Patellofemoral pain syndrome (PFPS) is a disorder related to peripatellar or retropatellar pain (5) and it is 1 of the most prevalent musculoskeletal conditions of the lower limb, with incidences estimated as high as 25% within the general population and 60% within the athletic population (28). Vastus medialis oblique (VMO) and vastus lateralis (VL) are the 2 principle muscles that work synergistically to stabilize the patella during dynamic knee extension. The ratio of VMO:VL has a theoretical ideal of 1:1 (24), and research has shown this ratio to be as low as 0.54:1 in people with PFPS (23). Any disturbance in the VMO:VL ratio, owing to a decreased medial pull, may lead to patella maltracking (1,15) and consequently inflammation, pain, premature cartilage degeneration, and ultimately PFPS (26). It has been suggested that reestablishing this imbalance can be achieved by strengthening exercises specifically targeting VMO. The existing evidence base supports this as a successful method of preventing and reducing PFPS (19,21,26), and current literature is flooded with research concerning the best exercises to preferentially activate VMO. Nonetheless, debate still remains as to an agreed “gold standard” exercise and controversy litters the evidence base.
Contradictory evidence surrounds the specific use of hip position to activate VMO. Hanten and Schulthies (14) suggested that the addition of hip adduction is crucial to selectively activate VMO. Anatomic cadaver studies have shown that fibers from the VMO originate from the distal part of the adductor magnus and by contracting the adductors the VMO is provided with a “stable origin from which to contract” (14). Hip adduction also generates a stretch of the VMO fibers, which adjusts the length tension properties of the muscle and thus will increase the contraction force. In stark contrast, findings from Coqueiro et al (6) found that during a standard semi-squat exercise, there was a significant difference between VL and VMO electromyographic (EMG) activity in favor of VL. Different conclusions were made by Earl et al (11), who found that combining isometric hip adduction with a closed-chain mini-squat produced greater overall quadriceps activity compared to a standard mini-squat. Previous literature highlighted problems with failure to standardize adduction forces (6); therefore, interpretation of findings should be made with caution.
Open-chain leg extension exercises are often a routine method of selectively targeting VMO in clinical practice (13,20). There is little supporting evidence to verify its use, and within the available literature common consensus is far from conclusive. The established definition of an open kinetic chain exercise is a single joint movement that is performed in a nonweightbearing position with a free distal extremity. Conversely, closed-chain exercises are considered multijoint movements performed in a weightbearing or simulated weightbearing position with a fixed distal extremity (25). Although research is sparse, there are clinical situations when open-chain exercises may be the more appropriate treatment option. For example, those with a large body mass index (BMI) may be at risk of increased levels of load being placed through the joint and, because of reduced exercise tolerance, this may cause difficulty with following closed change programs. Additionally, open-chain knee extensions may be the only possible rehabilitation option for patients with cognitive difficulties, with multiple pathologies, or who are at risk of falls.
Laprade et al. (20) showed that VMO was more active than VL during knee extensions. Brownstein et al (3) reported that VMO was most active at 60 to 90 degrees of knee flexion in a nonweightbearing position and least active with the knee fully extended. Additionally, Tang et al (27) reported that in asymptomatic participants a VMO:VL ratio of greater than 1 was evident during open-chain knee extensions at 75 and 90 degrees of knee flexion. Patients with PFPS were also shown to have an improved ratio during the eccentric phase at 60 degrees, 75 degrees, and 90 degrees of knee flexion (3).Comparisons to other literature need to be made with caution because muscle force was measured via a dynamometer as opposed to EMG activity.
Current research has not yet explored the effectiveness of incorporating lunge exercises into the rehabilitation of VMO. The significance of such an exercise can only be hypothesized based on its biomechanics. Suggestions have been put forward that closed kinetic chain exercises, such as squatting or lunging, utilize both multiple-joint proprioceptive reactions and muscular cocontraction. For this reason they are assumed to be a more functional intervention than open-chain exercises (27). A recent study also suggested that a closed kinetic chain exercise with a short arc (<45 degrees) is the best exercise to strengthen the quadriceps muscles group because it induced the least joint reaction force (27). The study of Tang et al (27), however, showed that the greatest activation of VMO is actually achieved at 60 degrees of flexion in a weightbearing position. The authors used a squat-to-stand closed kinetic chain exercise with knee flexion angles ranging from 15 degrees to 90 degrees. Findings also showed that a ratio of greater than 1 was present at 60 degrees of knee flexion. This has been confirmed by other studies (17). For this reason, it can be anticipated that lunge exercise may be beneficial in the retraining of the VMO muscles.
