IMPROVEMENTS IN KNEE EXTENSION STRENGTH ARE ASSOCIATED WITH IMPROVEMENTS IN SELF-REPORTED HIP FUNCTION FOLLOWING ARTHROSCOPY FOR FEMOROACETABULAR IMPINGEMENT SYNDROME

Abstract

Background

Recovery of strength is critical for return to sport, and is a known predictor of functional outcomes in post-surgical orthopedic populations. Muscle weakness is a known impairment in patients with femoroacetabular impingement syndrome (FAIS) but whether improvements in muscle strength after arthroscopy are associated with improved hip function is unknown.

Hypothesis/Purpose

To examine the relationships between changes in hip and thigh muscle strength and self-reported function in athletes undergoing arthroscopy for FAIS.

Study Design

Single cohort descriptive and correlational study

Methods

Twenty-eight athletes underwent strength testing and completed the Hip Outcome Score Activities of Daily Living (HOS-ADL) and Sports (HOS-S) subscales prior to and six months after surgery. Isokinetic knee extension and flexion strength were measured using a Biodex dynamometer at 60 °/s and 300 °/s. Isometric hip abduction strength was measured using a custom dynamometer. Changes in strength, limb symmetry, and HOS scores were assessed using paired t-tests. Spearman's rank correlations were used to examine relationships between change in involved limb strength and change in HOS scores.

Results

Subjects were tested an average of 32 days before and 178 days after surgery. HOS-ADL and HOS-S subscales improved by a mean of 19.0 ± 21.1 and 23.8 ± 31.9, respectively, over time (p < 0.001). Hip abduction strength did not increase over time in either limb (p ≥ 0.27). Involved limb knee flexion and extension strength did not increase significantly over time (p-values: 0.10-0.48) with the exception of knee extension at 300 °/s (p = 0.04). Uninvolved limb knee extension strength at both velocities and knee flexion strength at 60 °/s improved significantly over time (p < 0.012). Increases in knee extension strength (60 °/s) of the involved limb were significantly correlated with improvements on the HOS-ADL (r = 0.431; 0 = 0.025) and HOS-S (r = 0.439; p = 0.025). There were no significant relationships between changes in involved limb hip abduction or knee flexion strength and HOS subscales (p≥0.123).

Conclusion

Improvements in knee extension strength were associated with improvements in self-reported hip function in athletes following arthroscopy for FAIS. Individuals with knee extension strength deficits prior to surgery may benefit from targeted knee extension strengthening during post-operative rehabilitation to improve functional outcomes.

Level of Evidence

Level III (non-randomized controlled cohort study)

INTRODUCTION

Femoroacetabular impingement (FAI) is increasingly identified as a significant cause of hip pain and disability in athletes due to advances in medical imaging and knowledge of structural hip pathologies.^1,2 FAI is characterized as an abnormal bony morphology of the femoral head-neck junction (cam deformity), the acetabulum (pincer deformity), or both (combined cam and pincer deformity), which commonly develops during adolescence^3,4 and can result in the premature development of hip osteoarthritis.^4-6 Surgical resection of the bony lesion is commonly performed using arthroscopic techniques after failure of non-operative management, and can result in favorable outcomes for the majority of affected athletic individuals.^2,5 Importantly, some athletes do not return to their previous level of function following arthroscopy, and efforts to identify modifiable impairments related to functional limitations are critical to maximizing positive outcomes.

