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Introduction

  1. Top of page
  2. Introduction
  3. Materials and methods
  4. Results
  5. Discussion
  6. AUTHOR CONTRIBUTIONS
  7. REFERENCES
  8. Supporting Information

Hip osteoarthritis (OA) is responsible for hip pain, stiffness, and dysfunction during activities of daily living and is the most common reason for a total hip replacement (1). It has been estimated that 3% of the adult population (2) and 8% of people ages >60 years (3, 4) are affected by hip OA. There is no known cure for OA and therefore, clinical management of hip OA largely focuses on alleviating pain and maximizing function (5–10). A thorough understanding of the musculoskeletal factors underlying dysfunction in hip OA is required to effectively achieve these goals.

There is consistent evidence for quadriceps muscle weakness in knee OA (11), with quadriceps strengthening exercise considered a core component of programs for the management of knee OA (12–16). A commonly held view among clinicians appears to be that lower extremity muscle weakness is also apparent in hip OA. However, compared to the knee, there is less literature on muscle strength in hip OA, and guidelines for therapeutic exercise prescription in hip OA tend to be based on expert opinion rather than supporting evidence (17, 18). It therefore remains unclear whether muscle weakness as observed in knee OA is evident in hip OA, and if so, which muscles are most affected.

The force generated by a muscle is largely a function of the muscle's physiologic cross-sectional area and the level of motor unit pool activation (19, 20), and weakness can result from one or both of these mechanisms. Another factor with the potential to influence muscle strength is muscle quality, which manifests as a reduction in muscle force per unit of muscle physiologic cross-sectional area, and can arise due to an increase in noncontractile material such as fat in the muscle, as reported for older compared to younger adults (21). If individuals with hip OA do exhibit muscle weakness, characterization of the mechanisms underlying this weakness is required to inform the development of best practice intervention programs to treat the weakness. The purpose of this systematic review and critical evaluation of the literature was to determine whether muscles of the affected legs of individuals with unilateral hip OA are weaker, smaller, and more inhibited than those of their contralateral leg and/or the legs of healthy controls.

Significance & Innovations

  • This review is the first to systematically evaluate evidence for muscle weakness and its underlying mechanisms in hip osteoarthritis (OA).

  • This review identified consistent evidence for muscle weakness and muscle atrophy in the affected leg in persons with unilateral hip OA relative to the contralateral leg and healthy control legs.

  • Overall findings of this review indicate the need to address atrophic muscle weakness in the clinical management of hip OA.

Materials and methods

  1. Top of page
  2. Introduction
  3. Materials and methods
  4. Results
  5. Discussion
  6. AUTHOR CONTRIBUTIONS
  7. REFERENCES
  8. Supporting Information

Search strategy.

One author (AL) searched 6 electronic scientific databases during March 2012 to identify articles in which muscle strength, muscle size, and/or muscle inhibition were assessed in participants with hip OA. These databases were CINAHL (from 1988), the Cochrane library (from 1800), Medline (from 1950), PsycINFO (from 1806), PubMed (from 1950), and Web of Science (from 1982). Keywords were grouped and searched in the title, abstract, and keyword fields using the conjunction “or.” The first group of search terms consisted of synonyms related to muscle strength, size, and inhibition. Subsequent search terms were used to confine the search to hip OA. Articles related to animals, children, and non–English language were excluded by filters, by a third group of terms, or manually. The search string is shown in Supplementary Appendix A (available in the online version of this article at http://onlinelibrary.wiley.com/doi/10.1002/acr.21806/abstract). Reference lists of all articles included in the final yield were manually searched to further ensure that all relevant articles were extracted.

Search process.

Articles identified in each search were downloaded into individual Endnote, version X3 (Thomson Reuters) files. Individual files were subsequently combined into a single file and duplicate records removed. Screening was performed by a single author (AL); the title and abstract of each record were first evaluated for inclusion with the full text inspected in cases where insufficient information was available in the title and abstract. Data extraction was performed by a single author (AL). Articles were included in the final yield if they involved assessment of peak muscle torque, size, quality, and/or inhibition and compared these muscle variables from the affected legs of individuals with unilateral hip OA with their contralateral leg and/or healthy control legs. Studies reported only in abstract form were excluded.

Data extraction, reduction, and analysis.

