This study was conducted between September and October 2015, during the off-season for elite youth baseball players in Korea. A total of 271 youth elite baseball players were included, excluding players who were in poor condition at the time of measurement, those undergoing surgery or rehabilitation due to musculoskeletal or neurological disorders, and those who voluntarily declared their withdrawal from the experiment. Additionally, none of the 271 players who agreed to participate in the experiment declared their intention to withdraw during the survey or muscle flexibility measurements. Their ages are between the ages of 8 and 16 (age 12.5 ± 1.9 years; height 157.6 ± 12.8 cm; weight 55.1 ± 14.7 kg; baseball career 3.2 ± 1.9 years; bat weight 25.3 ± 3.5 oz; bat length 31.5 ± 1.5 inches). Participants were divided into two groups based on the occurrence or absence of lifetime LBP (non-LBP group and LBP group). The non-LBP (NLBP) group consisted of 113 players (age 11.5 ± 1.8 years; height 151.4 ± 12.3 cm; weight 49.6 ± 13.8 kg; baseball career 2.6 ± 1.7 years; bat weight 23.8 ± 3.4 oz; bat length 30.8 ± 1.4 inches), while the LBP group consisted of 158 players (age 13.2 ± 1.6 years; height 162.1 ± 11.3 cm; weight 59.1 ± 14.0 kg; baseball career 3.7 ± 1.8 years; bat weight 26.3 ± 3.2 oz; bat length 32.0 ± 1.4 inches) <Table 1>. Since all subjects were minors, permission was obtained from coaches, parents, and elite youth baseball players. The study was authorized by the university’s Institutional Review Board (approval number: 28-64).

The questionnaire consisted of three sections. The first section included 11 items related to basic information (age: date of birth, grade, height, weight, baseball career, another career, main position, pitching side, batting side, bat weight, bat length). The second section included 22 items concerning training and practice situations (weight training: yes or no, time, days; core training: yes or no, time, days; defense practice time and days; batting practice time, days, and count; pitching practice time, days, and count; team practice time, individual practice time, total practice time, total practice days; total rest days; heavy bat swing practice, long bat swing practice; stretching: always or almost always, sometimes, or never). The third section consisted of 13 questions about past and current low back pain and other injuries (current LBP, lifetime LBP, degree of LBP, duration of LBP, frequency of recurrent LBP, absence from practice: yes or no, duration of absence; hospital treatment due to LBP, other injury or pain sites: shoulder, elbow, knee, ankle, foot). All participants who agreed to participate in the study completed and submitted the questionnaire.

Muscle flexibility was measured using passive methods (except for trunk rotation, which was measured actively) and photographed with a digital camera. The range of motion (ROM) and distance were calculated using ImageJ software (National Institutes of Health, Bethesda, MD, USA), which analyzes the images based on the secured photos. Furthermore, the data were analyzed by classifying them into the dominant side and the non-dominant side. Six measurements were performed, which included Fingertip to Floor Distance (FFD), Thomas Test, Hip Extension, Heel Buttock Distance (HBD), Straight Leg Raising (SLR), and Trunk Rotation. The detailed measurement methods are described below and shown in <Figure 1>.

(a) FFD: The FFD was calibrated by suspending a 1-meter plumb line from the ceiling and measuring the distance between the third fingertip and the top of the stand.

(b) Thomas Test: The angle of hip flexion was measured using a line parallel to the bed and the long axis of the femur as reference points.

(c) Hip Extension: The angle between a line parallel to the bed and the long axis of the femur was measured.

(d) HBD: Calibration was performed by attaching a 1-meter indicator to a vertical line from the ceiling. The measurement was taken by aligning the maximum protrusion of the heel and buttocks in a straight line.

(e) SLR: The angle between a line parallel to the bed and the long axis of the femur was measured.

(f) Trunk Rotation: The trunk rotation angle was calculated as the angle between the shoulder line, which was connected to two markers attached to landmarks on both the acromion and the horizontal line.

Low back pain was defined as an injury that resulted in a restriction or cessation of participation in baseball activities (practice and games) for more than one day due to an incident that occurred during baseball activities [13,14].

The items in the questionnaire that were left unanswered and the items that could not be accurately measured during the muscle flexibility assessment were treated as missing values, and statistical analysis was conducted thereafter. Variables were calculated using SPSS 26.0 (IBM SPSS Statistics for Windows, IBM Corp., Armonk, NY, USA). Independent t-tests (for continuous variables) and Pearson’s chi-square tests (for categorical variables) were conducted to compare differences in training volume, exercise volume, and physical characteristics between groups with and without low back pain. In all cases, the significance level was set at p < 0.05.

