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

Functional performance tests like the single-leg squat (SLS) are used to assess dynamic movement patterns and identify potential injury risks across all three planes of motion. Prior work links kinematic patterns, particularly at the knee, to lower-limb injury risk and supports SLS as a screening tool. However, findings on sex differences in SLS kinematics, especially trunk and pelvis motion, are inconsistent, and execution parameters (e.g., squat depth, limb positioning) vary widely across studies. Given reported higher rates of knee injuries in females and proposed sex-dependent biomechanical risk profiles (e.g., greater hip adduction and internal rotation), this study aimed to: (1) compare peak angles of the spine, pelvis, hip, knee, and ankle across planes during SLS between recreationally active females and males; and (2) examine how SLS depth relates to segment and joint kinematics within each sex. The authors hypothesized sex differences at all levels of the kinematic chain and sex-specific relationships between kinematics and squat depth.

Previous research has established SLS as valid and reliable for detecting abnormal lower-limb and trunk kinematics and assessing dynamic knee valgus and potential injury risk. Studies suggest females often exhibit greater hip adduction, internal rotation, and knee valgus, potentially contributing to patellofemoral pain and ACL injury risk. However, reports of sex differences in pelvis and trunk kinematics are equivocal, partly due to methodological differences: variations in SLS execution (arm and non-stance leg positions), inconsistent control or reporting of squat depth, and differing measurement points (e.g., fixed knee flexion angles vs. peak depth). Some studies indicate physically active individuals show better SLS performance than non-active individuals. There is a lack of comprehensive three-dimensional analyses encompassing spine, pelvis, and lower-limb joints that also account for squat depth, motivating the present study.

Participants: Fifty-eight healthy young adults (35 males, 23 females) from a university community, all reporting moderate to high physical activity via the IPAQ, with no musculoskeletal or orthopedic injury or lower-limb pain. Informed consent obtained; ethics approval KE-0254/322/2018. Experimental design: Kinematics were recorded at 100 Hz using an 8-camera Vicon 3D motion capture system with full-body Plug-in Gait markers. Participants were shown the task and instructed to look forward, descend slowly under control while maintaining heel contact and balance, with arms at sides. After a calibration trial and three practice attempts, each performed five repetitions of a single right-leg squat to self-selected maximal depth, held the bottom position for 3 s, then returned to start. Trials were repeated/excluded for loss of balance, incorrect non-stance leg position, or discontinuous movement. Data were collected in one session. Data processing: The middle three repetitions were averaged. Filtered marker trajectories were used to compute 3D segment (trunk and pelvis) and joint kinematics in BodyBuilder (Vicon). Angles were calculated using Euler sequences (flexion/extension, abduction/adduction, internal/external rotation). Positive conventions: spine (flexion, lateral flexion towards stance leg, rotation to side opposite stance leg); pelvis (anterior tilt, obliquity towards stance leg, rotation to side opposite stance leg); hip (flexion, adduction, external rotation); knee (flexion, varus/outward bend, internal rotation); ankle (dorsiflexion, inversion, internal rotation). Peak maximal and minimal angles during descent were analyzed. Squat depth: Defined as percent decrease in vertical S2 marker height from highest to lowest position relative to initial standing leg length based on S2 height: Δh = (hmax − hmin) / hmax × 100%. Leg length was the vertical distance from ground to S2 marker in the initial position. Statistical analysis: Normality assessed with Shapiro–Wilk. Between-sex comparisons of peak angles (max/min) in each plane used independent t-tests with Cohen’s d effect sizes. Pearson correlations assessed relationships between joint/segment angles and squat depth, and between squat depth and anthropometrics (body length, body mass, BMI). Significance at p < 0.05; results reported as mean, SD, and 95% CI. Reliability of repeated measures assessed with ICCs, interpreted as: <0.40 poor, 0.40–0.70 fair, 0.70–0.90 good, >0.90 excellent.