Sample sizes, participant characteristics (age, sex, body mass, and height), inclusion/exclusion criteria, and details of the experimental protocol were extracted for each study (Table 1). Data required to compute effect sizes for muscle strength and size comparisons between the affected and contralateral legs and the affected segment and healthy controls were obtained by digitizing data from Figures 5 and 1, respectively, in the study by Reardon et al (22). Three groups of studies by Grimaldi et al (23, 24), Rasch et al (25–27), and Suetta et al (28–30) contained data from the same cohorts; duplicate data from those studies published later chronologically were removed and the remaining data were pooled. Data were also pooled for the gluteus maximus (upper and lower gluteus portions) from the study by Grimaldi et al (23) and were pooled for muscle quality (men and women) from the study by Suetta et al (28).

Table 1. Summary of participant characteristics, inclusion criteria, and dependent variables for the included studies*
Author, year (ref.)ParticipantsAge, mean ± SD or range yearsInclusion criteria for hip OA groupMuscle strengthMuscle sizeMuscle qualityMuscle inhibition
  • *

    OA = osteoarthritis; MRI = magnetic resonance imaging; K/L = Kellgren/Lawrence score; HR = hip replacement; CT = computed tomography; US = ultrasound.

  • Sex of 2 participants that withdrew not stated.

  • Sex of 6 participants that withdrew not stated.

Arokoski et al, 2002 (36)N = 30 control (M) N = 27 OA (M)56.2 ± 4.9 56.3 ± 4.5Unilateral or bilateral OA with functional impairmentIsometric hip adduction, abduction, flexion, and extension Eccentric hip flexion and extension (60°/s and 180°/s)Cross-sectional area of thigh muscles from MRI  
Grimaldi et al, 2009 (24)N = 12 control (6 F, 6 M) N = 6 mild OA (3 F, 3 M) N = 6 advanced OA (3 F, 3 M)51.8 ± 9.7 46.5 ± 9.5 57.7 ± 6.7Unilateral OA Mild: K/L >1 to ≥2 Advanced: K/L ≥3 Volume of gluteus maximus and tensor fasciae latae from MRI  
Grimaldi et al, 2009 (23)N = 12 control (6 F, 6 M) N = 6 mild OA (3 F, 3 M) N = 6 advanced OA (3 F, 3 M)51.8 ± 9.7 46.5 ± 9.5 57.7 ± 6.7Unilateral OA Mild: K/L >1 to ≥2 Advanced: K/L ≥3 Volume of gluteus medius, gluteus minimus, and piriformis from MRI  
Klausmeier et al, 2010 (37)N = 10 control (5 F, 5 M) N = 23 OA (6 F, 17 M)59.9 ± 5.3 56.9 ± 6.5Unilateral late-stage OA scheduled for HR surgeryIsometric hip abduction   
Madsen et al, 1997 (38)N = 20 (F)70.8 ± 7.3Unilateral or bilateral late-stage OA scheduled for HR surgeryConcentric knee flexion and extension (30°/s and 180°/s)   
Rasch et al, 2007 (25)N = 22 (18 F, 4 M)67 ± 7Unilateral late-stage OA scheduled for HR surgeryIsometric hip flexion, extension, abduction, and adduction, and knee flexion and extensionCross-sectional area of hip, thigh, and shank muscles from CTRadiologic density from CT of hip, thigh, and shank muscles 
Rasch et al, 2009 (27)N = 20 (18 F, 4 M)67 ± 7Unilateral late-stage OA scheduled for HR surgery Cross-sectional area of lower back, pelvic, hip, thigh, and shank muscles from CT  
Rasch et al, 2010 (26)N = 20 (18 F, 4 M)67 ± 7Unilateral late-stage OA scheduled for HR surgeryIsometric hip flexion, extension, abduction, and adduction, and knee flexion and extension   
Reardon et al, 2001 (22)N = 19 (8 F, 11 M)49–76Late-stage OA scheduled for HR surgeryConcentric and eccentric knee extension (speed not stated)Muscle thickness of the quadriceps muscle group from US  
Rossi et al, 2006 (35)N = 11 (5 F, 6 M)74.4 ± 4.8Late-stage OA scheduled for HR surgeryConcentric knee flexion and extension (60°/s)   
Suetta et al, 2004 (30)N = 30 (19 F, 17 M)F: 60–86 M: 60–79Unilateral late-stage OA scheduled for HR surgeryConcentric knee extension (60°/s and 180°/s)   
Suetta et al, 2007 (28)N = 39 (20 F, 19 M)F: 60–86 M: 60–79Unilateral late-stage OA scheduled for HR surgeryIsometric knee extensionCross-sectional area of quadriceps muscle group from CTRadiologic density of quadriceps muscle group from CTSuperimposed twitch of quadriceps during maximal voluntary knee extension
Suetta et al, 2008 (29)N = 28 (15 F, 13 M)60–86Unilateral late-stage OA scheduled for HR surgeryConcentric knee extension (60°/s and 180°/s)Muscle thickness of quadriceps from US  

Risk of bias and methodologic quality assessment.

