Landing, squatting, and running tasks are often affected by valgus or varus dynamic forces on the knee joint (26). These forces may place excessive mechanical stress on the associated cartilage and ligaments of the tibiofemoral and patellofemoral joints, ultimately resulting in injuries or impaired movement efficiency (21,28). Patellofemoral pain, chondromalacia, and lateral subluxation or dislocation of the patella is often associated with delayed onset of activation in the vastus medialis oblique (VMO) relative to the vastus lateralis (VL), as well as a lower VMO/VL ratio in subjects with lower extremity instability. In this scenario, the patella may track laterally, resulting in uneven wear on the underlying cartilage and in knee pain (19,23,24).
The VMO is attached to the medial intramuscular septum of the adductor longus and adductor magnus; with the majority of the fibers arising from the tendon of the adductor magnus (25). Consequently, there have been numerous studies that have sought to identify exercises to selectively recruit the VMO in an effort to retrain this muscle, considering that quadriceps supplies the motive force for dynamic knee extension. During this action, the VMO and VL co-contract to stabilize the patella as it tracks within the patellofemoral groove (5,23,28,29). Previous studies noted that lower extremity exercises performed with an elastic band around the knee or with the hips externally rotated induced greater coactivation of the VMO/VL and hip abductors to enhance knee joint stability (3,5,17,26).
Moreover, excessive varus forces (i.e., hip abduction/knee adduction) have been associated with poor co-activation of VMO/VL and hip adductors (8,10). A strategy used to increase activation of the hip adductors is to hold a medicine ball between the knees while simultaneously performing exercises that involve extension of the knee joint such as the leg press (LP) (2,3). However, this practice has displayed inconsistent results in terms of VMO and VL activities (3,5,7,12). Song et al. (28) conducted a study that included 89 subjects with patellofemoral pain syndrome that were divided into 3 groups: (a) unilaterally open-kinetic chain LP with hip adduction using an elastic band fixed laterally in the machine support, (b) with neutral position, and (c) control. Each group performed 3 weekly training sessions during an 8-week resistance training program composed by 5 sets of 10 repetitions using 60% of 1 repetition maximum (1-RM) load. The authors observed similar increases in muscle thickness (i.e., ultrasonography) of the VMO and VL for both groups. Peng et al. (23) investigated the activity of the VMO and VL during a closed-kinetic chain LP exercise performed with submaximal and maximal hip isometric adduction. During the submaximal hip adduction, the subjects were instructed to squeeze a 6-kg medicine ball just enough to keep it from dropping while performing the LP; however, during maximal hip contraction, the subjects were instructed to vigorously and maximally squeeze and hold the medicine ball. Both conditions did not change the overall VMO-VL activation patterns. However, Coqueiro et al. (5) investigated 3 bilateral semi-squat exercise trials with maximum isometric hip adduction vs. 3 bilateral semi-squat exercise trials without hip adduction and noted significantly greater activity of the VMO and VL muscles during the semi-squat trials with maximum isometric hip adduction.
Regardless, with reference to the activity of the VMO-VL, these studies tested a specific point in the joint range of motion (3,23), isometric contractions (7), weight-bearing tasks (2,12), and unilateral knee extension exercises (1). Therefore, the application to real-world scenarios is limited. To date, it remains unclear as to whether the addition of implements such as a physioball or elastic band to the LP exercise would facilitate VMO/VL co-activation to preserve tibiofemoral and patellofemoral joint stability. Additionally, such investigation may help rehabilitation and training professionals during the prescription of exercise programs with the goal to optimize strength gains and reduce knee joint dysfunction. Therefore, the purpose of this study was to analyze the myoelectric activity of VL, VMO, rectus femoris (RF), and biceps femoris (BF), as well as the VMO:VL ratio during an open-kinetic chain 45° angled LP (LP45). The traditional LP45 technique was compared with 2 alternative LP45 exercise techniques that used a physioball and elastic band, respectively.