A total of 271 subjects were classified into the LBP group or the NLBP group according to the presence or absence of a lifetime history of LBP. There were 158 (58.3%) in LBP group and 113 (41.7%) in NLBP group. Compared to the NLBP group, the LBP group exhibited significantly higher age, height, weight, baseball career, bat weight, and bat length (all p < 0.001). However, there was no statistically significant difference in BMI between the two groups (p = 0.103) <Table 1>. The LBP group consisted of 103 middle school (MS) players and 55 elementary school (ES) players, while the NLBP group included 33 MS participants and 80 ES participants (p < 0.001).

The LBP group comprised 108 fielders and 45 pitchers, whereas the NLBP group consisted of 92 fielders and 18 pitchers (p = 0.014). There was no statistically significant difference in another career among two groups (p = 0.890).

In the NLBP group, 43.4% (n = 49) of players engaged in weight training, with an average training time of 22.5 minutes per day and 1.4 days per week. In contrast, 58.2% (n = 92) (p = 0.016) of players in the LBP group performed weight training, with an average time of 30.6 minutes per day (p = 0.030) and 2.0 days per week (p = 0.018). Regarding core training, 65.4% (n = 74) of the NLBP group participated, with an average time of 16.5 minutes per day, compared to 81% (n = 128) (p = 0.004) participation and 20.6 minutes per day (p = 0.022) in the LBP group. Additionally, defense practice days per week were slightly higher in the NLBP group (6.0 days) compared to the LBP group (5.8 days) (p = 0.008). However, the LBP group reported significantly higher batting counts (352.4 swings/day) compared to the NLBP group (261.4 swings/day) (p = 0.001). Pitching practice times per day were also longer in the LBP group (89.4 minutes) than in the NLBP group (76.0 minutes) (p = 0.048). Team practice times per day were 6.5 hours in the LBP group and 5.8 hours in the NLBP group (p < 0.001). Rest days were slightly fewer in the NLBP group (0.5 days) compared to the LBP group (0.7 days) <Table 1, 2>.

The LBP group showed significantly higher HBD NDS and DS (p < 0.001) compared to the NLBP group. Additionally, the SLR NDS angle (p = 0.034) of the LBP group was significantly lower than that of the NLBP group. On the other hand, no significant differences were observed between the two groups in FFD (p = 0.686), Thomas test NDS (p = 0.484), Thomas test DS (p = 0.201), Hip Extension NDS (p = 0.097), Hip Extension DS (p = 0.128), SLR DS (p = 0.315), Trunk Rotation NDS (p = 0.419), and Trunk Rotation DS (p = 0.192) <Table 1>.

The LBP group had a significantly higher prevalence of shoulder pain (n = 45, 28.5%) compared to the NLBP group (n = 19, 16.8%). Players with low back pain were 1.970 times more likely to experience shoulder pain (OR = 1.970, 95% CI = 3.597–0.576; p = 0.026). Similarly, elbow pain was reported in 70 players (44.3%) in the LBP group, significantly higher than the 31 players (27.4%) in the NLBP group. Players with low back pain were 2.104 times more likely to develop elbow pain (OR = 2.104, 95% CI = 1.252–3.535; p = 0.005). In contrast, there were no significant differences between the two groups regarding knee pain (OR = 1.567, 95% CI = 0.848–2.893; p = 0.150), ankle pain (OR = 1.595, 95% CI = 0.816–3.118; p = 0.170), or foot pain (OR = 1.421, 95% CI = 0.656–3.080; p = 0.371) <Table 2>.

We divided the participants into the LBP group and the NLBP group based on their history of LBP, and compared the differences in basic information, training, and practice volume between the two groups. The LBP group consisted of 153 participants (58.3%), which was larger than the NLBP group with 113 participants (41.7%). The LBP group showed statistically significant higher values in age, height, weight, years of baseball experience, bat weight, and bat length compared to the NLBP group (all p < 0.001). Additionally, the LBP group included 103 middle school players, while the NLBP group included 33 middle school players (p < 0.001). These results suggest that the prevalence of LBP tends to increase with age.