Risk of bias was assessed using a modified version of the Cochrane Collaboration's bias assessment tool (31) (Table 2). The original tool assesses bias across 6 domains: 1) sequence generation, 2) allocation concealment, 3) blinding to group assignment, 4) incomplete outcome data, 5) selective outcome reporting, and 6) other potential bias (26). Items 1 and 2 are specific to randomized controlled trials and were replaced by items assessing bias due to differences in recruitment protocols for hip OA and control groups and nonmatching of hip OA and control groups with respect to age, sex, height, and body mass. Item 3 was restricted to blinding of personnel and outcome assessment, since hip OA and control participants were inherently aware of to which group they belonged. Item 4 was not applicable due to the observational nature of the data extracted for this review and was replaced with an item assessing bias due to the OA and/or control samples not being representative of their respective subpopulations. Each item was classified as having either a high, low, or unclear risk of bias. For studies in which the variables of interest were compared between the index and contralateral legs, the risk of bias for items 1 and 2 was deemed not applicable. Risk of bias assessments were conducted independently by 2 authors (AL and PMM), with disagreements resolved by a consensus meeting. Agreement between raters, defined using the kappa statistic, was 0.40 for item 1, 0.64 for item 4, and 1.00 for items 2, 4, 5, and 6.

Table 2. Risk of bias for the included studies*
StudyRecruitment proceduresMatching of groupsBlinding of personnel and outcome assessmentParticipants representative of subpopulationSelective reportingOther potential bias
  • *

    OA = osteoarthritis; THR = total hip replacement; N/A = not applicable.

Arokoski et al, 2002 (36)High; OA participants recruited via newspaper advertisements and joint replacement waiting lists. Control participants randomly sampled from the communityLow; no significant differences in age, sex, or body mass, while OA participants were significantly taller than controls by <1 SD (P < 0.05)Low; blinding not reported but unlikely to bias the resultsHigh; OA participants were awaiting THR. Control participants randomly sampled from the communityLowLow; funding sources not disclosed but unlikely to affect results
Grimaldi et al, 2009 (23)Unclear; OA and control participants were recruited from community newspapers and via contact with medical practitioners, but no indication as to whether recruitment protocol differed between groupsLow; no significant differences in age, sex, height, or body massLow; blinding not reported but unlikely to bias the resultsLow; mild- and advanced-stage OA participants included. Control participants recruited from the communityLowLow; funding sources not disclosed but unlikely to affect results
Grimaldi et al, 2009 (24)As aboveAs aboveAs aboveAs aboveAs aboveAs above
Klausmeier et al, 2010 (37)High; OA participants recruited via joint replacement waiting lists. Control participants were a convenience sample recruited from the community and a single universityHigh; no differences in age and height. OA participants were heavier; however, strength was normalized for body mass. OA participants were 26% women, while control participants were 50% womenLow; blinding not reported but unlikely to bias the resultsHigh; OA participants were awaiting THR. Control participants recruited from the communityLowLow; authors reported no funding sources with potential to cause a conflict of interest
Madsen et al, 1997 (38)N/A; within-participant analyses onlyN/A; within-participant analyses onlyLow; blinding not reported but unlikely to bias the resultsHigh; OA participants were awaiting THRLowLow; no commercial funding
Rasch et al, 2007 (25)N/A; within-participant analyses onlyN/A; within-participant analyses onlyLow; blinding not reported but unlikely to bias the resultsHigh; OA participants were awaiting THRLowLow; no commercial funding
Rasch et al, 2009 (27)N/A; within-participant analyses onlyN/A; within-participant analyses onlyLow; blinding not reported but unlikely to bias the resultsHigh; OA participants were awaiting THRLowLow; no commercial funding
Rasch et al, 2010 (26)N/A; within-participant analyses onlyN/A; within-participant analyses onlyLow; blinding not reported but unlikely to bias the resultsHigh; OA participants were awaiting THRLowLow; no commercial funding
Reardon et al, 2001 (22)N/A; within-participant analyses onlyN/A; within-participant analyses onlyLow; blinding not performed but unlikely to bias the resultsHigh; OA participants were awaiting THRLowLow; no commercial funding
Rossi et al, 2006 (35)N/A; within-participant analyses onlyN/A; within-participant analyses onlyLow; blinding not reported but unlikely to bias the resultsHigh; OA participants were awaiting THRLowLow; funding sources not disclosed but unlikely to affect results
Suetta et al, 2004 (30)N/A; within-participant analyses onlyN/A; within-participant analyses onlyLow; blinding of MRI measurements. Nonblinding of other measures not reported but unlikely to bias the resultsHigh; OA participants were awaiting THRLowLow; funding sources not disclosed but unlikely to affect results
Suetta et al, 2007 (28)N/A; within-participant analyses onlyN/A; within-participant analyses onlyLow; blinding not reported but unlikely to bias the resultsHigh; OA participants were awaiting THRLowLow; funding sources not disclosed but unlikely to affect results
Suetta et al, 2008 (29)N/A; within-participant analyses onlyN/A; within-participant analyses onlyLow; blinding not reported but unlikely to bias the resultsHigh; OA participants were awaiting THRLowLow; funding sources disclosed but unlikely to affect results

