Introduction
Recently, many studies have analyzed muscle activity during different strength exercises (14,25,28,29,37). The superficial electromyographic (EMG) technique is often used to identify the participation of a muscle or muscle group in different performance techniques of many exercises (2-4,6,17,27,33). Exercises commonly used in a strength training program seem to be more interesting to analyze during those analyses (20,23,24,35,36,39).
The leg press (LP) is a multijoint (hip, knee, and ankle) exercise, its variations (low foot placement [LPL], high foot placement [LPH], and 45° [LP45]) are some of the most common exercises performed by athletes to enhance performance in sports (11,12). The hip and knee extension observed during concentric phase on LP is a very important motion for these individuals because it involves the activation of large muscle groups of the lower body. The conditioning of those muscles are directly related to improvement in running, jumping, and lifting for football, track and field, power lifting, and Olympic weightlifting athletes (10-12). Identifying how mechanical changes and different loads affect the activation pattern in hip and knee extensor muscles may improve physical performance in athletic and nonathletic populations (10-18).
Caterisano et al. (5) evaluated hip and knee extensor muscle activity performing squats at 3 ranges of motion. They found that gluteus maximus and vastus medialis activity was influenced by different mechanical changes in this exercise (partial, parallel, and full depth). Escamilla et al. (12) quantified the hip and knee extensors muscle activity during LPH and LPL exercises in different stance widths and foot positions. They found that the peak of EMG activity for the gastrocnemius muscle was greater during the LPL than during the LPH, indicating that mechanical changes could modify muscle activity pattern during the performance of LP exercises. However, only a single voluntary effort level was used, gluteus maximus muscle activity was not measured, and the LP45 was not performed during these analyses.
Although the mechanical changes during strength exercises variations can modify muscle activity pattern, studies have not quantified how mechanical changes affect the hip and knee extensor muscle activity pattern during LP exercises at different submaximum loads lifted (5,11,12). Furthermore, these studies have been done only with men (5,6,12,35-37). Thus, the specific purpose of this study was to analyze how mechanical changes and the loads lifted could modify the hip and knee extensor muscle activity in women during different LP exercises (LPL, LPH, and LP45). Based on the findings of Escamilla et al. (12), Caterisano et al. (5), Anders et al. (1), Lawrence and De Luca (26), and Woods and Bigland-Ritchie (38), we propose the hypothesis that muscle activity could differ during performance of the 3 LP exercises and that these differences would depend on the load lifted.
Methods
Experimental Approach to the Problem
Articles in academic journals and fitness periodicals have never examined how mechanical changes and loads lifted affect muscle activity during LP exercises. Thus, we used 3 of the most common LP variations to examine which positions adopted during LP exercises could elicit the highest level of electrical activity in 5 lower limb muscles. Electromyographic signals were collected from each muscle during performance of the different LPs using different submaximum (40% and 80%) effort levels. Acquisition of all EMG signals was performed on the same day for each subject. According to convention, the root mean square of the EMG signal (rmsEMG) was used to quantify the average level of electrical activity produced during each condition. The signals were normalized by the signal collected during the maximum repetition of the LP45 to reduce the effect of variations in signal amplitude among muscles and subjects (37). Comparisons were made among exercises at 2 submaximum effort levels. These procedures were designed to address the effectiveness of each exercise targeting specific muscles; however, some controversy regarding their relative efficacy and safety still exists.
Subjects
Fourteen healthy young women (physical education students) from the Federal University of Rio Grande do Sul (UFRGS) were selected for this study. The participants' mean age, height, percentage of lean body mass and fat mass (±SD) were 21.5 ± 1.6 years, 1.63 ± 0.06 m, 74.23 ± 3.63%, and 26.04 ± 3.79%, respectively. Subjects reported an average of 1 hour twice weekly for at least 6 months of strength training. All subjects had been performing LP exercises for at least 4 months. None of the subjects presented knee or hip injury or had undergone any knee or hip surgery before the study. The Ethics Committee of UFRGS approved the study. Before their participation, all subjects signed a university-approved informed consent form.
