Femoroacetabular impingement (FAI) is a common cause of hip pain among younger active adults (7,16). Impingement involves abutment of the proximal femur against the acetabular rim (16), typically in deep end range hip flexion, combined with hip adduction and internal rotation (16). FAI is common in athletes who move repeatedly into this motion (e.g., ice hockey, golf, tennis, football, and soccer) (29), and these individuals are considered at risk for hip osteoarthritis (1,7). Less hip range of motion (ROM) toward directions of impingement has been identified in symptomatic FAI (3,21), though evidence is limited during dynamic functional activities. Although only subtle alterations in hip joint biomechanics have been reported during gait (10,18,20,32), this task is typically pain free and does not usually involve hip motion that approaches the position where impingement might occur. It is reasonable to expect that differences in hip biomechanics may be more apparent in provocative functional tasks that require hip motion toward the impinging position, such as squatting.
Squatting is a demanding task often used for the evaluation of lower limb function (14). Findings are limited and conflicting regarding alterations in hip joint biomechanics during squatting in symptomatic FAI (5,22,24). When hip and pelvis motion were assessed during a maximum depth squat (24), reduced sagittal plane pelvis ROM (anterior pelvic tilt) was found in individuals with symptomatic FAI, although no alterations in hip kinematics were reported. The absence of differences at the hip may be because individuals with FAI alter squat mechanics in subtle ways (that may include individualized strategies) to avoid painful positions, which necessitates consideration for task design. Because of coupling, sagittal plane pelvis motion influences transverse plane motion of the femur (4). Taken together, we predicted that this would primarily involve changes in sagittal plane pelvic tilt and forward trunk lean to enable squatting to a low depth but limiting the requirement for hip flexion. Assessment of hip joint biomechanics during a constrained version of a squat that is designed to limit sagittal plane movement of the pelvis and trunk could expose alterations at the hip, which might otherwise be concealed, in individuals with symptomatic FAI. On the basis of previous work, it would be expected that not all individuals would deal with the challenge to perform a maximal squat in the same way. A constrained squatting task has not been investigated in this population.
Two studies reported decreased squat depth in individuals with symptomatic FAI compared with healthy controls in a task that necessitated heel contact with the ground (5,24). The altered mechanics underpinning this functional deficit are likely to involve the hip and adjacent joints (i.e., knee and ankle). Squat depth can be restricted by ankle dorsiflexion range (e.g., calf muscle length and talocrural joint range), which has been previously identified as reduced in preoperative symptomatic FAI patients (23). Thus, a task design to encourage deep hip flexion toward impingement and to moderate any ankle constraint may provide further insight into biomechanical alterations at the hip in individuals with symptomatic FAI.
This exploratory study compared individuals with symptomatic FAI (cam-type or combined cam plus pincer) and asymptomatic controls with no evidence of morphological FAI on hip imaging. The specific aims were twofold: first, to compare hip, pelvis, and trunk biomechanics between groups during an unconstrained deep squat where participants could use their preferred strategy to reach maximum depth and, second, to probe potential biomechanical differences between groups when the task was constrained to encourage deep hip flexion with constraint of sagittal plane pelvic tilt and forward trunk lean, and if differences are identified, to probe possible adaptations that enable completion of the task with consideration of individual participants.
This exploratory study used a convenience sample of participants who were enrolled in other studies (12,13). Fifteen individuals 18–35 yr old diagnosed with cam or combined FAI were recruited from the surgical records of an orthopedic surgeon (JO). Participants were scheduled for arthroscopic surgery for FAI and had persistent groin pain and activity limitation for at least 3 months, with no substantial improvement with conservative treatment. All participants tested positive for a clinical impingement test (presence of hip pain during passive hip flexion to 90°, followed by forced adduction and internal rotation) (30), and had definitive signs of FAI on imaging (x-ray and/or magnetic resonance imaging [MRI], alpha angle > 55° [cam FAI], and lateral center edge angle >39° and/or positive crossover sign [combined FAI]) (27,31). Potential participants were excluded if they had only pincer-type FAI given the different morphological presentation (excessive acetabular coverage in the absence of a bony deformity on the femoral head) and the older, predominantly female demographic of that group (15). Other exclusion criteria included (i) any history of hip surgery; (ii) moderate or severe radiographic osteoarthritis, as determined by the treating physician; (iii) lower limb injury/pain sufficient to limit function in the preceding month; or (iv) other forms of arthritis, diabetes, cardiac, or circulatory conditions that could limit everyday activities.
