to decrease muscle hardness to improve or maintain muscle condition. 5, 6 In addition, muscle pain isassociated with muscle hardness.7 These findings suggestthat decreasing muscle hardness may be useful for athletes
to enhance their performance by improving or maintaining
muscle condition and reducing muscle pain.
†Corresponding author, E-mail: nonaka@rehab.osakafu-u.ac.jp

Heat is effective for enhancing joint range of motion(ROM).8 Deep heating, including with microwave diathermy, ultrasound, and shortwave diathermy, is often used
before or during stretching to heat the muscle layer andincrease ROM. It has been reported that using ultrasoundtogether with stretching can increase ankle dorsiflexion
ROM.9-11 Furthermore, shortwave diathermy with stretching has been reported to increase ankle dorsiflexion
ROM12 and knee extension ROM.13 Deep heating increases muscle temperature, resulting in improved ROM. It is
obvious that ROM is related to muscle stiffness becausemuscle stiffness represents the resistance that appears
when the muscle is extended longitudinally. Therefore,
because muscle stiffness and muscle hardness are related,
we hypothesized that deep heating could decrease muscle
hardness.
Microwave diathermy treatment can be applied to a

10larger area than therapeutic ultrasound can, and physicaltherapists do not have to remain near the person being
treated. Therefore, we determined to use microwave diathermy as the deep heating modality in this study. Thepurpose of this study was to evaluate the changes in thesoleus muscleʼs hardness after a single deep-heating session using microwave diathermy. We hypothesized thatdeep heating with microwave diathermy would decrease
muscle hardness and that its effects on athletesʼ muscles
would be positive.
2 Methods
2.1 Trial Design
We performed a randomized, placebo-controlled pilot
trial in the present study, which was approved by the ethics committee of the Osaka Prefecture University (2012PT05).
2.2 Subjects
This was a randomized, placebo-controlled trial.
Twenty healthy adults (12 men and 8 women) participatedin this study. Their mean age, height, and weight were
20.9±0.9 years, 164.0±8.7 cm, and 57.7±8.4 kg, respectively. The exclusion criteria were previous ankle fractures
within the last 12 months and passive right ankle dorsiflexion of ＜0°with the right knee extended at 0°. After
screening, the subjects were randomly allocated to either
the deep heating group or the placebo group. All subjects
were informed of the studyʼs purpose, and they provided
informed consent.
2.3 Intervention
Subjects in the microwave group received a singlemicrowave diathermy treatment using a microwave therapy apparatus (2,450 Hz, ME-7200, OG Giken Co. Ltd,
Okayama, Japan). The probe of the apparatus was placed
10 cm above each subjectʼs right calf in the prone position,and the treatment was administered for 15 minutes at 100W. The placebo treatment was administered to subjects inthe placebo group by using the unplugged microwave device for 15 minutes on each subjectʼs right calf in theprone position. The room temperature was maintained at
23−25℃ during the experiment.
2.4 Outcome Measures
Deep temperature, muscle hardness, and passive
ROM were measured before the interventions.
Deep temperature was measured while the participant
was in the prone position using a deep temperature monitor (Core Temp CTM-205, Terumo Corp., Tokyo, Japan),and the probe was placed 5 cm distal from the right gastrocnemius muscle-tendon junction.
Soleus muscle hardness was measured using a muscle hardness meter (NEUTONE TDM-N1, TRY-ALL

Fig. 1

Measurement of muscle hardness

Corp., Chiba, Japan) in the prone position. The device hashigh repeatability and efficacy of the measurement ofmuscle hardness.14 Muscle hardness was measured 5 cm
distal from the gastrocnemius muscle-tendon junction,
while the right knee joint was extended at 0°and the rightankle joint was dorsiflexed at 0°(Fig. 1). Muscle hardness
was measured three times, and the mean value was used
for analysis.
Passive ROM of ankle dorsiflexion was measured byan analog goniometer. Passive ROM of ankle dorsiflexion
was measured while the participant was in the prone position, with the right knee flexed at 90°. To ensure the consistency of the passive ROM measuring conditions, theright ankle joint was dorsiflexed at 5 kgf using a handheld dynamometer (μTas F-1, Anima Corp., Tokyo, Japan). The ankle joint was held in a dorsiflexed position,and the ROM was measured. Ankle joint positioning andthe measurement of ROM were performed by the sameassessors to avoid measurement errors.
2.5 Blinding
Outcome assessors were blinded to the treatment orplacebo to eliminate bias. However, subjects could not beblinded, because they could feel their calf heat up or remain unchanged during the intervention.
2.6 Sample size
The sample size was calculated based on the effecton the muscle hardness (partial η2＝0.211) that was obtained from our preliminary study. It was calculated using
G*Power, version 3.1.7 (Statistical Power Analyses for
Windows and Mac, http://www.gpower.hhu.de), when α
was set at 0.05 and power was set at 0.95. A necessary sample size of 16 (8/group) was calculated. We also assumed
a reduction of 20 % because of the exclusion criteria.

