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Associations of Trunk Muscle Thickness and Muscle Strength in Healthy Male Adults

Background: Sincethetestof musclestrengthneedsaperson’smaximumexertion,theobjectiveassessmentof musclesizeisoften used. Among the imaging techniques for assessing muscle size, muscle thickness is easy to measure by ultrasonography. However, little is known about the associations of trunk muscle thickness and muscle strength. Objectives: This study aimed to investigate the correlation between trunk muscle thickness and muscle strength. Methods: Seventeen healthy male university students participated in this study (20.8 ± 0.6 years). Abdominal (rectus abdominis, external oblique, internal oblique, transverse abdominis) and low back (lumbar erector spinae, lumbar multiﬁdus) muscle thickness were measured at rest using B-mode ultrasonography. Maximum voluntary isometric trunk ﬂexion, extension, and rotation torque were measured using dynamometers. The relationships between all tested variables were investigated using the Pearson’s correlation coeﬃcient. Results: A signiﬁcant positive correlation was found between ﬂexion and extension torque (r = 0.664, P = 0.004), ﬂexion and right rotation torque (r = 0.565, P = 0.018), ﬂexion and left rotation torque (r = 0.641, P = 0.006), extension and right rotation torque (r = However, there was no signiﬁcant correlation between trunk muscle thickness and torque. Conclusions: Thisstudyprovidesbasicinformationabouttrunkmusclethicknessandmusclestrengthinhealthymaleadults. The associations of trunk muscle thickness and muscle strength could not be identiﬁed through our measurements.


Background
Muscle strength is needed for physical activity. Since the test of muscle strength needs a person's maximum exertion, the objective assessment of muscle size is often measured in sedentary people (1). Muscle volume which, is related to muscle cross-sectional area and thickness, is associated with muscle strength (2,3). Among the imaging techniques for assessing muscle size, the muscle thickness is the easiest to measure via ultrasonography. Previous reports showed that muscle thickness of the quadriceps femoris, triceps brachii, and forearm might be a predictor of knee and elbow extension, and handgrip strength, respectively (2,3). However, little is known about the associations between trunk muscle thickness and strength. The abdominal and low back muscles are relevant muscles that contribute to lumbopelvic postural control (4).

Objectives
This study aimed to investigate the correlations between trunk muscle thickness and strength in healthy male adults.

Participants
Seventeen male adults were included in this study. Participants consisted of healthy university student volunteers who were physically active although not athletes. Their mean (± standard deviation) age, height, weight, by the Ethics Committee at Kawasaki University of Medical Welfare (#18-080). Written informed consent was obtained from all subjects. The protocol for this study is consistent with the ethical guidelines of the 1975 Declaration of Helsinki.

Procedure
Muscle thickness (cm) of the right and left abdominal (rectus abdominis, external oblique, internal oblique, transverse abdominis) and low back (lumbar erector spinae, lumbar multifidus) muscles, at rest, was measured using B-mode ultrasonography (Noblus; Hitachi Ltd., Tokyo, Japan) with a 7 -3 MHz linear probe (L34; Hitachi Ltd., Tokyo, Japan). Subjects were positioned in the supine and prone posture in order to perform ultrasonography of the abdominal and low back muscles, respectively. Measurement sites were defined as 3 cm lateral to the umbilicus for the rectus abdominis muscle, 2.5 cm anterior to the midaxillary line and at the midpoint between the inferior rib and iliac crest for the external oblique, internal oblique, and transverse abdominis muscles, and approximately 4 cm and 2 cm lateral to the L3/4 spinous process for the lumbar erector spinae and multifidus, respectively (Figure 1). Measurement location of the lumbar erector spinae and multifidus muscle thickness were at midpoint of the lumbar erector spinae muscle fascia between the transverse processes of the 3rd and 4th lumbar vertebrae and at the intervertebral joint of the L3/4, respectively. Longitudinal image for the lumbar multifidus muscle and transverse image for the other muscles were measured. Measurements were performed by an experienced investigator with 7 years of experience in musculoskeletal ultrasound. The probe was held using the minimum pressure required to achieve a clear image. Measurement of abdominal muscle thicknesses was performed at the end of a relaxed expiration. The muscle images were collected twice. The average values of the two trials were used in this analysis.
Maximum voluntary isometric trunk flexion and extension torque (Nm) were measured using the trunk flexion/extension dynamometer (Isoforce GT-350; OG Wellness Co., Ltd., Okayama, Japan) ( Figure 2). Subjects were positioned in an upright sitting posture with the center of the sensor pad placed at the height of the inferior angulus of the scapula. The subject was fixed at different places (upper trunk, pelvis, anterior thigh) with straps. Maximum voluntary isometric trunk right and left rotation torque (Nm) were measured using the trunk rotation dynamometer (S-18034; Takei Scientific Instruments Co., Ltd., Niigata, Japan) which was custom developed using the trunk rotation training machine (Rotary torso FY-1032; NISHI Athletic Goods Co., Ltd., Tokyo, Japan), tension/compression type load cell (UNCLB-1kN; UNIPULSE Co., Ltd., Niigata, Japan), and strain amplifier (T.K.K.1268b; Takei Scientific Instruments Co., Ltd., Niigata, Japan) ( Figure 2). Calibration of the equipment was performed before each test according to the manufacture's recommendations. Before analysis, subjects performed some sub maximum exertion practice. The test order was randomized. Subjects were tested twice for each measurement. If the second value of torque was more than 110% of the first one, the torque was measured one more time (5). In 2 or 3 torque measurements, the highest value was used for analysis.