Fifteen asymptomatic healthy control participants, comparable with the FAI group with respect to age, sex, and leg dominance, with no history of hip/groin pain were recruited from the community. Control participants underwent an MRI of the hip to ensure absence of morphological FAI, via a 3-T scanner (Magnetom Trio syngo MR B17; Siemens, Munich, Germany) and a 16-channel body coil (coupled with a Siemens Spine array). Alpha angle was measured in the oblique sagittal plane (2,34); lateral center edge angle was measured in the coronal plane. The localizer sequence was used to correct for pelvic obliquity where necessary (34). Measurements were made using OsiriX imaging software (©Pixmeo SARL, Bernex, Switzerland) and Orthopaedic Studio v1.2 Plugin (Spectronic AB, Helsingborg, Sweden). Eligible participants had alpha angle <50° and center edge angles <40° (27,31).
Participants provided written informed consent. One control participant was excluded after they were unable to undergo MRI. The institutional human research ethics committee approved the study.
Participants underwent three-dimensional motion analysis while performing unconstrained and constrained squats wearing standardized footwear (Dunlop Volley, Pacific Brands, Australia). Participants diagnosed with bilateral FAI were tested on the more symptomatic side that was scheduled for surgery. Kinematic data were collected using a 12-camera motion analysis system (Vicon, MX, Oxford, UK) at 120 samples per second. Two embedded force platforms (AMTI Inc., Watertown, MA) collected ground reaction force data at 3000 Hz (to equate with electromyographic data not presented here). Reflective markers were applied to the trunk (anteriorly on the manubrium and posteriorly on the spinous processes of T2 and T10) (33), pelvis anterior superior iliac spine and posterior superior iliac spine, and bilaterally on the lower limbs (three-marker triad over the lateral thigh, three-marker triad over the lateral tibial shank, lateral femoral condyles, lateral malleoli, calcaneus, second and fifth metatarsal bases, and first and fifth metatarsal heads) in accordance with Besier et al. (9). Two additional markers were placed on the iliac crests. Medial femoral condyles and medial malleoli markers were included during an initial static standing trial to determine knee and hip joint center positions. Hip joint centers were estimated using a dynamic functional approach where standing participants performed flexion, abduction, circumduction, and extension of the leg (9). Knee joint centers and functional flexion/extension axes were defined from helical knee axes according to a functional approach where participants performed standing knee bends through a range of approximately 60° of knee flexion (9). The use of midsegment marker clusters to determine direct kinematic-based joint angles and moments, combined with functional methods to estimate hip and knee joint centers (9), have demonstrated superior reliability compared with some anatomical landmark based approaches (19).
For all squats, participants were instructed to stand with their feet parallel and shoulder width apart. Each foot was positioned on a firm foam wedge (Slant by OPTP, Minneapolis, MN) such that the heels of both feet were in approximately 30° of plantarflexion (Fig. 1A). The wedges were used to encourage deep hip flexion, unrestricted by ankle dorsiflexion range (e.g., calf muscle length and talocrural joint range). Participants were instructed to hold their straight arms to the front (horizontal) for balance and to maintain an even distribution of weight between feet. The squat included (i) descent from a standing position to the end of self-chosen available range, (ii) a 3-s hold, and (iii) ascent to the upright standing position. Participants were instructed to use their preferred strategy to squat as deeply as possible at a self-selected controlled pace. The constrained squat included the addition of a vertical pole positioned anterior to the trunk (Fig. 1B). Participants were instructed to maintain an upright trunk and descend parallel to the pole. The constrained squat was designed to minimize forward trunk lean and pelvic anterior–posterior tilt and, thus, encourage more isolated hip flexion. If forward trunk lean was observed by the tester during descent, additional trials were performed. After demonstrating proficiency with the squatting tasks, participants completed five trials for each squat type.