Experimental Approach to the Problem
To assess and compare the effects of an elastic band and physioball incorporated into the LP exercise technique, a randomized crossover design study was carried out with 6 testing sessions conducted on nonconsecutive days (Figure 1). The myoelectric activity of the RF, BF, VMO, and VL and the VMO:VL ratio were assessed in 3 protocols performed in a random order: TRAD—1 LP45 set performed using the traditional exercise technique, PBALL—1 LP45 set performed with a physioball held between the knee joints to promote isometric hip adduction, and PEB—1 LP45 set performed with an elastic band proximal to the knee joints to promote isometric hip abduction. Ten repetitions at 70% of a 10-RM load were performed in each protocol. Dependent variables measured were normalized myoelectric activity of the surface electromyographic (SEMG) signal recorded during each LP45 protocol for the VL, VMO, RF, and BF, and the normalized VMO:VL ratio.
Thirteen female college students (age: 22.5 ± 2.9 years; height: 164.5 ± 5 cm; body mass: 62.3 ± 9.9 kg) volunteered for this study. The following criteria were adopted for subject recruitment: (a) nonsmokers, (b) absence of any kind of cardiovascular or metabolic disease, (c) no joint or bone injury, and (d) absence of any ergogenic substance that could influence neuromuscular performance. Furthermore, all subjects reported no history of patellofemoral pain syndrome or trauma to the knee or other joints of the lower extremity during the time of study.
Before participation, subjects received a detailed description of the study procedures which were approved by a university ethics committee in accordance with the Declaration of Helsinki. Prior to data collection, the participants were informed about the experimental procedures, including the risks and benefits, and signed an informed consent before any of the tests were performed. A questionnaire regarding medical history, age, height, body mass, training characteristics, injury history, team volleyball experience, and performance level was completed before participation. The study was approved by the Ethics Committee of the Rio de Janeiro Federal University through the process: 46153515.3.0000.5257.
Maximal Voluntary Contraction
One week before the experimental sessions, subjects underwent a session to estimate their maximal voluntary contraction (MVC) at a 90° knee angle for the LP45 exercise (Life Fitness, Rosemont, IL, USA). Subjects performed 3 knee extension/flexion MVCs during 10 seconds against fixed resistance with the right leg only, separated by 20-second rest (16). Verbal encouragement was provided to promote higher forces in each trial. For the MVCs, data analyses were conducted over a 4-second window between the second and sixth seconds. The highest root mean square (RMS) value of the 3 MVCs was used for normalization purposes (15).
Ten Repetition Maximum Testing
During the second and third testing sessions, the 10-RM load was determined for each subject for the LP45 exercise (Life Fitness). The 10-RM load was defined as the maximum weight that could be lifted for 10 consecutive repetitions at a constant velocity of 4 seconds per repetition (2 seconds concentric and 2 seconds eccentric), but repetitions were still counted if the cadence slowed because of the effects of fatigue (22). The execution of the LP was standardized, and pauses were not permitted between the concentric and eccentric phases. A metronome (Metronome Plus, version 2.0; M&M System, Braugrasse, Germany) was used to help control the lifting cadence. If a 10-RM was not accomplished on the first attempt, the weight was adjusted by 4–10 kg and a minimum 5-minute rest was given before the next attempt. Only 3 trials were allowed per testing session. The test and retest trials were conducted on different days with a minimum of 48 hours between trials (18).
To reduce the margin of error in testing, the following strategies were adopted (20): (a) standardized instructions were provided before the test, so the subject was aware of the entire routine involved with data collection, (b) the subject was instructed on the technical execution of the exercises, (c) the researcher carefully monitored the position adopted during the exercises, (d) consistent verbal encouragement was given to motivate subjects for maximal repetition performance, and (e) the additional loads used in the study were previously measured with a precision scale.