The aims of this study were to investigate the effect of the tested exercises on the activation of VMO:VL and to ascertain which exercise preferentially activates VMO in relation to VL. The choice of exercises aims to compare 2 exercises that are commonplace in rehabilitation (i.e., open-chain leg extension and squat with isometric hip adduction) and add an exercise that has never been studied but is potentially a valuable addition to VMO strengthening. The lunge exercise may offer a simple alternative closed-chain exercise that is highly functional and, because of the increased weightbearing on the limb doing the exercise, may result in a substantial VMO contraction.
Experimental Approach to Problem
The muscle activities of the VMO and VL were measured in 22 healthy, asymptomatic subjects in 3 quadriceps muscle-strengthening exercises. Surface EMG signals were utilized to investigate the differences among the 3 exercises tested. This study was a repeated-measures design and took place in a university laboratory setting.
The electrical activity of VMO and VL was recorded using surface EMG electrodes (Biometrics Ltd, SX230, Gwent, United Kingdom), with a set interelectrode distance of 20 mm. Surface electrodes have been shown to be influenced by crosstalk from directly adjacent muscles (4), and the contracting muscle can potentially change the EMG signal as it may consequentially move the electrode. Farina et al. (12) reported that surface EMG is influenced by motor unit discharge rates and muscle fiber membrane characteristics. Therefore, participants were swabbed with alcohol wipes prior to electrode placement and BMI was limited to 30 (26). Results were recorded and filtered using Biometric software.
To enable normalization of the EMG findings, a maximal voluntary contraction (MVC) was carried out (3). The maximal voluntary contractions of VMO and VL were determined by using the mean average of 3 static quadriceps maximal isometric contractions at 45 degrees of knee flexion. A 30-second rest period between each contraction was included to eliminate possible fatigue affects. Prior to data analysis, all results were normalized by calculating them as a percentage of their MVC (%MVC). The average value of %MVC for each exercise was then calculated as the arithmetic mean of the 3 values. These data then were used to calculate the VMO:VL ratio.
EMG surface electrodes were placed on the muscle belly of VMO, 4 cm superior and 4 cm medial to the supromedial boarder of patella at approximately 55 degrees to the long axis of the femur. The VL electrode was placed 10 cm superior and 6-8 cm lateral to the superior lateral boarder of the patella and orientated 15 degrees to the vertical (16). A reference electrode was situated at the tibial tubercle to normalize background electrical activity. Electrodes were attached using double-sided tape, with additional taping over the whole electrode site. This was to ensure the best possible skin contact to limit skin impedance.
Twenty-two healthy asymptomatic subjects participated in this study (11 men, 11 women, age = 25.06 ± 4.67; height = 1.73 ± 0.09 m, mass = 65.57 ± 9.38 kg, BMI = 23.51 ± 2.70). Participants were excluded if their age was younger than 18 or older than 40; if they had any current or previous history of knee or lower limb injury (18); if they had any history of knee pain on ascending or descending stairs, squatting, kneeling, prolonged sitting, hopping, or jumping within the last 3 months (7,10); or if their BMI was larger than thirty (26). The study was undertaken in the months of March and April; however, this should not influence the repeatability of the study because the setting was in closed conditions much like an outpatient or ward setting. Participants had a range of fitness levels and training backgrounds. The study was approved by the Human Ethics Sub-Committee of the University of Plymouth, United Kingdom. All participants were older than 18 years old and were informed of the procedures, experimental risks, rationale, and their role. A consent form was signed prior to the investigation and participants were informed of their right to withdraw.
Prior to the study, participants performed a 5-minute submaximal warm-up using a cycle ergometer, working at a rating of 11 or 12 on the Borg rating of perceived exertion scale (2,9). The scale ranged from 6 to 20, where 6 meant “no exertion at all,” 20 meant “maximal exertion,” and 11 corresponded to “light work.” Patients were instructed to reflect on how heavy and strenuous the warm-up felt and must relate it to a combination of all sensations of physical stress, effort, and fatigue. The possibility of exercise induced injury was therefore limited.
This study investigated the following quadriceps strengthening exercises with no external additional weight.
- i. Open kinetic chain knee extension exercise
- The subject was seated on a raised platform with both feet off the floor and thighs supported to the popliteal fossa (Figure 1). A single leg extension was performed between the angles of 90 degrees to 0 degrees knee flexion (27).