The methodologic quality of each article was also assessed independently by 2 authors (AL and PMM) using a tool developed by Galna et al (32) (see Supplementary Table 1, available in the online version of this article at http://onlinelibrary.wiley.com/doi/10.1002/acr.21806/abstract). The tool consists of 14 questions that address issues of internal validity, external validity, and the ability of the methods to be replicated. Each question was scored of a total of 1, with scores of 0 and 1 indicating relatively low and high quality, respectively. Any discrepancies in reviewer scores were resolved via a consensus meeting.

In order to facilitate comparison of results from different studies, means and SDs for each outcome variable were extracted, and standardized effect sizes (Cohen's d) and their corresponding 95% confidence intervals (95% CIs) (33) were calculated. These data were shown as forest plots to facilitate visual comparison of findings from different studies. Bold error bars on the forest plots indicate statistically significant findings as reported in the original article, while grey error bars indicate pooled standardized effect sizes and their 95% CIs calculated in accordance with the study by Lipsey and Wilson (34).

Results

  1. Top of page
  2. Introduction
  3. Materials and methods
  4. Results
  5. Discussion
  6. AUTHOR CONTRIBUTIONS
  7. REFERENCES
  8. Supporting Information

Yield.

The search resulted in an initial yield of 1,022 unique articles. Of these, 93 articles assessed neuromuscular and/or biomechanical variables in hip OA. A total of 13 articles that compared muscle strength, size, quality, and/or inhibition between individuals with hip OA and healthy controls, or between the affected and contralateral legs in hip OA, were included in the final yield. This included 1 article obtained from manual searching (35). In total, 7 articles compared strength between the affected and contralateral legs in hip OA (22, 25, 26, 29, 36–38), 2 articles compared strength in hip OA and healthy controls (36, 37), 7 articles compared muscle size between the affected and contralateral legs in hip OA (22–25, 27, 29, 30), and 3 articles compared muscle size of the affected leg in hip OA with healthy control legs (23, 24, 36). Three articles compared muscle quality between the affected and contralateral legs in persons with hip OA (25, 27, 28), while 1 article compared muscle inhibition between the affected and contralateral legs in hip OA (28). A flow chart summarizing identification, screening, and eligibility of the articles is shown in Figure 1. The Preferred Reporting Items for Systematic Reviews and Meta-Analyses checklist is shown in Supplementary Appendix B (available in the online version of this article at http://onlinelibrary.wiley.com/doi/10.1002/acr.21806/abstract).

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Figure 1. Preferred Reporting Items for Systematic Reviews and Meta-Analyses flow diagram for the systematic review.

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Methodologic quality assessment and risk of bias.

