Smaller Size and Asymmetry of Trunk Muscles in Combat Sports Athletes With Lumbar Intervertebral Disc Degeneration

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 with the body coil 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 Pfirrman et al. This system uses characteristics of disc structure, distinction between the nucleus and annulus, MRI signal intensity, and intervertebral disc height for grading, and has been accepted as a standardized and reliable evaluation tool for assessment of MRI disc morphology. The assessment was blinded so as not to disclose any knowledge about the athlete's condition, such as the subjective functional disability level of LBP. When the 2 experienced orthopedic specialists had differing opinions on disc grades, the disagreements were debated and resolved with discussion.

The 151 participating athletes were divided into 2 groups: LDD and non-LDD. The LDD group included subjects 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. For further assessment, the most damaged disc of the 5 constituted the highest grade of LDD.

CSAs of trunk muscles Transverse MR spin-echo T1-weighted images were obtained at the L3-4 level parallel to the lumbar disc space (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. CSAs were calculated using image analysis software (Scion Image Beta 4.02, Scion Corp., Frederick, MD, USA), and grouped into 5 large areas because they had poorly defined borders. Each of the 5 areas was represented by the same 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 determine 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 includes the iliocostalis, longissimus, and spinalis. All CSAs were normalized by dividing the values by the athlete's body weight. This method was used because most of the combat sports adopt the weight category system, and a previous study reported the correlation between CSAs of the trunk muscles in athletes according to their body mass.