Abstract
Background: A common treatment for patients with patellofemoral pain syndrome (PFPS) is strength (force-generating capacity) training of the vastus medialis oblique muscle (VMO). Hip adduction in conjunction with knee extension is commonly used in clinical practice; however, evidence supporting the efficacy of this exercise is lacking.
Objective: The objective of this study was to determine the surplus effect of hip adduction on the VMO.
Design: This study was a randomized controlled trial.
Setting: The study was conducted in a kinesiology laboratory.
Participants: Eighty-nine patients with PFPS participated.
Intervention: Participants were randomly assigned to 1 of 3 groups: hip adduction combined with leg-press exercise (LPHA group), leg-press exercise only (LP group), or no exercise (control group). Training consisted of 3 weekly sessions for 8 weeks.
Measurements: Ratings of worst pain as measured with a 100-mm visual analog scale (VAS-W), Lysholm scale scores, and measurements of VMO morphology (including cross-sectional area [CSA] and volume) were obtained before and after the intervention.
Results: Significant improvements in VAS-W ratings, Lysholm scale scores, and VMO CSA and volume were observed after the intervention in both exercise groups, but not in the control group. Significantly greater improvements for VAS-W ratings, Lysholm scale scores, and VMO volume were apparent in the LP group compared with the control group. There were no differences between the LP and LPHA groups for any measures.
Limitations: Only the VMO was examined by ultrasonography. The resistance level for hip adduction and the length of intervention period may have been inadequate to induce a training effect.
Conclusions: Similar changes in pain reduction, functional improvement, and VMO hypertrophy were observed in both exercise groups. Incorporating hip adduction with leg-press exercise had no impact on outcome in patients with PFPS.
Patellofemoral pain syndrome (PFPS) is a common musculoskeletal problem of the knee. Patients with PFPS often have retropatellar or peripatellar pain.1 This pain is aggravated by activities that increase patellofemoral compressive force, such as walking up and down stairs, squatting, running, and prolonged sitting with bent knees.2–4
Patellofemoral pain syndrome is thought to be associated with lateral malalignment of the patella.5 Possible causes of this malalignment include hypotrophy or atrophy of the vastus medialis oblique muscle (VMO), changes that are commonly seen in patients with PFPS, and an imbalance or delay in the activation of the VMO relative to the vastus lateralis muscle (VL).5,6 Several cadaver studies have shown that the VMO fiber angle was 50 to 57 degrees off the long axis of the femur in the frontal plane, and the proximal vastus medialis muscle fiber orientation was approximately 15 degrees.7–10 Anatomically, the direction of VMO muscle pull was more horizontal. It also has been demonstrated in a cadaver study that all of the quadriceps femoris muscles, except the VMO, can extend the knee (regardless of the force applied) and that the role of the VMO is to maintain medial tracking of the patella during knee extension.11 An in vivo study12 showed that electrical stimulation of the VMO resulted in predominantly medial patellar pull. Because the VMO plays an important role in medial stabilization of the patella,11–13 any dysfunction may lead to reduced medial stabilization of the patella against the counterforce of lateral pull exerted by the VL and other quadriceps femoris muscle components.14,15
Numerous rehabilitation protocols have been described for treating people with patellofemoral problems. Quadriceps femoris muscle strengthening (increased force-generating capacity), especially that emphasizing the VMO, is generally considered to be a conservative treatment.14,16–19 Incorporating hip adduction with knee extension is a popular strategy for strengthening the VMO.14,16 This intervention takes into consideration the fact that the VMO is connected to the adductor magnus and longus muscles.20 Training of the adductor muscles uses this anatomical link to provide a more-stable proximal attachment and transfers physiological stretch to the VMO, thereby enhancing the contraction force.15,21
Because hip adduction exercise was found to selectively activate the VMO,22 numerous studies have examined the electrical activity of the VMO and VL with hip adduction during various knee extension exercises.15,21,23–29 Findings have been disparate. Selective recruitment of the VMO was reported in 2 studies where VMO activity was higher than VL activity following hip adduction with knee extension from a semi-squatting position in subjects who were healthy.21,28 In patients with PFPS, however, incorporation of hip adduction has been found to promote a more-balanced VMO/VL ratio29 or increased whole quadriceps femoris muscle activity.24,25
Hodges and Richardson21 and Grelsamer and McConnell30 have advocated incorporating hip adduction with prone-lying or squatting positions. These positions, however, are nonfunctional or weight bearing, where the tendency of increasing the dynamic Q-angle may exacerbate stress on the patellofemoral joint.31 Furthermore, some patients may not tolerate this position because the gradually increasing joint stress may aggravate the pain. Leg-press (LP) exercise, therefore, be used as a substitute to train patients in a functional position without aggravating symptoms.
