Relationship between trunk muscle strength and trunk muscle mass and thickness using bioelectrical impedance analysis and ultrasound imaging

Abstract

BACKGROUND:

Although trunk muscles are involved in many important functions, evaluating trunk muscle strength is not an easy task. If trunk muscle mass and thickness could be used as indicators of trunk muscle strength, the burden of measurement would be reduced, but the relationship between trunk muscle strength and trunk muscle mass and thickness has not been clarified.

OBJECTIVE:

The purpose of this study was to clarify the relationship between trunk muscle strength and trunk muscle mass by bioelectrical impedance analysis and trunk muscle thickness by ultrasound imaging in healthy adults.

METHODS:

One hundred and twenty-one healthy university students were included in this study. Trunk flexion/extension muscle strength and trunk muscle mass by bioelectrical impedance analysis, and trunk muscle thickness by ultrasound imaging were measured.

RESULTS:

Both trunk flexion strength and trunk extension strength were significantly correlated with trunk muscle mass and oblique and rectus abdominis muscle thickness. Multiple regression analysis showed that trunk extension muscle strength had an independent relationship with trunk muscle mass.

CONCLUSIONS:

This study demonstrated that trunk muscle mass or trunk muscle thickness can be used as an alternative means for evaluating trunk muscle strength, making the evaluation of trunk muscles less burdensome.

Keywords

Trunk muscle muscle strength bioelectrical impedance analysis ultrasound imaging

1. Introduction

The trunk muscles are involved in daily life activities such as getting up and maintaining posture; they also perform various important roles such as stabilizing the trunk prior to moving the limbs to achieve smooth movement of the limbs [1] and respiratory function [2]. It is important to evaluate the condition of the trunk muscles as it has been shown that strengthening exercises of the trunk muscles can reduce low back pain [3] and that training of the trunk extensors can maintain and improve the bone mass of the spinal column and reduce the risk of vertebral fractures [4,5]. Trunk muscle strength is often measured using manual muscle testing methods, isokinetic muscle testing devices, and hand-held dynamometry. However, these methods are risky for the elderly and for those with cardiovascular diseases. They require additional expensive and extensive equipment. Therefore, it is necessary to have a measurement method that is easy to use and can be used as an index of muscle strength without directly measuring muscle strength. In addition to neurological factors such as the recruitment of motor units, morphological factors, including muscle mass and cross-sectional area, are known to be related to muscle strength [6]. In general, the larger the mass and cross-sectional area of a muscle, the greater the muscle force and, conversely, the smaller the muscle force exerted by atrophy [6]. In recent years, bioelectrical impedance analysis (BIA) has been relied on as an evaluation method for the morphological component of muscles [7,8]. BIA estimates body composition by measuring the difference in conductivity due to differences in the biological properties of each tissue [7]. It is widely used because it is easy to measure and transportable compared to computed tomography and magnetic resonance imaging (MRI), which are often used to measure trunk muscle mass. However, it remains to be elucidated whether trunk lean mass measured using BIA reflects actual trunk muscle mass [9]. Nevertheless, we consider that if BIA muscle mass is useful as an index of trunk muscle strength, its use could reduce the burden of measuring trunk muscle strength in people with trunk-related diseases and symptoms, such as the elderly and people with back pain, which makes it a useful evaluation method.

In addition to BIA, muscle thickness assessed by ultrasound imaging (USI) is widely used as a method to easily evaluate the morphological component of the muscle. The advantages of USI are its ease of use, real-time imaging, and the ability to evaluate each muscle individually. There are many reports assessing trunk muscle thickness by USI [10–12], and trunk muscle thickness is often used as a component of trunk muscle assessment. However, the relationship between trunk muscle thickness and trunk muscle strength or muscle mass by BIA has not been adequately demonstrated. Therefore, understanding the relationship between trunk muscle strength and muscle mass and trunk muscle thickness by USI will clarify which muscles reflect muscle strength and muscle mass, which will help in training for trunk muscles. Therefore, the purpose of this study was to investigate the relationship between trunk muscle strength and trunk muscle mass measured by BIA and trunk muscle thickness measured by USI. This is based on the hypothesis that trunk muscle strength and trunk muscle mass measured using BIA are related and this measured muscle mass could serve as an indicator of trunk muscle strength.

