The significance of patellofemoral pain (PFP) as a medical

Transcription

1 DANIEL RAMSKOV, PT, MHSc 1 CHRISTIAN BARTON, PT, PhD 3-6 RASMUS O. NIELSEN, PT, MHSc 1,2 STEN RASMUSSEN, MD 2,7,8 High Eccentric Hip Abduction Strength Reduces the Risk of Developing Patellofemoral Pain Among Novice Runners Initiating a Self-Structured Running Program: A 1-Year Observational Study The significance of patellofemoral pain (PFP) as a medical condition is highlighted by reports that between 71% and 91% of individuals report chronic pain up to 20 years following initial diagnosis. 32,37,41 Additionally, PFP is one of the most common injuries of the lower extremity among runners. 9,21,43 The poor long-term prognosis 32,37,41 of PFP may be due to poor understanding of TTSTUDY DESIGN: Observational prospective cohort study with 1-year follow-up. TTOBJECTIVES: To investigate the relationship between eccentric hip abduction strength and the development of patellofemoral pain (PFP) in novice runners during a self-structured running regime. TTBACKGROUND: Recent research indicates that gluteal muscle weakness exists in individuals with PFP. However, current prospective research has been limited to the evaluation of isometric strength, producing inconsistent findings. Considering that hip muscles, including the gluteus maximus and medius, activate eccentrically to control hip and pelvic motion during weight-bearing activities such as running, the potential link between eccentric strength and PFP risk should be evaluated. TTMETHODS: Eight hundred thirty-two novice runners were included at baseline, and 629 participants were included in the final analysis. Maximal eccentric hip abduction strength was measured using a handheld dynamometer prior to initiating a self-structured running program. The diagnostic criteria to classify knee pain as PFP were based on a thorough clinical examination. Participants were followed for 12 months and training characteristics were gathered with a global positioning system. TTRESULTS: Results from the unadjusted generalized linear regression model for cumulative risk at 25 and 50 km indicated differences in cumulative risk of PFP between high strength, normal strength, and low strength (P<.05), with higher strength associated with reduced risk. TTCONCLUSION: Findings from this study indicate that, among novice runners, a level of peak eccentric hip abduction strength that is higher than normal may reduce the risk of PFP during the first 50 km of a self-structured running program. TTLEVEL OF EVIDENCE: Prognosis, level 4. J Orthop Sports Phys Ther 2015;45(3): Epub 27 Jan doi: /jospt TTKEY WORDS: anterior knee pain, chondromalacia, patella the biomechanical mechanisms contributing to PFP. 35 Identification of modifiable biomechanical risk factors for PFP development may allow for prevention and improved rehabilitation strategies for this debilitating condition. It is thought that lateral patellar tracking and subsequent PFP may develop due to greater dynamic knee valgus as a result of increased hip adduction and internal rotation during functional activities. 34 In support of this theory, prospective studies have reported greater hip internal rotation during a drop-jump landing task 5 and greater peak hip adduction during running 33 as risk factors for PFP development in the military and female runners, respectively. However, whether deficits in hip muscle function are risk factors for PFP development is currently unclear. Recent systematic reviews have reported global gluteal muscle weakness, 36 as well as delayed and shorter duration of gluteus medius muscle activation, during running and stair negotiation in individuals with PFP. 2 However, there is a paucity of prospective research in this area to distinguish between cause and effect. 36,38 1 Department of Public Health, Section of Sport Science, Faculty of Health Sciences, Aarhus University, Aarhus, Denmark. 2 Orthopaedic Surgery Research Unit, Science and Innovation Center, Aalborg University Hospital, Aalborg, Denmark. 3 Complete Sports Care, Melbourne, Australia. 4 Pure Sports Medicine, London, UK. 5 Centre for Sports and Exercise Medicine, Queen Mary University of London, London, UK. 6 Faculty of Health Sciences, La Trobe University, Melbourne, Australia. 7 Department of Clinical Medicine, Aalborg University, Aalborg, Denmark. 8 Department of Clinical Medicine, Aarhus University, Aarhus, Denmark. The study design, procedure, and informed consent procedure were presented to the Ethics Committee of Central Denmark Region (M ). The Danish Data Protection Agency approved the study. The authors certify that they have no affiliations with or financial involvement in any organization or entity with a direct financial interest in the subject matter or materials discussed in the article. Address correspondence to Daniel Ramskov, Dalgas Avenue 4, Room 334, 8000 Aarhus C, Denmark. Daniel.joergensen@ph.au.dk t Copyright 2015 Journal of Orthopaedic & Sports Physical Therapy journal of orthopaedic & sports physical therapy volume 45 number 3 march