To date, it remains unclear as to whether the addition of hip adduction to LP exercise would facilitate VMO hypertrophy and result in a better treatment outcome. In addition to the pain and functional scales that typically are used, ultrasonography provides a further quantitative measure for assessing muscle morphology in clinical trials involving patients with PFPS. Ultrasonography is a noninvasive and low-cost technique that is now extensively used for morphological investigations in the field of rehabilitation.32 The measurement validity of ultrasound compared with the magnetic resonance imaging or computed tomography gold standards has been reported to be acceptable.33,34 Indeed, although VMO strength cannot be directly assessed in vivo, morphological changes following exercise training can be both observed and quantified via ultrasonography and, thus, muscle force and excursion capability can be determined.35
The purpose of this study was to investigate the surplus effect of hip adduction to seated LP exercise on VMO morphology, pain, and function in patients with PFPS. We hypothesized that incorporation of hip adduction with leg-press training (LPHA) would result in more-beneficial effects on VMO hypertrophy, pain, and functional improvement compared with LP training alone.
Method
Setting and Participants
A total of 123 patients with a diagnosis of PFPS were referred to our kinesiology laboratory by an orthopedic surgeon (YFL). The inclusion criteria were: (1) experience of anterior or retropatellar knee pain after performing at least 2 of the following activities: prolonged sitting, stair climbing, squatting, running, kneeling, hopping and jumping, and deep knee flexing; (2) insidious onset of symptoms unrelated to traumatic accident; (3) presence of pain for more than 1 month; and (4) age of 50 years and under (to eliminate the possibility of osteoarthritis). In addition, participants had to exhibit at least 2 of the following positive signs of anterior knee pain during the initial physical examination: (1) patellar crepitus, (2) pain following isometric quadriceps femoris muscle contraction against suprapatellar resistance with the knee in slight flexion (Clarke's sign), (3) pain following compression of the patella against the femoral condyles with the knee in full extension (patellar grind test), (4) tenderness upon palpation of the posterior surface of the patella or surrounding structures, and (5) pain following resisted knee extension.
Participants were excluded if they had: (1) self-reported clinical evidence of other knee pathology; (2) patellar tendinitis or knee plica; (3) a history of knee surgery; (4) central or peripheral neurological pathology; (5) knee radiographic abnormalities (eg, knee osteoarthritis) or lower-extremity malalignment (eg, foot pronation); (6) severe knee pain (visual analog scale [VAS] score of >8); or (7) received nonsteroidal anti-inflammatory drugs, injections, or physical therapy intervention in preceding 3 months.
Of the 123 patients initially screened, 98 met the study inclusion criteria. Nine of the 98 recruited participants declined to participate before randomization. Therefore, a total of 89 participants were enrolled in this study. Sample size was calculated using a predetermined difference between treatment groups of 1.5 cm for worst pain on a 10-cm VAS. Assuming a standard deviation of 2 cm, at least 29 participants per treatment group were required to attain 80% power.
