Intra-Rater Reliability of the Shore Durometer in the Assessment of Upper Trapezius Muscle Hardness

 

Chung-Yoo Kim, Jong-Ho Kang*, Tae-Sung Park

Department of Physical Therapy, Catholic University of Pusan, 57, Oryundae-ro, Geumjeong-gu, Busan, Republic of Korea, KS012

 

 

ABSTRACT:

Background/ Objectives: This study aims to measure the reliability of the Shore durometer in assessing the hardness of the upper trapezius, providing the basis for muscle hardness measurement. Methods/Statistical analysis: The subjects of this study were 30 healthy adult males. After typing for 20 minutes, the subjects’ trapezius muscle hardness was measured while they were in a supine position. This process was repeated for six days. Their muscle hardness was measured three times per day, and the average value of the three measurements was used for analysis. To assess the reliability of the measured results, intra-class correlation coefficients were calculated. Findings: The result was that the intra-rater reliabilities of the left upper trapezius and of the right upper trapezius were 0.827 and 0.816, respectively. The Shore durometer has a high level of intra-rater reliability in measuring upper trapezius muscle hardness. Improvements/Applications: The Shore durometer can provide reproducible results of the upper trapezius muscle’s hardness when repetitively measured by the same measurer.

 

 

 

1. INTRODUCTION:

Muscle tone is the amount of tension appearing in muscles during rest. It is defined as the condition of continuous, passive, and partial muscle contractions or resistance against passive extension¹. Accordingly, muscle tone may be assessed through muscle hardness during palpation, passive joint movement, joint extensibility, and postural abnormality. The structure and shape of a muscle itself, spinal reflex, pyramidal tract, the cerebellar-vestibular system, the cerebral basal ganglia, and the brain stem reticular formation complexly engage in muscle tone². Feedbacks from the proprioceptive system are also involved in muscle tone². Therefore, muscle tone is difficult to quantify, and there are few methods to measure it. The Modified Ashworth Scale and Tone Assessment Scale are the methods most frequently used in the clinical field³, but objective evaluation using the subjective scales is difficult. Therefore, measurement using a quantitative method is necessary.

 

Although there are limited methods to measure muscle tone, diverse approaches (measuring activity level or hardness of muscle) have been attempted^4,5. Activity of the superficial or deep muscles may be measured in order to analyze muscle signals. Superficial muscle activity may be measured non-invasively. However, it is heavily influenced by the external environment, and precise measurement is difficult. In addition, deep muscles are hard to measure because invasive electrodes must be used. Hence, another simple method, other than measuring muscle activity, is needed.

 

Nagao et al.^’ study⁶ devised a durometer measuring muscle hardness and presented relevant research on the development of equipment to evaluate muscle tone. Previous research involved muscle tone measurement using rats⁷ and muscle tone measurement of humans⁸. However, research on the reliability of such methods and their basis has been lacking. Kim⁹ and Kerins et al. study¹⁰ performed research on the reliability of a myotonometer, but they utilized equipment with differences in its unit and size—unlike engineering hardness measurement equipment that is made of rubber or steel. Accordingly, this study aims to measure the hardness of a human muscle using the Shore durometer with international standard hardness and to analyze its reliability. In addition, this study intends to provide the ground for such a measurement method.

 

2. MATERIALS AND METHODS:

2.1. Subjects

The subjects of this study were 30 males in their 20s whose primary hand was the right hand. All the subjects were healthy with no experience of neurological or orthopedic injuries. They all listened to a detailed explanation on the purpose and method of this study and provided written consent to participate in this study. The procedures of this study complied with all the provisions of the Declaration of Helsinki. An assistant researcher performed all the procedures of this study.

 

2.2. Experimental process

The subjects performed typing before the hardness of their tense muscle was measured. According to the results of a study by Horikawa¹¹, the subjects’ muscle hardness increased after visual display terminal work. Accordingly, this study measured muscle harness after it was induced by typing. All the subjects performed typing work on a notebook for 20 minutes before their muscle hardness was measured. The experimental procedure is illustrated in Figure 1.

 

 

Figure 1. Design of study

 

2.3. Measurement of muscle hardness of upper trapezius

In this study, a Shore C type durometer was used to measure the hardness of upper trapezius as shown in Figure 2. The subjects lied in a supine position on the treatment table and relaxed their whole bodies. The measured muscles were the bilateral upper trapezius muscles. The area between the seventh cervical vertebra spinous process and acromion was pushed vertically, and the fixed value was measured within one second after being pushed vertically. The average value of the three measurements from the same area was used for analysis¹². Measurements were taken repetitively for six days to measure the intra-rater reliability.

 

Figure 2. Shore durometer

 

2.4. Statistical processing

An intra-class correlation coefficient was derived to measure the intra-rater reliability of the repetitively measured hardness of the upper trapezius muscle. SPSS 12.0 program was employed for all statistical processing. The significance level was set to 0.05.