2 Current prospective research evaluating strength is limited to isometric strength, producing inconsistent findings. 5,13,44 These findings indicate that isometric hip strength may be reduced following the development of PFP, 36 but isometric measurement may be of limited value in predicting development of PFP. 44 Importantly, hip muscles, including the gluteus maximus and medius, activate eccentrically, not isometrically, to control hip and pelvic motion during weight-bearing activities such as running. 14,16,34,39 Although there is limited evidence in comparison to isometric evaluation, reduced eccentric hip abduction strength has been reported to exist in individuals with PFP. 4,8,23 Additionally, eccentric hip abduction strength correlates with symptoms and functional capacity in individuals with PFP. 23 However, the influence of eccentric hip abduction strength on PFP risk has not yet been evaluated. 38 The purpose of this study was to investigate whether running distance, measured in kilometers, until the development of PFP varied among novice runners with different levels of eccentric hip abduction strength. It was hypothesized that eccentric hip abductor weakness would be associated with an increased risk of developing PFP. METHODS Included at baseline, n = 832 Included in analysis, n = 629 Free of injury, n = 605 Diagnosed with PFP, n = 24 (8 bilateral) Legs included in analysis, n = 1258 Uninjured legs at end of follow-up, n = 1226 Study Design The Danish Novice Running (DANO-RUN) study was designed as an observational prospective cohort study with a 1-year follow-up. The original purpose of the study has been presented elsewhere, 24 and several other papers have been published from the DANO-RUN data set. 3,25-27,31 The study design, protocol, and informed consent were presented to the Ethics Committee of Central Denmark Region (M ), which waived the request of ethics approval based on the study s observational design. According to Danish law, observational studies do not require ethics committee approval. The Danish Data Protection Agency approved the study. Participants The DANO-RUN study included a sample of 933 participants. The present study included a subsample of 832 participants, after excluding 101 participants who underwent pilot testing of the method used for hip strength measurement. A flow chart of the 832 participants included in this analysis is presented in FIGURE 1. Informed written consent was provided by all participants prior to inclusion. Information about participation in the project was distributed through newspapers and posters. People interested in participation signed up by completing an online questionnaire, which contained questions regarding health, previous injuries, and participation in sports. Based on an initial screening of the questionnaires, eligible participants were contacted Excluded from analysis, n = 203 Injury other than PFP, n = 202 Missing baseline data, n = 1 Legs injured with PFP, n = 32 (13 right, 19 left) FIGURE 1. Flow chart from baseline to end of follow-up. Abbreviation: PFP, patellofemoral pain. by phone and given verbal information about the study. Individuals between 18 and 65 years of age, who had not experienced injuries in the lower extremity in the 3 months prior to signing up and had not been running more than a total of 10 km in the previous 12 months, were eligible for inclusion. The process of recruitment prior to the baseline investigation and reasons for participant exclusion have been presented elsewhere. 27 At baseline, participants were interviewed regarding health conditions and demographic data were collected. Individuals with high blood pressure (greater than 140 mmhg for systolic or greater than 90 mmhg for diastolic) required permission from their general practitioner to participate. All participants received the same neutral running shoe (Supernova Glide 3; adidas Group, Herzogenaurach, Germany) and were instructed to use the shoe during all running sessions. 154 march 2015 volume 45 number 3 journal of orthopaedic & sports physical therapy

3 Strength Testing Eccentric strength testing, referred to as a break test, 46,49 was performed by 3 testers (1 male, 2 female). The maximal eccentric hip abduction strength was measured with a handheld dynamometer (Commander PowerTrack II muscle dynamometer; JTECH Medical, Midvale, UT) following reliable methods as previously described. 45,47 Testing followed the protocol used by Thorborg et al 46 and was performed on both hips in random order. Prior to each test, the dynamometers were calibrated. Participants were placed in a sidelying position and instructed to stabilize themselves by holding on to the examination table with the hand of the side being tested. The participants were guided to place the pelvis in a 90 angle relative to the examination table, and the hip and knee of the limb not being tested were both placed in 90 of flexion. The limb being tested was fully extended, and the examiner lifted the limb of the participants into a neutral hip position in the frontal plane. The participants were instructed to abduct the hip with maximal force against the examiner s resistance, keeping the heel as the highest point, and performed a 3-second maximum voluntary isometric contraction, directly followed by eccentric hip abduction strength testing, while given a command to push. The test leg was moved through a range of approximately 30, from an estimated 20 of hip abduction into approximately 10 of adduction, while the participant continued to exert maximum effort. 