Overall, the risk of bias was relatively low across the included studies (Table 2). Of the 3 studies that compared muscle strength–related variables between the affected leg of persons with hip OA and controls, 2 (36, 37) were deemed to have a high risk of bias due to differences in recruitment; both recruited hip OA participants from hip surgery waiting lists and recruited controls from convenience samples or the community. The remaining study that included a between-subject analysis was deemed unclear about their recruitment of hip OA and control participants. The risk of bias due to nonmatching of hip OA and control participants on age, sex, mass, and height was deemed to be low for 2 of the studies that compared muscle strength variables between hip OA and control participants (23, 24, 36), but high for the remaining study, which did not match for sex (37). For all of the studies, blinding of personnel and outcome assessment was not performed; however, this was deemed to have a low risk of bias due to the objective nature of the assessment methods. With the exception of Grimaldi et al (23, 24), who recruited participants with both early- and late-stage hip OA, all of the studies were deemed to have a high risk of bias due to participants not being representative of the subpopulation from which they were sampled. The primary reason for this was selective recruitment of late-stage hip OA patients from joint replacement waiting lists. There was no evidence for selective reporting in any of the included studies. The risk of bias from other potential sources was also deemed low for all studies.

The findings from the methodologic quality appraisal of the articles included in the final yield are described in Supplementary Table 1 (available in the online version of this article at http://onlinelibrary.wiley.com/doi/10.1002/acr.21806/abstract). All of the studies scored a maximum value of 1 for questions relating to clear statement of the aim, clear description of key outcome variables, adequate methodologic detail, research questions adequately addressed in the discussion, and key findings logically interpreted and supported by results and references. Participant details were relatively well reported in all studies. Recruitment and sampling details were provided with limited information in 11 studies (22–30, 35, 38). Detailed inclusion and exclusion criteria were well described in 7 studies, 5 studies (22, 25–27, 38) provided only limited information, and 1 study (35) provided no information concerning inclusion and exclusion criteria. Two studies (36, 38) were performed using participants of a single sex and were therefore assumed to have controlled for sex as a covariate. The effect of body mass on strength was corrected for in 1 study using analysis of covariance (38). Reliability measurements were not reported in 2 studies (22, 38), while an additional 3 studies provided only limited information (28–30). Internal validity of methodology was not reported in any study. Clinical implications were discussed in 11 studies, but with limited detail in 6 studies (25–30). No articles were excluded on the basis of quality appraisal findings.

Participant and study characteristics.

Participant and study characteristics are summarized in Table 1. The mean age range for participants across all articles included in the review was 46–86 years. Participants consisted of men only for 1 study (36), women only in 1 study (38), and both sexes for the remaining studies. Isometric muscle strength was assessed in 5 studies (25, 26, 28, 36, 37), 4 studies evaluated only concentric strength (29, 30, 35, 38), 1 study assessed both concentric and eccentric strength (22), 1 study evaluated both isometric and concentric strength (29), and 1 study evaluated isometric and eccentric muscle strength (36). Muscle size was assessed using magnetic resonance imaging in 3 studies (23, 24, 36), computed tomography in 3 studies (25, 27, 28), and ultrasound in 1 study (22). Muscle inhibition was assessed using the interpolated twitch technique in 1 study (28), while muscle quality was assessed using radiologic density in 2 studies (25, 28).

Muscle strength.

Effect sizes for muscle strength between the affected and contralateral lower extremity in hip OA, and between the affected leg in hip OA and healthy control legs, are shown in Figure 2. Evidence for significantly lower muscle strength in the affected leg compared to the contralateral leg was reported in all 8 studies (22, 25, 26, 29, 30, 35, 36, 38). Two studies (36, 37) reported evidence for lower muscle strength in the affected leg in hip OA relative to healthy controls.

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Figure 2. Effect sizes for comparisons between the affected leg and the contralateral leg, and between the affected leg versus the healthy control leg, on measures of lower extremity muscle strength. Error bars show the 95% confidence interval. Bold and normally weighted error bars show statistically significant and nonsignificant findings, respectively, as reported in the source material. Grey error bars show the pooled effect size. Mean percentage differences are shown adjacent to the grey error bars. OA = osteoarthritis.

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Muscle size.

Effect sizes for muscle size between the affected and contralateral lower extremity in hip OA, and between the affected leg in hip OA and healthy controls, are shown in Figure 3. All 7 studies included in the review reported some evidence for significantly lower muscle size in the affected leg compared to the contralateral leg (22–25, 27, 29, 30). In contrast, the 3 studies (23, 24, 36) that compared muscle size between the affected leg of hip OA and healthy controls reported a lack of significant differences, with the exception of the gluteus medius in 1 study (24).

