Sex-based differences in hamstrings stiffness assessment in football players using ultrasound shear wave elastography

Introduction

Hamstrings are the muscles most commonly affected by muscle injury in football players (1,2). They involve one or more of the muscle-tendon group of the posterior thigh: semimembranosus (SM), semitendinosus (ST) and/or biceps femoris (BF). The prevalence of hamstring injuries in football has been observed to increase by 2–3% annually, despite the diligent efforts of the medical team (3). Hamstrings play a critical role in sprinting, kicking, and deceleration, making their mechanical properties an important consideration in athletic performance and injury risk. Muscle stiffness is a key factor in athletic injury prevention and performance optimization (4). Stiffness plays a critical role in transmitting forces efficiently during muscle contractions and absorbing and dissipating forces during eccentric lengthening.

Several imaging techniques are utilized to study hamstrings in football players. Magnetic resonance imaging (MRI) and conventional ultrasound provide valuable insights into the muscle architecture of the hamstrings and help assess the severity and extent of injuries. MRI is considered the gold standard for assessing hamstring injuries (5,6). However, the non-invasive determination of hamstrings stiffness through ultrasound elastography has gained substantial interest in recent years. Shear wave elastography (SWE) is a quantitative ultrasound application that enables the estimation of hamstrings’ elasticity by calculating the speed of shear wave propagation (7). Previous studies have utilized SWE to assess the stiffness of hamstring muscles during various conditions, such as submaximal isometric contractions, passive stretching, and different contraction types and intensities (8,9). This technique has provided valuable insights into the stiffness distribution among the hamstring muscles, changes in muscle stiffness due to exercise, injury, and recovery, as well as the relationship between shear wave velocity (SWV) and hamstrings morphology (10).

Despite the growing interest in the biomechanical properties of skeletal muscles, there are not many published studies specifically focusing on sex-based differences in hamstring stiffness among athletes using SWE (11,12). A previous correlation has been found between high hamstring stiffness and injury in males, especially in those with a history of hamstring strain (13). This study aimed to address this gap by conducting a comprehensive analysis of hamstring stiffness in male and female football players. Therefore, the aim of the study was to investigate differences in SWV between male and female footballers, employing an observational, prospective design in both dominant and non-dominant legs. It was postulated that SWV in hamstrings would be lower in the female hamstrings muscles compared to male hamstrings muscles, but no between-dominant leg differences would be present. By considering these sex-based and leg dominance-based differences in hamstrings muscle stiffness, it is possible to tailor training and prevention strategies to the individual, potentially enhancing performance and reducing the risk of injury among football players. We present this article in accordance with the STROBE reporting checklist (available at https://qims.amegroups.com/article/view/10.21037/qims-24-398/rc).

Methods

Study design

This was a prospective study conducted in the Department of Radiodiagnosis and Imaging. The study was conducted in accordance with the Declaration of Helsinki (as revised in 2013). The study received approval from the institutional ethics committee Clinical Research Ethics Committee (approval No. 2021.148). Informed consent was procured from all patients. The clinical data gathered encompassed patient age, gender, and leg dominance. Leg dominance was established by asking each participant to identify their dominant leg, which is defined as the leg they would naturally use to perform tasks requiring unilateral leg motion, such as kicking a ball. This information was collected prior to the ultrasound examination and recorded for both the dominant and non-dominant legs as part of the study protocol.

Male and female athletes participating in the football club were recruited to participate in the study during annual medical check-up in June 2022. Subject recruitment and ultrasound examinations were performed at the same hospital. The inclusion criteria were age ≥18 years, referral to mandatory annual medical check-up for elite competitions and participants were free of any lower limb musculoskeletal injury in the 6 weeks prior to the ultrasound scan.

Exclusion criteria for the study included age ≥40 years, known prior hamstrings surgery and the presence of lumbar radiculopathy or other central or peripheral neurologic deficits of the lower limb.

Imaging evaluation

US evaluation of hamstrings muscles

Ultrasound was performed using a commercial ultrasound system (LOGIQ E10 XDclear, GE Healthcare, Milwaukee, WI, USA), a linear 4–20 MHz transducer is required. In this study SWE elastography (imaging of tissue property-stiffness) was added to the B-mode imaging (imaging of acoustic impedance differences) which offers a more accurate ultrasound diagnosis for characterization hamstrings. All ultrasounds were performed by a single radiologist with 10 years of experience as a dedicated musculoskeletal radiologist.

B-mode ultrasound

Hamstrings muscles were evaluated with B-mode ultrasound to detect fiber disruption or unusual echogenicity areas that shown previous injury.