Procedures
In the first week, anthropometric measurements of the subjects were taken, and they also performed a 1 repetition maximum (1RM) voluntary test of the LP45, LPH, and LPL (Figures 1, 2, and 3, respectively) exercises. The anthropometric protocol consisted of height and weight measurements as well as body density assessment using the skinfold method suggested by Jackson et al. (21) to later body fat measurements according to Heyward and Stolarczyk (19).
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Figure 1: (A) LPL starting concentric position. (B) LPL final concentric position.
Figure 2: (A) LP45 starting concentric position. (B) LP45 final concentric position.
Figure 3: (A) LPH starting concentric position. (B) LPH final concentric postion.
In the second step, subjects performed the 1RM test randomly with the 3 LP exercises, using a protocol similar to that previously proposed by Glass and Armstrong (18), and it was limited to a maximum of 5 trials to reach 1RM. The rest period among trials was 2 minutes and 5 minutes among exercises in order to avoid problems related to muscle fatigue (32). Exercise cadence and range of motion were controlled by a Quartz metronome with 1-b·min−1 resolution and a Biometrics electronic goniometer (model TM 180) (12,36). Subsequently, the 1RM values were used to calculate the submaximum effort levels (40% and 80% of the 1RM). The starting concentric position to perform the exercises was set by a manual goniometer (CARCI), as 90° of knee flexion during the 3 LP exercises and 90°, 105°, and 125° of hip flexion during the LPL, LP45, and LPH, respectively (Figures 1A, 2A, and 3A). The foot position used was that considered the most comfortable for each subject. The final concentric position in all exercises was set as the full knee extension (Figures 1B, 2B and 3B) (12). An LP machine with high and low pedals (Taurus, Porto Alegre, Brazil) and an LP45 machine (Topline, porto Alegre, Brazil), both of invariable resistance, were used to perform the different LP exercises.
Data Collection
Subjects returned for data collection 1 week after the initial measurements. During this time, they were encouraged to keep their exercise routine. Myoelectric activity was obtained by bipolar (20-mm interelectrode distance) surface electrodes (Noraxon 272) placed longitudinally to the direction of the muscle fiber on the rectus femoris, gluteus maximus, vastus lateralis, biceps femoris (long head), and gastrocnemius (lateral head), following recommendations by Pincivero et al. (31) and Rainoldi et al. (34). The reference electrode was placed over the medial shaft of the tibia ~ 6-8 cm below the inferior pole of the patella. Before electrode placement, the skin area was shaved, abraded to reduce skin impedance, and cleaned with isopropyl alcohol. The EMG signal was obtained using an 8-channel electromyograph (model AMT-8 channel; Bortec, Calgary, Alberta, Canada) with a sample rate of 2000 Hz coupled to a Pentium desktop (200 MHz, 32 Mb RAM) fitted to a digital-analog conversion plate. The common mode rejection of the current system is 115 dB at 60 Hz with an input impedance of 10 gΩ. The electronic goniometer was positioned on the lateral epicondyle of the right knee in each subject to set the 90° knee flexion angle in the starting position of each exercise, as well as to distinguish the concentric (90°-180° full extension) and eccentric (180°-90°) phases during the signal interpretation (12).
The submaximum exercise protocol was performed randomly as well. First, the subjects performed the 1RM of the LP45. Subsequently, they performed 5 repetitions at 40% and 5 repetitions at 80% of 1RM in each LP exercise (LPL, LP45, and LPH). All exercise protocols were performed on the same day. During the exercise protocol, both concentric and eccentric phases were set at about 2 seconds each to reduce the acceleration effects on the resistance offered by the weight lifted. The rest period between exercises was the same allowed during the 1RM test. Data acquisition was started at the beginning of the first repetition and finished at the end of the fifth repetition. Between repetitions, subjects were instructed to stop for 1 second at the end of concentric phase to promote a clear separation between them. Finally, the subjects performed the 1RM in LP45 to verify the effect of fatigue on the EMG signal amplitude. The intraclass correlation coefficient between 1RM tests was at least 0.92 for all muscles, indicating no fatigue induced by the exercise protocol.