Reliability: Overall descent-phase ICC = 0.86 (good). Of all ICCs, 6.5% fair (0.40–0.70), 41.9% good (0.70–0.90), and 51.6% excellent (>0.90). Segment/joint-specific ICCs were generally good-to-excellent across planes (Table 2). Between-sex kinematic differences (selected significant results; females vs males): - Ankle: Males showed greater inversion (frontal plane) at min and max (min: 1.28° vs −0.79°, p = 0.001; max: 4.77° vs 2.80°, p = 0.003) and greater external rotation (transverse plane) with more negative minimal angle (−28.59° vs −21.64°, p = 0.01) and lower maximal angle (8.87° vs 1.44°, p = 0.004), indicating a more inverted and externally rotated foot in males. - Knee: Frontal plane showed more varus (outward bend) in males (max: 13.40° vs 4.93°, p < 0.001; min: 2.69° vs −3.43°, p < 0.001). - Hip: Females had greater adduction (max: 14.92° vs 10.43°, p = 0.006; min: 1.78° vs −1.80°, p < 0.001), less external rotation (max: 3.32° vs 10.07°, p = 0.02), and greater internal rotation (min: −4.00° vs 2.66°, p = 0.04). - Pelvis: Females had greater minimal anterior tilt (15.56° vs 12.95°, p = 0.04); males had greater minimal pelvic obliquity towards the non-stance side (−4.04° vs −2.36°, p = 0.02). Max pelvic transverse rotation was higher in males (2.95° vs 1.03°, p = 0.05, borderline). - Spine: Females exhibited more extension throughout the SLS in the sagittal plane (max: −13.43° vs −0.77°, p < 0.001; min: −18.18° vs −8.60°, p < 0.001) and less transverse rotation to the side opposite the stance leg (max: 1.78° vs 3.80°, p = 0.01). Males showed greater lateral flexion toward the stance leg (10.02° vs 7.37°, p = 0.05, borderline). SLS depth: Mean depth (% leg length) was similar between sexes (females 13.10% ± 3.20 vs males 15.00% ± 4.40; p = 0.08). Correlations between SLS depth and kinematics: - Females: Positive correlations with sagittal-plane maximal knee angle (r = 0.88, p < 0.001) and maximal ankle dorsiflexion (r = 0.53, p = 0.01). Minimal ankle dorsiflexion angle correlated negatively with depth (r = −0.50, p = 0.01). No significant depth relationships with hip, pelvis, or spine. - Males: Positive correlations with sagittal-plane maxima at ankle (r = 0.60, p < 0.001), knee (r = 0.87, p < 0.001), hip (r = 0.73, p < 0.001), and pelvis (r = 0.40, p = 0.02); additionally with knee transverse-plane max (r = 0.43, p = 0.01). No significant correlations with frontal-plane variables. Anthropometrics: No significant correlations between squat depth and body mass, height, or BMI in the full sample or by sex (all p > 0.05).

The study confirmed sex-dependent kinematic strategies during the single-leg squat. Females demonstrated greater hip adduction and internal rotation and less knee varus, alongside a more extended trunk posture, whereas males exhibited a more inverted and externally rotated foot, greater knee varus, and larger pelvis and trunk rotations. These findings align partly with prior literature indicating female profiles of hip adduction/internal rotation and knee valgus, while also highlighting differences in trunk and pelvic motions likely influenced by execution parameters and depth. Importantly, relationships between SLS depth and joint/segment kinematics differed by sex. In females, depth related primarily to sagittal-plane ankle and knee motion, suggesting reliance on distal joints to achieve greater depth. In males, depth correlated with a broader set of sagittal-plane variables (ankle, knee, hip, pelvis) and with knee transverse motion, indicating a more distributed, multi-joint strategy. Frontal and transverse plane kinematics were generally not depth-dependent, implying that sex differences in these planes persist across depths and should be considered during clinical assessment irrespective of depth achieved. Clinically, these results suggest that evaluations and interventions should account for sex-specific movement strategies: for females, targeting ankle and knee sagittal mechanics may be most relevant to depth, while for males, addressing coordinated hip–pelvis mechanics and transverse knee control may be important. The absence of associations between depth and anthropometrics indicates that technique and control, rather than body size, underpin depth-related kinematic changes in this cohort.

Males and females exhibited similar sagittal-plane lower-limb kinematics but different trunk sagittal motion during SLS, with pronounced sex differences across the frontal and transverse planes at multiple segments. Squat depth related to kinematics differently by sex—primarily ankle and knee sagittal mechanics in females, and a broader hip–knee–ankle–pelvis involvement in males. Clinicians and coaches should consider these sex-specific patterns and incorporate both kinematic analysis and depth assessment when evaluating SLS performance. Future research should include older and less active populations, assess limb dominance effects, standardize or systematically manipulate SLS execution parameters (including depth), and perform prospective studies to link these kinematic patterns to injury risk.

The sample included only healthy, young, physically active individuals; older and less active populations were not studied. Limb dominance was not evaluated, which may influence motor control and kinematics.