Study design
We performed two studies to fulfill the goals of this paper. The first study (Study 1) was designed to examine the association between COL5A1 rs12722 polymorphism and joint ROM and passive muscle stiffness. In the second study (Study 2), we investigated whether COL5A1 rs12722 polymorphism was associated with sports-related muscle injury. Study 2 was a part of the Japanese Human Athlome Project (J-HAP) in “Athlome Project Consortium” (21).
Study 1
A total of 363 healthy young adults (men, n = 231; women, n = 132) participated in Study 1. None of the participants had apparent neurological, orthopedic, or neuromuscular problems. None of the participants reported muscle soreness or fatigue in the lower limbs at the time of testing. Written consent was obtained from each participant. The procedure was approved by the ethics committee of the Juntendo University and National Institute of Fitness and Sports in Kanoya, and performed in accordance with the Declaration of Helsinki.
The PSLR (right leg) test, SR test, and passive muscle stiffness measurements were performed on each participant in a randomized order. Room temperature for all measurements was kept at 24 ± 2 °C to minimize potential effects of temperature-induced changes in tissue mechanical properties and participants’ sensation to muscle stretch. Prior to the measurements the participants were not allowed to warm-up or stretch. The procedures have been described in detail previously (14). Briefly, in the PSLR test, participants lay supine with their legs straight on an examining bed. The pelvis and non-testing (left) leg were secured to the bed. The right hip joint was passively flexed, with the knee straight, by an examiner until each participant felt pain in the hamstring. The PSLR ROM (i.e., hip flexion angle from the resting position) was measured using a digital inclinometer (MLT-100, Sakai Medical, Japan) attached to the right shank. In the SR test, participants sat on the floor with their head, back, and hip against a wall, knee fully extended, and soles of the foot positioned flat against an SR box (T.K.K.5111, Takei Scientific Instruments, Japan). They were then asked to bend forward slowly and reach forward as far as possible while keeping the knees extended and slid their hands along a digital measuring scale which was placed on the box to measure SR ROM.
In accordance with a previous study, passive shear modulus (a measure of stiffness, expressed in kPa) of the biceps femoris long head (BF), semitendinosus (ST), and semimembranosus (SM) of the right leg were measured using an ultrasound shear wave elastography scanner (Aixplorer, Supersonic Imagine, France). During the measurement of passive muscle stiffness, participants sat on a bench with their hip flexed 70° and the right knee fully extended. This hip joint angle was chosen based on a recent study (14), which aimed to define an angle where the hamstring could be stretched to a tensioned state without pain for all participants and the shear modulus could be quantified at a given joint angle in all participants. An ultrasound linear probe was positioned at 50% level of the thigh length (the distance between the greater trochanter and the lateral epicondyle of the femur). For each muscle, the probe orientation was adjusted to visualize the fascicles within the B-mode image. Care was taken not to press and deform the muscles while scanning. The subjects were instructed to fully relax the leg throughout the measurements. The images were acquired after ensuring a stable color distribution of shear modulus mapping for a few seconds. The measurements were performed three times for each muscle. To evaluate the stiffness of the overall hamstring, the shear modulus of three muscles were averaged. All measurements and analyses of the elastographic data were performed by experienced examiners (>3 years of practice). For each variable (i.e., SR test, PSLR test, and passive muscle stiffness), the average values of three measurements were used for subsequent analyses. All participants then completed a questionnaire that included information on regular stretching of the hamstring.
Study 2
Participants of study 2 were Japanese athletes of various sports (not limited to a specific discipline), recruited from March 2015 to November 2017. A total of 2181 participants were recruited. Written consent was obtained from each participant. The procedure was approved by the Ethics Committees of Juntendo University, Nippon Sport Science University, Tenri University, and the National Institute of Fitness and Sports in Kanoya, and performed in accordance with the Declaration of Helsinki.
