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) using isometric contractions at 3 set knee angles, 90° (1.571 rad), 150° (2.620 rad), and 175° (3.054 rad). Given that these are the only studies that evaluated the concurrent influences of these 2 factors on quadriceps' activity levels, we will begin our discussion with a comparison between the findings from this study and those presented by Signorile et al. (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), the increased muscle activity may have been due to the medial rotation of the leg causing an increase in tension along the fibers that run obliquely downward from the VL lateral to the medial side. These results differ from those reported by Signorile et al. (30) for the VL during isometric LE testing, where NEU produced greater electrical activity than TI at 90° (1.571 rad) and greater than TO at 150° (2.620 rad) knee angles. These knee angles are within the first and second 0.524 ROM used during this study. Our results showing that NEU produced greater activity than TO during the final 0.524 rad agree with their results, which noted that NEU produced greater electrical activity than TI or TO at a 175° (3.054 rad) knee angle.
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). They reported that 90° produced greater activity than 175° (3.054 rad) during NEU and TO and greater activity than 150° (2.620 rad) in neutral POS. They also stated that in the NEU and TO POS, 150° (2.620 rad) produced higher activity than 175° (3.054 rad). They noted that no differences were seen due to POS for any knee angle with the exception of 150° (2.620) rad, which generated a higher rmsEMG at NEU versus TI, but did not differ from TO.
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) who stated that this POS produced the highest electrical activity levels for the VL throughout participants' full ROM. However, our results differ from those reported by Signorile et al. (32) who reported no impact on EMG activity due to POS. The stronger agreement between the results reported by Stoutenberg et al. (33) compared with the earlier study by Signorile et al. (32) may have been the result of the use of a similar number of repetitions by thsi study and that of Stoutenberg et al. (33) compared with the 3 repetitions used in the earlier study (32). Additionally, the subjects in the study by Signorile et al. (32) were Division 1 competitive athletes, whereas this study included participants between the ages of 18 and 40 years, and the study by Stoutenberg et al. (33) used college-aged subjects, engaged in recreational resistance training. Our results also agree with the findings of Wheatley and Jahnke (38) who reported that the greatest level of activation of the VL was during medial rotation of the tibia when the knee was held at 1.57 rad and the isometric contraction was performed with the subject in a seated position. For the RF, our results indicating that the TO POS produces the highest level of activity among the 3 foot positions tested and is in agreement with the 2 previous studies that have examined the impact of POS on EMG activity during dynamic isoinertial exercise (30,33). Finally, our finding that POS had no impact on VM activity is in agreement with the results reported by Signorile et al. (30), but conflicts with the results reported by Stoutenberg et al. (33) indicating that the TI position produced significantly greater muscle activity than NEU.
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) and Browstein et al. (6) who indicated that the highest activity levels for this muscle were elicited between 90° (1.570 rad) and 100° (1.745 rad). Our results for the VM do support the contention that the levels of activity of this muscle should be greater at larger knee angles (13,39); however, the results of a number of studies argue against this hypothesis (1,34), and the studies that have provided direct electromyographic evidence are limited (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). In fact, the greatest contact forces at the 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). In addition, the greatest 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.
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). However, in ACL rehabilitation, therapists using the LE exercise may choose to avoid the final 0.524 rad ROM, where anterior shearing forces are the greatest and can lead to increased graft strain and disruption (13,39). Additionally, when addressing PFP, targeting selected superficial quadriceps muscles using different knee angles, foot positions, or combinations of the 2 can increase the effectiveness of the intervention. Our results indicate that for targeting the VMO when addressing PFP, the final 1.047 rad of LE is most effective regardless of foot position; however, when addressing imbalances between the vasti muscles, the relative training effect on the VL can be reduced using the TO position. Our results indicate that, given the data indicating that the final 0.524 rad ROM should be avoided during ACL rehabilitation, using the middle 0.524 rad ROM and alternating between the TI and TO positions will allow the greatest activation of the superficial quadriceps, while minimizing anterior shearing forces.
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).
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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