Muscular activation of the rectus femoris, vastus medialis and vastus lateralis muscles were measured using surface electromyography. Passive bipolar surface electrodes of 30-mm diameter (Blue Sensor, Ltd, Denmark, Ag/AgCl) were placed on the contracted belly of each of the muscles, in line with the muscle fibre direction, with an interelectrode distance of 1.5 cm. Before electrode placement, the area was cleaned with isopropyl alcohol, shaved and abraded in order to reduce skin impedance. Reference electrodes were also placed on a bony prominence on the right wrist. All electrodes remained in place until data collection was completed in the three intensities. Efforts were made to minimize crosstalk across the muscles of interest by selecting appropriate electrode size and interelectrode distance (Winter, 1996; Merletti, 1999). Manual muscle testing during pilot work confirmed that crosstalk in the muscles of interest was minimised as much as possible. Electromyographic activity was differentiated by pre-amplifiers and recorded via an on-line, cable ME6000 system (MEGA Electronics, LTD, Finland), with an input impedance of less than 1015/0.2 ohm/pF, a common mode rejection ratio at 60 Hz of greater than 110 dB, a noise level of 1.2 mV, a gain of 10 + 2% and a bandwidth range from 0 Hz – 500 Hz. Muscle activity was sampled at 1000 Hz via a 16bit DAQ-516 A/D card and stored on a Sony Vaio laptop computer using MegaWin software, version 1.2 (MEGA Electronics, LTD, Finland). The raw EMG data were filtered using a high and low pass Butterworth filter (5-500 Hz) and visually checked for artefacts, which were excluded from subsequent analysis. The EMG data were sampled at 100 ms with a 50% frame overlap. This moving average approach in which the time windows overlap ensures that the EMG curve follows the trend of the underlying rectified EMG, but without the variable peaks that are evident in the rectified EMG. This method has been recommended for use in dynamic contractions (Burden, 2008). On completion of the experimental protocol the Root Mean Square (RMS) of the filtered EMG signal was full-wave rectified and normalised as a % of the 1RM value for each exercise.
All EMG data collected were subsequently expressed as a percentage of this value and used in the analysis.
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2.5 Data Analysis
Any differences in muscle activation between the squat and leg extension and across the two intensities were investigated using a 2 (exercise) X 2 (intensity) ways, repeated measures analysis of variance (ANOVA). Where any significant differences were found, Bonferroni’s multiple comparisons were used to determine where these differences lay. Descriptive statistics were also calculated and the level for statistical significance was set at P < 0.05. The Statistical package for Social Sciences, version 13 (SPSS inc, Chicago, IL, USA) was used for all analysis.
3. RESULTS
Results from repeated measures ANOVA for each muscle group indicated significant exercise X intensity interactions for rectus femoris (F 1, 8 = 54.4, P<
0.01, partial Ș2 = 0.872), vastus lateralis (F1, 8 = 18.2, P< 0.01, partial Ș2 = 0.694) and vastus medialis (F1, 8 = 9.7, P< 0.05, partial Ș2 = 0.518) These indicated, for all muscle groups, that muscle activity at 90% of 1RM was similar for the squat and leg extension but at 30% 1RM, leg extension muscle activity was significantly lower than muscle activity during the squat.
In addition, there were significant main effects for exercise for rectus femoris (F1, 8 = 48.5, P< 0.01, partial Ș2 = 0.872), vastus lateralis (F 1, 8 = 8.1, P<0.05, partial Ș2 = 0.501) and vastus medialis (F1, 8 = 25.1, P< 0.01, partial Ș2 = 0.758) and significant main effects for intensity for rectus femoris (F1, 8 = 54.4, P< 0.01, partial Ș2 = 0.859), vastus lateralis (F1, 8 = 41.5, P< 0.01, partial Ș2 = 0.838) and vastus medialis (F1, 8 = 84.3, P< 0.01, partial Ș2 = 0.913). For all muscle groups muscle activity was greater during the squat exercise and at 90% 1RM compared to leg extension and 30%1RM respectively. Mean ± S.D. of normalised (%) root mean square EMG across intensities and exercises for the three muscles involved is presented in Table 1.