A modified Tegner activity scale (0 = disability, 10 = competitive sport at the professional level) was used to grade level of physical activity in all participants (35). An 11-point numerical rating scale (NRS) (0 = no pain, 10 = worst pain possible) was administered verbally to all participants immediately after each of the squatting tasks to quantify their hip pain during its performance. Physical function of the FAI group was assessed with the International Hip Outcome Tool (25) and the Copenhagen Hip and Groin Outcome Score (36) (0 = extreme hip and/or groin problems, 100 = no hip and/or groin problems).
Marker trajectories and ground reaction forces were low-pass filtered at 6 Hz using a dual-pass, second-order Butterworth filter. Spatiotemporal variables of descent speed (m·s−1), ascent speed (m·s−1), and squat depth were measured for each trial. The test-leg hip joint center was used to calculate velocities. Maximum depth was defined as the lowest point reached by the test-leg hip joint center during the squat subtracted from its maximum height from the floor while standing, and normalized as a percentage of leg length. Hip, pelvis, and trunk kinematics were measured for each trial using the UWA model programmed in Vicon Body Builder (9). The trunk segment was defined with a sternum only coordinate system in participants who exhibited a large amount of forward trunk lean at maximum depth (unconstrained task), which forced the manubrium marker out of view. Similarly, the additional iliac crest markers were used to define the pelvis if the anterior superior iliac spine markers became occluded (i.e., at unconstrained maximum depth). Pelvic angles were determined using a rotation–obliquity–tilt Cardan angle sequence (6). Inverse dynamics, also programmed in Vicon Body Builder (9), were used to calculate net external hip moments. Joint moment data were normalized to body weight (N) multiplied by body height (m) and expressed as a percentage (N·m/BW·HT [%]). One control participant's data (unconstrained squat) and one FAI participant's data (constrained squat) were excluded because of a malfunction of one of the force plates. Data analysis was undertaken using Matlab, version 2013a (The Mathworks, Inc., Natick, MA) and Microsoft Excel, version 14.4.1 (Microsoft Corporation, Redmond, WA).
Peak pelvis angles (expressed in relation to the laboratory coordinate system) and hip angles (measured as femur relative to pelvis) and ROM (measured as peak-to-peak excursion) in each plane (sagittal, frontal, and transverse) were calculated. Peak trunk angles (expressed in relation to the laboratory coordinate system) in the sagittal and frontal planes were calculated. All discrete spatiotemporal, kinematic, and hip moment data were averaged over the trials for each type of squat. All data were explored for normality. Spatiotemporal and demographic variables for each type of squat were examined for between-group differences using independent t-tests and Pearson's chi-square as appropriate. Between-group kinematic and kinetic comparisons were made using independent t-tests and Mann–Whitney U tests as required. When significant differences (P < 0.05) in squat speed were identified between the groups, an analysis of covariance was also performed using squat speed as a covariate. Within-participant variability was calculated by time normalizing the trial data to 100 points, then computing the SD of all trials for each of the 100 data points; participant variability for each task was expressed as the mean of these 100 SD. Between-group comparisons were made for variability using independent t-tests. Pearson's correlation coefficient (r) was used to explore associations between hip flexion and ipsilateral pelvic rise at the end of squat descent during the constrained squat. Statistical analyses were performed using the Statistical Package for the Social Sciences version 22 (IBM, New York, NY) using an alpha of 0.05.
Participant characteristics and clinical data are presented in Table 1. The FAI and control groups were comparable for age, height, sex, and BMI. Sporting activity level (Tegner activity scale) was significantly higher in the control group at the time of testing (mean difference = 1.4, P = 0.04). Both groups reported mean activity level in the “physical fitness, moderate to strenuous work” category. FAI participants reported an average pain score of 2 on the NRS after completing both the unconstrained (range 0–8) and constrained squats (range 0–6). There were no significant differences for patient pain between the tasks (P = 0.8). Eight participants with FAI (53%) reported bilateral symptoms concurrent with radiographic findings.