46 The test was repeated until the participant reached a difference in force plateau of less than 5% between 2 consecutive measurements. 46 The mean of the 2 consecutive measurements with a force plateau within 5% was used in the analysis. To avoid fatigue, participants rested for 1 minute between each measurement. By multiplying the strength values by the moment arm and dividing the results by body weight, eccentric hip abduction strength was normalized to limb length and body weight [normalized strength = (absolute strength moment arm)/body weight]. Previously, age and sex have been shown to be associated with maximal eccentric hip abduction strength, such that strength declined with increasing age and was greater in males than in females. 1,22 Maximal eccentric hip abduction strength was calculated with a regression equation that took sex and age into account, 36 with age expressed as years and sex as 0 for female and 1 for male. To differentiate the normalized eccentric hip abduction strength between normal strength and low or high strength, a root-meansquare error of was inserted: maximal eccentric hip abduction strength = [ (age 0.005) + (sex 0.215) ] Nm/kg. The calculated maximal eccentric hip abduction strength of the right and left legs was categorized as low strength, normal strength, or high strength. Reliability Assessment of Strength Testing Following the original data collection, reliability was assessed by 2 testers (1 male, 1 female) on a separate sample (n = 19; 13 male, 6 female). Testing was performed on the right leg on 2 nonconsecutive days. Intertester reliability showed an almost perfect strength of agreement (κ = 0.82) within day 1, moderate strength of agreement (κ = 0.47) within day 2, and moderate to substantial strength of agreement (κ = ) between days. Intratester between-day reliability showed a moderate strength of agreement (κ = 0.47). Patellofemoral Pain At baseline testing, injury was defined for each participant as any musculoskeletal complaint of the lower extremity or back caused by running, resulting in a restriction of running for at least 1 week. This definition of injury is a modified version of that used by Buist et al. 6 If a running-related injury was sustained, participants were instructed to use their personal web-based training diary to contact the medical team. The injured participant was contacted by telephone and an appointment for a clinical examination was made, preferably no later than 1 week after initial contact. Diagnosis was made after thorough examination by 1 or 2 of the 3 physical therapists with experience in diagnosing running-related injuries employed in the DANO-RUN study. The diagnostic criteria to classify knee pain as PFP were based on recommendations by Fredericson and Yoon. 15 For an injury to be diagnosed as PFP, a minimum of 2 of the following 4 tests from the clinical examination had to be positive: (1) palpation of medial and lateral borders of the patella with the knee in full extension and palpation with the patella displaced medially and laterally; (2) patellar compression test with the knee in full extension, compressing the patella against the femoral groove while moving the patella superiorly and inferiorly; (3) eccentric resistance test of the quadriceps muscle; and (4) isometric resistance test of the quadriceps muscle. In addition, these tests were supplemented by a more functional examination consisting of jumping on 1 leg and performing a deep squat with both legs. Tests were considered positive if the participant reported pain or palpation tenderness. If participants experienced pain when palpating the apex of the patella, and testing was negative for the 4 PFP criteria, they were diagnosed with patellar tendinopathy and excluded from further analysis. Furthermore, all participants with knee pain were palpated at the medial and lateral joint lines, and tests for meniscal and anterior cruciate, medial collateral, and lateral collateral ligament injury were performed to exclude other sources of symptoms. If further examination, including diagnostic imaging, was needed (eg, to rule out knee osteoarthritis), the participant was referred to an extensive medical examination at the Division of Sports Traumatology, Aarhus University Hospital (Denmark). journal of orthopaedic & sports physical therapy volume 45 number 3 march