An open-kinetic chain LP45 exercise was adopted in the present study, considering previous studies that indicated greater levels of RF and VMO-VL myoelectric activity vs. other LP models (7,13). To ensure that the full range of motion (ROM) was achieved, subjects received sensory feedback from a custom-made device positioned behind their lower limbs and oral feedback when the knee joint was at 90° of knee flexion and 125° of hip flexion during each LP45 repetition (Figure 2). The sensory device was calibrated for each subject using a manual goniometer (Sanny, São Paulo, Brazil) to normalize intersubject variability. The most comfortable foot position was adopted by each subject, which was marked with tape.
During the fourth through sixth testing sessions, subjects performed 3 protocols in random order: TRAD—1 LP45 set performed using the traditional exercise technique, PBALL—1 LP45 set performed with a physioball held between the knee joints, PEB—1 LP45 set performed with an elastic band proximal to the knee joints. Before performing the experimental protocols, each subject performed a warm-up set of 12 repetitions with 50% of the 10-RM load, and after a 2-minute rest interval, the experimental set was performed for 10 repetitions at 70% of the 10-RM load (18). Seventy percent of the 10-RM load was chosen for all experimental protocols in order to avoid the effects of fatigue on SEMG responses, considering the nonlinear force-electromyographic (EMG) relationships in muscles of mixed fiber composition (33).
For the TRAD protocol, subjects performed 1 LP45 set of 10 repetitions with no external implements, just as the LP45 would usually be performed. For the PBALL protocol, subjects performed 1 LP45 set of 10 repetitions with a physioball (Over Ball 23 cm; Tryex, São Paulo, Brazil) held isometrically between the medial condyles of the knee joints. The specific physioball was chosen based on predetermined categories associated with the hip size of each subject. The subject was instructed to isometrically adduct at the hip joints in order to fix the physioball between the medial condyles of the knees during all LP45 repetitions. For the PEB protocol, subjects performed 1 LP45 set of 10 repetitions with an elastic band (Band MiniStrong; Tryex) placed around the lateral condyles of the knee joints. The stiffness of the elastic bands was chosen based on color to provide a load of approximately 20% of a subject's body mass when fully extended in accordance with a previous study (27). The elastic bands were stretched 100% of the resting length to provide adequate tensile force.
There was a minimum of 48 hours between the experimental protocols. During each protocol, both concentric and eccentric phases were set at about 2 seconds each to reduce the acceleration effects and to mitigate the influence of the stretch-short cycle (Metronome Plus, version 2.0; M&M System). Surface electromyographic data acquisition of VL, VMO, RF, and BF muscles were started at the beginning of the first repetition and finished at the end of the 10th repetition.
The SEMG data were captured through passive bipolar surface electrodes (Kendal Medi Trace 200; Tyco Healthcare, Pointe-Claire, QC, Canada) with a recording diameter of 1 mm and a distance between the electrode centers of 1 cm. The surface electrodes were placed over the muscle bellies. The electrodes were connected to an analog-to-digital converter of 16 bits (M400F; Miotool, Porto Alegre, Brazil) and acquired with the assistance of proprietary software (Miotec Suite; Miotec, Curitiba, Brazil). The SEMG signals were amplified by 1.000 with a common mode rejection ratio of 100 dB. The signal was sampled at 1.000 Hz, and the signal was filtered through band pass at 10–450 Hz using a Butterworth 2 poles filter with order 4. The reference electrode was placed on the patella bone. A permanent marker was used to mark the location of the electrodes in the first test session for consistent electrode placement during subsequent sessions. After positioning of the electrodes, the impedance was checked and accepted when it was <5 kΩ (30).
Surface electromyographic data of the BF, VL, VMO, and RF muscles were evaluated during the LP45 exercise. Electrodes were placed as described below, according to the recommendations of Cram et al. (6). For the RF, the electrodes were placed half the distance between the anterior-superior iliac spine and the superior part of the patella. For the VL, the electrodes were placed two thirds the distance between the anterior-superior iliac spine and the lateral side of the patella. For the VMO, the electrodes were placed at an angle of approximately 55° to the long axis of the femur at a location that was over the muscle belly when the knee was in 60°of flexion. For the BF, the electrodes were at 50% on the line between the ischial tuberosity and the lateral epicondyle of the tibia. Before the placement of the electrodes, the areas were shaved and cleaned with alcohol.
