Association Between Lower Limb Kinematics During Single Leg Squat and Patient-Reported Outcomes After Hip Arthroscopy for Femoroacetabular Impingement Syndrome

Abstract

Background:

Typically, 3 months is an early postoperative time point at which patients with femoroacetabular impingement syndrome (FAIS) demonstrate clinically meaningful improvement, while 6 months represents an important midterm stage for recovery and return to sport.

Purpose:

To assess continuous changes in the kinematics of the involved and uninvolved lower limbs in patients with FAIS at 3 and 6 months after hip arthroscopy as well as to examine their relationships with International Hip Outcome Tool–33 (iHOT-33) and Copenhagen Hip and Groin Outcome Score Sport subscale (HAGOS-Sport) scores.

Study Design:

Cohort study; Level of evidence, 2.

Methods:

The study included 15 patients who underwent hip arthroscopy between March 2024 and November 2024 as well as a healthy control group of 15 participants, matched to the patients by sex, age, and body mass index, recruited from the community. All participants performed the single-leg squat (SLS), and kinematic data were collected using an 8-camera motion capture system. One-way analysis of variance was used to compare baseline, 3-month postoperative, and 6-month postoperative kinematic data of the FAIS group with those of the healthy control group. Post hoc comparisons were conducted using the Tukey test to determine between-group differences. Correlation analysis was performed to assess the relationship between changes in the hip flexion angle and changes in iHOT-33 and HAGOS-Sport scores.

Results:

At 3 months postoperatively, the involved limb hip flexion angle (P = .021), hip adduction angle (P = .0011), knee flexion angle (P = .016), and change in the center of mass (P = .0034) were reduced compared with the healthy control group. At 6 months postoperatively, no significant kinematic differences were observed between the FAIS and healthy control groups. Correlations between changes in the iHOT-33 score at 3 months and changes in the hip flexion angle at 6 months were positive (r = 0.64; P = .0094), and changes in the iHOT-33 score at 6 months were also positively correlated with changes in the hip flexion angle at 6 months (r = 0.53; P = .041).

Conclusion:

Patients with FAIS exhibited abnormal kinematics during the SLS at 3 months postoperatively, which showed improvement by 6 months. Furthermore, improvement in sagittal-plane angles of the hip at 6 months postoperatively was correlated with the iHOT-33 score, supporting our hypothesis. Clinicians should not overlook functional recovery after hip arthroscopy for FAIS.

Keywords

femoroacetabular impingement syndrome hip arthroscopy single-leg squat kinematics clinical outcomes

Femoroacetabular impingement syndrome (FAIS) is a morphological abnormality of the hip joint, commonly observed in young, physically active adults.^1,2,10,28 It is typically characterized by a triad of symptoms, clinical signs, and radiographic findings.¹² Corrective surgery for FAIS is typically performed arthroscopically to restore the normal femoral head-neck offset.¹⁴

Some studies^8,20 have reported that 3 months postoperatively represents an early time point at which patients with FAIS typically demonstrate the bulk of clinically meaningful improvement, while 6 months corresponds to an important midterm stage for recovery and return to sport. The single-leg squat (SLS) is an effective task for clinical assessments, biomechanical investigations, and evaluations of athletic performance.¹¹ Compared with the double-leg squat, the SLS imposes greater demands on stability and motor control, elicits more multiplanar (particularly frontal-plane) kinematic deviations, provokes stronger unilateral muscle activation, and exhibits higher clinical sensitivity; consequently, in postoperative functional assessments and rehabilitation, the SLS can detect unilateral deficits and compensatory strategies earlier and more precisely.⁵ Motion capture analysis can accurately characterize the impact of FAIS on hip function during activities of daily living.^6,17,19,27 Therefore, preoperative and early postoperative motion capture assessments of SLS kinematics in patients with FAIS can improve our understanding of their functional status and provide an evidence-based foundation for early rehabilitation. A previous study reported that, after hip arthroscopy at 9 months, patients with FAIS demonstrated a significant increase in sagittal-plane range of motion of the hip during the SLS.¹⁶ Similarly, there was a significant increase in sagittal-plane range of motion of the knee in patients with FAIS postoperatively. Furthermore, the changes in angles were significantly correlated with Hip Outcome Score–Sports Subscale and Hip Outcome Score–Activities of Daily Living scores.¹⁶ However, there remains a lack of data on clinical scores and functional performance during the early recovery period after hip arthroscopy for FAIS.