Individuals with FAI syndrome (FAIS) (i.e. those with pain and functional limitations associated with FAI morphology) commonly demonstrate hip muscle weakness^7,8 and altered kinematics of the involved hip during functional and sporting activities.^9,10 Compared to uninjured controls, subjects with FAIS demonstrate deficits in hip flexion, abduction, external rotation, and adduction strength.⁸ Furthermore, hip weakness and muscle performance deficits are hypothesized to contribute to altered kinematics, including increased hip adduction and internal rotation, during dynamic single leg tasks in subjects with FAIS.^9,10 Casartelli et al.⁷ found that at 2.5 years after hip arthroscopy, maximal voluntary contraction for all hip muscle groups had returned with the exception of hip flexion strength in a small cohort of patients. Although these results are favorable at two and a half years post-operatively, return to sport for an athletic population following hip arthroscopy may occur as early as six months post-operatively.^11-14

Thigh muscle weakness, including the knee flexors and extensors, may also be a critical factor in the recovery of function in patients with FAIS. Quadriceps femoris strength is a known predictor of functional outcomes in other post-surgical orthopedic populations including anterior cruciate ligament reconstruction¹⁵ and total hip replacement.¹⁶ Due to their role as major force producers in the lower extremity for both daily and sporting activities, deficits in thigh muscle strength may have a significant impact on function following hip arthroscopy. Importantly, adequate thigh muscle strength is highly relevant for athletes attempting to return to cutting, pivoting, and jumping sports.

Muscle strength symmetry is a common criterion for injured athletes to achieve prior to returning to sports because of its association with improved function,¹⁷ more symmetrical biomechanics,^18,19 and lower re-injury risk.²⁰ However, this hypothesis has not been validated in patients with FAIS, despite the fact that muscle weakness is characteristic of this painful syndrome. The relationship between muscle weakness and self-reported function in the short-term (i.e. within the first six months of surgery and during the course of supervised rehabilitation) needs to be quantified in order to help develop the most effective post-operative rehabilitation program. The purpose of this study was to examine the relationship between changes in hip and thigh muscle strength and changes in self-reported function in athletes with FAIS before and six months after hip arthroscopy. The primary hypothesis was that increases in hip abduction, knee extension, and knee flexion strength of the involved limb after surgery would be positively associated with improvements on the Hip Outcome Score (HOS) Activities of Daily Living (HOS-ADL) and Sports (HOS-S) subscales. The second hypothesis was that involved limb weakness in hip abduction and knee extension and flexion compared to the uninvolved limb prior to surgery would be resolved at the 6-month post-operative follow-up.

METHODS

Participants

Athletes who were undergoing hip arthroscopy for FAIS were recruited from a single orthopedic practice from April 2012 to December 2013. Athletes between the ages of 14 and 50 were eligible if they were diagnosed with FAIS per radiographic findings and clinical examination, and were regular participants in cutting, jumping, pivoting, or lateral movement activities for at least 50 hours per year prior to the onset of hip symptoms.²¹ Radiographic guidelines for FAIS included: alpha angle > 50 ° for cam impingement or presence of acetabular retroversion and/or coxa profunda for pincer impingement and minimal degenerative hip changes (Tönnis grade > 1). Potential subjects were excluded if they: (1) were diagnosed with osteopenia or osteoporosis, (2) were diagnosed with moderate to advanced osteoarthritis at the time of the procedure (Tönnis grade > 1 on radiographic examination), or (3) had a history of lumbar spine or lower extremity surgery (e.g. laminectomy or knee ligament reconstruction) or injury that required a period of immobilization or non-weight-bearing. Criteria for study inclusion were reviewed by the patient's orthopaedic surgeon (TE) and a licensed physical therapist (SD) prior to enrollment. A total of 42 subjects were recruited and informed consent was obtained from all participants in this study, in accordance with the study protocol, as approved by the The Ohio State University Institutional Review Board.

Surgical Procedures and Rehabilitation Program

All hip arthroscopies were performed by a board-certified orthopedic surgeon and included femoral and/or acetabular osteoplasty and/or labral repair. A standardized post-hip arthroscopy physical therapy guideline was used in The Ohio State University Wexner Medical Center (OSUWMC) Sports Medicine Physical Therapy clinics. These physical therapy guidelines were provided to patients undergoing rehabilitation at non-OSUWMC facilities to give to their physical therapist. The clinical guideline used for the current study was developed based on current literature^22-26 and is consistent with the clinical practice guideline for non-arthritic hip pain.²⁷ The rehabilitation guideline consists of four phases with subjective and objective criteria to progress through the phases. Initially, progression is limited by tissue healing and is later focused on subjective reports of pain and performance of objective tests that address strength and limb symmetry.