The mean amplitude of the RMS was assessed using the custom-written software Matlab 5.02c (MathWorks, Natick, MA, USA). The averaging window for the RMS was 100 milliseconds, and all reported values are the mean RMS over a predetermined sampling window from the onset to the end of each contraction. Only the signal of 8 central repetitions obtained at submaximum intensities was analyzed (repetitions 2–9 of 10 repetitions at 70% of 10 RM). This procedure was adopted to avoid problems with signal discrepancies regarding the inertia at the beginning of LP45 sets, and the possibility of fatigue in the last repetition (8,10).
Surface electromyographic data were collected for the entire (concentric and eccentric phases) LP45 set for each protocol. Myoelectric activity of the lower limb muscles analyzed during LP45 sets were expressed as a percentage relative to the largest RMS value of the SEMG signal obtained in the MVCs (100%) (15). The averaged integrated EMG activation of the VMO and VL muscle was calculated over the entire time interval (2 seconds) for the concentric and eccentric phases, respectively, and through every 15° of knee motion for each phase of the LP45 exercise using a similar approach as that of Toumi et al. (31). To display the change in the relative activity of VMO and VL muscles, the VMO:VL ratio was calculated.
Statistical analysis was performed using SPSS version 20.0 software (SPSS, Inc., Chicago, IL, USA). Statistical analysis was initially performed using the Shapiro-Wilk test of normality and homoscedasticity test (Bartlett criterion). All variables showed normal distribution and homoscedasticity. Descriptive statistics (mean and SD) from the 3 experimental protocols were used to represent the SEMG activity for each muscle. The intraclass correlation coefficient (), where MSb = mean-square between, MSw = mean-square within, and k = average group size, was calculated to determine the reproducibility of the testing and retesting 10-RM load and also the SEMG measures. One-way analysis of variance with repeated measures followed by Bonferroni post hocs was applied to determine significant differences or interactions between the type of implement (physioball and elastic band) on the SEMG signal of the 4 muscles analyzed. The effect sizes for the LP45 exercise (TRAD, PBALL, and PEB) were also calculated for the SEMG activity of RF, BF, VMO, VL, and VMO:VL ratio. Effect sizes between 0.20 and 0.49 (small), between 0.50 and 0.79 (moderate), and above 0.80 (large) were computed and classified (4). The value of p ≤0.05 was adopted for all inferential analyses.
The mean 10-RM load for the LP45 was 121.6 ± 34.8 kg which, at 70% of 10-RM, led to the experimental load of 78.6 ± 33.2 kg. The ICC for 10-RM test and retest was 0.91 and for the SEMG records was 0.96 (RF), 0.86 (VL), 0.91 (BF), and 0.97 (VMO). Significant increases in VMO myoelectric activity were noted under the PBALL compared with the PEB (p = 0.001, effect size = 2.32) and TRAD (p = 0.002, effect size = 1.20) (Figure 3; Table 1). Higher VMO activity was also noted under TRAD than under PEB (p = 0.001, effect size = 1.12). Greater VL activity was found under the PBALL than under TRAD (p = 0.0001, effect size = 2.42) and PEB (p = 0.0001, effect size = 2.50). However, no difference in VL activity was observed between the TRAD and PEB protocols. The PBALL protocol showed a significantly greater VMO:VL ratio during the concentric phase than the PEB (p = 0.001, effect size = 3.92) and TRAD (p = 0.001, effect size = 3.12) protocols.
Greater RF activity was observed under PEB than under TRAD (p = 0.001, effect size = 1.52) and PBALL (p = 0.001, effect size = 1.62) (Figure 4). However, no difference was observed between TRAD and PBALL protocols. No difference in BF activity was observed between protocols (p ≤ 0.05).