Participants were randomly allocated to 1 of 3 groups: a group that received hip adduction combined with leg-press exercise (LPHA), a group that received LP exercise only, or a group that received no exercise (control group). Ten participants later dropped out of the study due to personal factors (not knee pain) or work commitment. Seventy-nine participants completed the trial (27 in each exercise group and 25 in the control group, Fig. 1). The demographic data for the 3 study groups are presented in Table 1. There were no significant between-group differences for any of the demographic variables. None of the participants were engaged in regular sporting activities.
Flow chart demonstrating the progression of participants through the trial. LPHA=hip adduction with leg-press exercise, LP=leg-press exercise.
Demographic Data for Study Participantsa
Randomization
All volunteers were enrolled after providing written informed consent. The study was performed in a blind (nondiscriminatory) manner. A single physical therapist, unaware of the purpose of the study, was responsible for randomization and interventions. Stratified allocation was carried out with regard to the number of affected sides (unilateral or bilateral) and symptom severity (Lysholm scale scores ≥65 or <65). Participants were randomly assigned to the LP group, LPHA group, or control group in blocks of 9 (chosen through numbered opaque envelopes) and participated in 3 weekly exercise sessions for 8 weeks. Two assessment sessions were performed by another physical therapist (blinded to each patient's grouping) before and after the 8-week intervention.
Interventions
Simple LP exercise.
Leg-press exercise was performed unilaterally starting from 45 degrees of knee flexion to full extension using an EN-Dynamic Track machine.* Exercise within the functional range was considered safe for patients with PFPS.36 A blue Thera-Band† was tied to each patient's thigh (without resistance) to maintain consistent tactile stimulation among groups. Prior to the beginning of exercise training, the unilateral 1-repetition maximum (RM) strength of the lower extremity was determined by Odvar Holten Pyramid diagram37 with repetition-to-fatigue testing. Patients were unilaterally trained at 60% of 1 RM for 5 sets of 10 repetitions. The 1 RM was re-measured every 2 weeks, and the exercise intensity was adjusted accordingly. A 60-Hz metronome was used to control the exercise pace at 2-second concentric and eccentric contractions from 45 degrees of knee flexion to full extension. There were 2-second breaks between repetitions and 2-minute breaks between sets. Limbs were alternatively trained between exercise sets.
LPHA.
This exercise was performed as per the LP, except that a 50-N hip abduction force was applied to the distal one third of the thigh. This force was achieved by tying a blue Thera-Band to an arm of the EN-Dynamic Track machine (Fig. 2). Therefore, this exercise was a combination of LP and 50-N isometric hip adduction.
Hip adduction with leg-press exercise (a), with a close lateral view of the setup of resisted hip adduction via Thera-Band (b).
A hot pack was applied to the quadriceps femoris muscle for 15 minutes before exercise was commenced. After exercise completion, participants were asked to stretch the quadriceps, hamstring, iliotibial band, and calf muscle groups and were given a cold pack to apply for 10 minutes. Stretches were maintained for 30 seconds and were repeated 3 times for each muscle group. All study participants were asked not participate in any form of sport or exercise during the intervention period.
Control group.
Control group participants did not receive any exercise intervention, but were provided with health educational material regarding patellofemoral pain. They were advised not to perform or receive any exercise program or intervention. Neither tape nor brace was used. Exercise training was implemented after the 8-week control period.
Outcome Measurements
VAS pain assessment.
The VAS is a reliable, well-validated, and widely used tool for assessing knee pain.38–41 A 100-mm VAS was used to measure the worst pain (VAS-W) experienced in the previous week.
Functional evaluation.
The Lysholm scale was used to measure functional ability. The scale ranges from 0 to 100 points (with a score of 100 indicating maximal function) and was originally designed to evaluate symptoms and functions pertaining to knee injury.42 There are 8 components to this assessment: stair climbing (10 points), squatting (5 points), pain (25 points), presence of a limp (5 points), locking (15 points), instability (25 points), swelling (10 points), and the requirement of support when walking (5 points). The reliability, validity, and robustness of the Lysholm scale have been well documented.42–48 The Lysholm scale was found to correlate highly with the Kujala anterior knee pain scale (r=.86)45 and has been used as a knee function evaluation tool in patients with patellofemoral disorders.45,49–51
Measurement of VMO morphology.