All players were examined in prone position with their feet hanging off the edge of the table. During the analysis, the hamstring muscles were explored at mid-thigh, according to the ultrasound examination published by Balius that delineates the areas of interest of the hamstrings (14). In the middle third of the posterior compartment of the thigh, the SM and ST muscles are located medially, where the muscle mass of the SM is significantly greater than that of the ST. On the other hand, in the lateral part of the middle third of the posterior compartment of the thigh, the BF is located, using the sciatic as a reference point.

Ultrasound based on SWE

Hamstring SWV was assessed using an ultrasound scanner (LOGIQ S8 XDclear, GE Healthcare) in SWE mode (musculoskeletal preset, penetrate mode, persistence off), coupled with a linear multifrequency probe (9–15 MHz, GE Healthcare).

Players were positioned prone on the table maintaining both their knees and hips in a neutral alignment (0° of flexion). The probe was positioned on the hamstrings in accordance with the previously explained Balius ultrasound protocol examination, detecting the ST, SM and BF in the middle third of the posterior thigh compartment. Then the probe was set to acquire a longitudinal view of each muscle (10). Imaging in this plane correlates strongly with muscle biomechanical properties and is less sensitive to the inherent viscoelastic nature of the soft tissue (15). The same operator, musculoskeletal radiologist with over 10 years’ experience took several mesures to ensure the reliability and repeatability of our results. The operator was trained to apply consistent probe pressure, and the participants were instructed to maintain a relaxed state to minimize muscle activity during the measurements.

The SWV [in meters per second (m/s)] was assessed in each hamstrings muscle using dedicated SWE software from GE Healthcare on a GE Logiq E10. Initial color maps were selected to confirm that the SWV was within the range of detectable velocities, previously determined to be 0.0–12 m/s as indicated by the manufacturer. Subsequently, SWV was measured in meters per second by placing a circular region of interest (ROI) of 5 mm in diameter at the point of interest, in a shadow-free region and confirming that the ROI had color-fill in each hamstrings muscle for the ST, SM (Figure 1) and the BF (Figure 2). SWV was ascertained for each leg of each muscle. Each muscle (ST, SM, and BF) was measured three times consecutively in a single session. The data analysts who processed and interpreted the SWE data were blinded to the sex and leg dominance information.

Figure 1 Representative B-mode and SWE image of a posterior compartment of the medial thigh of a male football player. (A) Quality map is applied by the user to prevent areas with low measurement, white as maximum quality. (B) SWE image (color elastogram) of the same region shows predominatly low SWV on semimembranosus (1) and semitendinosus (2) (1.87, 1.79 m/sec, respectively). Red: hard consistency (7.1 m/sec), blue: soft consistency (0.0 m/sec), and green and yellow = intermediate consistency. SWE data were collected using an Logiq S8 R4 (General Electric, United States) with a L9–15-MHz linear transducer. SWE, shear wave elastography; SWV, shear wave velocity.

Figure 2 Representative B-mode and SWE image of a posterior compartment of the medial thigh of a male football player. (A) Quality map is applied by the user to prevent areas with low measurement, white as maximum quality. (B) SWE image (color elastogram) of the same region shows predominatly low SWV on biceps femoris (1) (1.36 m/sec). Red: hard consistency (7.1 m/sec), blue: soft consistency (0.0 m/sec), and green and yellow: intermediate consistency. SWE data were collected using an Logiq S8 R4 (General Electric, United States) with a L9–15-MHz linear transducer. SWE, shear wave elastography; SWV, shear wave velocity.

Data analysis

Data analysis was performed using the IBM SPSS Statistics 22.0 (IBM Corporation, New York, USA) Descriptive statistics (means and standard deviations) were calculated for each sex, considering both dominant and non-dominant legs across the three hamstring muscles. Each dependent variable was screened for normality using Shapiro-Wilk tests. Given the wide range of shear-wave value (m/s), sex differences were examine using independent t-tests or Mann-Whitney tests for each muscle.

In our study, we indeed measured the stiffness of the hamstring muscles (ST, SM, and BF), using SWE and analyzed the differences based on sex (male vs. female) and leg dominance (dominant vs. non-dominant) using t-test. Post hoc analysis was performed with the Bonferroni test. Significance was set at P<0.05. Cohen’s d was calculated to measure of effect size for the t-tests conducted. Effect sizes were interpreted as follows: small effect size (0.2); medium effect size (0.5) and large effect size (0.8).

Results

Subjects. Of a total of 32 potential patients according to selection criteria, 2 refused to participate. A total of 30 players, 15 females and 15 males participated in the study. All of these 30 football players participated consistently from the beginning to the end of the study. Demographics for male and female football players are in Table 1.