Testing Procedures

Self-reported outcomes and strength data were collected prior to surgery and six months post-operatively.

Self-Reported Outcome Measures

Subjects were asked to complete the Hip Outcome Score Activities of Daily Living (HOS-ADL) and Sports (HOS-S) subscales. The HOS-ADL subscale contains 19 items (17 scored) pertaining to basic daily activities, and the HOS-S subscale contains 9 scored items pertaining to higher-level athletic activities.²⁸ Each question is activity-based, and ranges from a score of 0 (unable to do) to 4 (no difficulty). Higher scores (maximum of 100%) on both subscales represent better function. The HOS is valid,²⁸ reliable,^29,30 and responsive to change^30,31 in this population. HOS-ADL and HOS-S outcome measures were completed pre-operatively and 6 months post-operatively.

Assessment of Knee Extension and Flexion Strength

Isokinetic strength testing was used to assess the concentric strength of the knee extensor and knee flexor muscle groups of both limbs by a single physical therapist. Subjects were seated upright on an isokinetic dynamometer (Biodex System 3, Biodex Medical Systems, Inc, Shirley, NY) with their hips and knees flexed to 90 °. Chair depth, height, placement, and length of attachment arm were adjusted for each subject and recorded for consistency between test sessions. Subjects were secured with straps at the distal shank, mid-thigh, pelvis, and trunk. All tests were performed on the uninvolved lower extremity first and range of motion was set from 100 ° to 10 ° of knee flexion. Two sets of five repetitions were performed at 60 °/second and then two sets of ten repetitions were performed at 300 °/second with 90-second rest breaks between tests. Subjects were instructed to “kick and pull through the whole range of motion as fast and hard as you can,” and then given two practice repetitions prior to each test set to ensure comfort with the task and proper strap fixation. Standardized verbal encouragement was provided to elicit maximal effort from each subject. Similar methods of assessing isokinetic thigh strength have been shown to be highly reliable (ICCs 0.82-0.91) and valid.³² Peak flexion and extension torque values (ft-lbs) were recorded for all testing sets; the peak torque value from both sets for each muscle group was normalized to body weight in pounds and used in the analyses. Limb symmetry index (LSI) values were calculated with the peak normalized torque value of the involved limb divided by the peak normalized torque value of the uninvolved limb and multiplied by 100.

Assessment of Hip Abduction Strength

Isometric hip abduction strength was assessed using a custom dynamometer with a strain gauge. All hip strength measurements were collected by the same licensed physical therapist (SD). Hip abduction strength was tested in the side-lying position with the subject's head, scapula, buttocks, and heels against the wall (Figure 1). The straps of the custom dynamometer were then placed just proximal to the femoral condyles. Prior to testing, subjects were instructed to place their hand lightly on the table for balance, while the examiner manually secured the position of the bottom/non-testing limb to the mat. The subject was instructed to gradually increase force over the verbal count of three then to produce a maximal voluntary contraction against the straps for five seconds. Subjects were given two practice trials and then performed two testing trials for each limb. The peak force value of the two trials was used for analyses. A regression equation based on standardized weight measurements was used to convert test output from millivolts to pounds. Data were then normalized to the subject's body weight in pounds. LSI was calculated as peak normalized force value of the involved limb divided by the peak normalized force value of the uninvolved limb and multiplied by 100. These testing methods have demonstrated good intra-rater reliability for the assessment of hip abduction strength in healthy controls in the OSUWMC Sports Medicine Biodynamics Laboratory (ICC [2,1] = 0.875, minimal detectable change = 2.1 lb).

Figure 1.

Hip abduction strength testing set up.