Vastus medialis oblique muscle morphology was assessed by ultrasonography (HDI 5000‡) with a 5- to 12-MHz broadband linear-array transducer (38 mm). The ultrasonographic measurements included VMO cross-sectional area (CSA) on the patellar-base level and VMO volume under the patellar-base level.52 All measurements were obtained while participants were lying on a bed, with both legs relaxed (feet were positioned in a frame to prevent leg rotation) and a thick padded towel placed underneath the knee to maintain resting position.
The longitudinal length of the patella (in millimeters) was determined from the upper border to the lower border with calipers. The VMO volume under the patellar base was approximated from a series of VMO CSAs using the trapezoidal rule.52 To obtain a valid calculation of VMO volume from the sonographic image, a custom-made holder was used to fix the probe.52 The holder was calibrated to quantify movement of the transducer by synchronizing with a scaled hub, which was turned in a full circle to mobilize the transducer by 1 mm from the proximal patellar base toward the distal patellar apex along a line perpendicular to the horizontal representing the upper border of the patella. The first VMO CSA was taken from the line passing through the patellar-base level (Fig. 3). Serial VMO CSAs were obtained every 2 mm until the VMO image on the visual display faded. To control for any potential confounding pressure exerted by the probe holder, gel was applied to the skin such that there was no direct contact between the probe and the skin.53 The image was carefully monitored by the examiner to ensure that the VMO was not being compressed (Fig. 3).
Measurement of vastus medialis oblique muscle cross-sectional area on patellar-base level.
The intraclass correlation coefficients for between-day test-retest reliability of VMO CSA and volume measurements were .96 and .94, respectively. The actual day-to-day differences (X̄±SD) were 0.02±0.30 cm2 and 0.06±0.68 cm3, respectively. The standard errors of measurement were 0.29 and 0.52, respectively.
Data Analysis
Data obtained from the most symptomatic knee were analyzed using SPSS version 11.0.§ Data were subjected to an intention-to-treat analysis and included all dropouts. Descriptive statistics (X̄±SD) were used to determine participant characteristics. Prior to statistical analysis, the Kolmogorov-Smirnov test was performed to assess the normality of continuous data. Normally distributed baseline demographic variables (age, body height, body weight, and body mass index) were compared by 1-way analysis of variance (ANOVA). Non-normally distributed variables (symptom duration) were compared by Kruskal-Wallis test with an alpha of .05. Sex and numbers of afflicted sides (bilateral versus unilateral) were compared by chi-square test with an alpha of .05. For each outcome variable measured, a 2 (preintervention and postintervention) × 3 (treatment groups: LPHA, LP, and control) 2-way mixed ANOVA was performed. When a significant 2-way interaction was detected, post hoc analysis was performed using Bonferroni adjustment (P<.008).
Results
All exercise intervention participants except one attended all scheduled exercise sessions. One participant in the LP group completed only half of the intervention and subsequently dropped out of the study due to work commitments (Fig. 1). The remaining study participant dropouts in both exercise groups completed the exercise programs but did not attend postintervention evaluations.
Results pertaining to VAS-W, Lysholm scale scores, VMO CSA, and VMO volume for the 3 groups before and after the 8-week intervention period are shown in Table 2. There were no significant baseline differences among the groups. The 2-way ANOVA for repeated measures revealed significant interactions for VAS-W, Lysholm scale scores, VMO CSA, and VMO volume. Post hoc analyses indicated that VAS-W, Lysholm scale scores, VMO CSA, and VMO volume significantly increased following intervention in the LP and LPHA groups (P<.008), but not the control group (Tab. 2). Furthermore, values pertaining to the VAS-W and Lysholm scale were significantly better in both exercise groups compared with the control group after intervention (P<.008) (Tab. 3). The values for VMO volume were significantly higher in the LP group compared with the control group after intervention (P<.008), whereas the between-group difference in VMO CSA did not reach the level of adjusted significance (P=.012). The values for VMO CSA and VMO volume were not different between the LPHA group and the control group after intervention (P=.046 and P=.02, respectively). No differences were detected between the LP and LPHA groups (Tab. 3).