Descent speed for the FAI group was significantly slower than controls during the unconstrained squat (P < 0.05). With one exception, participants with FAI did not differ significantly in a systematic manner from controls with respect to trunk, pelvis, or hip kinematics (Table 2). As predicted, but not statistically significant in the unconstrained task, there was a tendency for the FAI group to have greater forward trunk lean. This feature failed to reach significance because it was not adopted systematically for the group. One kinematic feature did reach significance; the FAI group had greater pelvic rise on the ipsilateral side (i.e., the pelvis did not descend as far on the ipsilateral side) than controls (adjusted mean difference = 2.3°, 95% confidence interval [CI] = 0.2°–4.5°, P = 0.04) (Table 2, Fig. 2) at maximum squat depth.
Participants with FAI demonstrated a significantly lower hip flexion moment than the control group (P = 0.02), but this was not statistically significant when adjusted for descent speed. There were no significant between-group differences for peak joint moments. Within-group variations (SD) were large across kinematic and kinetic variables (Table 2), particularly in the FAI group. Visual inspection of ensemble-averaged hip kinematic and kinetic curves (Fig. 2), time normalized over a squat cycle, demonstrated comparable spatial and temporal characteristics between groups.
As planned by the task design, forward trunk lean (mean difference = 24.4°, 95% CI = 9.0°–30.0°, P < 0.01) and anterior pelvic tilt (mean difference = 7.3°, 95% CI = 2.1°–12.4°, P = 0.01) were significantly less during the constrained squat than the unconstrained squat for all participants. There were no frontal or transverse plane kinematic differences for the trunk or pelvis between squat tasks.
There were no significant differences between groups for timing and amplitude measures of squatting during the constrained task (Table 2). When participants were restricted from using the trunk or sagittal pelvis motion to enable deep squatting, participants with FAI demonstrated greater ipsilateral pelvic rise than controls (mean difference = 2.4°, 95% CI = 0.6°–4.1°, P = 0.01) (Fig. 3) at maximum squat depth and across the entire task (mean difference = 1.7, 95% CI = 0.05°–3.3°, P = 0.04), indicating a more universal adoption of this strategy by the individual in the FAI group, when other options for compensation were restricted (i.e., forward trunk lean). With respect to the hip, participants with FAI were significantly more adducted/less abducted throughout the squat than controls (peak mean difference = 2.6°, 95% CI = 0.2°–5.1°, P = 0.04). This difference was significant at the beginning of the task, although inspection of Figure 3 implies a systemic difference in frontal plane angle between the groups throughout the squat cycle. Consistent with the design of the task, there were no between-group differences in trunk kinematics. The hip external rotation moment was significantly less in participants with FAI during descent than controls (mean difference = 0.2%, 95% CI = 0.01%–0.4%, P = 0.04) (Table 2, Fig. 3). There were no other significant between-group differences for hip kinematics or kinetics.
Although when all participants with FAI were considered together, there was no relationship between hip flexion at the end of squat descent and ipsilateral pelvic rise at the same time point (r = −0.09, P = 0.76), it was apparent that two participants reached a hip flexion angle that approximated that in the control group (30°–35° > FAI group mean). When these two participants were excluded from the analysis, a negative correlation was apparent that narrowly missed significance (r = −0.55, P = 0.06); less hip flexion corresponded to higher pelvic rise, and higher hip flexion corresponded to greater pelvic drop. The opposite tendency toward an association was found for the control group, whereby hip flexion at the end of descent corresponded to higher pelvic rise (r = 0.41, P = 0.15) (Fig. 4).
There were no differences in within-participant variability for any measures during the unconstrained squat. The participants with FAI had significantly less transverse plane hip angle variation (P = 0.04) than control participants during the constrained squat.
This exploratory cross-sectional study aimed to explore whether hip and pelvis biomechanics differ between individuals with and without symptomatic FAI during an unconstrained deep squat and a constrained squat designed to limit compensation by the pelvis and trunk. Findings demonstrate that without constraint, individuals with symptomatic FAI can squat to a depth comparable with healthy controls, albeit at a reduced speed, with few systematic differences in biomechanics between groups. Differences were more apparent when the performance of the task was constrained. When anterior pelvic tilt and forward trunk lean were constrained, individuals with symptomatic FAI demonstrated greater pelvic rise on the symptomatic side (which would reduce the demand for hip flexion) at maximum depth and across the task and maintained a more adducted hip position (which would be expected to approximate the hip is counterproductive for the pathology). This difference could be a compensatory strategy imparted to avoid end range flexion as the hip approaches a potentially provocative position.
