Association between low back pain and limited lower-extremity flexibility in elementary school-aged softball players: a cross-sectional study

Abstract

Background

Limited lower-extremity flexibility has long been debated as a physical risk factor contributing to low back pain in various populations. However, its impact on younger populations, particularly elementary school-aged softball players, remains unclear. This study aimed to explore the relationship between limited lower-extremity flexibility (iliopsoas, hamstrings, and quadriceps femoris) and low back pain in elementary school-aged softball players.

Methods

This cross-sectional study included 521 elementary school-aged softball players (469 males and 52 females; mean age, 10.7 years) who participated in annual medical evaluations from 2017 to 2019. Lower-extremity flexibility was assessed once during annual medical evaluations using the Thomas test for the iliopsoas, straight-leg-raise test for the hamstrings, and heel-to-buttock-distance test for the quadriceps. Additionally, low back pain during the previous season was evaluated through self-administered questionnaires distributed to participants and collected during medical check-ups. The prevalence of low back pain and its association with lower-extremity flexibility were investigated using univariate and multivariable analyses. In multivariable analysis, age, position, body mass index, and total practice time per week were included as explanatory factors.

Results

Overall, 16.9% (88/521) of the players experienced low back pain that required rest during the previous year. After adjustment for confounding factors (age, position, body mass index, and total practice time per week) with a logistic regression model, low back pain showed a significant association with quadriceps muscle tightness (odds ratio: 3.15, 95% confidence interval: 1.51–6.56, P = .002).

Conclusion

Quadriceps femoris tightness may be a significant risk factor for low back pain in elementary school-aged softball players. These findings underscore the importance of assessing quadriceps femoris tightness to prevent low back pain in elementary school-aged softball players.

Trial registrations

Not applicable.

Introduction

Low back pain (LBP) is a prevalent cause of activity limitations in individuals of all ages, including children actively involved in sports [1]. A recent systematic review with meta-analysis reported a pooled 1-year prevalence estimate of LBP among adolescent athletes ranging from 29 to 55% [2]. Notably, the occurrence of LBP during childhood is associated with an increased risk of chronic LBP in adulthood, highlighting the importance of early preventive research [2,3,4].

Several risk factors for LBP in young athletes have been identified, including older adolescent age, female sex, higher body mass index, increased sport volume/intensity, family history of LBP, and psychological distress [1, 4,5,6]. Among physical risk factors, limited lower-extremity flexibility, characterized by tightness in muscles such as the iliopsoas, hamstring, and quadriceps, has been extensively discussed as a contributor to LBP [7,8,9]. Limited flexibility often develops during school-age years owing to rapid bone growth, which creates excessive tension in the muscles and tendons of the lower extremities, subsequently reducing flexibility [10,11,12]. For example, tightness in the iliopsoas or quadriceps muscles can lead to biomechanical alterations such as anterior pelvic tilt and lumbar hyperlordosis, which increase stress on the posterior elements of the lumbar spine [7, 12]. Despite these findings, most previous studies have primarily focused on middle-to-late adolescent athletes, leaving the relationship between lower-extremity flexibility and LBP in younger populations poorly understood.

Baseball and softball are among the most popular sports for children in Japan and are characterized by unique biomechanical demands, such as pitching and batting, involving complex trunk rotational movements that may predispose players to LBP [13,14,15]. The prevalence of LBP in young baseball players has been reported to range from 3 to 35%, with an increasing prevalence observed as age advances [7]. Among softball players, injuries predominantly occur in the upper and lower extremities (30–35%), whereas injuries to the trunk including the lumbar region are less common (5%) [16]. While findings from baseball-related studies can provide valuable insights due to biomechanical similarities, epidemiological data on the prevalence and risk factors of LBP in young softball players are lacking [13]. For instance, lumbar spondylolysis, a common cause of LBP in adolescent athletes, has been shown to vary in prevalence depending on the sport involved. A retrospective case series of 1,025 non-elite adolescent athletes with various types of sports revealed that baseball had the highest risk of lumbar spondylolysis among males, while softball had the third highest risk among females [15]. Additionally, in high-school female fast-pitch softball pitchers, increased pitch volume has been associated with higher rates of reported LBP, suggesting that repetitive motion and high pitch counts may contribute to the development of LBP in softball players [17]. Similarly, in an investigation of injuries to collegiate female windmill softball pitchers, 17% of chronic/overuse injuries caused by repetitive pitching motions were to the lower back [14]. However, limited data are available on the prevalence and risk factors of LBP in elementary school-aged softball players, particularly in male athletes.