Independent t-tests were used to compare the SWV values of the hamstring muscles between male and female players, indicated a statistically significant difference in SWV for the non-dominant ST muscle between male and female players (P=0.02) (Table 2). Other comparisons across different muscles and sides did not reveal significant differences (P values ranged from 0.051 to >0.99). For the ST muscle, it found that on the dominant side, males had a mean speed of 3.8±1.5 m/s, while females had a mean speed of 2.9±0.7 m/s. However, the P value of 0.112 indicated that this difference was not statistically significant. On the non-dominant side, males had a mean speed of 4.1±1.2 m/s, while females had a mean speed of 2.8±0.7 m/s. The P value of 0.021 suggested a statistically significant difference. Cohen’s d for the ST muscle is approximately 0.77 for the dominant leg and 1.33 for the non-dominant leg. These values suggest a moderate to large effect size respectively, indicating a significant difference in stiffness between male and female football players in these contexts.

In the SM muscle, the dominant side showed males with a mean speed of 4.0±1.7 m/s and females with a mean speed of 3.0±1.0 m/s (P=0.072). On the non-dominant side, males had a mean speed of 4.1±1.4 m/s, while females had a mean speed of 3.3±1.0 m/s (P=0.187). Therefore, Cohen’s d for the SM muscle is approximately 0.717 for the dominant leg and 0.657 for the non-dominant leg. These values suggest a moderate effect size, indicating a noticeable difference in stiffness between the male and female football players for the SM muscle in both the dominant and non-dominant legs.

For the BF muscle, as shown in Table 2, the dominant side showed males with a mean speed of 3.1±1.3 m/s and females with a mean speed of 3.8±1.1 m/s (P=0.119). On the non-dominant side, males had a mean speed of 3.6±2.3 m/s, while females had a mean speed of 2.5±0.4 m/s (P=0.76). Cohen’s d for the BF muscle is approximately -0.58 for the dominant leg and 0.67 for the non-dominant leg. These values suggest a moderate negative effect size for the dominant leg (indicating females have higher stiffness) and a moderate positive effect size for the non-dominant leg (indicating males have higher stiffness).

Figure 3 illustrates the distribution of SWV measurements for the ST, SM, and BF muscles, highlighting the differences between male and female players across dominant and non-dominant legs. This figure provides a visual summary of the data trends and variability.

Figure 3 The box-plot represents the distribution of data for each hamstring muscle dominant and non-dominant for both males and females. The central line in the box represents the median of the data, while the box itself spans from Q1 to Q3. The whiskers extend to the minimum and maximum data points within 1.5 times the IQR. Data points outside this range are considered outliers and are represented as individual points. (A) Box-plot represents the distribution of data for semitendinosus dominant and non-dominant for both males and females. (B) Box-plot represents the distribution of data for semimembranosus dominant and non-dominant for both males and females. (C) Box-plot represents the distribution of data for bíceps femoris dominant and non-dominant for both males and females. Q1, the first quartile; Q3, the third quartile; IQR, interquartile range.

The post-hoc Bonferroni study showed statistically significant differences for SWE in the ST among the different subgroups in both females and males with the non-dominant leg (P=0.02; Bonferroni corrected significance level P=0.05) (Table 3).

Discussion

The purpose of this study was to identify sex-based differences in hamstrings stiffness (ST, SM and BF) in football players using shear wave ultrasound. The results of the study indicate significant differences in ST stiffness between male and female football players, as well as differences based on dominance (P value <0.05). The study utilized shear wave ultrasound to assess muscle stiffness, providing valuable insights into the biomechanics of the hamstrings in athletes.

The statement previous postulated that all SWV values for males are greater than females is not accurate. While this is true for the ST and SM muscles, it is not the case for the BF muscle. In our study, the dominant leg, the mean value for females (3.8) is greater than that for males (3.1). The mean stiffness values for the BF muscle in the dominant and non-dominant legs were greater in females compared to males. However, McPherson et al. demonstrated significantly greater hamstring stiffness for males than females at various leg positions in healthy male basketball players. Nevertheless, the stiffness of the BF muscle was notably different between the dominant and non-dominant limbs for females at the 40% position, with higher stiffness observed in the non-dominant limb (P<0.01), near the prone position, similar to our study (12).

Previous studies provide evidence of sex-based differences in ST biomechanical structure. Fournier et al. revealed sex differences in ST muscle fiber-type composition as illustrated by a faster phenotype and larger muscle size in men as compared with women (16). Additionally, the study by Yu et al. found that there were differences in muscle stiffness between males and females, with females outperforming males in terms of flexibility. The muscle stiffness of the hamstring muscles was larger in males than in females at all time points (P<0.001). These findings collectively suggest that there are indeed sex-based differences in ST stiffness, influenced by muscle fiber-type composition and functional performance (17).