Three previous studies have compared unconstrained deep squatting between individuals with symptomatic FAI and pain-free controls (5,22,24). Hip biomechanics data have been conflicting between studies. One study showed no differences (24), another study showed greater hip adduction angle and internal rotation moment in symptomatic FAI (22), and the third study showed a smaller hip internal rotation angle and mean hip extensor moment in symptomatic FAI (5). Although all studies included individuals with cam-only FAI, sample sizes ranged from 7 to 15, task designs varied, and marker models differed. Similar to Lamontagne et al. (24), we found no difference in hip biomechanics during an unconstrained deep squat. However, our data do not support the secondary finding from the aforementioned study regarding lesser squat depth in FAI. Data from our cohort suggest that individuals with symptomatic FAI can reach a squat depth comparable with controls without FAI when permitted to use an unconstrained strategy. Several issues could explain the difference. We used a wedge to encourage deep hip flexion unrestricted by ankle dorsiflexion range, given the reduced ankle dorsiflexion previously reported in symptomatic FAI (23) and the potential for limitation at the ankle to influence other joints in the lower limb kinematic chain, including the hip. Thus, reduced squat depth during a task where heel contact is maintained with the ground (5,24) may be confounded by a limitation at the ankle. Further, depth measurement or group characteristics (cam-only patients, 10 yr older, and unknown physical activity levels) may contribute to differences.
Our finding of reduced descent speed may be related to pain. Most but not all of the participants in our FAI group reported some minor pain during the task (mean = 2/10 on NRS, range = 0–8). They may have slowed the squatting speed when moving toward the impingement position to reduce the potential for pain or in anticipation of pain.
Given that participants from both groups were instructed to use their preferred squatting strategy in the unconstrained task, the resulting kinematic data are highly variable, which might account for some of our nonsignificant findings, particularly for hip internal rotation angle, which demonstrated a trend toward significance and has been previously reported as less in FAI (5). Large SD values in both groups imply that different strategies are used to execute the task. Greater within-group SD values observed in the FAI group compared with the control group at the trunk and pelvis in the sagittal plane imply individual solutions aimed at reducing the burden on the hip while maintaining functional performance.
Less sagittal plane pelvis ROM has been identified in symptomatic FAI (5,20,24) and proposed as a risk factor for symptoms (26). Kinematics of the femur and pelvis are coupled. Recent data for healthy individuals demonstrated 1.2°–1.6° of internal femur rotation for each 5° of anterior pelvic tilt (4). Participants in our FAI cohort did not demonstrate less pelvis ROM in the sagittal plane, perhaps because of variability in squatting strategies between participants (i.e., large within-group SD), our small sample size (albeit comparable with previous studies), or the placement of the ankle in a plantar flexed position. Nevertheless, our findings and these aforementioned studies relating to a deep unconstrained squat do not show systematic differences between individuals with symptomatic FAI and those free from the condition, which might relate to contribution from the pelvis of varying degrees depending on task design (5,22,24).
In our constrained squat, anterior pelvic tilt and forward trunk lean were reduced in both groups, which confirm that we were successful in limiting the contribution of these segments to the squat. When constrained in this manner, individuals with FAI placed the hip in a more adduction/less abduction position. This equated to a 3° difference, which was statistically significant at the beginning of the task, although the ensemble curves suggest a systemic difference in frontal plane angle between the groups throughout the squat cycle. Greater adduction supports the findings of Kumar et al. (22), who reported greater peak hip adduction in FAI and presented values and frontal plane curves similar to our cohort. These similarities may be explained by their task design, which allowed for participants to lift their heels while squatting (comparable with the addition of a wedge). Given that individuals with FAI were in more hip adduction at the beginning of the task, this finding could reflect a habitual posture in those with FAI, which is likely to be unfavorable given that adduction is a direction of impingement. This alteration may be a consequence of hip abductor weakness (previously reported in FAI [11,13]) or as a mechanism to reduce the demands on this muscle group. The FAI group also exhibited a lesser hip external rotation moment during descent, which may be attributed to weakness (17) or imbalance across the rotators (13), but again relevant for control into the impinging position.