In the present study, we compared traditional LP45 exercise technique with the additional effect of holding a physioball between the knee joints (e.g., isometric adduction) or placing an elastic band around the knee joints (e.g., isometric abduction) on the myoelectric activity of the RF, BF, VMO, and VL and the VMO:VL ratio. The key findings of the current study were the higher VL and VMO activities under the PBALL protocol than the TRAD and PEB protocols. A greater VMO:VL ratio was also noted under the PBALL protocol during the concentric phase than the TRAB and PEB protocols. However, RF activity was significantly higher under PEB than under TRAD and PBALL. These results are in disagreement with previous studies which did not find differences in lower limb muscle activity adopting different implements (i.e., physioball, elastic band) during the LP exercise (1,12,13,23,28,32).
Patellofemoral pain, chondromalacia, and lateral subluxation or dislocation of the patella are often associated with delayed onset of activation in the VMO relative to the VL, as well as a lower VMO/VL ratio in subjects with lower extremity instability. In this scenario, the patella may track laterally, resulting in uneven wear on the underlying cartilage and knee pain (19,23,24). For this reason, the restoration of the VMO-VL muscle balance through emphasis on increasing VMO activity during exercise has been explored in the scientific literature (3,5,13,23,28). In the current study, the greatest VMO and VL activities were noted under the PBALL (i.e., isometric hip adduction) than the TRAD and PEB (i.e., isometric hip abduction) protocols. However, these findings were in disagreement with previous studies which indicated that the addition of isometric hip adduction had no effect on the relative activation of the VMO and VL in healthy subjects (3,23). This contrast may be associated to the methodological differences among studies such as the open or closed chain exercises adopted, training experience, external load, and health status.
For instance, Song et al. (28) investigated the effects of an 8-week resistance training program in subjects with patellofemoral pain syndrome who performed a unilateral LP with or without isometric hip adduction (LP augmented with a blue elastic band fixed laterally in a wall). The authors observed that there was no additive beneficial effect of adopting hip adduction (concomitant with the LP exercise) on pain evolution or VMO hypertrophy. Similarly, Boling et al. (3) concluded that VMO amplitude and the VMO:VL ratio were not influenced by performing isometric hip adduction during a dynamic squat exercise through a range of 0°–90° of knee flexion in healthy subjects (e.g., 10 repetitions against their body weight).
However, results consistent with the present study were reported by Peng et al. (23), who observed that isometric hip adduction during closed-kinetic chain LP promoted a more balanced VMO:VL ratio during specific portions of the range of motion when compared with the traditional LP. Increases in the VMO/VL ratio were noted during the concentric phase (0.78–0.95 at 30°–15°) and during the eccentric phase (0.62–0.78 and 0.73 to 0.88 at 0°–15° and 30°–45°). Furthermore, at deeper knee flexion angles (90°–75°), the VMO/VL ratio increased from 0.93 to 1.04 during the concentric phase. These data were similar to those found in the current study (1.24) and support previous SEMG studies that showed that the ideal ratio of the VMO:VL was 1:1 in knee extension exercises for the asymptomatic knee joint (3,5). This greater balance between the VMO and VL muscles may be partially explained by the anatomical linkage between the hip adductor and VMO muscle where the hip adductor may help to provide a more stable proximal attachment and transfers physiological stretch to the VMO, thereby enhancing its activation level.
On the other hand, the different fiber types present in the VL, VMO, and RF and external load (e.g., 70% of 10 RM) may account for the observed uniformity in SEMG normalized values in the current study. Biopsy studies demonstrated a significantly lower proportion of type II fibers in the VMO than in the VL and RF muscles (31). According to Johnson et al. (14), the VL muscle consisted of approximately 45% type I and 55% type II fibers. Thus, selective recruitment of type II fibers at increasing force levels (80% of 1 RM) in the VL might be responsible for increasing the EMG signal at the high effort level. Thus, coordination patterns might be different from the high to moderate effort levels (11). This finding, combined with the elastic band and physioball results for the LP45 exercise, indicates a specific activity pattern for the quadriceps muscles mainly influenced by technique variations (5,10), range of motion (3,23), and effort level (21,24,28).