Data Analyses

Data was analyzed with SPSS version 20 (SPSS Inc., Chicago, IL, USA). Assumptions of parametric testing were assessed with histograms, scatterplots, and Shapiro-Wilk tests. Spearman's rank correlation coefficients were calculated to quantify the relationships between the change scores in normalized hip and thigh strength and HOS subscales. Paired t-tests were used to assess change in involved and uninvolved limb strength, limb symmetry, and self-reported measures over time. Frequency counts were used to describe the number of subjects who demonstrated clinically significant weakness before and after surgery, and how many underwent a clinically significant change in strength over the same period. Clinically significant weakness of the involved limb was operationally defined as at least a 10% deficit on the involved limb compared to the uninvolved limb. Since recommendations on adequate strength symmetry in this population have not been defined,^22,27 our operational definition of clinically significant weakness was based on return to activity strength criteria for patients who have undergone knee ligament reconstruction.³³ A clinically significant change in strength was operationally defined as at least a 10% change³⁴ from pre-operative to post-operative testing. Statistical significance was set a priori at α = 0.05.

RESULTS

All of the 42 enrolled subjects completed pre-operative testing for knee extension and knee flexion strength testing. Twenty-eight of the 42 enrolled athletes returned for six-month post-operative testing and were included in the final analysis (Table 1). Of those lost to follow-up, one elected to not have surgery, six elected to drop out of the study prior to post-operative testing, one refused to return for testing, five had contralateral hip surgery, and one had an unrelated medical condition that precluded post-operative testing. Subjects in the final sample (N = 28) did not differ significantly from subjects in the initial sample (N = 42) in age, sex distribution, BMI, time from pre-operative testing to surgery, or HOS subscale scores (p>0.05). Of the 28 subjects who participated in both testing sessions, three subjects did not complete either pre- or post-operative strength testing due to testing device malfunction and one subject could not complete pre-operative strength testing due to time limitations.

Table 1.

Demographic characteristics

	Pre-Operative Testing	Six Month Post-Operative Testing
Sex
Female	22	15
Male	18	13
Involved Limb
Right	20	15
Left	20	13
Age (years, means ± SD) (min-max)	24.9 ± 9.9 (15-49)	25.7 ± 10.4 (15-49)
Mass (kg, means ± SD)	70.9 ± 13.4	72.5 ± 13.1
BMI (kg/m2, means ± SD)	24.6 ± 3.9	24.8 ± 3.4
Time from Testing to Surgery (days, means ± SD)	24 ± 21	-
Time from Testing to Surgery (days, means ± SD) with drop-outs*	30 ± 41
Time from Surgery to Testing (days, means ± SD)	-	180 ± 32

Abbreviations: kg, kilograms; m, meters

^*One individual who dropped out of the study prior to six month testing completed a protracted bout of conservative treatment (physical therapy included) prior to having surgery

The HOS-ADL scores increased significantly from pre-operative to six-month post-operative assessments (mean ± SD; pre-operative: 68.9 ± 18.0, six-month post-operative: 88.1 ± 14.2; p < 0.001). Similarly, the HOS-S scores significantly increased from pre-operative to six-month post-operative assessments (pre-operative: 50.82 ± 21.70; six-month post-operative: 74.7 ± 21.8; p < 0.001).

Muscle strength asymmetry was identified in a significant portion of subjects prior to arthroscopy. Clinically significant weakness of the involved limb (<90% LSI) was found in 27.5% of the subjects for knee extension, 22.5% for knee flexion, and 30.6% for hip abduction. Conversely, an LSI of >110% was identified in 12.5% of subjects for knee extension, 22.5% for knee flexion, and 8.3% for hip abduction strength.

Isometric hip abduction strength did not change in either limb following hip arthroscopy (Table 2). Uninvolved limb isokinetic knee extension strength improved significantly over time at both velocities; however, only involved limb knee extension strength at 300°/second, not 60°/second, improved significantly over time (Table 3). Bilateral isokinetic knee flexion strength did not change over time (P ≥ 0.48) with the exception of a significant increase in the uninvolved limb at 60°/second (Table 3).