2. Materials and methods

All measurements were conducted at Kyoto Tachibana University, Yushinkan. Data were collected between October 2019 and December 2019. The participants were healthy university students who were recruited for the study. Those with a history of surgery, metal implants in the body, pain that interfered with daily life, or typical physical disabilities such as motor paralysis were excluded. The participants gave written consent for “voluntary participation” after the purpose and methods of the study were explained to them orally and in writing. This study was conducted with the approval of the Research Ethics Committee of Kyoto Tachibana University (Approval No.: 20-09).

The measurement items were trunk flexion muscle strength, trunk extension muscle strength, trunk muscle mass, and muscle thickness of rectus abdominis (RA), external oblique abdominis (EO), internal oblique abdominis (IO), transverse abdominis (TrA) and multifidus muscle (ML). Each measurement was performed by the same examiner who was experienced in performing these measurements. A hand-held dynamometer (HHD) (μ-TasF1, ANIMA Inc. Tokyo, Japan) was used to measure trunk flexion muscle strength (Fig. 1a). The participants were instructed to lie in the supine position with 90° knee flexion and the plantar surface of the foot placed on the bed and the upper limbs folded at the abdomen. The HHD was placed on the midline of the trunk at the level of the third intercostal space and fixed with a belt (Traction belt 270 cm, Erler Zimmer GmbH & Co. KG, Lauf, Germany). The pelvis was fixed to the bed with a belt similar to that used for the HHD below the superior anterior iliac spine. A break of 2 min was taken between each measurement, and the average of three measurements was used as the representative value.

Fig. 1.

Measurement of trunk muscle strength and mass. (a) Measurement of trunk flexion muscle strength using hand-held dynamometry; (b) measurement of trunk extension muscle strength using back muscle strength meter (the solid line is perpendicular to the floor and the dotted line is the line of the trunk, the angle between them is 30°); (c) measurement of trunk muscle mass using the bioelectrical impedance analysis body composition analyzer.

A digital back muscle strength meter (Back-D: Takei Scientific Instruments Co., Ltd, Tokyo, Japan) was used to measure trunk extension muscle strength (Fig. 1b). The participants were instructed to stand barefoot on the back muscle strength meter, and the length of the chain of the back muscle strength meter was adjusted to place the trunk in a 30° forward tilt position and the upper limbs in a drooping position. For the measurement, the participants were instructed to raise their upper body, taking care not to flex the elbow or knee joints. A break of 2 min was taken between measurements, and the average of the two measurements was used as the representative value.

A BIA body composition analyzer (Inbody 470: Inbody Japan Inc., Tokyo, Japan) was used to measure trunk muscle mass. The measurement was performed by standing barefoot on the analyzer and grasping the sensors with the upper limbs (Fig. 1c).

Muscle thickness measurement was performed using an USI system (LOGIQ e, GE Healthcare Japan, Tokyo, Japan) with a linear probe (10 MHz) in B mode. RA, EO, IO, TrA, and ML were tested on the right side. To avoid changes in muscle thickness due to probe pressure, imaging was performed using a hard-type gel (LOGIQLEAN, GE Healthcare Japan, Tokyo, Japan) that prevented direct contact between the probe and the skin. The TrA, IO, and EO measurements were acquired based on the method of Misuri et al. [13], the center of the probe was positioned midway between the costal margin and the iliac crest, along the right anterior axillary line, and short-axis images were obtained (Fig. 2a). For the RA, the center of the probe was placed at 1∕4 region from the umbilicus on the line connecting the umbilicus and midpoint between the costal margin and the iliac crest, and a short-axis image was acquired (Fig. 2b). For the measurement of ML thickness, the method proposed by Sions et al. [14] was used as a reference. The center of the probe was positioned 2 cm to the right of the fourth lumbar spinous process with the patient in the prone position, and a short-axis image was acquired (Fig. 2c). Subsequently, the image analysis program (ImageJ ver1.52) was used to measure the muscle thickness of each muscle using the center perpendicular line of the ultrasound image as an index (Fig. 3a–c).

Fig. 2.

Location of probes for ultrasound imaging measurements. (a) Measurement site for oblique abdominis muscle and transverse abdominis muscle (the solid line is the anterior axillary line); (b) rectus abdominis muscle (the solid line is the line connecting the umbilicus and midpoint between the costal margin and the iliac crest); (c) multifidus muscle (the solid line is the line connecting the spinous processes).