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Figure 3. Effect sizes for comparisons between the affected leg and the contralateral leg, and between the affected leg versus the healthy control leg, on measures of muscle size. Error bars show the 95% confidence interval. Bold and normally weighted error bars show statistically significant and nonsignificant findings, respectively, as reported in the source material. Grey error bars show the pooled effect size. Mean percentage differences are shown adjacent to the grey error bars. OA = osteoarthritis; TFL = tensor fasciae latae; psoas = psoas muscle group; vasti = vastus muscle group.

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Muscle quality.

Two studies assessed the effect of hip OA on muscle quality, with both reporting significantly lower radiologic density within the affected leg compared with the contralateral leg. Rasch et al (27) reported that all 8 leg muscles/muscle groups in the affected leg had lower radiologic density than those in the contralateral leg, whereas Suetta et al (28) reported significantly lower radiologic density within the quadriceps muscle group of the affected leg than the contralateral leg (Figure 4).

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Figure 4. Mean effect sizes for comparisons between the affected leg and the contralateral leg on measures of muscle quality. Error bars show the 95% confidence interval. Bold and normally weighted error bars indicate statistically significant and nonsignificant findings, respectively, as reported in the source material. Grey error bars show the pooled effect size. Mean percentage differences are shown adjacent to the grey error bars. OA = osteoarthritis; vasti = vastus muscle group; psoas = psoas muscle group.

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Muscle inhibition.

One study assessed the effect of hip OA on muscle inhibition (28). Suetta and colleagues compared voluntary activation of the vastus lateralis, vastus medialis, rectus femoris, biceps femoris, and semitendinosus between the affected and contralateral legs of subjects with hip OA. The affected leg exhibited significantly lower levels of muscle activation of the vastus lateralis (effect size −1.51; 95% CI −2.23, −0.79) and vastus medialis (effect size −0.98; 95% CI −1.65, −0.30) in men and of the rectus femoris (effect size −1.99; 95% CI −2.75, −1.23) in women compared to the contralateral leg (26). The pooled effect size across all of the muscles assessed was −1.39 (95% CI −1.60, −1.19).

Discussion

  1. Top of page
  2. Introduction
  3. Materials and methods
  4. Results
  5. Discussion
  6. AUTHOR CONTRIBUTIONS
  7. REFERENCES
  8. Supporting Information

This systematic review incorporated findings from 13 cross-sectional studies that evaluated lower extremity muscle strength, size, quality, and/or inhibition in individuals with hip OA. The main findings of the review were strong evidence for muscle weakness and moderate evidence for smaller muscles of the affected leg in persons with hip OA compared with their contralateral leg and/or the legs of healthy controls. There was also some evidence for reduced muscle quality defined by radiologic density in the affected leg compared with the contralateral leg of hip OA participants, and for muscle inhibition within the affected leg of persons with hip OA compared with their contralateral leg and/or healthy control legs.

The 8 included studies that compared muscle strength between the affected and contralateral legs in persons with OA provided generally consistent evidence for weakness of the osteoarthritic leg (22, 25, 26, 28, 29, 35, 36, 38). The greatest reductions in strength of the affected leg compared with the contralateral leg were found for the hip and knee flexors and extensors (22, 25, 29, 30, 35, 36, 38). The reduction in muscle strength was less consistent for the hip adductors and abductors; Rasch et al (25) reported that both hip adductors and abductors were weaker for the affected leg compared with the unaffected leg, while Arokoski et al (36) reported no differences between legs. It should be noted that Arokoski et al (36) included individuals with both unilateral and bilateral hip OA, with the leg exhibiting the lower radiographic grade designated as the control leg for persons with bilateral disease. This heterogeneity of control leg hip OA status may have biased their results toward a null result when comparing adduction and abduction strength between legs. Only 2 studies compared muscle strength between legs with hip OA and healthy control legs, with both reporting relatively large effect sizes (i.e., >1.0) for lower hip abduction strength in persons with hip OA compared with healthy controls (36, 37). Inconclusive evidence was found for the remaining muscles assessed; however, all of the effect sizes were in the direction of lower strength of the affected leg of persons with hip OA compared with healthy controls. It is likely that these differences would have been greater if strength had been normalized for body mass, which is typically greater in hip OA compared to controls. Leg dominance was reported in 1 study only (23, 24) and has the potential to confound results; however, there is no clear evidence that hip OA is more prevalent in the dominant/nondominant leg or that strength within the dominant leg is different from that of the nondominant leg (39–41). Overall, there were no major differences in muscle strength findings between the hip, knee, and ankle joints, suggesting that hip OA may result in a generalized weakening of the affected leg muscles, rather than a localized weakening of the hip musculature per se.