Table 2.

Normalized isometric hip abduction strength before and six months after hip arthroscopy

	Pre-Operative (N = 26)	Six Months Post-Operative (N = 28)	p-value
Involved Limb	0.30 ± 0.10	0.28 ± 0.11	0.27
Uninvolved Limb	0.31 ± 0.09	0.29 ± 0.09	0.29

*Values are represented as peak isometric force (lbs) ± standard deviation (normalized to body weight)

Table 3.

Thigh muscle isokinetic strength before and six months after hip arthroscopy

Involved Limb	Pre-Operative	Six months Post-Operative	p- value
60 °/s Extension	0.71 ± 0.15	0.74 ± 0.20	0.10
60 °/s Flexion	0.38 ± 0.10	0.39 ± 0.11	0.48
300 °/s Extension	0.42 ± 0.12†	0.44 ± 0.11	0.04
300 °/s Flexion	0.30 ± 0.09†	0.31 ± 0.07	0.48

Uninvolved Limb
60 °/s Extension	0.75 ± 0.16	0.80 ± 0.18†	0.012
60 °/s Flexion	0.38 ± 0.10	0.42 ± 0.10†	0.006
300 °/s Extension	0.43 ± 0.11	0.47 ± 0.11†	0.0004
300 °/s Flexion	0.31 ± 0.08	0.31 ± 0.07†	0.77

*Values are presented as peak torque (ft-lbs) ± standard deviation (normalized to body weight)

^†N = 27, Bold indicates statistically significant difference

There were no significant changes in mean LSI over time for any of the muscle groups tested (Figure 2). Approximately one-third of subjects demonstrated no clinically meaningful change (>10% increase or decrease) in isometric hip abduction strength of the involved limb over the testing period; 27% demonstrated an increase and 38% showed a decrease compared to pre-operative values. For involved knee extension at 60°/second, 61% demonstrated no clinically meaningful change in strength, 32% of subjects demonstrated a clinically meaningful increase and the remaining 7% showed a clinically meaningful decrease compared to pre-operative values. Involved limb knee flexion strength at 60°/second did not demonstrate a clinically meaningful change in 61% of subjects; 25% demonstrated a clinically meaningful increase, and 14% demonstrated a clinically meaningful decrease compared to pre-operative values.

Figure 2.

Limb symmetry indexes (%) of peak strength values for hip abduction, knee flexion, and knee extension pre- and post-operatively. No statistically significant changes (P ≥ 0.115). Bars represent standard deviation values. Abbreviations: SLR, straight leg raise; 6 mo, 6 month post-operative; abd, abduction; ext, extension; flex, flexion; s, second.

Increases in involved limb knee extension strength at 60°/second were significantly correlated with improvements in both the HOS-ADL (r = 0.431; p = 0.025; Figure 3) and HOS-S scores (r = 0.439; p = 0.025; Figure 3). Changes in uninvolved knee extension strength at both velocities were not correlated with increases in HOS-ADL or HOS-S scores (p ≥ 0.253; Figures 3 and 4). There were no statistically significant relationships between changes in hip abduction and knee flexion strength in either limb with changes in HOS-ADL (p ≥ 0.139) or HOS-S (p ≥ 0.123) scores.

Figure 3.

Change in Normalized Knee Extension Strength at 60 deg/s and HOS-ADL and HOS-S over Time. Torque measured in ft-lbs and normalized to subject weight (lbs). Abbreviations: deg, degrees; s, second.

Figure 4.

Change in Normalized Knee Extension Strength at 300 deg/s and HOS-ADL and HOS-S over Time. Torque measured in ft-lbs and normalized to subject weight (lbs). Abbreviations: deg, degrees; s, second.