Therefore, the aim of this study was to assess continuous changes in the kinematics of the involved and uninvolved lower limbs in patients with FAIS at 3 and 6 months after hip arthroscopy as well as to examine the correlations between changes in hip flexion angles and changes in International Hip Outcome Tool–33 (iHOT-33) and Copenhagen Hip and Groin Outcome Score Sport subscale (HAGOS-Sport) scores. It was hypothesized that abnormal kinematic patterns would be observed during the SLS at 3 months postoperatively, with significant improvements by 6 months, reaching levels comparable to those of a healthy control group. Furthermore, changes in sagittal-plane hip angles were expected to be associated with improvements in both iHOT-33 and HAGOS-Sport scores.

Methods

Participants

After institutional review board approval, patients who underwent hip arthroscopy from March 2024 to November 2024 at our institute were selected for SLS analysis. The inclusion criteria comprised patients aged 18 to 50 years who were diagnosed with FAIS based on clinical symptoms and radiographic findings.¹² The exclusion criteria were as follows: (1) previous lower limb surgery; (2) concomitant hip conditions, including hip osteoarthritis with Tönnis grade >1, avascular necrosis, Legg-Calvé-Perthes disease, osteoid osteoma, synovial chondromatosis, pigmented villonodular synovitis, and developmental dysplasia of the hip (lateral center edge angle <18°); (3) lower extremity injuries within the past month; (4) other forms of arthritis, diabetes, or heart disease that limit daily activities; and (5) refusal to participate in follow-up. Ultimately, 15 patients with FAIS were enrolled in the study. All patients were asked to return to our clinic for follow-up at 3 and 6 months postoperatively, where they underwent motion capture testing. Participants of the healthy control group were recruited from the university community and matched with patients for sex, age, and body mass index.

Data Collection

Patient characteristics, including age at surgery, sex, affected side, height, weight, body mass index, and duration of symptoms, were recorded. Radiographic examinations were performed preoperatively to obtain the alpha angle in the Dunn view, the lateral center edge angle in the anteroposterior view, and the Tönnis grade in the anteroposterior view, as described in previous studies.^18,22,23

Patient-reported outcome measures, including the iHOT-33 and HAGOS-Sport, were used to assess hip function.^4,7,13 iHOT-33 and HAGOS-Sport scores were routinely collected preoperatively and at 3 and 6 months postoperatively via telephone or during outpatient visits. In addition, kinematic data collected included peak hip flexion angle, peak hip adduction angle, peak knee flexion angle, and change in the center of mass (ΔCOM) of the involved and uninvolved limbs during the SLS.

SLS Task

All participants were required to wear fitted swimming trunks and remain barefoot during testing. A total of 37 reflective markers were attached to the participants, which were placed on the lateral and medial malleoli, heel, midpoint of the second metatarsophalangeal joint, first metatarsophalangeal joint, fifth metatarsophalangeal joint, lower one-third and upper one-third of the tibia, lateral lower one-third of the leg, tibial tuberosity, medial and lateral femoral condyles, anterior thigh, lateral thigh, anterior superior iliac spine, posterior superior iliac spine, highest point of the iliac crest, acromion, and right scapula. All participants were marked by the same examiner to ensure that the testing results were not affected by interrater variability. All SLS tasks were performed at a self-selected speed to better reflect movements in a clinical setting. Participants were instructed to “squat as low as possible while maintaining full contact between your foot and the force plate at all times.” No depth targets were used. A familiarization session was provided before data collection, which included a demonstration of the task by the researcher. Participants completed 3 practice trials before data collection commenced.²¹ Kinematic parameters of the lower limbs during the SLS were obtained via an 8-camera infrared high-speed motion capture system (Nexus T40; Vicon) at a sampling frequency of 100 Hz (Figure 1).