Fig. 3.

Region of interest in ultrasound imaging. (a) Oblique abdominis muscle and transverse abdominis muscle; (b) rectus abdominis muscle; (c) multifidus muscle. TrA, transverse abdominis muscle thickness; IO, internal oblique abdominis muscle thickness; EO, external oblique abdominis muscle thickness; RA, rectus abdominis muscle thickness; ML, multifidus muscle thickness. Arrows indicate muscle thickness.

For statistical analysis, Pearson’s product rate correlation coefficient was used to assess the association between trunk flexion muscle strength, trunk extension muscle strength, trunk muscle mass, and muscle thickness. In addition, a multiple regression analysis (forced imputation method) was conducted with trunk flexion muscle strength, trunk extension muscle strength, and trunk muscle mass as dependent variables, and the items with significant correlations in the simple correlation analysis as independent variables. The covariates were age, sex, and body mass index (BMI), which is an index of body size, in accordance with previous studies [7]. SPSS Statistics version 24.0 (Japan IBM, Tokyo, Japan) was used for statistical analysis, and the significance level was set at 5%.

3. Results

There were 121 participants (61 males [age: 20.3 ± 0.7 years, height: 171.6 ± 5.1 cm, weight: 62.5 ± 6.4 kg, BMI: 21.2 ± 1.7 kg/m²], 60 females [age: 19.9 ± 1.1 years, height: 158.0 ± 5.4 cm, weight: 51.1 ± 5.1 kg, BMI: 20.5 ± 1.8 kg/m²]). The measurements from the BIA and USI are shown in Table 1. Correlation analysis showed that trunk flexion muscle strength and trunk extension muscle strength were significantly correlated with trunk muscle mass (Table 2). Trunk flexion muscle strength, trunk extension muscle strength, and trunk muscle mass were all significantly correlated with muscle thickness of IO, EO, and RA (Table 2). BMI and ML thickness were significantly correlated with trunk extension muscle strength and trunk muscle mass (Table 2). Age and TrA muscle thickness were not correlated with trunk flexion muscle strength, trunk extension muscle strength, or trunk muscle mass (Table 2).

Table 1
Characteristics of the study participants

Age (years) 20.12 ± 0.97

BMI (kg/m²) 20.88 ± 1.78

Trunk flexion muscle strength (Nm) 10.48 ± 4.99

Trunk extension muscle strength (Nm) 87.88 ± 27.11

Trunk muscle mass (kg) 20.02 ± 3.87

TrA (cm) 0.39 ± 0.22

IO (cm) 0.91 ± 0.25

EO (cm) 0.86 ± 0.26

RA (cm) 0.98 ± 0.28

ML (cm) 2.87 ± 0.37

Data are presented as the mean ± standard deviation. BMI, body mass index; TrA, transverse abdominis muscle thickness; IO, internal oblique abdominis muscle thickness; EO, external oblique abdominis muscle thickness; RA, rectus abdominis muscle thickness; ML, multifidus muscle thickness.

Table 2

Correlation coefficients for trunk muscle strength and trunk muscle mass assessed by bioelectrical impedance analysis

	Trunk flexion muscle strength	Trunk extension muscle strength	Trunk muscle mass
Age	−0.05	0.16	0.05
BMI	0.05	0.18^∗	0.38^∗
Trunk muscle mass	0.69^∗	0.87^∗	—
TrA	−0.09	−0.03	0.03
IO	0.52^∗	0.49^∗	0.55^∗
EO	0.44^∗	0.48^∗	0.47^∗
RA	0.49^∗	0.61^∗	0.64^∗
ML	0.13	0.24^∗	0.35^∗

^∗Statistically significant correlation (p < 0.05). BMI, body mass index; TrA, transverse abdominis muscle thickness; IO, internal oblique abdominis muscle thickness; EO, external oblique abdominis muscle thickness; RA, rectus abdominis muscle thickness; ML, multifidus muscle thickness.

In the multiple regression analysis, three items were extracted as significantly related factors when trunk flexion muscle strength was the dependent variable: sex, BMI, and EO muscle thickness (Table 3). Four items were extracted as significantly related factors when trunk extension muscle strength was used as the dependent variable: age, BMI, trunk muscle mass, and RA muscle thickness (Table 3). The two items extracted as significantly related factors when trunk muscle mass was used as the dependent variable were sex and BMI (Table 3). All variance inflation factors were less than 10, and there was no multicollinearity among the independent variables.