DISCUSSION

In the present study, the relationships between changes in hip abduction, thigh muscle strength, and self-reported hip function in athletes with FAIS were evaluated before and after hip arthroscopy. The findings partially supported the a priori hypothesis that strength gains in the involved limb would be associated with increases in both ADL and sports-related self-reported hip function. Only mean involved limb knee extension strength at 300°/second improved over time this was not associated with improvements in self-reported hip function. Despite a lack of significant improvement in the group mean knee extension strength at 60°/second, there was a positive association between improvements in low velocity knee extension strength and self-reported hip function. This relationship has not been previously identified in patients with FAIS, and adds important knowledge to the current understanding of muscle function impairments in athletes with FAIS.

Weakness of hip musculature is commonly identified in individuals with painful, non-arthritic hip pathology.^7,8,31 In this study, mean hip abduction strength did not improve following hip arthroscopy and improvements in hip abduction strength were not associated with improvements in self-reported hip function. In a recent small case control study, individuals with FAIS demonstrated long-term improvements in hip abduction strength and post-operative strength values comparable to healthy individuals.⁷ Hip abduction strengthening is a major focus during early phases of rehabilitation (in order to improve gait mechanics) as well as late phase rehabilitation to normalize lumbo-pelvic and lower extremity control with higher level functional activities.^25,26 The subjects in this study did not demonstrate the expected hip abduction strength gains of the involved limb following arthroscopy and rehabilitation. This may be due in part to the short-term follow-up time of this study (six months post-operative) as compared to the two and a half years post-operative follow-up in the study by Casartelli et al.⁷ Furthermore, the lack of controls in this study make it impossible to determine whether bilateral strength impairments, which are common to individuals with chronic hip joint pain,³⁶ were characteristic of this sample. Future work should investigate the time course of recovery of normal, symmetrical muscle strength in individuals with FAIS.

Assessment of thigh muscle weakness in patients with hip pathology has previously been limited to older adults with hip osteoarthritis.^16,35 Quadriceps femoris weakness was associated with greater functional disability in a case-control study of individuals with mild to moderate hip osteoarthritis³⁵ and also predicted poorer post-operative function in a small prospective cohort early after total hip arthroplasty.¹⁶ The current findings show that improvements in knee extension strength, but not knee flexion strength, are also related to improvements in self-reported hip function in young, athletic individuals with FAIS. Improvements in normalized muscle strength were expected following arthroscopy due in part to post-operative physical therapy that included lower extremity strengthening. However, the only significant improvements in involved limb strength was in mean knee extension strength at 300 °/second. Late stage rehabilitation, which includes plyometric exercises to prepare for return to sport, may explain the changes in high-velocity force production of the knee extensors. In contrast, improvements in low velocity knee extension strength likely requires isolated, high load quadriceps strengthening; this is not currently included in the standard clinical guidelines used in this study. Importantly, evaluation of individual strength data at both velocities revealed significant variability in muscle performance improvements; only one-third of subjects demonstrated significant gains in knee extension strength at 60 °/second. Though this subgroup was not significantly weaker than the rest of the group prior to surgery, their improvements in knee extension strength over the study period corresponded with an average improvement of 32.5 points on the HOS-ADL score as compared to their counterparts, who improved an average of 12.2 points. Though the influence of quadriceps muscle weakness on function cannot be determined from this study, the positive association between improved quadriceps strength and hip function indicate that evaluation and targeted treatment of quadriceps weakness could be beneficial for patients undergoing hip arthroscopy for FAIS.

Over half of the cohort demonstrated no clinically significant change in knee flexion strength in the involved limb, while one-quarter showed an improvement in strength, and the remaining subjects showed a decline in strength. In sum, these data indicate that longer-term follow-up, comparison to a healthy control group, and method of strength testing are important to fully understanding the strength impairments of this population. These results also indicate that it may take longer than six months for athletes to regain functional strength for hip and thigh musculature at a time that is pivotal for return to sport.