Figure 1.

Demonstration of the single-leg squat.

Surgical Technique

Primary hip arthroscopy was performed by a fellowship-trained surgeon (Y.X.) with over 10 years of experience in hip arthroscopy using the standard supine approach described in previous studies.²⁴ After anesthesia and traction, the anterolateral, anterior, and midanterior portals were established. A thorough examination of the central compartment was conducted through capsulotomy between the portals to assess the acetabular rim, acetabular labrum, articular cartilage, and ligaments. After the examination, synovectomy was performed on the inflamed synovium. Acetabuloplasty was carried out for pincer deformities. If a labral tear was observed, labral repair was performed using a multiple suture anchor technique if remaining fibrous tissue was sufficient to restore the sealing function of the labrum. If remaining fibrous tissue was of poor quality or insufficient to restore the sealing function, labral reconstruction was performed using an autologous iliotibial band. Additionally, chondroplasty was performed for partial-thickness chondral lesions and cartilage flaps. After treating the central compartment, the arthroscope was moved to the peripheral compartment, where femoroplasty was performed to address cam deformities. Finally, the capsule was routinely repaired.

Rehabilitation Protocol

All patients followed the same instructions for rehabilitation. Isometric contraction and passive range of motion exercises were allowed during the first 2 days after surgery. Partial weightbearing exercises to restore range of motion and regular gait were permitted from day 3 to week 3. Walking with full weightbearing was permitted at week 4. Muscle strength exercises and dynamic balance exercises with full weightbearing were permitted at week 6. Patients could gradually resume activities and return to sport based on their tolerance.

Data Analysis

All kinematic data were processed via Visual3D software (C-Motion). The 3-dimensional coordinates of all the markers were smoothed via a Butterworth low-pass filter with a cutoff frequency set at 10 Hz.

Lower limb segment coordinate systems were established based on marker positions. The hip joint center was calculated according to the methodology described by Reize et al.²⁵ We computed the peak flexion angles for the hip and knee joints, the peak adduction angles for the hip joint, and the ΔCOM on both the involved and uninvolved limbs. ΔCOM was defined as the change in the center of mass from squat initiation to the lowest point. Squat initiation was defined as the moment when the center of mass deviated by 3 standard deviations from the resting position, and the lowest point was defined as the minimum center of mass value.

Statistical Analysis

All statistical analyses were performed using Prism (Version 10.1.2; GraphPad Software). We performed 1-way analysis of variance (ANOVA) to compare SLS discrete variables of the FAIS group preoperatively and at 3 and 6 months postoperatively with the healthy control group. Within the FAIS group, comparisons across time points were conducted using repeated-measures ANOVA, with all post hoc pairwise comparisons performed using the Tukey test. Additionally, Pearson correlation analysis was employed to assess the relationship between the changes in hip flexion angles and the changes in iHOT-33 and HAGOS-Sport scores. Statistical significance was set at an alpha of .05.

Based on post hoc analysis of peak hip flexion angle, the study employed a mixed design in which the preoperative and postoperative comparisons were treated as within-participant repeated measures. Repeated-measures ANOVA yielded a partial η² of 0.22 (equivalent Cohen f = 0.53). With an alpha of .05 and target power of 0.80, and assuming an intraparticipant correlation of r = 0.50 among repeated measures, power analysis indicated that approximately 9 participants in the repeated-measures arm were required to detect the observed effect. The control group was an independent healthy control. For comparisons of the FAIS group versus the healthy control group using one-way ANOVA, a partial η² of 0.24 (equivalent Cohen f = 0.57) was used, which required at least 10 participants per group. Consequently, enrolling 15 participants per group ensured the desired statistical power.

Results

As shown in Figure 2, a total of 15 patients were included in the FAIS group. Participant data of the FAIS and healthy control groups are presented in Table 1. Baseline information did not show statistically significant differences (P > .05).

Figure 2.

Flowchart of patient selection.