Statistical Analysis
Statistical analyses were performed using SPSS Statistics V.23 (IBM Inc, Chicago, IL). The reliability of the measured values of the muscle thickness and torque between the 1st and 2nd measurements were examined by the intraclass correlation coefficient (ICC) (1,2) and (1, 1)

Results
The mean ± standard deviation of representative values and the correspondent reliability are listed in Table 1.
The correlation coefficients between muscle thickness and torque values are presented in Table 2. There were significant positive correlations with the torque. However, there was no significant correlation between trunk muscle thickness with torque.

Discussion
This study investigated the associations between trunk muscle thickness and strength in healthy male adults. In this study, ICC values between the 1st and 2nd measurements in the muscle thickness were almost perfect. Teyhen et al. (7) found mean rectus abdominis, external oblique, internal oblique, and transverse abdominis muscle thickness of 244 healthy males (21.8 ± 3.9 years) to be 1.41 ± 0.24, 0.80 ± 0.24, 1.04 ± 0.23, and 0.39 ± 0.24 cm, respectively.    measured at the transverse process of the 3rd lumbar vertebrae. These results indicated that measurements of muscle thickness in this study might be valid.
In this study, significant correlations were observed with torque. These results indicated that the differences in muscle strength among subjects could be detected by torque measurements. However, trunk muscle thickness was not associated with torque, which might be due to the compound movement of synergist muscles (10). Whiler et al. (10) could not demonstrate associations between gluteus medius and maximus muscle thickness and strength for the reasons mentioned above. Sasaki (13) showed that trunk flexion, extension, and rotation torque of 7 healthy males (ranging from 21 to 26 years old) to be 122.9 ± 28.9, 260.3 ± 65.1, and 119.3 ± 32.5 Nm, respectively. Our data showed higher values in the trunk flexion and extension, lower values in the trunk rotation. In this study, extremities muscle might contribute to the tasks of trunk flexion and extension. Though the subject was fixed at different places (upper trunk, pelvis, anterior thigh) with straps, the hip flexion and extension could contribute to the testing of trunk flexion and extension torque, respectively. On the other hand, in this study, the contribution of the trunk rotation muscles might not be sufficiently transmitted to the dynamometer, since sub- jects were not fixed at several places. Therefore, the associations between muscle size and torque might be limited by our measurements, though ICC values between 1st and 2nd measurements in the torque were almost perfect.
The limitation of this study was ambiguity in determining that the subjects generated maximum voluntary force for each measurement. The subjects did not undergo a preliminary leaning, which could have an influence on the reliability of torque values (14). The previous study demonstrated a learning effect on isometric trunk flexion strength between different testing days (15). Other architectural parameters such as cross-sectional area or muscle volume measured by magnetic resonance imaging and computer tomography should be used in future studies. The sex difference remains unclear. Moreover, the sample size might have been a limitation.
This study provides basic information about trunk muscle thickness and muscle strength in healthy male adults. The associations of trunk muscle thickness and muscle strength could not be detected by our measurements.
Tadanobu Suehiro; analysis and interpretation of data: Hiroshi Ishida, Tadanobu Suehiro, and Susumu Watanabe; drafting of the manuscript: Hiroshi Ishida; critical revision of the manuscript for important intellectual content: Hiroshi Ishida, Tadanobu Suehiro, and Susumu Watanabe; statistical analysis: Hiroshi Ishida; administrative, technical, and material support: Hiroshi Ishida and Tadanobu Suehiro; study supervision: Hiroshi Ishida.

Conflict of Interestes:
The authors have no conflict of interests.
Ethical Approval: This study was approved by the Ethics Committee at Kawasaki University of Medical Welfare (#18-080). The protocol for this study is consistent with the ethical guidelines of the 1975 Declaration of Helsinki.
Funding/Support: Kawasaki University of Medical Welfare Scientific Research Fund was received for this study.
Patient Consent: Written informed consent was obtained from all subjects.