Therefore, the present cross-sectional study aimed to explore the association between and LBP and limited lower-extremity flexibility in male and female elementary school-aged softball players aged 6 to 12 years. We hypothesized that limited flexibility of the iliopsoas, hamstrings, and quadriceps muscles is significantly associated with LBP in this population.

Methods

Study design

This study was a cross-sectional study evaluated the association between limited lower-extremity flexibility and LBP in elementary school-aged softball players. Data were collected through annual medical evaluations conducted between October and December from 2017 to 2019. Conforming to the Declaration of Helsinki's principles, this research received the endorsement of our institution's Research Ethics Committee (identification numbers 2063, 2064). Written informed consent from participants and their parents/guardians was obtained before their enrollment in the study.

Participants

A total of 555 players from 35 elementary school softball teams in Fukushima Prefecture, Japan, participated in the medical check-ups. After excluding three participants who regularly visited a hospital for lower-extremity problems (two participants with avulsion fracture in the anterior superior iliac spine and one participant with medial collateral ligament injury in the knee) and 31 participants with missing data, 521 players (469 males and 52 females; mean age, 10.7 years; 86 pitchers and 435 fielders; mean softball experience, 2.6 years) were included in the analysis (Fig. 1). Only data from the most recent year of participation were used for players who underwent multiple evaluations.

Fig. 1

Study flow chart

Medical check-ups

Medical check-ups included physical examinations and self-administered questionnaires. The questionnaires were written in Japanese hiragana letters, which were easily comprehensible to younger participants, and included sex, age, playing position, years of softball experience, and weekly total practice duration (in hours). The questionnaires were distributed to the participants before the day of the physical examination and were completed with parental assistance when necessary. Questionnaires were collected on the day of the physical examination. Players who assumed the role of pitchers were classified exclusively under the pitcher category regardless of their participation in other positions. Anthropometric data, including height and weight, were measured on the day of physical examination.

Lower-extremity flexibilities tests

Lower-extremity flexibility was evaluated using well-established methods of physical examination (Fig. 2). The measurements were performed between 9 am and 12 pm during the medical check-ups. The medical check-ups took place either in an indoor conference room or a gymnasium, and measurements of the lower extremities were conducted on both sides.

Fig. 2

Lower-Extremity Flexibility Assessment. [A Negative result in Thomas test; B Positive result in Thomas test; C Straight leg raise test; D Negative result in the Heel-to-buttock distance test; E Positive result in the Heel-to-buttock distance test]

Thomas test

The Thomas test was performed to assess iliopsoas tightness. Participants lie on their back on an examination table. An examiner passively flexed one hip, bringing the knee towards their chest in order to flatten the lumbar spine and stabilize the pelvis; a positive result indicated by opposite hip flexed and the knee lifted off the examination Table [7, 8]

Straight leg raise test

Straight-Leg-Raise (SLR) test was performed to assess hamstring tightness. Participants were instructed to lie supine on an examination table and to passively elevate a leg, with their hip and knee completely extended. The SLR angle was measured using a goniometer and was deemed to be limited if the elevation angle was < 70°, indicating hamstring tightness [7, 8]. The testing procedure applied in this study ensured accurate SLR angle measurement by providing suitable pelvic stabilization throughout the test, and the ankle was also immobilized in a relaxed position during the test to limit the influence of the gastrocnemius muscle [18].