In J-HAP, the history of sports-related injuries was assessed by questionnaire (8). In the present paper, we focused on non-contact muscle injury diagnosed by medical practitioners. The number of days that elapsed from the date of injury to the date of the athlete’s return to usual training was asked to evaluate the severity of injury via the questionnaire. Injury severity was categorized into one of four levels based on the number of days until return: minimal (1-3 days), mild (4-7 days), moderate (8-28 days), and severe (>28 days) (22). If a participant had a history of multiple muscle injuries, data on the most severe injury was used. In addition, precise information on the primary sport, playing years, and competition level was obtained using the questionnaire. Participants who had (1) no Japanese ancestry, (2) missing or invalid questionnaire data, or (3) less than 3 years playing experience in their primary sport were excluded. The final sample size in the analyses of history of muscle injury and severity of muscle injury were 1559 and 186, respectively.
Genotyping analysis
Total DNA was isolated from the saliva of all participants in both Study 1 and 2 with an Oragene® DNA Collection Kit (DNA Genotek, ON, Canada) and quantified using a NanoDrop 8000 UV-Vis Spectrophotometer (Thermo Fisher Scientific, DE, USA) or Eppendorf Bio Photometer Plus (Eppendorf, Tokyo, Japan). DNA samples were stored at 4 °C until use. The samples were analyzed for the rs12722 polymorphism in the 3′-UTR of COL5A1 using a TaqMan SNP Genotyping Assay (Assay ID: C____370252_20) and LightCycler® 480 System (Roche Molecular Systems, Mannheim, Germany) or StepOneTM Real-Time PCR System (Thermo Fisher Scientific). Five microliters of the genotyping mixture contained 2.5 μL TaqMan® GTXpressTM Master Mix (2×) or TaqMan® Universal Master Mix II (2×), 0.0625 μL TaqMan® SNP Genotyping Assay mix (40×), 1.4375 μL sterilized water, and 1 μL genomic DNA (10 ng/μL). Two to four negative controls were included on each plate. Genotypes were called based on TaqMan® assays results using LightCycler® 480 SW (version 1.5, Roche Molecular Systems) or StepOneTM software (version 2.3, Thermo Fisher Scientific). 380 randomly selected samples were genotyped in duplicate for the rs12722 polymorphism, and we confirmed that the genotyping results perfectly agreed between duplicates.
Statistical analysis
Data are expressed as mean ± standard deviation (SD). Statistical significance was set at P < 0.05. Statistical analyses were performed using JMP Pro version 12 (SAS Institute, USA). The Hardy-Weinberg equilibrium of the rs12722 polymorphism was assessed using χ2 test. For the data of study 1, comparisons between two groups (sex, regular stretching habit, genotype [T-dominant and T-recessive models]) were conducted using χ2 test or unpaired Student’s t-test as appropriate. For comparisons under an additive model, the Cochran-Amitage trend test or Spearman correlation test was used as appropriate. Additionally, in order to examine whether the genotypes are associated with the phenotype variables independently of sex and regular stretching, analysis of covariance (ANCOVA) and multiple regression analysis were employed for T-dominant and T-recessive models and for an additive model, respectively. For the data of study 2, comparisons between the muscle injury and no muscle injury groups were conducted by χ2 test or unpaired Student’s t-test. Logistic regression analysis was applied to investigate the associations between rs12722 polymorphism and history of muscle injury with adjustment for playing years and main sport (track & field or the others). Odds ratios (OR) and 95% confidence intervals (CI) were calculated under the T-dominant genetic model. Association of the rs12722 polymorphism with severity of muscle injury was assessed by ordinal regression analysis. When the sample size of the TT genotype of the rs12722 polymorphism was less than five, statistical analyses were not conducted for T-recessive and additive models. Using the data of our previous study (8), we calculated the necessary samples size to detect the expected difference in passive muscle stiffness between the T allele carriers and CC genotype carriers (α = 0.05, power = 0.8, difference in group means = 3.5, within group SD = 7.4, and the ratio of CC genotype carriers to T allele carriers = 2.3) and to detect the association between the T allele of the rs12722 polymorphism and a history of muscle injury with an OR of 2.0 (α = 0.05, power = 0.8, probability of T allele carriers = 0.317, and the ratio of control to case subjects = 8.9). The critical sample sizes were estimated to be 168 and 733, respectively.