4 Assessment of Kilometers at Risk At baseline, participants were instructed to run as much as they wanted for the following 12 months. Because no prescriptions or guidelines regarding training distance, duration, intensity, or frequency were given, the running program was considered a self-structured one. Training distance was measured in kilometers using a global-positioning-system (GPS) watch (Forerunner 110 M; Garmin Ltd, Schaffhausen, Switzerland). Global positioning has previously been reported as a valid and reliable method to objectively measure training distance among runners, with a low percentage of measurement error (6.2% or less). 29,48 Participants were instructed to upload training data to an internet-based training diary developed by the research group ( If problems with the GPS device occurred during the 12 months, participants were instructed to upload the missing training session manually by reporting running distance and time spent running. Participants were also instructed to contact the DANO- RUN study group through their diary to solve the problem and ensure that the GPS device worked correctly. During and after 1-year follow-up, data quality control was performed. In the case of suspicious data, the participant was contacted to verify that the data uploaded to the homepage were correct. If the participant did not respond to this contact, the participant was censored at the time of the initial uploading of the suspicious data. As an example of suspicious data, a person was contacted if he or she ran 300 km in 1 running session. Assessment of Confounders Prior to data analysis, we hypothesized that the following variables could potentially confound the association between maximal hip abduction strength and risk of PFP: age, sex, weight, height, and previous activity level. Participants were categorized into groups based on their hip abduction strength. During categorization, the participant s age, sex, weight, and height were accounted for, to ensure that these variables did not bias the estimates. In addition, previous activity level may bias the estimates, because persons with low activity levels may have reduced hip strength, possibly leading to an increased risk of PFP development. Hence, participation in other sports was included in the adjusted analysis as a dichotomous variable (yes/no) based on the response to the question at baseline: Do you participate in weightbearing sport activities on a regular basis (greater than 4 hours per week)? Statistical Analysis The injury proportion as a function of distance was estimated using the Kaplan-Meier curve. Time to injury at 25, 50, 100, 250, and 500 km was analyzed using cumulative running distance as the time scale. To correct for potential dependence between the 2 legs, injury risk difference was analyzed by performing a generalized linear regression using the pseudo-values method allowing for clusters. 19 The unit of analysis was the individual leg, and each participant was considered as 1 cluster (with 2 legs). When estimating risk differences by the pseudo-values method, at least 10 events per variable included in the analysis had to be present to avoid violation of the statistical assumptions. 17 Because hip strength was categorized into 3 groups, at least 20 cases of PFP were needed to perform a valid crude analysis. In addition, the remaining injuries above the 20 required for the crude analysis allowed for only 1 confounder to be included in the adjusted analysis. Participants were right censored in the sense that they contributed with kilometers at risk until any of the following events occurred: pregnancy, disease, lack of motivation, non running-related injury causing a permanent cessation of running, unwillingness to attend clinical examination in case of injury, the use of a shoe other than the neutral shoe, or the end of the 1-year follow-up. If 1 leg sustained an injury, the other leg was still followed until injury or the end of the follow-up. The number of legs needed to increase their strength from low to high to avoid 1 occurrence of PFP was calculated as 1/ cumulative risk difference. Differences were considered statistically significant at P<.05, and estimates are presented with 95% confidence intervals (CIs). All analyses were performed using Stata/SE Version 12 (StataCorp LP, College Station, TX). RESULTS A total of 629 participants were included in the analysis, after excluding 202 individuals who sustained a running-related injury other than PFP and 1 individual with missing baseline data. Of the 629 participants, 605 remained free of injury and 24 were diagnosed with PFP (FIGURE 1). TABLE 1 provides the demographic characteristics of all participants, injury-free participants, and participants diagnosed with PFP. The normalized mean eccentric hip abduction strength for women was 1.38 Nm/kg (95% CI: 1.34, 1.42) for the right leg and 1.44 Nm/ kg (95% CI: 1.41, 1.48) for the left leg. On average, men had a normalized strength of 1.62 Nm/kg (95% CI: 1.57, 1.66) for the right leg and 1.64 Nm/kg (95% CI: 1.60, 1.68) for the left leg. Of the 24 runners diagnosed with PFP, 8 sustained bilateral injuries (a total of 32 legs with PFP, 13 in the right knee and 19 in the left knee) (FIGURE 1). The 1258 legs included in the analyses ran a total of km until injury or censoring. Results from the unadjusted generalized linear regression model for the cumulative risk at 25 and 50 km showed significant differences in cumulative risk of PFP between high strength, normal strength, and low strength (P<.05). At 100, 250, and 500 km, no significant differences in risk were found between groups (TABLE 2). Comparisons of the risk difference between each group showed a statistical trend for lower risk of injury in the high-strength group compared 156 march 2015 volume 45 number 3 journal of orthopaedic & sports physical therapy