As noted above, greater RF activity was observed in the current study for the PEB protocol than for the TRAD and PBALL protocols. For the RF, the fact that it is a biarticular (hip and knee) muscle may explain these differences (10). When comparing different types of LP models, it can be verified that the horizontal LP performed with a high foot placement increases in the hip flexion angle, which stretches the BF and gluteus maximus and shortens the RF (7,10). This condition could impair the RF mechanism that shortens this muscle, resulting in a strength deficit because, in that position, the RF would not be at a favorable length to increase force production. However, in the LP45 exercises, the RF would not be as shortened thus increasing its force production capacity (7). In this sense, the result observed in the present study could be associated to the level of concentration on producing horizontally directed force (hip abduction) induced by the elastic band stretching specifically during the eccentric phase (10). Secondly, greater valgus force induced by elastic band during the LP45 exercise may promote an augmentation in RF role as a hip stabilizer.
Similar to Escamilla et al. (10), in the current study, the LP45 exercise promoted greater myoelectric activity of the quadriceps (RF, VMO, and VL) muscles. However, in the present study no difference in BF activity was noted between protocols. This lower level of BF activity might be associated to an increase in gluteus maximus activity as previously reported in the scientific literature during the LP45 exercise (7,9,13). During LP45 exercise a greater angle of hip motion could increase gluteus maximus activity when compared with the other LP models. In the starting position, the greater hip flexion angle observed could be favorable to gluteus maximus force production, minimizing the role of BF as hip extensor. In addition, this high gluteus maximus activity noted during the LP45 exercise might also be associated to the deficit caused by the RF and gastrocnemius muscles during the final hip flexion angle due to the length-tension relationship (12,24).
There were some limitations in the current study worth noting. Discrepancy in the amplitude of surface SEMG signals should always be interpreted with caution since they may also be associated with changes in either the shape of the intracellular action potential, volume conductor, sarcolemmal properties of the muscle fibers, or differences in subcutaneous tissue thickness (13). Therefore, it is possible that the increased resistance to adduction and internal rotation forces, provided by the physioball in the PBALL condition or to abduction and external rotation forces provided by the elastic bands in the PEB condition, led to a decrease in the recruitment of the musculature which is responsible for abduction, adduction, and external and internal rotation of the hip. This possibility and the potential implications of this are beyond the scope of this investigation. Finally, since the current investigation was cross-sectional in nature rather than longitudinal, the longer term implications of training in the manner described above in the PBALL or PEB conditions, either from the perspective of the alterations noted in quadriceps muscle activation or the potential alterations in hip musculature recruitment, cannot be answered from these data.
On the other hand, this research not only adds to the existing debate concerning the use of external implements (e.g., physioball, elastic band, medicine balls) to stabilize knee joint against valgus and varus forces but also highlights that there is a requirement to investigate which exercise and training variables preferentially alter the VMO:VL ratio, considering that both muscles work synergistically to stabilize the patella during dynamic lower extremity tasks. Therefore, additional research is needed to examine the longer term implications of these results as well as potential influences on recruitment of the hip musculature.
Based on our findings, preferential activation of VMO by adding isometric hip adduction (e.g., physioball) to the open-kinetic chain LP45 exercise was supported, presenting ratios of the VMO:VL higher than 1:1 when performed with submaximal loads (70% of 10 RM). In addition, our results showed that an elastic band (e.g., hip abduction) can also increase RF activity (e.g., hip stabilizer function) compared with the traditional technique without an elastic band. Therefore, the physioball and elastic band can be used during the LP45 exercise performed with submaximal loads with the goal to optimize the knee joint stability due to the increase in overall quadriceps activation against excessive varus and valgus forces, respectively.