Participants with FAI demonstrated a 2° greater pelvic rise on the symptomatic side (i.e., they did not descend as low on that side) at the end of descent during the constrained squat. Although small, it could be interpreted to suggest a compensatory strategy to avoid end range hip flexion and thus impingement, although the clinical relevance of a difference this size is unknown. The compensations were emphasized when anterior pelvic tilt and forward trunk lean are constrained, which perhaps removed some of the variety for solutions naturally adopted, preventing the identification of differences in the unconstrained task. Although our data do not show lesser hip flexion in all FAI patients, an exploratory analysis highlighted that most individuals with FAI raise the pelvis on the symptomatic side, which would allow for a deeper squat with less flexion at the hip joint. This adaptation may be a consequence of hip morphology, pain, or an avoidance strategy imparted as the hip approaches the symptomatic position. Although it remains unclear why two individuals with FAI could achieve deep hip flexion without using this strategy, and in fact applied a strategy more similar to individuals with healthy hips, it could relate to the size (smaller than typical) or location (posterior instead of the more common anteriosuperior) of their femoral lesion. Future investigations should consider subgrouping patients based on FAI morphology.
An interesting observation was the reduced variability in participants with FAI with respect to transverse plane hip angle. Reduced variability may suggest that individuals with healthy hips have the flexibility to vary femur rotation even when pelvic tilt is constrained, whereas those with symptomatic FAI do not or cannot vary their femur position when the hip is forced into deep flexion in the absence of pelvic tilt. Lesser variability in kinematics has been demonstrated previously in individuals with other conditions (37) and could be an attempt to constrain the task (less variation could reduce the potential for error) with a benefit for joint health in the short term. However, there may also be long-term structural implication if areas of the joint are consistently under- or overloaded, or if the load is not adequately shared across the joint.
Characterization of the physical alterations in individuals with symptomatic FAI is needed to understand the condition, including its development and progression—ultimately with the goal to develop and optimize effective conservative management programs. The results of this exploratory study add to the growing body of evidence identifying the biomechanical alterations related to symptomatic FAI. Our findings suggest that individuals with symptomatic FAI can squat to a depth comparable with healthy controls, regardless of task design, but impart an altered biomechanical strategy to achieve functional performance when compensation by the pelvis and trunk is constrained. Although the implications of these findings for symptoms and function are not yet clear, they may suggest a compensatory strategy with possible long-term consequences (e.g., increased load on adjacent joints or the contralateral hip; sustained muscle weakness leading to suboptimal joint mechanics). Such strategies may have particular relevance for athletes who repeatedly perform tasks with similar demands (i.e., receiving a serve in tennis, baseball catching, and ice hockey goalkeeping) in sports where FAI has been identified (29).
Physical alterations (e.g., altered lower limb biomechanics during walking and stair climbing) have been identified in individuals with symptomatic FAI postoperatively (10,32). One interpretation is that preoperative treatments addressing functional abnormalities in these patients may have scope to improve surgical outcomes. However, investigation of larger cohorts is required. A larger patient group could provide insight into any sex-related alterations during squatting. An interlimb investigation, particularly in a unilateral FAI cohort, warrants further investigation. A standardized squat design has benefits for use across future studies as it appears to reduce some variation between individuals and expose task differences in a more systematic manner. Further, a standardized design may help achieve the goal of identification of targets for treatment in individuals with symptomatic FAI. Individuals with symptomatic FAI who experience higher levels of pain during the task may exhibit more pronounced compensations than those we observed. Hip muscle flexibility was not assessed, although it could inherently influence some of the findings presented here. Interpretation of this study is limited by the small sample size, the cross-sectional design, and the inclusion of bilateral FAI patients. A further limitation is the use of multiple statistical comparisons, which have the potential to increase the risk of type 1 error. A statistical correction was not performed due of the exploratory nature of this study (8,28).