5 TABLE 1 TABLE 2 to the normal-strength group after 25 km (P =.06; risk difference, 0.6%; Demographic Characteristics* Variable All (n = 629) Injury Free (n = 605) PFP (n = 24) Sex, n Male Female Age, y 36.6 ± ± ± 10.8 BMI, kg/m ± ± ± 4.4 Weight, kg 80.1 ± ± ± 15.6 Height, m 1.75 ± ± ± 0.1 Abbreviations: BMI, body mass index; PFP, patellofemoral pain. *Values are mean ± SD unless otherwise indicated. Data are presented for all participants and stratified by PFP status during the project. All variables were measured or reported prior to or at the baseline investigation. Crude Cumulative RDs for Running Injury According to Maximal Hip Abduction Strength* Distance/Normalized Strength Legs, n RD, % SE P Value 25 km.04 High strength ( 1.3, 0.1) Normal (reference) Low strength ( 1.1, 5.0) km.03 High strength ( 1.7, 0.1) Normal (reference) Low strength ( 1.4, 4.8) km.48 High strength ( 2.6, 4.1) Normal (reference) Low strength ( 1.3, 5.4) km.25 High strength ( 2.5, 8.6) Normal (reference) Low strength ( 1.5, 6.7) km.90 High strength ( 9.1, 5.7) Normal (reference) 22 0 Low strength ( 7.0, 5.7) Abbreviations: RD, risk difference; SE, standard error. *Analyses are presented for 25, 50, 100, 250, and 500 km. The reference risk (95% confidence interval) for normal strength was 0.6% ( 0.01%, 1.2%) at 25 km, 0.9% (0.5%, 1.7%) at 50 km, 1.4% (0.4%, 2.4%) at 100 km, 2.7% (1.1%, 4.3%) at 250 km, and 6.6% (1.7%, 11.6%) at 500 km. A total of 1258 legs were analyzed; there were 32 injured legs and km at risk. Values in parentheses are 95% confidence interval. Chi-square test for difference between all 3 groups. Results from the analysis after 500 km should be interpreted with caution, owing to low counts in the 3 exposure groups. 95% CI: 1.3%, 0.1%) and a significant lower risk of injury in the high-strength group compared to the normal-strength group after 50 km (P =.03; risk difference, 0.9%; 95% CI: 1.7%, 0.1%). No significant differences were found between the high-strength group and the normal-strength group after 100, 250, or 500 km (TABLE 2). Comparison of the low-strength group and normal-strength group resulted in nonsignificant results (P>.05) at 25, 50, 100, 250, and 500 km (TABLE 2). The Kaplan-Meier curve for the 3 exposure groups visualizing the injury proportion as a function of kilometers is presented in FIGURE 2. Based on the crude estimate, the number of legs needed to increase their strength from low to high at 25 km and at 50 km to avoid 1 occurrence of PFP was 38. In the chi-square analysis across the 3 exposure groups adjusted for other weight-bearing sports activities, the same statistical trends were apparent after 25 (P =.08) and 50 km (P =.06), whereas no associations were found after 100 (P =.63), 250 (P =.38), and 500 km (P =.33). At 50 km, the cumulative risk difference between those with high strength compared with those with normal strength was 0.9% (95% CI: 1.7%, 0.05%; P =.036), whereas the difference between low strength and normal strength was 1.8% (95% CI: 1.2%, 4.8%; P =.24). DISCUSSION The findings of previous studies have been inconsistent regarding the relationship between isometric hip abduction strength and the risk of PFP development. 5,13,44 This is the first study, to our knowledge, to evaluate the link between eccentric hip abduction strength and risk of PFP development. We hypothesized that reduced eccentric hip abduction strength would increase the risk of PFP development. This hypothesis was supported by the differences in cumulative risk across groups, but only at 25 and 50 km. Findings also suggest that higher eccentric hip abduction strength may be associated with re- journal of orthopaedic & sports physical therapy volume 45 number 3 march

6 Injury Survival High duced risk of PFP among novice runners in the first 50 km of their self-structured running regime. Though adjustment for participation in other weight-bearing activities at inclusion weakened the association and the relationship at 25 and 50 km (P values ranged from.08 to.06), the results may be considered clinically significant because the estimates did not change considerably compared to those of the crude analysis. 40 The association between higher eccentric hip abduction strength and reduced risk of PFP should be considered in the design of prevention programs, particularly during the first 50 km of running. The results of this study suggest that eccentric gluteal strengthening exercises prior to commencing a self-structured running program may reduce the risk of developing PFP. However, further research exploring this possibility and the most appropriate exercise prescription (dose, frequency, duration, and type [weight bearing or non weight bearing]) is needed before clear clinical recommendations can be made. No significant relationship between Kilometers at Risk Normal FIGURE 2. Kaplan-Meier curves of the proportion of the injury-free participants in each of the 3 exposure groups (high strength, normal strength, and low strength) according to different kilometers at risk. It can be visualized that more individuals in the low-strength group seem to sustain injuries during the first 50 km, whereas no difference seems to exist beyond this distance. Censored participants were assumed to be representative of the sample until the time they were censored. They were included in the analysis until the first point of kilometers at risk analyzed, to the right of the time of censoring (right censored). Low eccentric hip abduction strength and PFP risk was found beyond 50 km of running. One possible explanation for this may be that participants with low strength at baseline improved their strength during the first 50 km of running, after which their risk of PFP was similar to those who had high strength at baseline. Additional measurements of hip abduction strength during follow-up are recommended for future studies to explore this possibility. In addition to eccentric hip abduction strength, PFP risk might also have been influenced by other factors. 