Heel-to-buttock distance test

Heel-to-buttock distance test (HBT) was performed to assesses quadriceps tightness. Participants lay in the prone position, with their knee in a state of passive flexion; a positive result indicated quadriceps femoris tightness if the heel failed to make contact with the buttock [7, 10].

Data collection

Fifteen well-trained board-certified physiotherapists, who all attended two workshops prior to study commencement, carried out all procedures. The examiners administering the Thomas test, SLR test, and HBT were not informed about the participants' LBP status or the outcomes of other physical examinations. Additionally, all physical examinations were conducted in a standardized manner to reduce any potential measurement errors. In addition, the second or third author, both of whom are board-certified orthopedic surgeons, oversaw standardized physical examinations conducted in workshops and medical check-ups. The Thomas test was conducted in 2017 and 2019, whereas the SLR test and HBT were performed each year from 2017 to 2019. In 2018, the number of physiotherapists participating in annual medical evaluations was insufficient to include the Thomas test in the protocol.

Low back pain evaluation

Self-administered questionnaires were used to evaluate previous and seasonal episodes of LBP in the previous year. The assessment of previous LBP episodes was conducted using the following inquiry: “Have you ever felt pain in your lower back?” (0 = “No”; 1 = “Yes”). LBP episodes within the last year were assessed by asking the following question: “Have you experienced pain in your lower back within the previous year?” (0 = “Not at all”; 1 = “I felt low back pain and rested from practice for less than 1 week”; 2 = “I felt low back pain and rested from practice for 1 to 4 weeks”; 3 = “I felt low back pain and rested from practice for more than 4 weeks”). LBP during the season was defined as any answer from 1 to 3. LBP-related interference in playing softball was ascertained by asking the following question: “Have you felt pain or discomfort in your lower back while playing softball? Please select all applicable answers.” (1 = “With hitting”; 2 = “With throwing”; 3 = “With fielding”; 4 = “With running”). Pain during lumbar flexion or extension (spinal sign) was evaluated on the day of medical check-ups to assess the characteristics of current LBP. The participants were instructed to assume a relaxed stance with their feet positioned shoulder-width apart. From this position, the participants performed maximal lumbar spine flexion, followed by maximum lumbar spine extension with their legs held straight. If the participants reported localized pain between the costal margins and superior gluteal folds during the test, this was recorded as a positive indication of LBP. The examiners who conducted the spinal sign assessment on the day of medical check-ups were unaware of the results obtained from the assessments of lower-extremity flexibility.

Statistical analysis

The primary analysis consisted of subjects who had complete data. Descriptive statistics were computed to provide a summary of the participants' initial characteristics. Continuous data were presented as means with standard deviation (SD), whereas dichotomous or categorical data were expressed as proportions. The relationship between LBP and limited lower-extremity flexibility on each side was evaluated using the chi-squared test. Multivariable logistic regression analysis was conducted to investigate the association between limited lower-extremity flexibility and LBP. The following variables were analyzed as explanatory factors: age exceeding the median value of the study population, player position, total practice time per week surpassing the median value of the study population, body mass index ≥ 18 kg/m2, and limited lower-extremity flexibility on either side. Odds ratios (ORs) and 95% confidence intervals (CIs) were also calculated. Statistical analyses were performed using JMP software version 16.1.0 (SAS Institute, Cary, NC, USA). All tests were two-sided, and statistical significance was set at a P value < 0.05.

Results

A total of 521 players (86 pitchers and 435 fielders), with a mean age of 10.7 years and a standard deviation (SD) of 1.1 were included in the study. Table 1 summarizes the participants’ demographic data. Out of 521 participants, 469 (90.0%) were male. The participants had an average softball experience of 2.6 years (SD: 1.4) and an average total practice time of 11.2 h per week (SD: 4.3). The Thomas test was not performed in 2018 due to insufficient availability of trained physiotherapists during that year. As a result, the Thomas test was conducted in 2017 and 2019 (362 players), whereas the SLR test and HBT were performed each year from 2017 to 2019 (521 players).