Limb symmetry index (LSI) is a useful calculation to assess post-operative outcomes and is widely utilized as one component of return to sport criteria in other orthopedic populations.²⁰ The mean LSI for knee extension, flexion, and hip abduction strength in this cohort demonstrated no clinically meaningful (<90%) deficits prior to or following arthroscopy. There were also no significant changes in LSI over the testing period for any of the muscle groups tested despite significant improvements in uninvolved limb strength for knee extension and knee flexion. These findings may be explained by the bilateral muscle weakness that is common to this clinical population.^7,36 In a group of young adults with chronic hip joint pain, weakness of the hip rotators and abductors was identified in both the involved and uninvolved hips.³⁶ In sum, these data indicate that LSI alone may not be adequate to capture the muscle strength impairments in both limbs in this population. Bilateral pre-operative weakness may be a result of reduced activity levels for a prolonged period following the onset of symptoms and prior to diagnosis and surgical treatment for FAIS. While LSI is one important marker of effective rehabilitation and muscle recovery, limb-specific changes may be most relevant to the recovery of function in this population.

Current practice guidelines for the post-operative treatment of non-arthritic hip pain are limited to expert opinion.^22,27 Hip arthroscopy improves function in the majority of carefully selected patients^5,12,13,37 but some athletes do not successfully return to their previous level of activity.^5,12,37 The goal of this study was to determine whether increases in hip abduction and knee extension and flexion muscle strength would be associated with improvements in self-reported function in a cohort of athletes following hip arthroscopy for FAIS. These data indicate a positive association between involved limb knee extension strength and hip function in athletes following arthroscopy for FAIS. Though the causal link between increased knee extension strength and better hip function cannot be determined from the current study, future studies should examine the effect of progressive strengthening, including exercises targeted at quadriceps weakness, on return to activity outcomes in this population.

There are several limitations to the current study. This study was a secondary analysis of a larger observational study evaluating biomechanics in individuals with FAIS and was therefore not powered to detect changes in muscle strength. The subject attrition rate of 30% was not anticipated, and the second arthroscopy rate was high in this sample (12%). While demographic data were not different between the initial and final sample population, attrition may have influenced the findings of this study. Assessing only hip abduction strength was a significant weakness of this study. The custom hip strength dynamometer used in this study was part of a larger observational study evaluating biomechanics in individuals with a variety of lower extremity conditions and its design did not allow inter-limb strength testing of other muscle groups; thus it cannot be determined from this study whether improvements in strength of other hip muscle groups may be more strongly related to improvements in self-reported hip function. Different isokinetic testing speeds or the use of isometric knee extension/flexion testing may have yielded different results; however, the use of both low and high velocity assessments is consistent with clinical practice at OSUWMC for patients with hip pain. Pain during hip and thigh muscle testing also occurred in some individuals that may have adversely impacted their torque output. The lack of an uninjured control group limits the ability to identify bilateral muscle weakness, which has recently been identified in patients with chronic non-arthritic hip pain.³⁶ Although relevant to return to activity timeframes, the short-term follow-up in this small cohort further limits the generalizability of these data. Finally, adherence to the clinical guideline by the physical therapists providing care to these subjects and subject compliance to the rehabilitation program were not documented as they were not the focus of the current study. Both could have impacted patient-reported hip function and strength testing results.

CONCLUSION

Improvements in knee extension strength at 60°/second, but not knee flexion or hip abduction strength, were related to improvements in self-reported hip function in a cohort of athletic patients with FAIS. Strengthening programs that include targeted knee extensor strengthening during post-operative rehabilitation may improve functional outcomes, especially for patients who demonstrate the poorest function and greatest knee extension strength deficits pre-operatively, but this hypothesis warrants evaluation. Future investigation is needed to further identify modifiable lower extremity musculoskeletal impairments that can influence functional outcomes after hip arthroscopy in order to inform evidence-based clinical care for in this population of athletes.