The study’s results align with previous research on muscle stiffness assessment using ultrasound SWE (18-20). Römer et al. have demonstrated the application of shear wave ultrasound in assessing muscle stiffness in adolescent football players, highlighting its potential as a diagnostic tool for detecting early lateral imbalances and contributing to the prevention of acute and chronic injuries in young athletes (21). Moreover, the study by Medes et al. aimed to evaluate the stiffness of hamstring muscles during isometric contractions in healthy individuals, utilizing ultrasound-based SWE. They found that both biceps femoris long head (BFlh) and ST shear moduli increased nonlinearly with contraction intensity. Notably, the ST demonstrated the highest stiffness index for most contraction intensities (P<0.02). This finding is particularly interesting as it suggests that the ST may be more susceptible to strain and potential injury during intense physical activity, such as football (9). Furthermore, research by Crawford et al. has investigated differences in ultrasound shear wave speed (SWS) between uninjured and injured limbs following hamstring strain injuries, SWS was lower in the injured limb compared to the contralateral limb at time of injury [uninjured–injured limb difference: 0.23 (0.05, 0.41) m/s, P=0.006] offering insights into the potential use of shear wave ultrasound for monitoring muscle injury and detecting structural changes associated with muscle strain injuries (10).

Overall, the findings of this study shed light on the sex-based differences in hamstrings stiffness assessment in football players, emphasizing the importance of considering sex and dominance when evaluating musculoskeletal properties in athletes. According these findings, El-Ashker et al. have discerned sex-related differences in joint-angle-specific functional ratios of the hamstrings and quadriceps muscles. The research findings indicate significant differences in muscle strength and flexibility between non-dominant leg in female football players (22). Additionally, research findings indicate that there are indeed significant differences related to non-dominant leg in hamstring injuries in male football players. Studies have shown that a significantly greater extent of structural hamstring muscle injuries was found in the dominant leg compared with the non-dominant leg. In fact, football players demonstrated a 1.6 times greater risk of injury to the dominant limb [95% confidence interval (CI): 1.3–1.8], especially in hamstring muscle (RR 1.3; 95% CI: 1.1–1.4) (23,24).

The current study has several limitations. First, the sample size was small (30 players), a larger and more diverse sample could provide a more comprehensive understanding of sex-based differences in hamstring stiffness among football players, potentially leading to more robust and generalizable conclusions. Nevertheless, the sample size was limited due to our specific focus on football players who participate in international competition. Second, the exact degree of stiffness remains uncertain without a histological correlation serving as a standard of reference for the hamstrings. Third, other factors such as muscle geometry, fiber orientation, and intramuscular pressure may influence SWV. To reduce the potential impact of these variables on the measurements, it meticulously selected the probe angle to afford a longitudinal view, which is less susceptible to the inherent viscoelastic characteristics of the soft tissue. Fourth, concerning the absence of cut-off values for SWE measures of stiffness in hamstrings muscles, the outcomes of this study appear to be consistent with studies previously published (9). Fifth, the study should exclusive focus on the mid-third of the thigh, neglecting the proximal and distal portions of the hamstrings. While this focus was intentional, given that the mid-third of the thigh is the most frequently injured location in football players, it does limit the comprehensiveness of our findings.

Conclusions

This study revealed significant sex-based differences in hamstring stiffness among football players, as measured by SWE. Notably, the ST muscle showed significant SWE differences, particularly in the non-dominant leg of both sexes. However, no statistical differences were found in the SM and biceps muscles. These findings underscore the potential of SWE as a non-invasive tool for assessing muscle stiffness, which could inform injury prevention strategies. Future research should explore the implications of these differences on injury risk and performance.

Acknowledgments

Funding: None.

Footnote

Provenance and Peer Review: With the arrangement by the Guest Editors and the editorial office, this article has been reviewed by external peers.

Reporting Checklist: The authors have completed the STROBE reporting checklist. Available at: https://qims.amegroups.com/article/view/10.21037/qims-24-398/rc

Conflicts of Interest: Both authors have completed the ICMJE uniform disclosure form (available at https://qims.amegroups.com/article/view/10.21037/qims-24-398/coif). The special issue “Advances in Diagnostic Musculoskeletal Imaging and Image-guided Therapy” was commissioned by the editorial office without any funding or sponsorship. The authors have no other conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. The study was conducted in accordance with the Declaration of Helsinki (as revised in 2013). The study received approval from the institutional ethics committee Clinical Research Ethics Committee (approval No. 2021.148). Informed consent was procured from all patients.

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.

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