Table 1 Summary of players’ data

Furthermore, 88 players (16.9%) experienced LBP in the previous year (Table 2). LBP with required rest occurring in 79 players (15.2%) in grade 1 (< 1 week of rest) and 9 players (1.7%) in grade 2 (1–4 weeks of rest). No players required rest for more than 4 weeks. LBP-related interference in playing softball was recorded while throwing in 29 (5.6%) players and was significantly more common among pitchers (14.0%, 12/86) than among fielders (3.9%, 17/435) (P = 0.0002).

Table 2 Characteristics of Low Back Pain

LBP was associated with iliopsoas tightness on both the throwing and non-throwing arm sides (P = 0.003 and P = 0.010, respectively; Table 3) and with quadriceps tightness on the non-throwing arm side (P = 0.013) in the univariate analysis. Further analyses were conducted by categorizing the players into pitchers and fielders. LBP was associated with quadriceps tightness on the non-throwing arm side in pitchers (P = 0.009) and with iliopsoas tightness in both the throwing and non-throwing arm sides in fielders (P = 0.003 and P = 0.024, respectively).

After adjustment for confounding factors such as age (≥ 11 years or not), players position, body mass index (≥ 18 kg/m²), and total practice time per week (≥ 10 h per week) with LBP using a logistic regression model, quadriceps tightness showed a significant association with LBP (OR: 3.15, 95%CI: 1.51–6.56, P = 0.002; Table 4).

Table 3 Univariate analysis of the association between low back pain and limited lower-extremities flexibility

Table 4 Multivariable analysis of the association between low back pain and limited lower-extremity flexibility

Discussion

The present study highlights a significant association between quadriceps femoris tightness and LBP in elementary school-aged softball players. Unlike previous studies that predominantly focused on middle-to-late adolescent athletes, our findings provide novel insights into a younger population, where flexibility interventions may have a greater preventive impact.

While various studies have examined the relationship between lower-extremity muscle tightness and LBP, research involving elementary school-aged children remains scarce. For instance, a cross-sectional study involving 130 athletic patients aged 8–17 years (mean age:14 years) with LBP reported that hamstring and quadriceps tightness significantly correlated with lumbar stress fractures [19]. Similarly, prospective cohort studies involving 335 baseball players aged 15–16 years and 69 soccer players aged 12–14 years identified hamstring tightness as a risk factor for LBP and symptomatic lumbar bone stress injuries [8, 20]. These studies, however, focused on older adolescent athletes with more advanced skeletal maturity. Our findings indicate that quadriceps femoris tightness significantly contributes to LBP, even in athletes aged 6–12 years, supporting the hypothesis that early biomechanical changes predispose young players to overuse injuries. Interestingly, the iliopsoas and hamstrings, which have been implicated in other studies, did not show a significant association with LBP in our cohort. This discrepancy may be attributed to differences in sport-specific demands or to the younger age of participants. Aligning with our findings, previous study conducted on 1215 young baseball players aged 6–16 years revealed significant associations between LBP and quadriceps femoris tightness only in players aged 11–14 years [7]. Because the participants in the present study were of a similar age group, with a mean age of 10.7 years (SD: 1.1). We believe that quadriceps femoris tightness should be noted in this age group.

