Introduction
Resistance exercises are commonly used to strengthen the muscles of the quadriceps group during training and rehabilitation. Among the major concerns faced by therapists, athletic trainers and strength coaches are low quadriceps strength and imbalances in strength and activation patterns between vastus medialis oblique (VMO) and vastus lateralis (VL) muscles, which may potentially lead to patellofemoral pain and dysfunction. Clinicians maintain that when the patella is surrounded by unbalanced forces that cause lateral patella tracking, the results are abnormal stresses on the subchondral bone resulting in pain and injury. These forces can be caused by poor coordination between the medial and lateral pull produced by the VMO and VL, respectively, combined with tightness of the lateral retinaculum and iliotibial band (12 patellofemoral joint (12,24 7,35,36
The benefits of closed chain kinetic exercises and open chain kinetic exercises (OKC) are well established in the literature (17,41,42 13,39 35 19,21
Given the discussions concerning the effectiveness and safety of using limited ROM to selectively target the VMO and VL, researchers have also examined the use of different foot positions during LE. These studies have included analyses during isometric (36–38 9,10,18 32
To our knowledge, however, only 2 studies have used electromyographic (EMG) analyses to quantify VMO versus VL activity levels during isoinertial LE training. The initial study by Signorile et al. (32 33 30
Providing clinicians, including therapists, strength coaches, or athletic trainers with the capacity to combine ROM and POS for more effective targeting of the vasti muscles may allow safer and more effective protocols for reducing and assessing anterior knee pain (11
Methods
Experimental Approach to the Problem
To investigate the impacts of foot position and ROM on the selected activities of the VL, vastus medialis , and rectus femoris (RF) during the LE exercise, participants performed three 8-repetition sets through their full ROM with their feet in the toes in, neutral, or toes in position. Each repetition was divided into three 0.524 rad segments marked by pulses resulting from closures of magnetic reed switches. Electromyographic activities of the RF and vasti muscles were collected throughout each set. The patterns of foot positions were randomly assigned, and recovery periods of 10 minutes were provided between sets. Separate 3 (ROM) by 3 (foot position) by 8 (repetition) repeated measures ANOVA were used to assess differences in the activities of the 3 superficial quadriceps muscles, and least square mean analyses with a Bonferroni adjustments were used to ascertain the source of those differences.
Subjects
Twelve men and 12 women (aged 18–40 years), who were currently training at least twice a week and had a minimum of 6 months of lifting experience, participated in the study. Physical characteristics of the subjects, who met these criteria and participated in the study, are presented in Table 1 . Before admission into the study, all potential subjects completed a medical questionnaire form, which was used to screen for previous or preexisting injuries. To qualify for the study, applicants needed to be in good health with no knee problems, such as tendinitis, instability, or previous surgical repairs, which could have affected their performance or put them at undue risk for injury. The University of Miami Institutional Review Board for the Protection of Human Subjects approved all procedures before testing, and subjects were required to sign an informed consent before participation. The study complied with the principles laid down in the Declaration of Helsinki. A flow diagram illustrating the study design is presented in Figure 1 .
Table 1: Physical characteristics of subjects.*
Figure 1: Study flow diagram.
Lifting Protocols
Bilateral LEs were performed on a Cybex VR LE machine (CYBEX International, Inc., Medway, MA, USA) with the backrest secured at 1.75 rad. The bilateral LE was chosen over the single leg movement because it provided a more comfortable position for varying foot positions and is a more commonly used exercise than the unilateral LE. Subject testing and data collection occurred during a single laboratory session. This ensured that the collected EMG data were not affected by electrode placement or preparation or environmental variations. All exercises were preceded by a movement-specific warm-up on the Cybex machine consisting of 5 repetitions (REP) at 50% of the subject's reported 8RM. After the warm-up set, each subject performed 1 or 2 additional sets to establish his or her 8RM. Because subjects were experienced weight-trainers, they were able to establish their 8RM within 2 testing sets. Subjects were provided a 5-minute recovery period between 8RM attempts and received a minimum of 10 minutes of additional recovery before the start of testing. During this time, the subject was prepared for EMG data collection.