35,51 The training characteristics (running volume, speed, and frequency) of the first 50 km might also have influenced the injured participants risk of developing PFP. 30 Of 629 participants in the present study, 24 were diagnosed with PFP. This proportion of injured participants is low compared to that of previous studies in military recruits and other runners. 18,21 However, the participants in the current study completed a self-structured running program, which resulted in lower activity levels compared to those of other studies. 6,42 Of the 629 participants, 36 did not exceed 10 km, 148 did not exceed 50 km, and 242 did not exceed 100 km in total running distance by the 1-year follow-up. Had the participants been forced to follow a standardized running program and thereby expose themselves to a greater amount of running or run too much, too soon, we hypothesize that the incidence of PFP development would have increased. Three previous studies have evaluated the prospective link between hip strength and PFP, with inconsistent results. 5,13,44 Specifically, Boling et al 5 reported increased hip external rotation strength as a risk factor in military recruits, Finnoff et al 13 reported stronger hip abductors and weaker hip external rotators as risk factors, and Thijs et al 44 reported hip muscle strength not to be a risk factor for the development of PFP. However, these studies all evaluated isometric strength and not eccentric strength. Thijs et al 44 was the only one of these studies to investigate novice runners. In addition to different measures of strength (isometric versus eccentric), differences in statistical analysis between the study by Thijs et al 44 and the current study also require consideration. Data were analyzed in a case-control design, without the use of survival analysis, in the study by Thijs et al. 44 Therefore, it was only possible to analyze data at the end of follow-up, and the time (distance in the present study) at risk was not taken into account. In the present study, an important finding was that the influence of low hip strength on development of PFP ceased with accumulated total distance above 50 km during the followup period. Despite the prospective link between low eccentric hip abduction strength and PFP, it is important to consider that 38 legs were needed to increase strength from low to high to avoid 1 injury (assuming a causal relationship). From a preventive perspective, 38 legs may be considered high; however, this highlights 158 march 2015 volume 45 number 3 journal of orthopaedic & sports physical therapy

7 the nature of PFP as multifactorial and that eccentric hip abduction strength may only be a small piece of the puzzle. To optimize the number needed to treat when designing prevention programs, additional established risk factors, such as quadriceps weakness, 5 delayed vastus medialis oblique muscle activity, 50 excessive hip internal rotation, 5 and excessive hip adduction, 33 should also be considered. Recently, exercise interventions targeting hip muscle strength have been reported to both effectively reduce pain and increase isometric hip abduction strength The findings of this study provides further support to hip-strengthening programs in the management of PFP, particularly if aiming to prevent recurrence. Importantly, the link between higher eccentric strength and reduced PFP risk in the current cohort of runners indicates that future research and clinical hip-strengthening protocols should aim to increase eccentric hip abduction strength to optimize outcomes. The strengths of this study include a prospective design, large sample size compared to that of similar studies, 5,13,44 a long follow-up (1 year), and accurate and valid quantification of training distance using GPS. 26,29 All participants completed their run sessions using the same neutral running shoe, with no differentiation in foot type used to tailor footwear prescription. Previous analysis of the same cohort 26 indicated that foot type had no influence on injury risk. However, the present study also has several limitations that should be considered. The incidence of PFP (32 legs) in this study allowed the inclusion of just 1 variable in the adjusted analysis to avoid violation of the assumptions behind the statistical model. 19 As a consequence, other potential confounders, including previous running experience, how rapidly the runners changed their training distance over time, and previous injury, were not adjusted for. 28 Reliability of the eccentric testing protocol was not evaluated prior to the start of the present study, but rather following the original data collection. A weighted kappa analysis showed varying strength of agreements, primarily moderate. 