112.7 Data Analysis
All continuous data were expressed as the mean ±standard deviation. Statistical analyses were performed
using SPSS for Windows, version 14.0 (SPSS Inc., Chicago, USA). The characteristics and baseline data, whichwere the pre-intervention data, between the deep heating
and placebo groups were compared using the unpaired
t-test for quantitative data and the chi-squared test for
qualitative data. A two-way repeated measures analysis ofvariance (ANOVA), one between groups (two levels: intervention and control) and one within groups (two levels:pre-intervention and post-intervention), was used to determine the interaction effect. When interactions were found,a simple main effect test was performed as a lower-level
ANOVA to interpret how the values from each groupchanged, or did not change, from pre-intervention topost-intervention. Statistical significance was set at p ＜0.05. The 95 % confidence interval (CI) was calculated for
all the results.
3 Results
Twenty healthy subjects were randomly allocated tothe deep heating group (n＝10; six men, four women) or
placebo group (n＝10; six men, four women) (Fig. 2). No
significant differences in characteristics and baseline levels were detected between the groups (Table 1).
The mean change in the deep temperature of the calf
after the intervention was 4.4 ℃±1.7 ℃ (95% CI, 3.4 ℃to 5.3 ℃) in the deep heating group and −0.3 ℃±1.1 ℃(95 % CI, −1.2 ℃ to 0.7 ℃) in the placebo group. For themeasurement of deep temperature of the calf, an interaction effect between time and group was observed (p ＜0.001), suggesting a difference between the deep heatinggroup and the placebo group (Table 2).

Table 2

Fig. 2

Flowchart of participation in the present study

Table 1 haracteristics and baseline demographics ofCsubjects

Deep temperature, soleus muscle hardness, and ankle dorsiflexion12The mean change in passive ROM of ankle dorsiflexion after the intervention was 2.6°
±1.8°(95 % CI, 1.3°to3.9°) in the deep heating group and 0.3°
±2.1°(95% CI,−1.0°to 1.6°) in the placebo group. For the measurementof passive ROM of ankle dorsiflexion, an interaction effect between time and group was observed (p ＝ 0.015),suggesting a difference between the deep heating groupand the placebo group. When examining the results by using a simple main effect test, it was observed that there
was an increase in passive ROM for the deep heatinggroup. In contrast, no change in passive ROM of ankle
dorsiflexion was observed in the placebo group (Table 2).
The mean change in soleus muscle hardness after theintervention was −56 mN±40 mN (95 % CI, −84 mN to− 28 mN) in the deep heating group and 3 mN ± 44 mN(95 % CI, −25 mN to 31 mN) in the placebo group. For
the measurement of soleus muscle hardness, an interaction
effect between time and group was observed (p＝0.006),
suggesting a difference between the deep heating groupand the placebo group. When analyzing the results by using a simple main effect test, it was observed that there
was a decrease in muscle hardness for the deep heatinggroup. In contrast, no change in soleus muscle hardness
was observed for the placebo group (Table 2).
4 Discussion
The present study reports a novel result, namely thatmuscle hardness decreases after deep heating using microwave diathermy. The results supported our hypothesis.
The findings suggest that deep heating may improve ormaintain muscle condition by reducing muscle hardness.
Muscle hardness decreased after microwave diathermy irradiation. Static stretching over a single weeks andover 5 weeks increased passive ankle dorsiflexion ROM
while reducing gastrocnemius muscle hardness.5, 6 These
results were similar to our findings. The changes in ROM
and muscle hardness after stretching may be induced byan increase in intramuscular connective tissue extensibility, resulting in reduced muscle stiffness. On the otherhand, we considered that the mechanisms underlying thechanges in ROM and muscle hardness caused by deep
heating using microwave diathermy are different fromthose obtained when using stretching. Similar to our findings, passive dorsiflexion ROM increased after heating
using short-wave diathermy.15 The mechanisms underlying this phenomenon are not that the increased ROM wasinduced by changes in extensibility of the connective tissue but by changes in the mechanical stiffness of the muscle tissue, by changes in the afferent nerve activity, or by
a combination of both. Although an increase in the extensibility of the connective tissue is needed to increase