Data Analysis
The signal registered during 1RM of the LP45, and only the signal of 3 central repetitions obtained at submaximum intensities were analyzed. This procedure was adopted to avoid problems with signal discrepancies regarding the inertia at the beginning of exercises, as well as the possibility of fatigue in the last repetition (12). The EMG signal collected was analyzed on SAD32 (32 bits, 2.61.05 mp version) software, developed at the Mechanic Measurements Laboratory of UFRGS. For segmentation and quantification of the EMG signal, the goniometer's curves were used to identify and to separate the concentric and eccentric phases. Raw EMG signals were band-pass filtered (Butterworth 5th order) at 20-500 Hz following the recommendations of DeLuca (7). To examine the EMG signals in the time domain, the raw signals were processed through an rms calculation. The rmsEMG average was obtained from the 3 central submaximum effort repetitions of the LPL, LP45, and LPH, and the rmsEMG was obtained from maximum repetition (1RM) of the LP45 for each muscle during the concentric phase. This treatment was similar to that proposed by Escamilla et al. (12) and Pincivero et al (31). The rmsEMG mean collected at 40% and 80% was normalized using the value collected from different muscles during the 1RM of the LP45 (100%). This procedure was adopted due to limitations on normalization by isometric actions (7). Muscle activity was compared in the concentric phase between exercises at 40% and 80% of the 1RM.
Statistical Analyses
The Shapiro-Wilks statistical test was used to determine the data normality. According to the result, repeated-measures analyses of variance comparing the exercises (LP45, LPH, and LPL) for each intensity (40% and 80% of 1RM) were applied to the 5 muscle activity values to verify the differences in muscle activity. Subsequently, to determine the source of the significance, Bonferroni's post hoc test was used. All statistical procedures were adopted by using the SPSS 11.0 package for Windows. Significance was set at p ≤ 0.05.
Results
Figure 4 shows the rmsEMG normalized mean values of muscle activity among the 3 exercises at 40% and 80%. At moderate effort levels, rectus femoris and gastrocnemius activity was greater (p < 0.05) during the LP45 and LPL than during the LPH (Figure 4A). At high effort level, the rectus femoris and vastus lateralis (quadriceps) were more active (p < 0.05) in during the LPL than the LPH. The rectus femoris and gastrocnemius were more active (p < 0.05) during the LP45 and LPL than during the LPH. However, during the LPH exercise, gluteus maximus activity was greater (p < 0.05) than during the LPL (Figure 4B). No statistical difference was observed in biceps femoris activity among the 3 exercises (Figure 4).
Figure 4: Root mean square electromyographic values of the muscle activity of rectus femoris (RF), vastus lateralis (VL), biceps femoris (BF), gastrocnemius (GAS), and gluteus maximus (GM) muscles during different types of leg press exercises at a 40% effort level (adifference from LPH, p < 0.05) (A) and during different types of leg press exercises at an 80% effort level (bdifference from LPL, p < 0.05) (B).
Discussion
In the present study, we examined young women performing 3 different LP exercises at 2 submaximum effort levels. The EMG data were analyzed in order to compare muscle activity among exercises. The principal differences found in muscle activation patterns are related to the mechanical changes and effort levels required (40% and 80% of 1RM) during these exercises.
At the moderate effort level (40%), we found that rectus femoris and gastrocnemius activity during the performance of the LP45 and LPL was greater than during the LPH exercise. However, the same result was found at the high effort level (80%). It means that the activity patterns of rectus femoris and gastrocnemius were different among these exercises and did not depend on the effort level required. This was probably due to mechanical changes during the performance of these exercises.