Table 1

Characteristics of Study Cohort ^a

	FAIS (n = 15)	Healthy Control (n = 15)	P Value
Sex, male/female	9/6	9/6	>.99
Involved limb, right/left	6/9	—	—
Dominant side, right/left	15/0	15/0	>.99
Age, y	30.00 ± 5.44	27.93 ± 5.08	.41
Body mass index, kg/m²	22.64 ± 2.87	22.72 ± 2.91	.94
Duration of symptoms, mo	35.38 ± 31.06	—	—
Alpha angle, deg	67.79 ± 9.43	—	—
Lateral center edge angle, deg	31.65 ± 9.38	—	—
Tönnis grade			—
0	14	—
1	1	—
Visual analog scale score	3.53 ± 1.06	—	—
Follow-up, mo	6.09 ± 0.22	—	—

Data are shown as No. or mean ± SD. FAIS, femoroacetabular impingement syndrome.

The iHOT-33 score of the FAIS group at 3 months postoperatively showed no significant difference from baseline (P > .05), but at 6 months postoperatively, the score was significantly higher than that at both 3 months (P = .014) and baseline (P < .001). Regarding the HAGOS-Sport, at 6 months postoperatively, the score was significantly better than that at 3 months (P = .049); however, there were no significant differences in the HAGOS-Sport score between 3 months postoperatively and baseline or between 6 months postoperatively and baseline (P > .05) (Figure 3).

Figure 3.

Continuous changes in International Hip Outcome Tool–33 (iHOT-33) and Copenhagen Hip and Groin Outcome Score Sport subscale (HAGOS-Sport) scores before and after surgery in the femoroacetabular impingement syndrome (FAIS) group. ns: P > .05; *P < .05; **P < .01; ***P < .001.

At 3 months postoperatively, the peak hip flexion angle of the involved limb was significantly lower than that of the healthy control group (56.96°± 21.13° vs 82.59°± 17.81°, respectively; P = .021). Similarly, the peak hip adduction angle of the involved limb at 3 months postoperatively was significantly inferior to that of the healthy control group (4.41°± 3.92° vs 11.23°± 5.64°, respectively; P = .0011). The peak knee flexion angle of the involved limb at baseline (64.24°± 20.39° vs 77.42°± 8.49°, respectively; P = .016) and at 3 months postoperatively (63.37°± 13.80° vs 77.42°± 8.49°, respectively; P = .016) was significantly reduced compared to the healthy control group. At 6 months postoperatively, there were no significant differences between the involved limb and the healthy control group in peak hip flexion angle (71.54°± 22.45° vs 82.59°±17.81°, respectively; P = .061), peak hip adduction angle (7.62°± 4.35° vs 11.23°± 5.64°, respectively; P = .130), and peak knee flexion angle (70.27°± 14.79° vs 77.42°± 8.49°, respectively; P = .30). Additionally, no statistically significant differences were observed in peak hip and knee joint angles of the uninvolved limb at baseline, 3 months, and 6 months compared to the healthy control group (Figure 4).

Figure 4.

Comparison of peak angles at baseline, 3 months postoperatively, and 6 months postoperatively between the involved and uninvolved limbs of the femoroacetabular impingement syndrome (FAIS) and healthy control groups. ns: P > .05; *P < .05; **P < .01; ***P < .001.

At 6 months postoperatively, the peak hip flexion angle of the involved limb was significantly higher than that at 3 months (71.54°± 22.45° vs 56.96°± 21.13°, respectively; P = .0074). Additionally, the peak hip flexion angle of the uninvolved limb at 6 months postoperatively was significantly greater than that at 3 months (78.29°± 18.79° vs 69.31°± 21.00°, respectively; P = .028). The peak hip adduction angle of the involved limb at 3 months postoperatively was significantly lower than that at baseline (4.41°± 3.92° vs 10.32°± 5.19°, respectively; P = .0005) and at 6 months (4.41°± 3.92° vs 7.62°± 4.35°, respectively; P = .0034). Moreover, the peak hip adduction angle of the involved limb at 6 months was significantly less than that at baseline (7.62°± 4.35° vs 10.32°± 5.19°, respectively; P = .013), while the peak knee flexion angle of the involved limb at 6 months postoperatively was significantly higher than that at 3 months (70.27°± 14.79° vs 63.37°± 13.80°, respectively; P = .049) (Figure 5).