20 The fact that the original testers did not perform the reliability study and the results of the kappa statistics should be taken into consideration when interpreting the results. However, all 3 original testers were experienced, and 101 participants were used to pilot the testing procedure. The 3 physical therapists completing strength assessment in this study were both male and female. Based on reports by Thorborg et al 45 of reduced interrater reliability and systematic bias between testers of different sexes when measuring isometric hip strength, this may confound results. A paired t test of the current reliability measurements did not reveal any systematic bias between testers of different sexes on the within-day measurements on day 1 (P =.51) or on day 2 (P =.13). The testers collecting the original data and the testers collecting the reliability data all had experience in performing the specific measurement (greater than 30 measurements performed). The testers in the study by Thorborg et al 45 received only 1 hour of instruction and performed 6 different measurements. This difference in experience might influence the measurements obtained and explain the difference in findings. Another limitation was the use of multiple physical therapists to perform the evaluation and diagnosis of injured participants, which might have led to inconsistencies. However, during the study, all efforts were made to ensure standardization of the diagnosis, with close collaboration between the physical therapists, and each clinician was highly experienced. Additionally, in most cases, the injured runners were diagnosed through a clinical examination without the use of diagnostic imaging, which is a standard approach for PFP. Despite this, there is still some risk that misclassification might have occurred. As an example, individuals above 60 years of age diagnosed with PFP might have had osteoarthritis in the knee. However, misclassification is likely to be evenly distributed across exposure groups, preventing bias in relation to current findings. The current study evaluated only 1 potential risk factor, eccentric hip abduction strength, for this multifactorial condition. More research is needed to identify additional risk factors associated with PFP and how they interact together. If possible, such studies should be designed as prospective cohort studies with a considerably greater sample size than the 629 included in the present analysis, to allow for inclusion of more variables into statistical models. Additionally, the next logical step from this study design is to implement and evaluate interventions designed to address risk factors such as reduced eccentric hip abduction strength. This design should also continuously monitor strength development through follow-up measurements and the development of symptoms using the method proposed by Clarsen et al. 7 Monitoring strength development and symptoms of PFP could provide insight into the interaction between strength and pain sensation in runners developing PFP. CONCLUSION Findings from this study indicate that among novice runners, a level of peak eccentric hip abduction strength higher than normal reduces the risk of PFP. Considering this, exercise interventions designed to treat and prevent PFP should include strategies to increase eccentric hip abduction strength. An overall increase in peak eccentric hip abduction strength may be associated with reduced risk of PFP development among novice runners during the first 50 km of their self-structured running regime. Further research is needed to understand the relationship between eccentric hip abduction strength and other PFP risk factors, and to identify the most effective exercise protocols to improve strength. t journal of orthopaedic & sports physical therapy volume 45 number 3 march

8 KEY POINTS FINDINGS: Higher peak eccentric hip abduction strength may be associated with reduced risk of PFP development among novice runners. IMPLICATIONS: Exercises targeting the hip abductors may be incorporated in training programs for novice runners to prevent the development of PFP. CAUTION: Confounding variables might have affected the results. This is the first study, to our knowledge, to evaluate and identify eccentric hip abduction strength as a risk factor for PFP. Further research, taking into account confounding variables, is needed to confirm this association and to understand the interaction of eccentric hip abduction strength with other PFP risk factors. ACKNOWLEDGEMENTS: Medical student Solvej Videbæk Andersen is greatly acknowledged for her contribution to the assessment of reliability. REFERENCES 1. Andrews AW, Thomas MW, Bohannon RW. Normative values for isometric muscle force measurements obtained with hand-held dynamometers. 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Relationship among eccentric hip and knee torques, symptom severity and functional capacity in females with patellofemoral pain syndrome. Phys Ther Sport. 