 

3. RESULTS:

3.1. General characteristics of the subjects

Table 1 shows the subjects’ average age, height, weight, and body mass index.

 

Table 1. General characteristics of the subjects

 

3.2. The hardness of upper trapezius after typing task

Table 2 and figure 3 displays the hardness of the upper trapezius muscle of the subjects measured for six days.

 

 

Table 2. Hardness of upper trapezius after typing task

	Day 1	Day 2	Day 3	Day 4	Day 5	Day 6
Left	19.81±0.66	19.87±0.71	19.87±0.54	19.97±0.70	20.01±0.47	19.84±0.39
Right	20.04±0.61	20.02±0.71	19.87±0.55	20.04±0.61	20.11±0.48	19.93±0.55

 

Unit: HC (0~100)

Figure 3. Hardness of upper trapezius after typing task

3.2. Intra-rater reliability of the hardness of the upper trapezius muscle

The intra-rater reliability of the upper trapezius muscle hardness in this study’s subjects was 0.827 (95% Cl 0.710 to 0.908) in the left upper trapezius muscle and 0.816 (95% Cl 0.671 to 0.902) in the right upper trapezius muscle as shown in Table 3

                  

Table 3. Intra-rater reliability of the hardness of the upper trapezius muscle

Upper trapezius	Intraclass correlation	95% Confidence Interval		Sig
		Lower Bound	Upper Bound
Left	0.827	0.710	0.908	0.000*
Right	0.816	0.691	0.902	0.000*

 

4. DISCUSSION:

This study measured muscle hardness to evaluate muscle tone using a durometer. This study tested the reliability of the Shore durometer to establish the basis for using it to measure muscle hardness. In this study, intra-rater reliability of the Shore durometer was obtained with the hardness of the left upper trapezius muscle at 0.827 and the hardness of the right upper trapezius muscle at 0.816. This method is meant to evaluate the reproducibility of clinical tests. The intra-class correlation coefficient was measured using test-retest reliability; the intra-class coefficient ranges from zero to one. When the value is 0.75 or higher, the reliability is good; when it is lower than 0.75, the reliability is poor to moderate. According to Domholdt¹⁴, reliability was very high when the intra-class correlation coefficient was 1 to 0.90, high when it was 0.89 to 0.70, moderate when it was 0.69 to 0.50, low when it was 0.49 to 0.26, and very low when it was 0.25 or lower. Portney and Wakins¹⁵ viewed an intra-class correlation coefficient of 0.5 or lower as poor reliability, an intra-class correlation coefficient of 0.51 to 0.75 as moderate reliability, and an intra-class correlation coefficient of 0.75 or higher as good reliability. According to Fleiss’ guidelines¹⁶, reliability of the Shore durometer in this study was excellent; based on Nunnally and Bernstein’s guidelines¹⁷ that intra-class correlation coefficient of the tool developed should be at least 0.7, the durometer in this study is deemed to have a high level of reliability. Therefore, the Shore durometer is considered to have a high level of intra-rater reliability in measuring the hardness of the upper trapezius muscle.

Previously used equipment lacked reliability studies⁶, and equipment tested for reliability did not meet international unit standards^9,10. According to the standard of the American Society for Testing and Materials (ASTM D2240), the Shore type A to R durometer determines the hardness of different materials, such as rubber, cells, gel type, and plastic. Accordingly, the Shore durometer has adequate probing elasticity and characteristics to measure muscle hardness. Compared to other durometers, the Shore durometer’s advantage is it has units and a standard that correspond with the international standard. Research by Im and Jung’s study¹³ employed a Shore durometer of the OO class to measure Koreans’ skin hardness. Shore durometer of the OO class was also used to measure the hardness of the human body. Other types of shore durometer may be used effectively as long as the elasticity of the probe is not excessive. Also, as shown in Fig. 4, the units can be substituted for each hardness meter. Future studies will contribute to the standardization of human body hardness measurement using shore durometer of each class.

 

Figure 4. Shore hardness scales

(ref : https://albrightsilicone.com/wp-content/uploads/2015/01/ durometer_with_logo_small_580.jpg)

Muscle hardness by the Shore durometer is measured by a rater; therefore, the result may vary according to the rater’s strength, angle, and level of skill. In this study, repetitive measurement of upper trapezius muscle hardness by a trained assistant researcher for six days resulted in reproducible values. The results of this study are worthy of being able to provide reproducible values even with short training time. However, it only involved intra-rater reliability related to reproducibility. Future research will provide a scientific evidence on the Shore durometer to evaluate muscle tone by measuring inter-rater reliability and validity of the Shore durometer in comparison with other muscle hardness and tone measurement tools.

 

5. CONCLUSION:

According to the results of this study, the Shore durometer of the C type had a high level of intra-rater reliability and exhibited a reproducible outcome in muscle hardness measurement.

 

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Received on 05.08.2018          Modified on 15.10.2018

Accepted on 08.12.2018        © RJPT All right reserved