For the rectus femoris, the fact that it is a biarticular (hip and knee) muscle can explain these differences (10-12). Escamilla (10) suggested that the greater rectus femoris activity found during monoarticular exercises (knee extension) compared to biarticular exercises (LP and squat) for the lower limbs can be explained by its biarticular function. When comparing different types of LP, it can be verified that during the LPH at the starting position, the high foot placement increases the hip flexion angle (the biceps femoris and gluteus maximus are stretched and the rectus femoris is shortened). This could impair the rectus femoris mechanism that shortens this muscle. Thus, it would result in a strength deficit because, in that position, the rectus femoris would not be at a favorable length to increase force production (8,9,30). On the other hand, in the LP45 and LPL exercises, the rectus femoris would not be as shortened, thus increasing its force production capacity. Our result was different from that of Escamilla et al. (12). Probably the different hip ankle positions used during LPH and LPL performance can explain these differences. For the gastrocnemius muscle, similar results were previously reported by Escamilla et al. (12), although only the LPL and LPH were compared in their study. These results were explained by the subjects' different ankle joint positions adopted during the 3 exercises. During the LPH, the subject's ankle is positioned at a greater degree of plantar flexion compared to its position during the LP45 and LPL. This causes a shortening of the gastrocnemius muscle (17), which can impair the mechanics of the gastrocnemius during this exercise, suggested again by the relationship between force production and muscle length (force-length curves) (8,9,30).
On the other hand, at the high effort level (80%), we found that vastus lateralis activity was greater during the LPL than during the LPH and that gluteus maximus activity was greater during the LPH than during the LPL. It means that activity patterns of the vastus lateralis and gluteus maximus were different between these exercises, depending on the effort level required. It probably was a result of mechanical changes in the arrangement of the loads lifted while performing these exercises.
A specific requirement of the vastus lateralis can occur during the LPL, but it happened only at the high effort level, confirming the different result obtained from that found by Escamilla et al. (12). Woods and Bigland-Ritchie (38) found nonlinear force-EMG relationships in muscles of mixed fiber composition. They suggested that at the low to moderate effort level, low threshold units would be selectively recruited, while at the high effort level, high threshold units would be responsible for increasing the EMG signal. According to Johnson et al. (22), the vastus lateralis muscle consisted of approximately 45% of type I fibers and 55% of type II. Thus, selective recruitment of type II fibers at increasing force levels (80%) in the vastus lateralis may still be responsible for increasing the EMG signal at the high effort level. It suggests that coordination patterns are different from the high to moderate effort levels (1,25). This finding combined with the rectus femoris results indicates that the quadriceps muscle group (rectus femoris and vastus lateralis) appears to require more mechanical changes during the LPL than the LPH, indicating a specific activity of these muscles mainly at the high effort level (80%).
For the gluteus maximus, this finding supports the suggestions of Caterisano et al. (5) that a greater angle of hip motion could increase gluteus maximus activity during LPH performance. Compared to the other exercises, in the starting position, the greater hip flexion angle observed could be favorable to gluteus maximus force production (8,9,30). We suggest that the high gluteus maximus activity during the LPH helps the deficit caused by the rectus femoris and gastrocnemius muscles. However, this result was found only at the high effort level. According to Johnson et al. (22), the gluteus maximus muscle consisted of approximately 40%-70% of type I fibers and 30%-60% of type II fibers. Thus, selective recruitment of type II fibers at increasing force levels in the gluteus maximus may still be responsible for the increased EMG signal at the high effort level. Again, it suggests that coordination patterns are different from the high to moderate effort levels (1,26). No difference was observed between exercises in biceps femoris activity, in agreement with the results found by Escamilla et al. (12).
In conclusion, the results presented suggest that the mechanical changes in LPL, LP45 and LPH performance affect coordination activity patterns in women's lower limb muscles. The differences can be related to the load lifted (effort level) during these exercises.
Practical Applications
LP exercises (LPL, LPH, and LP45) are commonly performed in strength training programs. Due to the fact that the primary purpose of the LP exercises is the development of increased strength during knee and hip extension simultaneously, identifying the participation of the different muscles involved in these exercises at different loads is very important to coaches, athletes, and general people. The results of our study indicate that when a load at 40% of 1RM is selected, LPL and LP45 are recommended to strengthen the rectus femoris and gastrocnemius muscles. When a load at 80% of 1RM is selected, the exercises recommended to strengthen the rectus femoris and gastrocnemius are the same. To strengthen the quadriceps (rectus femoris and vastus lateralis) muscle, we recommend performing the LPL exercise. On the other hand, to strengthen the gluteus maximus muscle, the LPH exercise should be performed.
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