Table 3

Multiple regression analysis with trunk muscle strength and trunk muscle mass as dependent variables

		Partial regression coefficient	Standard partial regression coefficient	Intentional confirmation rate	95% confidence interval	Variance inflation factor
Trunk flexion muscle strength	Age	−0.44	−0.09	0.16	−1.07 ∼ 0.18	1.10
	Sex	4.24	0.43	<0.01	1.40 ∼ 7.07	6.09
	BMI	−0.52	−0.19	0.01	−0.90 ∼ −0.14	1.39
	Trunk muscle mass	0.33	0.25	0.10	−0.06 ∼ 0.72	6.88
	IO	2.24	0.11	0.14	−0.74 ∼ 5.22	1.68
	EO	2.90	0.15	0.03	0.31 ∼ 5.48	1.36
	RA	0.03	<0.01	0.98	−2.85 ∼ 2.91	1.93
	Adjusted R ²	0.59
Trunk extension muscle strength	Age	2.76	0.10	0.02	0.39 ∼ 5.12	1.12
	Sex	8.73	0.16	0.11	−2.08 ∼ 19.54	6.24
	BMI	−2.27	−0.15	<0.01	−3.73 ∼ −0.81	1.43
	Trunk muscle mass	4.89	0.70	<0.01	3.42 ∼ 6.36	6.92
	IO	−1.37	−0.01	0.81	−12.58 ∼ 9.85	1.68
	EO	5.98	0.06	0.23	−3.73 ∼ 15.70	1.36
	RA	12.46	0.13	0.03	1.49 ∼ 23.43	1.98
	ML	−4.98	−0.07	0.14	−11.53 ∼ 1.58	1.27
	Adjusted R ²	0.80
Trunk muscle mass	Age	0.21	0.05	0.16	−0.08 ∼ 0.51	1.10
	Sex	6.04	0.78	<0.01	5.26 ∼ 6.82	2.02
	BMI	0.43	0.20	<0.01	0.27 ∼ 0.60	1.16
	IO	0.16	0.01	0.82	−1.26 ∼ 1.58	1.68
	EO	0.72	0.05	0.25	−0.50 ∼ 1.94	1.34
	RA	1.30	0.09	0.06	−0.07 ∼ 2.67	1.92
	ML	−0.30	−0.03	0.48	−1.13 ∼ 0.53	1.26
	Adjusted R ²	0.85

BMI, body mass index; IO, internal oblique abdominis muscle thickness; EO, external oblique abdominis muscle thickness; RA, rectus abdominis muscle thickness; ML, multifidus muscle thickness.

4. Discussion

This study examined the relationship between trunk muscle strength and trunk muscle mass measured by BIA, and muscle thickness measured by USI. This study also sought to verify the usefulness of trunk muscle mass measurement by BIA as an index of trunk muscle strength and to determine which muscles have the greatest influence on trunk muscle strength and trunk muscle mass.

The simple correlation analysis showed that muscle strength in both trunk flexion and extension were significantly correlated with trunk muscle mass. This suggests that trunk muscle mass may be an indicator of muscle strength in both trunk flexion and extension. Fujimoto et al. [9] have shown that the cross-sectional area of muscle by MRI correlates with the muscle mass of the trunk obtained from the lean mass of the trunk by BIA, and the assessment of trunk muscle mass by BIA is considered to be useful for the evaluation of the structural component of muscle. In a previous study, it has been reported that measurements of trunk lean mass by BIA is correlated with measurements of trunk lean mass by dual-energy X-ray absorptiometry (DXA), which is considered the gold standard for body composition assessment [15], although measurements of trunk lean mass by BIA is higher than that obtained by DXA [7,9]. Therefore, one should be cautious when using BIA measurements of trunk lean mass as a direct measure of trunk muscle mass. In the current study, the results of the multiple regression analysis suggest that trunk muscle mass was independently associated with trunk extension muscle strength, and the standard partial regression coefficient suggested that trunk muscle mass was an important factor. In addition, RA muscle thickness was independently related to trunk extension muscle strength. In the simple correlation analysis, trunk extension muscle strength was related to the muscle thickness of the muscles functioning during trunk flexion, such as the RA, EO, and IO. The RA, EO, and IO do not have moment arms in the direction of trunk extension and are not directly involved in the extension movement of the trunk [16]. Therefore, it is possible that the thickness of the RA, EO, and IO were related to the strength of the trunk extension because these muscles need to work in antagonism with the trunk extensor muscle groups to maintain posture. Additionally, the greater the power output of the trunk extensors, the greater the power output of the trunk flexor that is required, and the thickness of the trunk flexor muscles was related to the strength of the trunk extension muscles. Fujimoto et al. [9] reported that there was a correlation between trunk muscle mass and the cross-sectional area of the erector spinae muscles by BIA, but not of the MLs; this suggests that, among the trunk extensors, the erector spinae muscles are more related to trunk muscle mass than the MLs are. However, in this study, the thickness of trunk extensors was measured only for the ML and not for the erector spinae muscles such as the longissimus and iliocostal muscle or the latissimus dorsi muscle [16], which has a large moment arm in the direction of trunk extension. Therefore, it is necessary to measure the muscle thickness of other trunk extensors in order to investigate the relationship between muscle thickness and trunk extensor strength and trunk muscle mass.