Electromyography
Because the purpose of the study was to examine the level of EMG activity in the 3 large superficial quadriceps muscles (VL, RF, and VMO), a bipolar surface electrode configuration was chosen to maximize the reception area, while controlling the potential for cross talk among the muscles examined. Surface electrodes have been specifically recommended for this application. Andreassen and Rosenfalck (2 22
The location of each muscle's motor point was determined using a low-voltage stimulator delivering a series of 5-millisecond pulses at 5 pulses per second. Voltage was progressively reduced from approximately 50 V until only 1 point elicited a response. This point was then used as the motor point for that muscle. After motor points were located, the skin surface distal to each point was shaved, lightly abraded, and cleaned with alcohol to remove dead surface tissues and oils, which might reduce signal fidelity. Disposable Ag/AgCl pregelled disk surface electrode pairs (Marquette Medical Systems, Jupiter, FL, USA) were positioned immediately distal to each motor point, 1.5 cm apart and parallel with the underlying muscle fibers as determined by the pennation of the muscle. EMG data were collected from the quadriceps muscles of each subject's right leg, and a reference electrode was placed on the head of the fibula of the same leg.
Raw EMG signals were recorded using a wireless telemetry system (Noraxon USA, Inc., Scottsdale, AZ, USA), and the quality of the signals was visibly assessed for each muscle before data collection. The Noraxon system used to collect the EMG data had an input impedance of 2 Megaohms (MΩ) and a common mode rejection ratio of 100 db. The gain was set at 2000 with band pass filtering set between 5 and 500 Hz. Signals were sampled at a speed of 1024 Hz, digitized using a 16-bit A/D converter (DataPac, Laguna Beach, CA, USA) and stored on a laboratory computer.
Data Collection
After electrode preparation, each subject performed bilateral LE at 70% of his or her 8RM. The ROM for the LE movement was individually established for each subject. A magnet was attached to the upper edge of the top plate on the moving weight stack. Two magnetic closure (reed) switches, interfaced with 5 V power sources, were placed on the frame of the LE machine at each end of the subject's ROM. One reed switch was placed on the frame of the LE machine directly across from the magnet, when the subject was seated in the machine with the weight plates resting on one another. The angle of knee flexion was measured at this point using a mechanical 12-inch International Standards of Measurement goniometer. The subject was then asked to maximally extend his or her legs, and a second reed switch was placed on the machine frame at this position to mark the upper end of the subject's ROM. Two additional reed switches were placed on the LE machine at 0.524 rad and 1.047 rad of knee flexion from the horizontal plane. A schematic diagram of the LE data collection is presented in Figure 2 . Mean values for the knee angle corresponding to the beginning and end points of ROM for the male and female subjects and the entire sample are presented in Table 2 .
Figure 2: Schematic diagram of the collection system.
Table 2: Beginning and ending knee angles of subjects.*
During each REP, whenever the magnet passed a reed switch, a 5-V spike was produced and sent to the analog-to-digital board of the EMG collection system. These spikes were used to separate the different angles at which EMG data were collected throughout the concentric portion of the LE (Figure 3 ). The reed switch marking the end ROM was interfaced with a second 5-v power source and a buzzer, so an audible signal was produced. This signal indicated to the subject that he or she had reached the end of the concentric ROM. The spikes collected at the beginning and end of each concentric contraction allowed us to accurately mark the concentric portion of the event for EMG analysis.
Figure 3: Illustration of a raw electromyography signal with 5-V spikes used to delineate specific ranges of motion.
Three foot positions (POS) were tested during each testing session: (a) toes neutral (NEU) with no medial or lateral rotation, (b) toes in (TI) with toes internally or medial rotated as far as the subject could comfortably be maintained by the subject throughout all REP, and (c) toes out (TO) with toes externally or laterally rotated as far as the subject could comfortably be maintained throughout all REP. No attempt was made to dictate a specific foot angle for the TI or TO POS because this would be dependent on each individual's ROM and his or her ability to maintain that position during performance of the LE. The order of the POS was randomly selected to reduce the impact of any order effect. One set of 8 REP was performed for each POS. A REP was not counted unless it was performed throughout the individual's full ROM. An acceptable REP began with a momentary resting of the weight plates and ended, when the magnet affixed to the weight stack passed the reed switch eliciting an audible signal. Subjects were instructed not to reverse the direction of movement at the top of the ROM until the audible signal was heard and to provide a momentary stop at the bottom of the ROM. Subjects were given a 5-minute recovery period between sets. Consistent instructions were used to motivate the subjects during testing. Data were collected throughout the entire ROM and later divided into the concentric and eccentric portions of each REP for analysis.