- ii. Double leg squat with isometric hip adduction exercise (closed kinetic chain)
- This exercise was achieved by compression of a folded pillow placed between the medial joint lines of the knees. The exercise was performed with the back flat against a wall while squatting to 45 degrees (6) (Figure 2). The adduction force was maintained throughout the entire exercise, and a pillow was chosen to maximize comfort because there was concern that a more rigid object could lead to discomfort, thus inhibiting muscle activity and affecting electrode placement.
- iii. Lunge exercise (closed kinetic chain)
- A lunge exercise was performed with the measured leg foremost in a stride stance with both knees fully extended. Feet were hip-width apart and both feet were angled forward. The measured knee was flexed to 45 degrees (Figure 3), followed by the return to full extension while maintaining the knee in a neutral alignment over the second metatarsal. The rear knee remained in full extension throughout the exercise, and both heels remained in contact with the floor.
The dependent variables in this study were the %MVC of VMO and VL of the exercised tested and the VMO:VL ratio of each exercise. If the VMO:VL ratio was greater than 1, it implied that the VMO had a higher muscle activity than that of VL and pulled the patella medially. A repeated-measures analysis of variance test was used to examine any differences in these variables among the 3 exercises. A post hoc least significant difference test was performed with the level of significance set at p = 0.05. The consistency of the dependent variables over the 3 trials within each subject was determined by using the intraclass correlation coefficient (ICC) (1,3).
The mean ICCs (1,3) of the dependent variables were 0.94 ± 0.02, indicating that there were no significant differences in the variables among the 3 trials. It is concluded that the results obtained were highly repeatable, allowing generalization of results. The power of this study was 0.62 and the effect size was found to be 0.87, which was sufficiently large enough to reveal any significant differences in this study. There was no significant difference in muscle recruitment and VMO:VL ratios between males and females for 3 exercises (Table 1, p > 0.05).
Activation of VMO
Double leg squat exercise showed a significantly greater activation of VMO in comparison with the open kinetic chain knee extension exercise (p = 0.015) and lunge exercise (p = 0.005) (Table 1, Figure 4). There were no significant differences between the double leg squat with isometric hip adduction exercise and the lunge exercise in activating VMO (p = 0.602).
Activation of VL
The open kinetic chain knee extension exercise and double leg squat exercise showed significantly greater activation of VL in comparison with the lunge exercise (p = 0.001 and p = 0.036) (Table 1, Figure 4). However, whereas the open-chain knee extension showed a greater VL activation than the double leg squat exercise, this was not significant (p > 0.05).
Both the double leg squat and lunge exercises showed a significantly greater VMO:VL ratio (p = 0.019 and p = 0.045) than open-chain knee extension exercise (Table 1). No significant difference was seen between double leg squat and lunge exercises in the VMO:VL ratio. Open-chain knee extension exercise was shown to preferentially activate VL resulting in a ratio of 0.72:1 (Table 1).
This study has added to the existing debate concerning the best exercise to selectively activate VMO in relation to VL. The conclusions drawn have allowed us to reject the null hypothesis and make possible recommendations for practice. Results have shown that the double leg squat with isometric hip adduction exercise produced a significantly greater activation of VMO than the other 2 exercises tested. These findings are in line with a previous study (14), which also concluded that this was the most effective exercise for activation of VMO. Coqueiro et al (6) reported that double leg squat exercises increased activation in both VMO and VL, thereby producing a ratio closer to the idealized ratio of 1:1 (24). This is in agreement with our findings because VMO and VL both achieved a significant increase in activity. Coquerio et al (6) showed no significant preferential activation of VMO over VL; however, our study did show that the double leg squat with isometric hip adduction exercise did preferentially activate VMO (Table 1), producing a ratio of greater than 1.
The open kinetic chain knee extension exercise was shown to produce significant activation of VL instead of VMO, resulting in a VMO:VL of less than 1. This concurs with the findings of Tang et al (27), in which, during an isokinetic open kinetic chain knee extension exercise, a VMO:VL ratio of less than 1, 15 to 60 degrees was reported. Therefore, it can be anticipated that open-chain knee extension exercises may have little beneficial effect or even an adverse effect on PFPS rehabilitation because the decreased VMO:VL ratio 0.72:1 suggests that open kinetic chain knee extension exercise may induce excessive lateral tracking of the patella.
The pathology of PFPS is generally accepted to result from VMO weakness and disruption of the VMO:VL ratio (19,21,26). The high muscle activity and preferential VMO:VL ratio (1.14:1) generated by the double leg squat with isometric hip adduction exercise showed that this could potentially be a key exercise in the PFPS rehabilitation. The double leg squat with isometric hip adduction exercise will be useful in restoring quadriceps strength with a desirable VMO activation when there is a large deficit in VMO muscle activity in patients with PFPS.
