Sex-dependent differences in single-leg squat kinematics and their relationship to squat depth in physically active individuals

Functional performance tests, such as the single-leg squat (SLS), are valuable tools for assessing dynamic movement patterns and identifying potential injury risks. The SLS is widely used in rehabilitation, sports medicine, and orthopedics, serving as a screening tool to evaluate movement system integrity and predict lower limb injury risk. Previous research has shown that kinematic patterns, particularly at the knee, are associated with injury risk. However, existing literature presents equivocal results regarding sex differences in SLS kinematics, particularly concerning pelvis and trunk motion, due to variations in SLS performance and lack of controlled squat depth. This study aimed to compare SLS kinematics in recreationally active females and males, focusing on peak angles of spine, pelvis, and lower limb joints in three planes of motion, and to investigate the relationship between these kinematics and squat depth. The hypothesis was that sex-dependent kinematic differences exist across the kinematic chain during SLS, and that the relationship between kinematics and squat depth varies by sex.

Numerous studies have examined lower extremity kinematics and muscle activity during closed kinetic chain exercises like squatting, jumping, and landing in both healthy and patient populations. These studies highlight the importance of assessing movement in all three planes of motion to evaluate range of motion, strength, and proprioception. The SLS, a commonly used functional performance test, has proven valid, reliable, and useful in detecting abnormal lower limb and trunk kinematics. While the reliability of the SLS can depend on the experience of the investigator, objective methods like optical motion capture systems provide high-precision, three-dimensional analyses. Sex differences in kinematics have been proposed as risk factors for lower limb injuries in female athletes, who have higher rates of knee injuries than males. Studies evaluating sex differences in SLS kinematics have yielded inconsistent results, possibly due to variations in SLS execution and uncontrolled squat depth. Previous research has identified parameters like increased hip adduction and medial-lateral knee displacement as predictors of knee dysfunction, often observed more frequently in females. However, a comprehensive three-dimensional analysis of the entire kinetic chain (spine, pelvis, lower limbs) relative to squat depth is lacking.

Fifty-eight healthy young adults (35 men, 23 women) with moderate to high physical activity levels participated. Exclusion criteria included musculoskeletal injuries or lower limb pain. Kinematic data were collected at 100 Hz using an eight-camera 3D motion capture system (Vicon), with markers placed on anatomical landmarks according to a full-body Plug-in gait model. Participants performed five repetitions of a right-leg SLS to maximal depth while maintaining balance, with three middle trials analyzed. Three-dimensional segment and joint kinematics were computed using BodyBuilder software. Angles were calculated using the Euler angle method, with positive values indicating flexion, adduction, and external rotation (depending on the joint). Squat depth was defined as the percentage change in S2 marker height from maximum to minimum position relative to maximum height. Data normality was assessed using the Shapiro-Wilk test; independent t-tests compared sex differences in peak joint angles. Pearson correlation analyzed relationships between joint angles, squat depth, and anthropometric variables. Intraclass correlation coefficients (ICCs) assessed the reliability of repeated measures. Ethical approval was obtained from the Medical University of Lublin Bio Ethics Committee.

Good reliability (ICC ≥0.70) was demonstrated for most kinematic variables. Sex differences were observed in several joint angles (Table 3). Males exhibited greater minimal and maximal frontal plane ankle angles and lower transverse plane angles, suggesting a more inverted and externally rotated foot. Males also showed greater knee varus (outward bending) in the frontal plane. Females demonstrated greater hip adduction and internal rotation, less hip external rotation, and greater pelvic anterior tilt at minimal angle. Females had more extended spines throughout the SLS and less spine rotation. In males, squat depth correlated positively with maximal sagittal plane angles of ankle, knee, hip, and pelvis. In females, squat depth correlated positively only with maximal sagittal plane knee and ankle angles and negatively with minimal ankle dorsiflexion (Table 4). No significant correlations were found between squat depth and anthropometric variables (Table 5). Figure 3 visually displays the kinematic differences between sexes in all three planes of motion.

The findings support the hypothesis that males and females employ different motor strategies during SLS. Sex differences were particularly evident in the frontal and transverse planes of lower limb joints and the sagittal plane of the spine. The results are partially consistent with previous literature, with some discrepancies possibly attributed to differences in SLS execution protocols (e.g., hand placement, non-stance leg position, squat depth). The present study's finding that females demonstrated a more upright posture aligns with previous research suggesting that they may rely more on quadriceps, potentially increasing ACL injury risk. The study is the first to analyze the relationship between SLS depth and three-dimensional kinematics, revealing that while average squat depth was similar between sexes, the correlation with kinematics differed. In males, a more complex motion involving several joints was linked to squat depth, while in females, the relationship was more limited to sagittal plane ankle and knee kinematics. These findings suggest that while sex differences in frontal and transverse plane kinematics are observable even at small squat depths, women may not necessarily lack control of hip and knee stabilizing muscles as depth increases. Limitations include the lack of older or less physically active participants, and the absence of limb dominance assessment.

This study provides a detailed three-dimensional kinematic analysis of SLS, revealing significant sex-dependent differences across the kinetic chain. Clinicians and coaches should consider these sex-specific kinematic patterns when evaluating SLS performance. Squat depth influences kinematics differently in males and females, emphasizing the need to consider both depth and kinematics in clinical SLS evaluations. Future research should incorporate a broader participant population (age, activity level) and investigate the long-term clinical implications of observed kinematic differences.

The study's limitations include a relatively young and physically active participant sample, limiting generalizability to other populations. The lack of limb dominance assessment might have influenced motor control. Future research should expand the participant pool to include individuals of varying ages and activity levels, and incorporate limb dominance assessment for a more comprehensive understanding of SLS kinematics.