4. DISCUSSION
The main aim of this study was to investigate quadriceps muscle activity during the squat and leg extension at high and low training intensities. Prior studies that have compared muscle activation patterns between the squat and leg extension have been limited in number and the only study that appears to have examined this issue has used the first and last repetitions of each exercise at values corresponding to 10RM as their dependant variables (Signorile et al., 1994). Furthermore, in the study by Signorile et al. (1994) these values were not normalised. The use of non- normalised EMG values in this study does not provide a relative reference point that is consistent across muscles, exertion and participants (Marras and Davis, 2001) and subsequently call into question the security of Signorile and co-workers’
conclusions. Conversely, the present study compared normalised muscle activity of
the squat and leg extension at high and low intensities. This is an important consideration when prescribing exercise for strength training, rehabilitation and prehabilitation as differing levels of muscle activation across exercises and intensities could provide guidance to practitioners as to which exercises to use, which intensities may best be suited for a particular programme of objective (e.g.
rehabilitation) and whether supplemental exercises might be needed for additional strength development.
Table 1. Mean ± S.D. of normalised (%) Root Mean Square EMG of the Rectus Femoris, Vastus Lateralis and Vastus Medialis during the squat and leg extension exercises at high and low intensities.
The main novel findings of this study were the significant exercise by intensity interactions in muscle activation for all three muscles investigated. These interactions indicated that for the rectus femoris, vastus lateralis and vastus medialis, muscle activity was significantly greater during the squat than the leg extension at 30% 1RM. However, there were no significant differences in rectus femoris, vastus lateralis and vastus medialis muscle activity during the squat and leg extension at 90% 1RM. These results provide some support for assertions made by Signorile et al. (1994) that supplemental leg extension exercises are required when athletes use the squat in an exercise programme in order to prevent muscular imbalances in the quadriceps muscle group as similar levels of muscle activation were found in the vastus medialis and vastus lateralis at 90% 1RM for both exercises. The results of this study also refute claims recently made by Dionisio et al. (2006) that muscle activity is greater in the vastus medialis and vastus lateralis compared to rectus femoris when performing the squat.
The results of this study demonstrate that as resistance exercise intensity Squat
Rectus Femoris Vastus Lateralis Vastus Medialis
M S.D. M S.D. M S.D.
30% 1RM 79.2 10.9 79.3 6.7 71.9 7.6
90% 1RM 113.9 19.5 102.6 20.6 107.6 18.2 Leg Extension
Rectus Femoris Vastus Lateralis Vastus Medialis
M S.D. M S.D. M S.D.
30% 1RM 33.4 3.9 44.6 12.4 41.8 7.6 90% 1RM 109.2 22.8 101.8 19.8 100.3 17.2
Duncan et al. 67
increases so does muscle activation. This supports a range of prior studies that have documented increased muscle activation with increased resistance exercise intensity across a range of intensities and exercises (Lagally et al., 2002; Lagally et al., 2004; Duncan et al., 2006). In addition, the significant exercise main effect indicated that overall, muscle activation was greater during the squat than the leg extension. This finding is also in line with prior research (Signorile et al., 1994) and is not surprising given that the absolute load for the squat, exceeded that of the leg extension at both 30% and 90% 1RM.
In relation to rehabilitation and/or prehabilitation the results of this study may be important as they indicate that, at lower intensities, the squat is associated with greater muscle activation than the leg extension. Therefore, if increased quadriceps muscle activation is the goal of a training/rehabilitation programme this is maximised at low intensities by performing the squat. However, both the squat and the leg extension elicit similar muscle activation profiles at high intensity. This at least partially supports the notion that closed chain kinetic exercises such as the squat may be more influential in aiding recovery of anterior and posterior joint injuries where low intensity exercise is often prescribed (Palmitier et al., 1991;
Signorile et al., 1994). Additionally, these findings may also have relevance for the general public in providing an insight into muscle activation across exercises and intensities and so provide an insight into prehabilitative strategies that could reduce the risk of lower-body injury (Caterisano et al., 2002).
Future studies are also warranted to confirm the findings of this study.
Research that examines a broader range of exercise intensities may be influential in helping develop effective training and rehabilitation programmes as would examining any differences in the timing of muscle activation across exercises and intensities. In addition, the participants in this study were all familiar with both the squat and the leg extension exercises. Further study of muscle activation patterns in participants who are unfamiliar with these exercises may also help shape strategies for rehabilitation.
References
Basmajian, J., Harden, T. and Regeons, E., 1972, Integrated actions of the four heads of the quadriceps femoris: An electromyographical study. Anatomical Records,172, pp.15-20.
Boyden, G., Kingman, J. and Dyson, R., 2000, A comparison of quadriceps electromyographic activity with the position of the foot during the parallel squat.Journal of Strength and Conditioning Research,14, pp. 379-382.
Burden, A., 2008, Surface electromyography. In: Biomechanical Evaluation of Movement in Sport and Exercise, edited by Payton, C. and Bartlett, R., (London: Routledge), pp. 77-102.