Humberto Miranda thanks the Fundação Carlos Chagas Filho de Amparo à Pesquisa do Estado do Rio de Janeiro (FAPERJ).
1. Bolgla LA, Shaffer SW, Malone TR. Vastus medialis activation during knee extension exercises: Evidence for exercise prescription. J Sport Rehabil 17: 1–10, 2008.
2. Boling MC, Bolgla LA, Mattacola CG, Uhl TL, Hosey RG. Outcomes of a weight-bearing rehabilitation program for patients diagnosed with patellofemoral pain syndrome. Arch Phys Med Rehabil 87: 1428–1435, 2006.
3. Boling MC, Padua DA, Blackburn JT, Petchauer M, Hirth C. Hip adduction does not affect VMO EMG amplitude or VMO: VL ratios during a dynamic squat exercise. J Sport Rehabil 15: 195–205, 2006.
4. Cohen J. Statistical Power Analysis for the Behavioral Sciences. Hillsdale, NJ: Laurence Erlbaum Associates, 1988.
5. Coqueiro KR, Bevilaqua-Grossi D, Berzin F, Soares AB, Candolo C, Monteiro-Pedro V. Analysis on the activation of the VMO and VLL muscles during semisquat exercises with and without hip adduction in individuals with patellofemoral pain syndrome. J Electromyogr Kinesiol 15: 596–603, 2005.
6. Cram JR, Kasman GS, Holtz J. Introduction to Surface Electromyography
. Gaithersburg, MD: ASPEM, 1998.
7. Da Silva EM, Brentano MA, Cadore EL, De Almeida AP, Kruel LF. Analysis of muscle activation during different leg press exercises at submaximum effort levels. J Strength Cond Res 22: 1059–1065, 2008.
8. Escamilla RF. Knee biomechanics of the dynamic squat exercise. Med Sci Sports Exerc 33: 127–141, 2001.
9. Escamilla RF, Fleisig GS, Zheng N, Barrentine SW, Wilk KE, Andrews JR. Biomechanics of the knee during closed kinetic chain and open kinetic chain exercises. Med Sci Sports Exerc 30: 556–569, 1998.
10. Escamilla RF, Fleisig GS, Zheng N, Lander JE, Barrentine SW, Andrews JR, Bergemann BW, Moorman CT III. Effects of technique variations on knee biomechanics during the squat and leg press. Med Sci Sports Exerc 33: 1552–1566, 2001.
11. Gonzalez-Izal M, Malanda A, Navarro-Amezqueta I, Gorostiaga EM, Mallor F, Ibanez J, Izquierdo M. EMG spectral indices and muscle power fatigue during dynamic contractions. J Electromyogr Kinesiol 20: 233–240, 2010.
12. Hertel J, Earl JE, Tsang KK, Miller SJ. Combining isometric knee extension exercises with hip adduction or abduction does not increase quadriceps EMG activity. Br J Sports Med 38: 210–213, 2004.
13. Irish SE, Millward AJ, Wride J, Haas BM, Shum GL. The effect of closed-kinetic chain exercises and open-kinetic chain exercise on the muscle activity of vastus medialis oblique and vastus lateralis
. J Strength Cond Res 24: 1256–1262, 2010.
14. Johnson MA, Sideri G, Weightman D, Appleton D. A comparison of fibre size, fibre type constitution and spatial fibre type distribution in normal human muscle and in muscle from cases of spinal muscular atrophy and from other neuromuscular disorders. J Neurol Sci 20: 345–361, 1973.
15. Kalmar JM, Cafarelli E. Central excitability does not limit postfatigue voluntary activation of quadriceps femoris. J Appl Physiol 100: 1757–1764, 2006.
16. Kendall FP, McCreary EK, Provance PG, Rodgers MM, Romani WA. Muscles, Testing and Function With Posture and Pain. Baltimore, MD: Williams & Wilkins, 2005.
17. Laprade J, Culham E, Brouwer B. Comparison of five isometric exercises in the recruitment of the vastus medialis oblique in persons with and without patellofemoral pain syndrome. J Orthop Sports Phys Ther 27: 197–204, 1998.