Individuals with symptomatic FAI can squat to a depth comparable with controls without FAI, regardless of task design. Biomechanical differences are evident at the hip and pelvis when the task is constrained. The ipsilateral pelvic rise is likely a compensatory strategy to avoid end range flexion as the hip approaches impingement. These biomechanical alterations may put additional stress on adjacent regions and have relevance in the clinical management of individuals with symptomatic FAI.
Funding was provided by a Program Grant from the National Health and Medical Research Council (NHMRC) of Australia (ID631717). K. B. is supported by a Principal Research Fellowship (APP1058440) from the NHMRC. R. H. (FT130100175) is supported by a Future Fellowship from the Australian Research Council, and P. H. is supported by a Senior Principal Research Fellowship (APP1002190) from the NHMRC. Results of this study are presented clearly, honestly, and without fabrication, falsification, or inappropriate data manipulation.
The authors declare no conflict of interest. Results of this study do not constitute endorsement by the American College of Sports Medicine.
1. Alshameeri Z, Khanduja V. The effect of femoro-acetabular impingement on the kinematics
of the hip joint
. Int Orthop
2. Anderson LA, Peters CL, Park BB, Stoddard GJ, Erickson JA, Crim JR. Acetabular cartilage delamination in femoroacetabular impingement. Risk factors and magnetic resonance imaging diagnosis. J Bone Joint Surg Am
3. Audenaert EA, Peeters I, Vigneron L, Baelde N, Pattyn C. Hip morphological characteristics and range of internal rotation in femoroacetabular impingement. Am J Sports Med
4. Bagwell JJ, Fukuda TY, Powers CM. Sagittal plane pelvis
motion influences transverse plane motion of the femur: kinematic coupling at the hip joint
. Gait Posture
5. Bagwell JJ, Snibbe J, Gerhardt M, Powers CM. Hip kinematics
in persons with and without cam femoroacetabular impingement during a deep squat
task. Clin Biomech (Bristol, Avon)
6. Baker R. Pelvic angles: a mathematically rigorous definition which is consistent with a conventional clinical understanding of the terms. Gait Posture
7. Beck M, Kalhor M, Leunig M, Ganz R. Hip morphology influences the pattern of damage to the acetabular cartilage: femoroacetabular impingement as a cause of early osteoarthritis of the hip. J Bone Joint Surg Br
8. Bender R, Lange S. Adjusting for multiple testing—when and how? J Clin Epidemiol
9. Besier TF, Sturnieks DL, Alderson JA, Lloyd DG. Repeatability of gait data using a functional hip joint
centre and a mean helical knee axis. J Biomech
10. Brisson N, Lamontagne M, Kennedy MJ, Beaulé PE. The effects of cam femoroacetabular impingement corrective surgery on lower-extremity gait biomechanics. Gait Posture
11. Casartelli NC, Maffiuletti NA, Item-Glatthorn JF, et al. Hip muscle weakness in patients with symptomatic femoroacetabular impingement. Osteoarthritis Cartilage
12. Diamond LE, Wrigley TV, Bennell KL, Hinman RS, O'Donnell J, Hodges PW. Hip joint
biomechanics during gait in people with and without symptomatic femoroacetabular impingement. Gait Posture
13. Diamond LE, Wrigley TV, Hinman RS, et al. Isometric and isokinetic hip strength and agonist/antagonist ratios in symptomatic femoroacetabular impingement. J Sci Med Sport
14. Flanagan S, Salem GJ, Wang MY, Sanker SE, Greendale GA. Squatting exercises in older adults: kinematic and kinetic comparisons. Med Sci Sports Exerc
15. Frank JM, Harris JD, Erickson BJ, et al. Prevalence of femoroacetabular impingement imaging findings in asymptomatic volunteers: a systematic review. Arthroscopy
16. Ganz R, Parvizi J, Beck M, Leunig M, Nötzli H, Siebenrock KA. Femoroacetabular impingement: a cause for osteoarthritis of the hip. Clin Orthop Relat Res