The LBP group had a higher rate of engaging in weight training compared to the NLBP group (58.2% vs. 43.4%; p = 0.016), as well as a tendency for greater average daily duration (30.6 minutes vs. 22.5 minutes; p = 0.030) and weekly frequency (2.0 vs. 1.4; p = 0.018). Furthermore, the LBP group reported a higher rate of participation in core training than the NLBP group (81% vs. 65.4%; p = 0.004), with a greater average daily training time (20.6 minutes vs. 16.5 minutes; p = 0.022). Both weight training and, in particular, core training is commonly performed for the prevention and rehabilitation of low back pain, and it is well established that athletes need to strengthen their core muscles to prevent injuries [15]. According to Silfies et al., LBP is particularly prevalent among many pitchers, and trunk stability is crucial for alleviating it [15]. Additionally, Watanabe et al. reported that a reduction in core stability is a potential risk factor for high school baseball players [17]. These findings suggest that the higher rate of core training in the LBP group is likely due to its purpose of preventing and rehabilitating low back pain.

The LBP group had fewer days of defensive practice per week compared to the NLBP group (5.8 days vs. 6.0 days; p = 0.008), but they showed significantly higher values in daily swing counts (352.4 swings vs. 261.4 swings; p = 0.001), daily pitching practice time (89.4 minutes vs. 76.0 minutes; p = 0.048), daily team training time (6.5 hours vs. 5.8 hours; p < 0.001), and weekly rest days (0.7 days vs. 0.5 days; p = 0.39). These results may be subject to memory bias due to the questionnaire-based design, and while our findings do not specifically establish causality for the occurrence of LBP, they suggest that excessive practice time and inadequate recovery periods may be related to the onset of LBP in youth athletes. Previous studies have indicated that youth baseball players are more vulnerable to overuse injuries due to various factors, including skeletal immaturity and excessive training volume [18,19].

We compared the flexibility of the lower extremities and trunk based on the history of low back pain (LBP) among elite youth baseball players. Compared to the NLBP group, the LBP group showed higher values in both the non-dominant side (NDS) (3.3 cm vs. 6.4 cm) and dominant side (DS) (3.9 cm vs. 7.3 cm) (p < 0.001). These findings indicate reduced flexibility in the quadriceps, suggesting that quadriceps tension is related to LBP. According to a report by Camp et al. [20], increased tension in the anterior femoral flexors is likely to induce LBP. Additionally, while this study did not compare left and right sides, the flexibility of the hamstring on the dominant side (HBD) tended to be lower than that on the non-dominant side (NDS). Furthermore, compared to the NLBP group, the LBP group showed a lower straight leg raise (SLR) angle on the non-dominant side (78.4° vs. 76.0°) (p = 0.034). This indicates lower flexibility in the non-dominant side hamstring, suggesting that hamstring tension is related to LBP. Previous studies have shown that a reduction in hamstring flexibility and increased tension are strongly associated with LBP [21,22]. Victora & Vieira reported that a decrease in lower limb flexibility may lead to changes in pelvic and lumbar spine movement, which could be a major factor in causing LBP [23]. Therefore, to prevent low back pain in youth baseball players, managing the flexibility of the quadriceps and hamstrings is essential.

Players in the LBP group had an 11.7% higher prevalence of shoulder pain compared to those in the NLBP group (28.5% vs. 16.8%), and the likelihood of experiencing shoulder pain was 1.970 times greater in the LBP group than in the NLBP group (OR = 1.970, 95% CI = 3.597–0.576; p = 0.026). Furthermore, players in the LBP group had a 16.9% higher prevalence of elbow pain compared to those in the NLBP group (44.3% vs. 27.4%), and the likelihood of experiencing elbow pain was 2.104 times greater in the LBP group than in the NLBP group (OR = 2.104, 95% CI = 1.252–3.535; p = 0.005). Our results indicate that players with a history of LBP have a higher incidence of shoulder and elbow pain compared to those without LBP. According to a report by Sekiguchi et al., lower extremity and trunk injuries may lead to shoulder and elbow injuries [24], and LBP has been shown to be significantly associated with elbow and/or shoulder pain in youth baseball players [25]. These findings suggest that elbow and shoulder injuries may be important concerns for youth baseball players with LBP.

This study had several limitations. First, it was a retrospective study based on questionnaires, which may introduce recall bias. Second, LBP was not classified into non-specific and specific types for evaluation. Third, we did not perform a comparison of physical flexibility between the group with current LBP and the group without a history of LBP. Nevertheless, this study provides valuable information on the physical characteristics, practical training-related data, and injuries in other body regions of elite youth baseball players based on their history of LBP. As such, it offers significant evidence for future research on injury prevention.