Comparison Between Preintervention and Postintervention Changes of Pain, Function, and Vastus Medialis Oblique Muscle (VMO) Morphology for the Hip Adduction With Leg-Press Exercise (LPHA) Group, the Leg-Press Exercise (LP) Group, and the Control Groupa
Comparison Among Group Changes of Pain, Function, and Vastus Medialis Oblique Muscle (VMO) Morphology for the Hip Adduction With Leg-Press Exercise (LPHA) Group, the Leg-Press Exercise (LP) Group, and the Control Groupa
The pretest-posttest effect size in the LPHA group ranged from 0.48 to 0.77 for VMO morphology and 0.76 to 1.10 for VAS-W and Lysholm scale scores, and the corresponding values for the LP group were 0.71 to 0.75 and 0.89 to 0.95, respectively. When comparing the effect between the LPHA and control groups, effect size values were 0.78 for VAS-W, 1.12 for the Lysholm scale scores, and 0.56 to 0.77 for VMO CSA and VMO volume. These effect sizes were consistently smaller than those associated with the LP group and control group comparison, where effect size values were 0.92 for VAS-W and 0.75 to 0.77 for VMO CSA and VMO volume. An exception was Lysholm scale scores (effect size=1.01). However, there was no significant difference in improvement for any variable between the LP and LPHA groups.
Discussion
Strengthening of the knee extensor via hip adduction is a very common therapeutic approach for treating people with patellofemoral pain.14,16 The effects of 8-week LP and LPHA exercise interventions on pain reduction, functional improvement, and VMO hypertrophy were comparable, indicating that there is no additive beneficial effect of incorporating hip adduction with LP exercise. This finding may be attributed to the fact that during simple LP exercise, the hip adductor magnus and longus muscles are simultaneously activated to stabilize hip movement.54
This is the first study, to our knowledge, that investigated the clinical effects of adding hip adduction to LP exercise for the management of PFPS. Limited knowledge might be gained if change of the training position would result in different outcomes. However, the effects of exercise training on pain reduction in the current study can be considered clinically significant based on a previous report that a VAS change of 1.5 mm in patients with PFPS is the minimal difference to be considered clinically important.38 These findings are in accordance with the results of previous studies regarding pain reduction where various quadriceps femoris muscle retraining exercise protocols were used.49,55–59 Functional performance (as indicated by Lysholm scale scores) following our exercise intervention improved by approximately 11 points. This finding is similar to those of a previous study in which Lysholm scale scores increased from 67.6±6.4 points to 81.1±9.4 points after 6 weeks of isokinetic training.49 Unlike the VAS, the minimal change in the Lysholm scale scores representing a clinically relevant improvement in functional status is yet to be determined. Out of 100 points, a score of 95 to 100 indicates excellent function, a score of 84 to 94 indicates good function, a score of 65 to 83 indicates fair function, and a score of <65 indicates poor function.42 Overall, the patients in our study exhibited significant functional improvement from a level of fair to good following LP or LPHA training. Functional improvements in our study were significantly correlated with reductions in VAS-W (r=−.451, P<.005). This finding is comparable to that from a previous report (r=−.424, P=.009).49 It must be borne in mind that the results of intention-to-treat may even be underestimated.