the tissueʼs temperature from 40 ℃ to 45 ℃,16 our results
showed that the muscleʼs temperature after microwave
diathermy reached about 37 ℃. Therefore, it is thought that
our findings might not be induced by the increase in theextensibility of the connective tissue but by the changes in
the mechanical stiffness of the muscle tissue, by thechanges of the afferent nerve activity or by a combinationof both as reported previously.15 Accordingly, deep heating
using microwave diathermy would decrease muscle stiffness, resulting in increased passive ROM and decreased
muscle hardness due to the aforementioned mechanisms.
Consequently, microwave diathermy may be a useful
method for improving or maintaining muscle condition
and reducing muscle pain.
There are certain limitations of the present study. The
individuals assessing muscle hardness and ROM could not
be blinded to treatment, because the skin turned red after
heating when using microwave diathermy. In addition, thesubjects could not be blinded to the treatment, because
they could feel their calf heat up or remain unchanged
during the intervention. Therefore, bias could not be completely excluded.
Decreased muscle hardness is expected to prevent
muscle injury and improve or maintain muscle condition.5Muscle pain is associated with muscle hardness,7 which
decreases when muscle pain is reduced.17 Therefore, physical therapists could use deep heating treatment for athletes to prevent muscle injury, improve or maintain muscle condition, and reduce muscle pain in clinical settings.
However, future studies are needed to determine whether
deep heating would have these effects for athletes.
5 Conclusion
The purpose of this study was to evaluate changes insoleus muscle hardness after deep heating. Deep heating
using microwave diathermy for 15 min decreased soleus
muscle hardness. Our results suggest that deep heating using microwave diathermy is effective for improving ormaintaining muscle condition and reducing muscle pain.
References
1 Murayama M, Nosaka K, Yoneda T, et al. (2000)
Changes in hardness of the human elbow flexor muscles after eccentric exercise. Eur J Appl Physiol, 82:
361-367.
2 Murayama M, WatanabeK, Kato R, et al. (2012) Association of muscle hardness with muscle tension dynamics: a physiological property. Eur J Appl Physiol,
112: 105-112.
3 Gennisson JL, Cornu C, Catheline S, et al. (2005)
Human muscle hardness assessment during incre-134567891011

mental isometric contraction using transient elastography. J Biomech, 38: 1543-1550.
Fischer AA (1987) Clinical use of tissue compliancemeter for documentation of soft tissue pathology.
Clin J Pain, 3: 23-30.
Akagi R, Takahashi H (2013) Acute effect of staticstretching on hardness of the gastrocnemius muscle.
Med Sci Sports Exerc, 45: 1348-1354.
Akagi R, Takahashi H (2014) Effect of a 5-week static stretching program on hardness of the gastrocnemius muscle. Scand J Med Sci Sports, 24: 950-957.
Mense S, Gerwin RD (2010) Introduction, “Musclepain: understanding the mechanisms” (Mense S, Gerwin D, editors). Springer, New York, pp.1-16.
Bleakley CM, Costello JT (2013) Do thermal agentsaffect range of movement and mechanical properties
in soft tissues? A systematic review. Arch Phys Med
Rehabil, 94: 149-163.
Draper DO, Anderson C, Schulthies SS, et al. (1998)
Immediate and residual changes in dorsiflexion rangeof motion using an ultrasound heat and stretch routine. J Athl Train, 33: 141-144.
Knight CA, Rutledge CR, Cox ME, et al. (2001) Effect of superficial heat, deep heat, and active exercise
warm-up on the extensibility of the plantar flexors.
Phys Ther, 81: 1206-1214.
Wessling KC, DeVane DA, Hylton CR. (1987) Effects of static stretch versus static stretch and ultra-12

13

14

15

16

17

sound combined on triceps surae muscle extensibility
in healthy women. Phys Ther, 67: 674-679.
Peres SE, Draper DO, Knight KL, et al. (2002)
Pulsed shortwave diathermy and prolonged long-duration stretching increase dorsiflexion range of motion more than identical stretching without diathermy.
J Athl Train, 37: 43-50.
Draper DO, Castro JL, Feland B, et al. (2004) Shortwave diathermy and prolonged stretching increase
hamstring flexibility more than prolonged stretchingalone. J Orthop Sports Phys Ther, 34: 13-20.
Tachibana K, Ueki N, Uchida T, et al. (2012) Randomized comparison of the therapeutic effect of acupuncture, massage, and Tachibana-style-method on
stiff shoulders by measuring muscle firmness, VAS,pulse, and blood pressure. Evid Based Complement
Alternat Med, 2012: 989705.
Robertson VJ, Ward AR, Jung P (2005) The effect ofheat on tissue extensibility: a comparison of deep andsuperficial heating. Arch Phys Med Rehabil, 86: 819825.
Lehmann JF, De Lateur BJ. (1990) Therapeutic heat,“Therapeutic heat and cold. 4th ed.” (Lehmann JF,editor). Williams & Wilkins, Maryland, pp.417-581.
Kanai S, Taniguchi N, Okano H (2011) Effect ofmagnetotherapeutic device on pain associated withneck and shoulder stiffness. Altern Ther Health Med,
17: 44-48.