This is the first study to look at the biomechanics of a lunge exercise. The VMO:VL ratio of the lunge exercise was closest to the idealized 1:1 ratio (24). Although this exercise has been shown not to produce as much muscle activity as the double leg squat with hip adduction exercise, this exercise may be advantageous in the initial stage of PFPS rehabilitation when sufficient VMO muscle strength has not been regained yet and the priority is to establish a balanced patella tracking. Little research has been undertaken regarding different stages of rehabilitation; therefore, this is an area that warrants further investigation.
Attempts were made to ensure rigorous and sound methodology throughout the study. Exercises and electrode placement were strictly standardized, and confounding variables (fatigue effect, skin impedance, and learning effect) were identified and accounted for in the study design, alongside strict inclusion exclusion criteria, to maximize external validity. There were a number of limitations within the study. First, all the participants are asymptomatic and findings therefore cannot be generalized to patients with PFPS. The numerous independent variables that relate to musculoskeletal rehabilitation such as pain inhibition, secondary complications, previous functional level, and stage of rehabilitation could potentially and significantly alter results. Future study will therefore require the investigation of symptomatic patients to enhance both the rigor and the external validity of the research. Second, the protocols used for electrode placement allowed for no variation in gender and size. Therefore, it cannot be certain that we sampled fibers from the correct muscles; however, this is a common flaw throughout the literature (8). This is clinically significant because recent research showed that EMG pads positioned at mid-thigh level, opposed to closer to the muscles insertion, detected lower signal strength owing to the close proximity of EMG pads positioning at mid-thigh level than a lower signal strength would be detected owing to its close proximity to the muscle's point of innervation (29). Finally, the current study only examined the maximum EMG activity for each exercise. Further study may wish to examine total muscle activity during the exercise. In addition, comparing concentric and eccentric phases of activity may provide interesting results.
This study investigated the exercises with no external additional weight. This ensured standardization and functionality. However, it could be beneficial to study the effect of external load on these specific VMO exercises because it is common consensus that an external load will cause muscle fatigue more rapidly than if no load was added. This in turn may be of interest because knowledge of fatigue rates could assist the practitioner in exercise prescription.
This research not only adds to the existing debate concerning PFPS, but also highlights that there is a requirement to investigate which exercise can preferentially alter the VMO:VL ratio and how the muscles respond to overloading and long-term rehabilitation. Fatigue has detrimental effects on muscle performance; therefore, considering the specific fatigue rates of VMO and VL is vitally important when designing exercise protocols for patients with PFPS (22). No research, as yet, has been undertaken to examine the fatigue rates of the quadriceps using exercises that specifically target VMO. Such knowledge would allow the development of appropriate exercise protocols regarding number of repetitions, load, and intensity. This study has been the first to investigate the effects of the lunge exercise on VMO and VL activity and is an area that requires further study in both symptomatic and asymptomatic participants.
In summary, this study showed that double leg squat with isometric hip adduction exercise produces significantly greater VMO activation than both the lunge exercise and the open-chain knee extension exercise. Furthermore, the double leg squat with isometric hip adduction exercise and the lunge exercise produced a significantly higher VMO:VL ratio than that of the open-chain leg extension exercise. In our study, the open-chain knee extension exercise was shown to preferentially activate VL instead of VMO and therefore may not be suitable for patients with PFPS.
One of the main factors leading to patellofemoral pain is the muscle imbalance between the VMO and VL, leading to an excessive lateral tracking of the patella. Because the VMO and VL demonstrated a lower level of activation during the lunge exercise, the lunge exercise will be useful in the initial stage of PFPS rehabilitation when sufficient VMO muscle strength has not been regained yet and the priority is to establish a balanced patella tracking. The higher muscle activity and preferential VMO:VL ratio generated by the double leg squat with isometric hip adduction exercise will be very useful in further reestablishing the correct tracking of the patella and strengthening of the VMO muscles in the later stage of PFPS rehabilitation. The results of this study provide the scientific basis for the use of the lunge exercise in devising a closed-chain exercise protocol for VMO strengthening with a lunge exercise potentially commencing rehabilitation and progressing to a squat with isometric hip adductions as a later stage addition. This research potentially assists the practitioner in selecting the most appropriate exercises at the correct stage of rehabilitation, thereby reducing the severity of PFPS and decreasing rehabilitation times of patients with patella maltracking. Further research is vital to distinguish the effects of these exercises on symptomatic participants.
No benefits in any form have been or will be received from a commercial party/grant body related directly or indirectly to the subject of this manuscript.
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