The most consistently reported mechanism underlying the observed muscle weakness in hip OA was decreased muscle size (atrophy). Overall, moderate evidence was found for smaller muscles in the affected leg compared with the contralateral leg in persons with hip OA; however, there was considerable variability across muscles. Consistently strong evidence was reported for reduced quadriceps muscle size in the affected leg compared with the contralateral leg of persons with hip OA, with all included studies reporting significant differences and moderate to large effect sizes (22, 25, 27, 28). Conversely, moderate evidence exists for no difference in hip abductor (i.e., gluteus minimus, gluteus medius, and/or tensor fasciae latae) size between the affected and contralateral legs, with 3 of 4 studies failing to identify a statistical difference between extremities. Findings from the studies that compared muscle size between individuals with hip OA and controls suggested that no difference in muscle size exists between the affected legs of hip OA participants and healthy control legs (23, 24, 36). Two of the 3 studies that compared muscle size between hip OA and controls reported significantly greater body mass for hip OA participants (23, 24). In the general population, muscle mass and body mass are positively related (42), and it is therefore possible that the lack of differences in muscle mass between hip OA and controls in this review was confounded by the greater body mass of patients with hip OA compared to control participants. With the exception of Grimaldi and colleagues (23, 24), who divided their hip OA participants into mild and advanced subgroups, studies recruited persons with advanced hip OA. The findings of Grimaldi et al (23, 24) that the gluteus medius, piriformis, and gluteus maximus were smaller in persons with advanced- compared to mild-stage hip OA suggest that muscle volume may be negatively associated with radiologic severity of hip OA and therefore, caution should be used when extrapolating muscle volume findings from this review to individuals with mild to moderate hip OA.

Both studies that investigated muscle quality in hip OA reported a large negative effect of OA on muscle radiologic density, with lower density in the affected leg than in the contralateral leg (25, 27). It has been proposed that a 1 Hounsfield unit difference in radiologic density is positively associated with an ∼1% difference in fat content (43), which suggests that the lower radiologic density of the affected leg reported in these studies (4–14%) corresponds to greater levels of intramuscular fat. A greater percentage of intramuscular fat within the muscles of the affected leg compared to the contralateral leg, and the associated reduction in contractile protein, would likely contribute to the lower strength of the osteoarthritic leg compared with the contralateral leg. Increased intramuscular fat content has also been identified as a risk factor for OA through its influence on metabolic pathways (44). It is not possible to ascertain whether the differences in muscle quality between the affected and contralateral legs of individuals with hip OA result from reduced radiologic density of the affected leg and/or increased radiologic density of the contralateral leg. While the pooled effect size estimates indicate a large effect of hip OA on muscle quality, these data are from 2 studies with relatively small sample sizes. Additional research comparing muscle quality in hip OA and healthy controls is required to confirm and better understand the effect of hip OA on muscle quality.

Only 1 study (28) assessed muscle inhibition in hip OA, through a comparison of maximum voluntary activation of the quadriceps group for the affected leg compared with the contralateral leg. The results from this study were inconclusive; the affected leg in men exhibited greater levels of inhibition for 2 of the 5 muscles assessed (vastus lateralis and vastus medialis), while women exhibited greater levels of inhibition for the rectus femoris only. There was, however, a nonsignificant trend toward greater levels of inhibition for the affected leg compared with the contralateral leg. Further research is therefore required to determine whether individuals with hip OA exhibit muscle inhibition. Furthermore, current knowledge is limited to the quadriceps group, and future studies focusing on a range of hip muscles will provide a more comprehensive understanding of muscle inhibition in hip OA.