Conversely, trunk flexion muscle strength was not independently related to trunk muscle mass, but EO muscles thickness was independently related to it. The EO has a larger physiological cross-sectional area than those of the RA and TrA [17], increases the flexion moment arm [18], and is known to work in concert with the RA during curl up [19]. Therefore, it is possible that the activity of the EO was also involved in the measurement of trunk flexion muscle strength in the curl-up position used in this study, suggesting that the greater the muscle thickness, the greater the trunk flexion torque. Among the abdominal muscles, only the TrA did not show a significant correlation with trunk muscle strength and trunk muscle mass. The TrA is morphologically the smallest abdominal muscle, with fibers running horizontally, and it has been reported that its contribution to moment production in the direction of trunk flexion and spinal stabilization is inferior to that of other trunk muscles [20,21]. For this reason, it is speculated that TrA muscle thickness was not related to trunk muscle strength or trunk muscle mass in this study.

One of the limitations of this study is that only morphological items were analyzed as factors involved in trunk muscle strength in this study, and neurological factors were not reflected. In the future, it is recommended to examine indices of trunk muscle strength that combine both neurological and morphological factors. Another limitation is that the participants in this study were healthy young adults. It is well known that changes in trunk muscle strength and muscle mass occur with aging [22,23], and the results of this study may differ from the results in the elderly. Although the relationship between trunk muscle mass by BIA and the cross-sectional area of trunk muscles by MRI has been reported in the elderly [24], the relationship between trunk muscle strength and trunk muscle mass by BIA has not been fully investigated. Regarding the assessment of trunk muscle strength, it is often difficult to measure trunk muscle strength in the elderly from the viewpoint of risk, and it is assumed that BIA assessments will prove to be effective when targeting the elderly; therefore, validation in the elderly is necessary in the future. Furthermore, it is known that both the quantity and quality of trunk muscles change due to disease conditions such as low back pain [25], hence it is necessary to verify the relationship and characteristics of trunk muscle strength and trunk muscle mass by BIA, and trunk muscle thickness by USI in people with diseases and disabilities.

5. Conclusions

In this study, we investigated the relationship between trunk muscle strength and trunk muscle mass using BIA and trunk muscle thickness using USI in healthy adults. The results of this study suggest that there is a correlation between trunk muscle strength and trunk muscle mass, and we demonstrated that trunk muscle mass or trunk muscle thickness can be used as an alternative means for evaluation of trunk muscle strength, making the evaluation of trunk muscles less burdensome.

Footnotes

Acknowledgements

The authors would like to thank Editage () for English language editing.

Conflict of interest

The authors declare no conflicts of interest.

Funding

This work was supported by JSPS KAKENHI grant number JP20K19298.

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Age (years)	20.12 ± 0.97
BMI (kg/m²)	20.88 ± 1.78
Trunk flexion muscle strength (Nm)	10.48 ± 4.99
Trunk extension muscle strength (Nm)	87.88 ± 27.11
Trunk muscle mass (kg)	20.02 ± 3.87
TrA (cm)	0.39 ± 0.22
IO (cm)	0.91 ± 0.25
EO (cm)	0.86 ± 0.26
RA (cm)	0.98 ± 0.28
ML (cm)	2.87 ± 0.37