Figure 5.

Comparison of peak angles at baseline, 3 months postoperatively, and 6 months postoperatively between the involved and uninvolved limbs of the femoroacetabular impingement syndrome (FAIS) group. ns: P > .05; *P < .05; **P < .01; ***P < .001.

At 3 months postoperatively, the ΔCOM of the involved limb was significantly smaller than that at baseline (0.14 ± 0.072 vs 0.18 ± 0.069, respectively; P = .0047) and 6 months (0.14 ± 0.072 vs 0.19 ± 0.082, respectively; P = .0124) as well as that of the healthy control group (0.14 ± 0.072 vs 0.23 ± 0.062, respectively; P = .0034) (Figure 6).

Figure 6.

Comparison of changes in the center of mass at baseline, 3 months postoperatively, and 6 months postoperatively between the involved and uninvolved limbs of the femoroacetabular impingement syndrome (FAIS) group. ns: P > .05; *P < .05; **P < .01; ***P < .001.

There was a significant positive correlation between the change in iHOT-33 scores at 3 months postoperatively and the change in peak hip flexion angles at 6 months postoperatively (r = 0.64; P = .0094). Additionally, a moderate positive correlation was observed between the change in iHOT-33 scores at 6 months postoperatively and the change in peak hip flexion angles at 6 months postoperatively (r = 0.53; P = .041). There was no significant correlation between the change in peak hip flexion angles at 6 months postoperatively and the change in HAGOS-Sport scores at 3 months (r = 0.17; P = .52) or at 6 months (r = −0.093; P = .74) (Figure 7).

Figure 7.

Relationship between changes in hip flexion angles and changes in International Hip Outcome Tool–33 (iHOT-33) and Copenhagen Hip and Groin Outcome Score Sport subscale (HAGOS-Sport) scores in the femoroacetabular impingement syndrome (FAIS) group.

Comparisons between the involved and uninvolved limbs of the FAIS group at each time point, and their comparisons with the healthy control group, are shown in Appendix Figure A1.

Discussion

The most important finding of this study was that patients with FAIS exhibited a certain degree of recovery in SLS kinematics at 6 months postoperatively and that changes in the hip flexion angle were significantly associated with improvements in clinical outcomes. At 3 months postoperatively, patients still exhibited an abnormal squatting pattern, characterized by reduced hip and knee flexion angles, decreased hip adduction angles, and diminished squatting depth, which may be attributed to early postoperative muscle atrophy and joint inflammation.^29,30 At 6 months postoperatively, both hip and knee flexion angles showed significant improvements and were comparable to those of the healthy control group. Furthermore, we found that changes in the hip flexion angle were significantly associated with improvements in clinical symptoms, suggesting a potential relationship between functional recovery and symptomatic outcomes. This finding highlights the importance of early postoperative rehabilitation interventions, and long-term follow-up studies are warranted to further investigate the dynamic relationship between functional improvement and symptom changes in patients with FAIS. However, we did not find a correlation between these changes and HAGOS-Sport scores in patients with FAIS. This suggests that the subjective responses to exercise at 6 months postoperatively do not align with observed functional recovery compared to preoperatively. Consequently, we infer that, although overall function has largely recovered, a longer time may be required for a full return of athletic or dynamic exercise capacity.

The changes in hip adduction angles were also quite significant. Previous studies have shown that the hip adduction angle is significantly correlated with improvement in clinical symptoms. After standardized physical therapy, smaller hip adduction angles during SLS tasks were associated with greater improvements in the modified Harris Hip Score (r = −0.67; P < .01).¹⁵ Research by Brown-Taylor et al³ also confirmed a significant correlation between greater biomechanical deficits in the coronal plane and poorer scores on the iHOT-33 (r = −0.36; P = .01). The experiment conducted by Harris-Hayes et al¹⁵ also confirmed that, after physical therapy, the hip adduction angle during SLS tasks was reduced (–2.6°± 7.6°). However, our study posits that these changes may be related to underlying alterations in muscle strength and joint loading. A computational simulation study⁹ comparing the effects of gluteus medius strength training and modified hip movement pattern training on patients with developmental dysplasia of the hip indicated that movement pattern training significantly reduced overall hip loading (joint reaction force) by decreasing the hip adduction angle (usually by ~10°). This suggests that reductions in the hip adduction angle significantly lessen the load on the hip joint, thereby improving patient function and pain symptoms, particularly for those with chronic hip pain. Patients postoperatively may alleviate hip loads by decreasing hip adduction.

