Experimental design
To confirm our hypotheses, CSAs of the trunk muscles and physical characteristics were measured by using various devices. Also, LDD was assessed in the way hereinafter prescribed. We compared the asymmetry between left and right sides and size of trunk muscles in collegiate male combat sports athletes with and without LDD.
Participants
A group of 151 collegiate male combat sports athletes, including 50 wrestlers and 101 judokas, participated in this study. The combat sports athletes were selected from the trained athletes attending the Nippon Sport Science University in Japan. All of the participants were Japanese collegiate high-level athletes who volunteered for the study. All athletes regularly spent approximately 4 h per day (2 times a day, 6 days a week) practicing their combat sports. The purpose of this study and protocol were explained to all athletes and their coaches, and signed informed consent was obtained prior to their participation. The study was approved by the Ethical Committee of the university.
Physical characteristics
Anthropometric data of the athletes were recorded (height to the nearest 0.1 cm and body weight to the nearest 0.1 kg). Body mass index (BMI) was calculated as body weight in kilograms divided by height in square meters (kg/m2). Moreover, the age and combat sports experience of each athlete were investigated.
Assessment of LDD
The athletes lay on a bed in the MR imaging unit in a comfortable and relaxed supine position. MR imaging was performed with a 0.3-T MR using surface coils in the supine position (AIRIS II, Hitachi, Tokyo, Japan). T2-weighted fast spin-echo imaging was used to obtain sagittal images of the lumbar spine and intervertebral discs (repetition time, 3000 ms; echo time, 112 ms; matrix, 256 × 265; field of view, 320 mm; slice thickness, 10 mm).
All MR images, taken at 5 lumbar intervertebral disc levels from the first lumbar (L1) vertebra to the first sacral vertebra (S1), were independently evaluated by 2 experienced orthopedic specialists in a random order using a grading system for LDD assessment. Using a comprehensive grading system for LDD, discs were classified into 5 grades, as described by Pfirrmann et al. (2001). This system uses characteristics of disc structure, distinction between the nucleus and annulus, MRI signal intensity, and intervertebral disc height for grading. This comprehensive grading system for LDD has been accepted as a standard (Pfirrmann et al. 2001) and reliable evaluation tool for assessment of MRI disc morphology (Hangai et al. 2009; Koyama et al. 2015; Kulling et al. 2014; Min et al. 2010; Salamat et al. 2016). The assessment was blinded so as not to disclose any knowledge about the athlete’s conditions. When the 2 experienced orthopedic specialists had differing opinions on disc grades, the disagreements were debated and discussed until a resolution was reached.
The 151 participating athletes were divided into 2 groups: LDD and non-LDD. The LDD group included participants with at least 1 abnormal disc from L1–2 to L5–S1 of grade III, IV, or V. The non-LDD group included subjects with 5 normal discs of grade I or II.
CSAs of trunk muscles
Transverse MR spin-echo T1-weighted images were obtained at the L3–4 level parallel to the lumbar disc space in order to minimize inter-participant differences in anatomical curvature of the lumbar spine (Fig. 1a, repetition time, 760 ms; echo time, 20 ms; matrix, 256 × 265; field of view, 320 mm; slice thickness, 5.0 mm). The image was traced onto paper and the traced image was then transferred to a computer in order to measure CSAs (Shown in Fig. 1b). CSAs were calculated using image analysis software (Scion Image Beta 4.02, Scion Corp., Frederick, MD, USA), and grouped into 5 large areas because the individual muscles had poorly defined borders. Each of the 5 areas was represented by the sum of the CSA on the left and right sides of the transverse image (rectus abdominis, obliques, psoas, quadratus lumborum, and lumbar erector spinae plus multifidus). The 5 areas were summed to obtain the total area. Three of the CSAs included multiple muscles: obliques, psoas, and lumbar erector spinae). Oblique muscles comprise the internal and external obliques and transversus abdominis. Psoas muscles comprise the psoas major and minor muscles. The lumbar erector spinae comprises the iliocostalis, longissimus, and spinalis. All CSAs were also normalized by dividing the values by the athlete’s body weight. This method was used in the previous study (Peltonen et al. 1998), in order to indirectly eliminate differences in their lean body mass.
Data analysis
All statistical analyses were evaluated using PASW Statistics 18 software (SPSS Japan Inc., Tokyo, Japan) and R (version 3.3.0) for Windows. The grade of LDD at each disc level was analyzed using the Friedman test, followed by the Wilcoxon signed rank-sum test for multiple comparisons. Means, standard deviations (SD), and 2-sided 95 % confidence intervals (95 % CI) were calculated, and the data were expressed as the mean ± SD (95 % CI). The physical characteristics and CSAs of the athletes were compared between the LDD and non-LDD groups using an unpaired Student’s t test. Moreover, a paired t test was employed for comparisons of CSAs between the left and right sides as an asymmetrical difference. The estimation was based on an effect size of 0.5, alpha level of 0.05, and a power (1 − β) of 0.80. Statistical analysis was performed by G*power (Faul et al. 2007). The level of statistical significance was adjusted based on p < 0.05.