17. Harris-Hayes M, Mueller MJ, Sahrmann SA, et al. Persons with chronic hip joint
pain exhibit reduced hip muscle strength. J Orthop Sports Phys Ther
18. Hunt MA, Guenther JR, Gilbart MK. Kinematic and kinetic differences during walking in patients with and without symptomatic femoroacetabularimpingement. Clin Biomech (Bristol, Avon)
19. Kainz H, Carty CP, Modenese L, Boyd RN, Lloyd DG. Estimation of the hip joint
centre in human motion analysis: a systematic review. Clin Biomech (Bristol, Avon)
20. Kennedy MJ, Lamontagne M, Beaulé PE. Femoroacetabular impingement alters hip and pelvic biomechanics during gait Walking biomechanics of FAI. Gait Posture
21. Kubiak-Langer M, Tannast M, Murphy SB, Siebenrock KA, Langlotz F. Range of motion in anterior femoroacetabular impingement. Clin Orthop Relat Res
22. Kumar D, Dillon A, Nardo L, Link TM, Majumdar S, Souza RB. Differences in the association of hip cartilage lesions and cam-type femoroacetabular impingement with movement patterns: a preliminary study. PM R
23. Lamontagne M, Brisson N, Kennedy MJ, Beaulé PE. Preoperative and postoperative lower-extremity joint and pelvic kinematics
during maximal squatting of patients with cam femoro-acetabular impingement. J Bone Joint Surg Am
. 2011;93(2 Suppl):40–5.
24. Lamontagne M, Kennedy MJ, Beaulé PE. The effect of cam FAI on hip and pelvic motion during maximum squat
. Clin Orthop Relat Res
25. Mohtadi NG, Griffin DR, Pedersen ME, et al. The development and validation of a self-administered quality-of-life outcome measure for young, active patients with symptomatic hip disease: the International Hip Outcome Tool (iHOT-33). Arthroscopy
. 2012;28(5):595–605; quiz 6–10.e1.
26. Ng KC, Lamontagne M, Adamczyk AP, Rakhra KS, Beaulé PE. Patient-specific anatomical and functional parameters provide new insights into the pathomechanism of cam FAI. Clin Orthop Relat Res
27. Nötzli HP, Wyss TF, Stoecklin CH, Schmid MR, Treiber K, Hodler J. The contour of the femoral head-neck junction as a predictor for the risk of anterior impingement. J Bone Joint Surg Br
28. Perneger TV. What's wrong with Bonferroni adjustments. BMJ
29. Philippon M, Schenker M, Briggs K, Kuppersmith D. Femoroacetabular impingement in 45 professional athletes: associated pathologies and return to sport following arthroscopic decompression. Knee Surg Sports Traumatol Arthrosc
30. Philippon MJ, Maxwell RB, Johnston TL, Schenker M, Briggs KK. Clinical presentation of femoroacetabular impingement. Knee Surg Sports Traumatol Arthrosc
31. Pun S, Kumar D, Lane NE. Femoroacetabular impingement. Arthritis Rheumatol
32. Rylander J, Shu B, Favre J, Safran M, Andriacchi T. Functional testing provides unique insights into the pathomechanics of femoroacetabular impingement and an objective basis for evaluating treatment outcome. J Orthop Res
33. Simic M, Hunt MA, Bennell KL, Hinman RS, Wrigley TV. Trunk lean gait modification and knee joint load in people with medial knee osteoarthritis: the effect of varying trunk lean angles. Arthritis Care Res (Hoboken)
34. Stelzeneder D, Hingsammer A, Bixby SD, Kim YJ. Can radiographic morphometric parameters for the hip be assessed on MRI? Clin Orthop Relat Res
35. Tegner Y, Lysholm J. Rating systems in the evaluation of knee ligament injuries. Clin Orthop Relat Res
36. Thorborg K, Hölmich P, Christensen R, Petersen J, Roos EM. The Copenhagen Hip and Groin Outcome Score (HAGOS): development and validation according to the COSMIN checklist. Br J Sports Med
37. van den Hoorn W, Bruijn SM, Meijer OG, Hodges PW, van Dieën JH. Mechanical coupling between transverse plane pelvis
and thorax rotations during gait is higher in people with low back pain. J Biomech