Caterisano, A., Moss, R., Pellinger, T., Woodruff, K., Lewis, V., Booth, W. and Khadra, T., 2002, The effect of back squat depth on the EMG activity of 4 superficial hip and thigh muscles. Journal of Strength and Conditioning Research,16, pp. 428-432.
Dionisio, V. C., Almeida, G. L., Duarte, M. and Hirata, R. P., 2006, Kinematic, kinetic and EMG patterns during downward squatting. Journal of Electromyography and Kinesiology. Epub ahead of print.
Duncan, M.J., Al-Nakeeb, Y. and Scurr, J., 2006, Perceived exertion is related to muscle activity during leg extension exercise. Research in Sports Medicine, 14, pp. 179-189.
Earle R. W. and Baechle, T. R., 2000, Resistance training and spotting techniques.
In: Essentials of Strength Training and Conditioning, edited by Baechle, T.R.
and Earle, R.W., (Champaign, IL: Human Kinetics), pp. 343-392.
Edwards, L., Dixon, J., Kent J., Hodgson, D. and Whittaker, V., 2008, Effect of shoe heel height on vastus medialis and vastus lateralis electromyographic activity during sit to stand. Journal of Orthopaedic Surgery and Research, 3, 2 (Online). Available from: http://www.josr-online.com/content/3/1/2.
Farina, D., 2006, Interpretation of the surface electromyogram in dynamic contractions. Exercise and Sports Science Reviews,34, pp. 121-127.
Galtier, B., Buillot, M. and Vanneuville, G., 1995, Anatomical basis for the role of vastus medialis muscle in femoro-patellar degenerative athropathy.
Anatomical and Radiologic Anatomy,17, pp. 7-11.
Kraemer, W. J., Ratamess, N. C., Fry, A. C., and French, D. N., 2006, Strength testing: Development and evaluation of methodology. In Physiological Assessment of Human Fitness, edited by Maud, P.J. and Foster, C., (Champaign, IL: Human Kinetics), pp. 119-150.
Lagally, K. M., Robertson, R. J., Gallacher, K. I., Goss, F. L., Jakicic, J. M., Lephart, S. M., McCaw, S. T. and Goodpaster, B., 2002, Perceived exertion, electromyography, and blood lactate during acute bouts of resistance exercise.
Medicine and Science in Sports and Exercise,34, pp. 552-559.
Lagally, K. M., McCaw, S. T., Young, G. T., Medema, H. C. and Thomas, D. Q., 2004, Ratings of perceived exertion and muscle activity during the bench press exercise in recreational and novice lifters. Journal of Strength and Conditioning Research,18, pp. 359-364.
Lieb, F. J. and Perry, J., 1971, Quadriceps function: An electromyographical study under isometric conditions. Journal of Bone and Joint Surgery,53-A, pp. 749- 758.
Marras, W. S. and Davis, K. G., 2002, A non-MVC EMG normalization technique for the trunk musculature. Journal of Electromyography and Kinesiology,11, pp. 1-9.
McCaw, S. T. and Melrose, D. R., 1999, Stance width and bar load effects on leg muscle activity during the parallel squat. Medicine and Science in Sports and Exercise,31, pp. 428-436.
McConnell, J., 1986, The management of chondromalacia pattalae: A long term solution. Australian Journal of Physiotherapy,32, pp. 215-223.
Merletti, R., 1999, Standards for reporting EMG data. Journal of Electromyography and Kinesiology (All Volumes).
Palmitier, R. A., Kai-Nan, A., Scott, S. G. and Chao, E. Y. S., 1991, Kinetic chain exercise in knee rehabilitation. Sports Medicine,11, pp. 402-413.
Signorile, J. F., Kwiatkowski, K., Caruso, J. F. and Robertson, B., 1995, Effect of
Duncan et al. 69
foot position on the electromyographical activity of the superficial quadriceps muscles during the parallel squat and knee extension. Journal of Strength and Conditioning Research,9, pp. 182-187.
Signorile, J. F., Weber, B., Roll, B., Caruso, J. F., Lowensteyn, I. and Perry, A. C., 1994, An electromyographical comparison of the squat and knee extension exercises. Journal of Strength and Conditioning Research,8, pp. 178-183.
Wheatley, M. and Jahnke, W., 1951, Electromyographical study of the superficial thigh and hip muscles in normal individuals. Archives of Physical Medicine, 32, pp. 508-515.
Winter, D., 1996, EMG interpretation. In: Electromyography in Ergonomics, edited by Kumar, S. and Mital, A., (London: CRC Press), pp. 109-125.
CHAPTER SIX