Electromyographic Analysis Procedures
Recorded EMG signals from each muscle were analyzed using DATAPAC 2K2 software (Run Technologies, Mission Viejo, CA, USA). The root mean square of the EMG signal (rmsEMG) was used as a measure of average muscle activity for each muscle throughout the concentric phase of each REP under each testing POS. The rmsEMG signal was normalized (NrmsEMG) using the highest recorded value seen for each individual muscle during any of the 3 testing POS, and all comparisons were made using NrmsEMG.
Statistical Analyses
Data analyses included separate 3 (ROM) by 3 (POS) by 8 (REP) repeated measures ANOVA to examine differences in NrmsEMG within each of the 3 superficial quadriceps muscles. When statistically significant differences were detected, individual least square means (LS means) analyses with a Bonferroni adjustment were used to ascertain the source of those differences. All analyses were performed using the SAS 8.0 statistical package (SAS Institute Inc., Cary, NC, USA).
Results
Vastus Lateralis For the VL, significant POS by ROM (p < 0.001) and REP by ROM (p = 0.007) interactions were detected. A post-hoc analysis of the POS by ROM interaction revealed that the EMG activity produced during the NEU and TO positions increased linearly across knee angles, while the EMG activity for the TI position was the greatest in the middle 0.524 rad ROM. The electrical activity produced in the TI position was significantly greater than the TO and NEU positions during the first 0.524 rad and middle 0.524 rad ROM. Additionally, over the final 0.524 rad ROM, only the NEU position was significantly different than the TO position (Figure 4 ). The post-hoc analysis of the REP by ROM interaction revealed that no significant differences existed between the middle and final 0.524 rad ROM over the course of the 8 REP. However, the first 0.524 rad ROM was significantly lower than the middle and final 0.524 rad ROM during each REP (Figure 5 ).
Figure 4: Normalized vastus lateralis rms electromyography for the neutral, toe in, and toe out positions across designated ranges of motion. *Significantly different than the neutral position (p ≤ 0.05). †Significantly different than toe out position (p ≤ 0.05).
Figure 5: Normalized vastus lateralis rms electromyography for the 0–0.524 rad, 0.525–1.047 rad, and 1.047–1.570 rad positions across repetitions. *Significantly different than 1.047–1.570 rad (p ≤ 0.05). †Significantly different than 0.525–1.047 rad (p ≤ 0.05).
Rectus Femoris
For the RF, significant main effects were observed for REP (p < 0.0001) and POS (p = 0.0009). Mean EMG activity of the RF progressively increased across REP (Figure 6 ). For POS, the TO position generated significantly higher electrical activity than either the NEU (p < 0.001) or TI (p < 0.001) positions (Figure 7 ).
Figure 6: Normalized rectus femoris root mean square electromyography for the toe in, neutral, and toe out positions. *Significantly different than the neutral and toe in positions (p < 0.0001).
Figure 7: Normalized rectus femoris root mean square electromyography across repetitions. *Significantly higher than the preceding repetition (p ≤ 0.05).
Vastus Medialis A significant REP by ROM interaction (p = 0.02) was found for the VMO. A post-hoc analysis revealed that no significant differences existed between the middle and final 0.524 rad ROM over the course of the 8 REP; however, the electrical activity during the first 0.524 rad ROM was significantly lower than that of the middle 0.524 rad for REP 6 and 8 and the final 0.524 rad for REP 1, 2, 3, 4, 6, and 8 (Figure 8 ).
Figure 8: Normalized vastus medialis root mean square electromyography for 0–0.524 rad, 0.525–1.047 rad, and 1.047–1.570 rad across repetitions. *Significantly different than 1.047–1.570 rad (p ≤ 0.05). †Significantly different than 0.525–1.047 rad (p ≤ 0.05).