18. Maia MF, Willardson JM, Paz GA, Miranda H. Effects of different rest intervals between antagonist paired sets on repetition performance and muscle activation. J Strength Cond Res 28: 2529–2535, 2014.
19. Malloy PJ, Morgan AM, Meinerz CM, Geiser CF, Kipp K. Hip external rotator strength is associated with better dynamic control of the lower extremity during landing tasks. J Strength Cond Res 30: 282–291, 2016.
20. Miranda H, Maia Mde F, Paz GA, Costa PB. Acute effects of antagonist static stretching in the inter-set rest period on repetition performance and muscle activation. Res Sports Med 23: 37–50, 2015.
21. Netreba A, Popov D, Bravyy Y, Lyubaeva E, Terada M, Ohira T, Okabe H, Vinogradova O, Ohira Y. Responses of knee extensor muscles to leg press training of various types in human. Ross Fiziol Zh Im I M Sechenova 99: 406–416, 2013.
22. Paz GA, Robbins DW, Oliveira CG, Bottaro M, Miranda H. Volume load and neuromuscular fatigue during an acute bout of agonist-antagonist paired-set versus traditional-set training. J Strength Cond Res 2015. June 2. [Epub Ahead of Print].
23. Peng HT, Kernozek TW, Song CY. Muscle activation of vastus medialis obliquus
and vastus lateralis
during a dynamic leg press exercise with and without isometric hip adduction. Phys Ther Sport 14: 44–49, 2013.
24. Pereira GR, Leporace G, Chagas Dd, Furtado LF, Praxedes J, Batista LA. Influence of hip external rotation on hip adductor and rectus femoris myoelectric activity during a dynamic parallel squat. J Strength Cond Res 24: 2749–2754, 2010.
25. Phornphutkul C, Sekiya JK, Wojtys EM, Jacobson JA. Sonographic imaging of the patellofemoral medial joint stabilizing structures: Findings in human cadavers. Orthopedics 30: 472–478, 2007.
26. Rath ME, Walker CR, Cox JG, Stearne DJ. Effect of foot type on knee valgus, ground reaction force, and hip muscle activation in female soccer players. J Sports Med Phys Fitness 56: 546–553, 2015.
27. Skovdal Rathleff M, Thorborg K, Bandholm T. Concentric and eccentric time-under-tension during strengthening exercises: Validity and reliability of stretch-sensor recordings from an elastic exercise-band. PLoS One 8: e68172, 2013.
28. Song CY, Lin YF, Wei TC, Lin DH, Yen TY, Jan MH. Surplus value of hip adduction in leg-press exercise in patients with patellofemoral pain syndrome: A randomized controlled trial. Phys Ther 89: 409–418, 2009.
29. Tang SF, Chen CK, Hsu R, Chou SW, Hong WH, Lew HL. Vastus medialis obliquus
and vastus lateralis
activity in open and closed kinetic chain exercises in patients with patellofemoral pain syndrome: An electromyographic study. Arch Phys Med Rehabil 82: 1441–1445, 2001.
30. Tarata MT. Mechanomyography versus electromyography
in monitoring the muscular fatigue. Biomed Eng Online 2: 3, 2003.
31. Toumi H, Poumarat G, Benjamin M, Best TM, F'Guyer S, Fairclough J. New insights into the function of the vastus medialis with clinical implications. Med Sci Sports Exerc 39: 1153–1159, 2007.
32. Walker S, Peltonen H, Avela J, Hakkinen K. Kinetic and electromyographic analysis of single repetition constant and variable resistance leg press actions. J Electromyogr Kinesiol 21: 262–269, 2011.
33. Zwambag DP, Freeman NE, Brown SH. The effect of elbow flexor fatigue on spine kinematics and muscle activation in response to sudden loading at the hands. J Electromyogr Kinesiol 25: 392–399, 2015.