The impact of quadriceps femoris tightness on the musculoskeletal system of elementary school-aged softball players may be greater than that of iliopsoas and hamstring tightness. This study employed the HBT, which is commonly utilized for assessing knee joint range of motion and quadriceps femoris tightness, and excluded players who were regularly referred to a hospital for lower-extremity problems. Moreover, because age, body mass, and practice volume could influence both the muscle tone and LBP, these factors were therefore included in the multivariable analysis [7, 21]. Previous studies identified quadriceps tightness as a potential risk factor for musculoskeletal pain in areas such as the knees, elbows, and shoulders of juvenile athletes [22, 23]. Cross-sectional studies targeting young baseball players aged 7–14 years and football players aged 10–15 years reported an increase in quadriceps tightness with age, corresponding to skeletal maturation of the tibial tuberosity, whereas hamstring tightness remained stable or decreased [23, 24]. A significant association between quadriceps tightness and LBP had also been highlighted in baseball players aged 11–14 years, which was not observed in the case of iliopsoas and hamstring tightness [7]. As indicated by biomechanical studies, the performance of windmill softball pitchers is strongly related to the angular extension velocity of the knee in the drive leg (throwing arm side) [25] and the resistant force on the ground in the stride leg (nonthrowing arm side) during pitching motion [26]. Consequently, overuse injuries to the knee have been reported among windmill pitchers [14]. Repetitive windmill pitching may contribute to increased stress on the quadriceps femoris muscle on both sides, resulting in tightness.

Quadriceps tightness can elicit an anterior pelvic tilt, leading to lumbar lordosis and increased posterior force on the lumbar spine elements. This can cause an excessive mechanical stress on the pars interarticularis, thereby increasing the risk of LBP associated with lumbar spondylolysis, which is more prevalent among young softball players [15]. Additionally, mechanical or muscular LBP, commonly referred to as posterior overuse syndrome or hyperlordotic LBP, may be a prevalent cause of LBP in this particular population [27, 28]. In our study, the incidence of LBP upon lumbar extension was three times higher than that of LBP upon lumbar flexion on check-up evaluations, indicating that increased force on the posterior lumbar spine elements contributes to the occurrence of LBP [27, 28]. However, within the context of our cross-sectional study, we were unable to establish a causal relationship between quadriceps femoris tightness and LBP. LBP plausibly restricts lower-extremity flexibility owing to reduced physical activity levels.

Strengths and limitations

Unlike previous studies that predominantly focused on middle-to-late female softball players, this study provides novel insights by focusing on elementary school-aged softball players, including male athletes. To the best of our knowledge, the present study is the first to investigate the association between lower-extremity tightness and LBP in male and female softball players in this age group. Standardized examination protocols conducted by board-certified physiotherapists further strengthen the reliability of our findings.

However, several limitations should be acknowledged. First, the reliance on self-reported data from participants aged 6–12 years, which might have been influenced by varying degrees of parental involvement, might have introduced imprecision in data collection, including in the assessment of the total amount of weekly practice, potentially leading to recall bias. Second, despite accounting for the hip and knee range of motion, the measured values of quadriceps femoris tightness using the HBT might have been influenced by the muscle tone in the gluteal area and the knee joint range of motion. To address this potential issue, we excluded participants who regularly sought medical treatment for lower-extremity problems. Third, imaging analyses were not conducted, leaving uncertainty regarding the cause of LBP pathology. Fourth, our study focused exclusively on elementary school-aged softball players and included a limited number of female participants, thereby limiting the generalizability of our results to other sport activities, different age groups, and female athletes. Finally, the sample size was small, further limiting the generalizability of the findings.

Practical applications

Our findings underscore the importance of incorporating flexibility training programs into the routines of young athletes. Stretching exercises targeting the quadriceps femoris, combined with core strengthening and postural education, may help mitigate the risk of LBP. Early interventions are particularly crucial in elementary school-aged players, as their musculoskeletal systems are still developing and may respond more effectively to preventive measures.

Conclusions

Our epidemiological findings suggested the association between quadriceps tightness and LBP in elementary school-aged softball players. Players with quadriceps tightness may exhibit increased susceptibility to LBP owing to disruptions in the kinetic chain, as compared with those without such tightness. Our analysis has the potential to contribute to the provision of appropriate support for young athletes.