2011;12: ptsp Nielsen R, Ramskov D, Sørensen H, Lind M, Rasmussen S, Buist I. Protocol for the danorun study: a 1-year observational follow up study on running related injuries in 1000 novice runners [abstract]. Br J Sports Med. 2011;45: bjsm Nielsen RO, Bertelsen ML, Parner ET, Sørensen H, Lind M, Rasmussen S. Running more than three kilometers during the first week of a running regimen may be associated with increased risk of injury in obese novice runners. Int J Sports Phys Ther. 2014;9: Nielsen RO, Buist I, Parner ET, et al. Foot pronation is not associated with increased injury risk in novice runners wearing a neutral shoe: a 1-year prospective cohort study. Br J Sports Med. 2014;48: org/ /bjsports Nielsen RO, Buist I, Parner ET, et al. 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9 in 254 injured novice runners. PLoS One. 2014;9:e journal.pone Nimon G, Murray D, Sandow M, Goodfellow J. Natural history of anterior knee pain: a 14- to 20-year follow-up of nonoperative management. J Pediatr Orthop. 1998;18: Noehren B, Hamill J, Davis I. Prospective evidence for a hip etiology in patellofemoral pain. Med Sci Sports Exerc. 2013;45: MSS.0b013e d2 34. Powers CM. The influence of abnormal hip mechanics on knee injury: a biomechanical perspective. J Orthop Sports Phys Ther. 2010;40: jospt Powers CM, Bolgla LA, Callaghan MJ, Collins N, Sheehan FT. Patellofemoral pain: proximal, distal, and local factors 2nd International Research Retreat, August 31-September 2, 2011, Ghent, Belgium. J Orthop Sports Phys Ther. 2012;42:A1-A54. jospt Ramskov D, Pedersen MB, Kastrup K, et al. Normative values of eccentric hip abduction strength in novice runners: an equation adjusting for age and gender. Int J Sports Phys Ther. 2014;9(1): Rathleff MS, Rasmussen S, Olesen JL. [Unsatisfactory long-term prognosis of conservative treatment of patellofemoral pain syndrome]. Ugeskr Laeger. 2012;174: Rathleff MS, Rathleff CR, Crossley KM, Barton CJ. Is hip strength a risk factor for patellofemoral pain? A systematic review and metaanalysis. Br J Sports Med. 2014;48(14): Snyder KR, Earl JE, O Connor KM, Ebersole KT. Resistance training is accompanied by increases in hip strength and changes in lower extremity biomechanics during running. Clin Biomech (Bristol, Avon). 2009;24: clinbiomech Stang A, Poole C, Kuss O. The ongoing tyranny of statistical significance testing in biomedical research. Eur J Epidemiol. 2010;25: Stathopulu E, Baildam E. Anterior knee pain: a long-term follow-up. Rheumatology (Oxford). 2003;42: Taunton JE, Ryan MB, Clement DB, McKenzie DC, Lloyd-Smith DR, Zumbo BD. A prospective study of running injuries: the Vancouver Sun Run In Training clinics. Br J Sports Med. 2003;37: bjsm Taunton JE, Ryan MB, Clement DB, McKenzie DC, Lloyd-Smith DR, Zumbo BD. A retrospective case-control analysis of 2002 running injuries. Br J Sports Med. 2002;36: Thijs Y, Pattyn E, Van Tiggelen D, Rombaut L, Witvrouw E. Is hip muscle weakness a predisposing factor for patellofemoral pain in female novice runners? A prospective study. Am J Sports Med. 2011;39: org/ / Thorborg K, Bandholm T, Schick M, Jensen J, Hölmich P. Hip strength assessment using handheld dynamometry is subject to intertester bias when testers are of different sex and strength. Scand J Med Sci Sports. 2013;23: org/ /j x 46. Thorborg K, Couppé C, Petersen J, Magnusson SP, Hölmich P. Eccentric hip adduction and abduction strength in elite soccer players PUBLISH Your Manuscript in a Journal With International Reach JOSPT offers authors of accepted papers an international audience. The Journal is currently distributed to the members of APTA s Orthopaedic and Sports Physical Therapy Sections and 33 orthopaedics, manual therapy, and sports groups in 26 countries who provide online access either as a member benefit or at a discount. As a result, the Journal is now distributed monthly to more than individuals around the world who specialize in musculoskeletal and sports-related rehabilitation, health, and wellness. In addition, JOSPT reaches students and faculty, physical therapists and physicians at more than 1,500 institutions in 56 countries. Please review our Information for and Instructions to Authors at in the Info Center for Authors and submit your manuscript for peer review at matched controls: a cross-sectional study. Br J Sports Med. 2011;45: org/ /bjsm Thorborg K, Petersen J, Magnusson SP, Hölmich P. Clinical assessment of hip strength using a hand-held dynamometer is reliable. Scand J Med Sci Sports. 2010;20: org/ /j x 48. Townshend AD, Worringham CJ, Stewart IB. Assessment of speed and position during human locomotion using nondifferential GPS. Med Sci Sports Exerc. 2008;40: dx.doi.org/ /mss.0b013e bc2 49. Tyler TF, Nicholas SJ, Campbell RJ, McHugh MP. The association of hip strength and flexibility with the incidence of adductor muscle strains in professional ice hockey players. Am J Sports Med. 2001;29: Van Tiggelen D, Cowan S, Coorevits P, Duvigneaud N, Witvrouw E. Delayed vastus medialis obliquus to vastus lateralis onset timing contributes to the development of patellofemoral pain in previously healthy men: a prospective study. Am J Sports Med. 2009;37: org/ / Waryasz GR, McDermott AY. Patellofemoral pain syndrome (PFPS): a systematic review of anatomy and potential risk factors. Dyn Med. 2008;7:9. org/ / MORE INFORMATION journal of orthopaedic & sports physical therapy volume 45 number 3 march