Because the Lysholm scale is not a PFPS-specific scale, we made a further subanalysis of the stair-climbing and squatting items. Initially, 93% and 82% of the exercise intervention participants had difficulty performing the stair-climbing and squatting tasks, respectively. After exercise intervention, 50% to 52% of the patients achieved maximal scores for stair climbing and squatting. Our findings of decreased pain and increased functional capacity agree with previous research demonstrating that patients with PFPS were able to perform significantly more step-ups, step-downs, and squats before pain onset after quadriceps femoris muscle training.59 The relationship between quadriceps femoris muscle strength and locomotor function in patients with PFPS has been documented previously by Powers and Perry.60 Leg-press exercise training, especially in the eccentric contraction mode, is better suited for individuals with PFPS who demonstrate weaker eccentric than concentric quadriceps femoris muscle strength.61 Given the improvements in 1 RM (from 90±30 kg to 145±50 kg in the LPHA group and from 89±33 kg to 138±51 kg in the LP group), it is not surprising that both exercise groups exhibited significant improvements in the stair-climbing and squatting scores on the Lysholm scale (r=.347, P<.05).
Folland and Williams62 concluded that the primary morphological adaptation after resistance exercise is related to an increase in the CSA of the whole muscle and individual muscle fibers (caused by an increase in myofibril size and number). After both LPHA and LP exercise intervention in our study, VMO hypertrophy was observed as the result of training because the amount of improvement was greater than the measurement error. This is the first study using noninvasive ultrasonography to investigate changes in VMO morphology in patients with PFPS following LPHA or LP training. We speculate that this positive outcome may be partially a consequence of VMO hypertrophy. As all patients were given hot packs before exercise and were instructed to stretch and apply cold packs after exercise, it is possible that the positive outcomes may not have been due to LP exercise alone. A previous study63 demonstrated that centralization of the patella can result from VMO strengthening and stretching procedures. Further research is needed to determine whether patellar alignment or tracking is altered with VMO hypertrophy.
We found that LPHA did not result in further beneficial effects compared with LP exercise alone. It is possible that hip adductor activation during LP training54 subtracted from the effect of additional isometric hip adduction. However, it also is possible that isometric hip adduction does not preferentially recruit the VMO.15,23–25,27 Further study is warranted to clarify this issue.
This study had several limitations. Only the VMO was examined by ultrasonography after exercise intervention, and the VL and other components of the quadriceps femoris muscle were not assessed. Additional investigation is warranted to examine morphological changes in individual quadriceps femoris muscles. A second potential limitation was the use of 50 N as the force level for hip adduction. This force level may have been inadequate to induce a training effect. We also note that outcomes were assessed following an 8-week exercise intervention period. Determining how these outcomes change with a longer-term intervention period would be of interest.
Conclusion
The findings suggest that an 8-week exercise program involving simple LP training (from 45° of knee flexion to full extension) and stretching can induce significant VMO hypertrophy, improve knee function, and reduce pain in patients with PFPS. We found that adding 50 N of hip adduction to LP exercise had no further beneficial effects on outcome compared with LP exercise alone after an 8-week intervention in patients with PFPS.
Footnotes
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Ms Song, Dr YF Lin, Dr DH Lin, and Ms Jan provided concept/idea/research design. Ms Song and Ms Jan provided writing. Ms Song and Mr Wei provided data collection. Ms Song and Dr DH Lin provided data analysis. Ms Jan provided project management, facilities/equipment, and institutional liaisons. Dr YF Lin and Dr DH Lin provided participants. Mr Wei and Ms Yen provided clerical support. Dr YF Lin provided consultation (including review of manuscript before submission).
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The study protocol was approved by the Research Ethics Committee of National Taiwan University Hospital.
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An oral presentation of the results of this study was given at the International Congress of the World Confederation for Physical Therapy; June 5, 2007; Vancouver, British Columbia, Canada.
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↵* Enraf-Nonius BV, Vareseweg 127, 3047 AT, Rotterdam, the Netherlands.
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↵† The Hygenic Corp, 1245 Home Ave, Akron, OH 44310-2575.
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↵‡ Advanced Technology Laboratories, 22100 Bothell Everett Hwy, Bothell, WA 98041-3003.
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↵§ SPSS Inc, 233 S Wacker Dr, Chicago, IL 60606.
- Received June 23, 2008.
- Accepted February 3, 2009.
- American Physical Therapy Association
References
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