While all included studies were deemed to be of a low/moderate risk of bias and of sufficient methodologic quality, a number of methodologic issues warrant discussion. Two (36, 37) of 3 case–control studies included in this review employed different recruitment protocols for the hip OA and control participants. Furthermore, with the exception of the studies by Grimaldi et al (23, 24), all of the studies recruited hip OA patients from joint replacement waiting lists. As a result, findings from these studies reflect those with late-stage hip OA, which needs to be taken into account when interpreting the findings reported here. Furthermore, 2 studies included participants with unilateral and/or bilateral hip OA (36, 45), which tends to attenuate between-leg differences in outcome measures. There was also a substantial sex imbalance in the case–control study by Klausmeier et al (37), with men to women ratios of 17:6 and 1:1 for the hip OA and control groups, respectively. Despite this, their hip OA participants had substantially lower hip abduction strength than the sex-matched control group. While no studies reported blinding of personnel or outcome assessment, the quantitative nature of the measurement procedures employed minimizes the risk of bias due to nonblinding.

The main limitation of this systematic review was the relatively small number of articles included in the final yield. This was particularly the case for muscle quality and muscle inhibition, for which only 1 and 2 articles, respectively, were included. There was also considerable variation in the inclusion/exclusion criteria of the studies, particularly in terms of disease severity and whether clinical and/or radiographic definitions of hip OA were used. Interpretation and synthesis of future findings would be enhanced by a more detailed description of inclusion/exclusion criteria and standardization of disease classification. The studies included in this review also exhibited differences in experimental protocols and specific outcome variables across the measures of muscle strength and size. Strength of hip, knee, and ankle musculature was measured during isometric (25, 26, 30, 36, 37), concentric (22, 29, 35, 38), and/or eccentric (22, 36) contractions; however, contraction mode did not have a discernible effect on the findings. Furthermore, no studies have examined the possible effects of co-contraction, intrinsic muscle properties such as the muscle force/length/velocity relationships, or tendinous factors on muscle force production in hip OA to date. Future studies aiming at tracking the progression of and/or improving strength and muscle function in hip OA should consider monitoring changes in neural, muscular, and tendinous factors in order to determine the relative contributions of these factors to changes in function (46).

There are several important clinical implications that arise from the findings of this review. First, individuals with end-stage hip OA have been reported to unload their hip during gait, and bone mineral density in the proximal femur is positively correlated with hip joint moments generated during gait (47). Muscle weakness arising from pain-mediated disuse atrophy could therefore be a mechanism underlying bone loss in hip OA that limits surgical options and negatively affects long-term outcomes following total hip replacement. Furthermore, although gait function following total hip replacement is improved, postsurgical gait function is positively correlated with presurgical gait function (48). The implication from this finding is that early intervention is required to avoid disuse atrophy in the affected leg and maximize gait function in hip OA. Given that muscle weakness persists following total hip replacement (49, 50), lower extremity muscle strengthening both pre- and postoperatively is recommended. Conversely, interventions that unload the hip, such as use of walking aids, need to be evaluated in terms of their likely negative effect on muscle strength and gait function in addition to their effect on pain. Evidence for the efficacy of exercise programs to improve outcomes in hip OA is sparse (10, 51), and therefore high-quality trials are required to optimize exercise program design.

The existing literature suggests that unilateral hip OA is characterized by generalized muscle weakness of the affected leg. The mechanisms underlying muscle weakness are multifactorial, and include, in order based on strength and amount of available evidence, a combination of reduced muscle size (atrophy), muscle inhibition, and decreased muscle quality. The findings of this review suggest the need to address the issue of muscle weakness in the clinical management of hip OA.

AUTHOR CONTRIBUTIONS

  1. Top of page
  2. Introduction
  3. Materials and methods
  4. Results
  5. Discussion
  6. AUTHOR CONTRIBUTIONS
  7. REFERENCES
  8. Supporting Information

All authors were involved in drafting the article or revising it critically for important intellectual content, and all authors approved the final version to be published.

REFERENCES

  1. Top of page
  2. Introduction
  3. Materials and methods
  4. Results
  5. Discussion
  6. AUTHOR CONTRIBUTIONS
  7. REFERENCES
  8. Supporting Information
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Supporting Information

  1. Top of page
  2. Introduction
  3. Materials and methods
  4. Results
  5. Discussion
  6. AUTHOR CONTRIBUTIONS
  7. REFERENCES
  8. Supporting Information

Additional Supporting Information may be found in the online version of this article.

FilenameFormatSizeDescription
ACR_21806_sm_AppendixA.doc26KSupplementary Appendix A
ACR_21806_sm_AppendixB.doc58KSupplementary Appendix B
ACR_21806_sm_SupplTable1.doc104KSupplementary Table 1

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