We found that only the hip flexion angle of the uninvolved limb was significantly greater at 6 months postoperatively than at 3 months, while no other angles of interest showed statistically significant differences. This suggests that, in the early postoperative period, function of the uninvolved hip may decline to a certain extent. The arthroscopic hip procedure may have an impact on joint function of the contralateral lower extremity, but this effect appears to be minimal and tends to recover by 6 months postoperatively. This may be explained by the fact that surgical-side pain or discomfort prompts patients to offload the operated limb, forcing the contralateral limb to bear increased loads or adopt altered movement patterns, which may impair joint range of motion and muscle coordination on that side. In addition, unilateral injuries or surgery can induce the reorganization of central motor control. As far as we know, few studies have reported functional changes in the uninvolved lower limb; future research should take this into account.

External moments refer to the mechanical loads exerted on the joint by the external environment. During the SLS, external moments may not fully reflect muscle activity around the joint, as the synergistic contraction of muscles (such as the gastrocnemius and tibialis anterior) may obscure variations in external moments. Therefore, the applicability of external moments in the SLS context may be limited.²⁶ Integrating electromyography data may provide a more comprehensive assessment of the biomechanical characteristics of the SLS. Future research should take this into consideration.

Limitations

First, this study is limited by a single-surgeon cohort; thus, generalization to the broader population should be approached with caution. Second, this study did not investigate kinetics in depth primarily because we did not perform electromyography, which limits our ability to cautiously interpret joint reaction force outcomes. Finally, the follow-up period in this study was relatively short; future work should incorporate a longer term follow-up.

Conclusion

Footnotes

Appendix

As shown in Appendix Figure A1, at baseline, the peak hip flexion angle differed significantly between the involved and uninvolved limbs (P = .010). At baseline, the peak knee flexion angle differed significantly between involved and uninvolved limbs (P = .021) and between the involved limb and healthy controls (P = .028). At 3 months postoperatively, the peak hip flexion angle differed significantly between involved and uninvolved limbs (P = .015) and between the involved limb and healthy controls (P = .012). At 3 months, the peak hip adduction angle differed significantly between involved and uninvolved limbs (P = .0002) and between the involved limb and healthy controls (P = .0006). At 3 months, the peak knee flexion angle differed significantly between involved and uninvolved limbs (P = .0013) and between the involved limb and healthy controls (P = .0023). At 6 months, the peak hip flexion angle differed significantly between involved and uninvolved limbs (P = .046), and the peak hip adduction angle differed significantly between involved and uninvolved limbs (P = .043). At 6 months, the peak knee flexion angle differed significantly between involved and uninvolved limbs (P = .0059).

Final revision submitted January 3, 2026; accepted February 23, 2026.

One or more of the authors has declared the following potential conflict of interest or source of funding: National Key Research and Development Program of China (Key Technologies Research and Development Program, 2024YFC3606704), National Key Research and Development Program of China (Key Technologies Research and Development Program, 2023YFC2410803), Peking University Third Hospital (BYSYZD2023011), Peking University Third Hospital Fund for Interdisciplinary Research(BYSYJC2024026),National Natural Science Foundation of China (82372496), Natural Science Foundation of Beijing Municipality (F252057) funded this study. AOSSM checks author disclosures against the Open Payments Database (OPD).

AOSSM has not conducted an independent investigation on the OPD and disclaims any liability or responsibility relating thereto.

Ethical approval for this study was obtained from Peking University Third Hospital (No. M2024138).

ORCID iDs

Yuang Hao

Yan Xu

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