Discussion
This is the first study, to our knowledge, examining levels of EMG activity of the superficial quadriceps muscles using specific ROM and POS during a set of isoinertial bilateral LE; however, the combined effects of ROM and POS have been examined in one other study by Signorile et al. (30 30
The results of this study show that to maximally activate the VL, both ROM and POS must be considered. When the LE was performed with the ROM limited to the initial 1.047 rad, TI produced the highest electrical activity levels. As we noted in an earlier article (31 30
Our results indicating that for targeting the RF, ROM was not a consideration and that TO generated the highest activity also differs considerably from those reported by Signorile et al. (31
Finally, when comparing our results for the VMO to those of Signorile et al. (30 30
The differences between the results reported by Signorile et al. (30 4,8 3 1,15 14 29 23,28
Comparisons of our results to other studies that have examined POS during dynamic contractions are feasible, but cannot address the potential interactions that are evidenced by our results. For the VL, our results showing that the TI POS produced the highest activity levels during the initial 1.047 rad are in agreement with the results reported by Stoutenberg et al. (33 32 33 32 33 32 32 33 38 30,33 30 33
The impact of knee angle alone on the EMG activities of the superficial quadriceps during this study can also be examined relative to results reported in previous studies; however, these comparisons must be considered recognizing the influence of foot position on muscle activity at specific knee angles. For example, considering the VL, no definitive statement can be made because optimal knee angles cannot be determined without considering foot position. On examining the impact of knee angle on RF activity, comparisons are also difficult because this study detected no impact of ROM on EMG activity. These results differ from those of Wheatley and Jahnke (38 6 13,39 1,34 5,20,38
Finally, the patterns of increasing activity seen across repetitions regardless of knee angle or foot position are to be expected given the correlation between increasing levels of fatigue and EMG amplitude increases reported during repeated submaximal contractions (27
When considering the use of LE to balance VM and VL forces, based on structural kinematics, researchers have advocated movement in the final 0.524 rad because in this position, the patella rests completely above the intercondylar groove against a suprapatellar fat pad resulting in little or no contact forces (16,35 patellofemoral joint have been shown to occur between 1.047 and 1.571 rad of flexion, when only 30% of total patella area is in contact with the femur leading to excessive contact pressure (25 patellofemoral joint stress is between 0 and 0.524 rad, plateauing between 0.524 and 1.047 rad, and steeply declining from 1.047 to 1.571 rad. These findings, in conjunction with our results, provide evidence that if targeting the VMO for reduction of patellar dysfunction, the optimal benefit would occur over the final 1.047 rad ROM and that the POS is not a factor. However, if the objective is to reduce the activity of the VL during this training, the TO POS should be used.
Practical Applications
Open chain kinetic exercises exercises are used in physical therapy clinics and athletic training facilities during the late phases of rehabilitation after major reconstructive surgery of the anterior cruciate ligament (ACL) and to address patellar malalignment that may lead to patellofemoral pain and dysfunction. Several studies have shown the efficacy of using OKC, such as the LE, as a part of this rehabilitation process (i.e., ACL reconstruction) (26,33 13,39
In conclusion, clinicians, with the use of the proper combination of ROM and POS, can apply our results to design exercise OKC protocols specifically for patients with either PFP or in the late stages of rehabilitation after ACL reconstruction surgery. The results from this study will allow clinicians to use suggested TI or TO positions to selectively activate the vasti muscles. In many OKC protocols, the focus on individual muscle activation of the superficial quadriceps muscles could be improved. The ability to target these muscles using a combination of ROM and varying foot positions supports our hypothesis. The generalizability of our results may be limited by the use of a young, healthy sample with no history of knee or ankle injury. This may affect both the exercise patterns and the EMG responses during LE. We suggest a similar study be conducted with patients presenting PFP. Finally, these results are specific to the LE exercise and may not be applicable to other open or closed kinetic chain knee exercises (40
Acknowledgments
The authors would like to thank A. Cole Meyers and Hyung-pil Jun, Research Assistants at the Department of Kinesiology and Sport Sciences of the University of Miami, for their help in preparing the Introduction and Discussion sections of this article.
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