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Electromyographic Activity of the Pectoralis Major and Anterior Deltoid

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Electromyographic Activity of the Pectoralis Major and Anterior Deltoid Muscles During Three Upper-Body Lifts

Elizabeth A. Welsch, Michael Bird, and Jerry L. Mayhew
Exercise Science Program, Truman State University, Kirksville, Missouri 63501

ABSTRACT
Welsch, E.A., M. Bird, and J.L. Mayhew. Electromyographic activity of the pectoralis major and anterior deltoid muscles during 3 upper-body lifts. J. Strength Cond. Res. 19(2): 449–452. 2005.—The purpose of this study was to examine the differences in activation levels and times of activation for the pectoralis major and anterior deltoid when performing the concentric phase of 3 upper-body lifts. Twelve college-age men and women with various degrees of lifting experience performed 3 repetitions using the 6 repetition maximum in a barbell bench press, dumbbell bench press, and dumbbell fly while being monitored for electromyographic activity in both muscles. Motor unit activation of both muscles was not significantly different during all 3 lifts. However, dumbbell flys had significantly less relative time of activation than did barbell or dumbbell bench presses. Therefore, dumbbell flys may be better suited as an auxiliary lift, whereas barbell and dumbbell bench presses may be used interchangeably in training programs. The compatibility of the barbell and dumbbell bench presses may aid lifters in overcoming training plateaus by alternating exercises for the same muscle groups.
Key Words: bench press, dumbbell flys, neural activation

Introduction
Among the exercises used in heavy-resistance training programs, the bench press is one of the most popular upper-body lifts (2, 3). This lift is typically performed lying supine on a bench using a lifting machine, a barbell, or dumbbells. It is understood to be an essential exercise for developing the chest muscles, particularly both heads of the pectoralis major and the anterior deltoid (2). Several studies have suggested that shoulder-stabilizing muscles are more active during the bench press when free weights are used rather than when machine weights are used (6, 7). No consensus has been reached, however, concerning the difference in muscle activation of prime movers between dumbbell and barbell bench press exercises.

One method of evaluating the muscle activation during a lift is through the use of electromyography (EMG). Measuring the integrated EMG (IEMG) signal quantifies the amount of motor unit activity in the muscle recorded from electrodes placed over the belly of the muscle. The IEMG studies on the bench press have not revealed significant differences in the motor unit activity during inclined and declined bench press exercises compared with the conventional flat-bench exercise (2). However, these study investigators observed the exercises performed only on a Smith machine, which may alter the bar path and change the neural activation of the muscles involved.

Many avid strength trainees use various lifts designed to isolate the pectoralis major with the idea that exercise variety will aid the development of the muscle and perhaps produce greater growth stimulus. Although most lifters use the barbell bench press to develop the chest and anterior shoulder muscles, the dumbbell bench press and bent-arm flys have been used under the assumption that they isolate the pectoralis major and more fully incorporate the anterior deltoid during the concentric phase of the lift.

Determining the extent to which these supplemental exercises may activate pectoralis major and anterior deltoid musculature and identifying their ability to promote strength development compared with the traditional barbell bench press would be beneficial. Greater understanding of the amount and duration of the electrical activity of the primary muscles in these variations of free-weight bench presses could aid in exercise selection among lifters who may be experiencing slowed progress or plateaus in their strength training programs. The purpose of this study, therefore, was to examine the differences in activation levels and times of activation for the pectoralis major and anterior deltoid during the concentric phase of a barbell bench press, dumbbell bench press, and dumbbell fly.

Methods

Experimental Approach to the Problem
To evaluate the activity level of the pectoralis major and the anterior deltoids during 3 different chest exercises, EMG activity was monitored by surface electrodes placed over the motor activation points of these muscles during the concentric phase of each lift. Peak activation of each muscle during any given 100-millisecond interval was determined from EMG. Relative time of activation of a muscle was the percentage of time the muscle was active compared with the total time of the concentric phase. The loads used were typical of those used by a strength trainee during the execution of these lifts.

Subjects
Twelve college-age men and women with at least 1 year of lifting experience (mean ± SD = 2.5 ± 1.5 years) were recruited for this study. This study was approved by the Institutional Review Board for the Protection of Human Subjects, and each participant signed an informed consent document before testing. Physical characteristics of the subjects are given in Table 1
.
Participants had been involved in regular resistance training 2 to 4 days per week for the preceding 6 months with both barbell and dumbbell bench presses. In addition, most had used a bent-arm fly as a supplemental exercise at various times in their program but not on a regular basis. All participants were screened to exclude those with injuries that could be aggravated by lifting or those unable to successfully complete the lifts according to specific testing guidelines. Participants had not performed any arm exercises for a minimum of 24 hours before testing. Each participate reported being at least 3 hours postabsorptive at test time.

Before testing, all participants gained familiarity with the barbell and dumbbell bench presses and the dumbbell fly by performing them under the guidance of the primary investigator. One or more familiarization periods were given, depending on the participant's ability to control the barbell or dumbbell in the same path on each repetition. The path of the barbell and dumbbell during the bench presses was as close to the same vertical path as possible above the midpoint of the sternum. The bent-arm fly required slightly more familiarization and needed approximately 3 sessions to allow the participants to maintain a proper angle at the elbow while following a similar path on each repetition. Following familiarization, each participant determined his or her 6 repetition maximum (6RM) in each exercise. A 6RM was chosen because it was similar to typical training levels yet did not predispose the individual to excess lifting fatigue, since fatigue has been shown to increase the IEMG signal (4).

Testing Procedure
On a separate day from the 6RM testing, the EMG was recorded from 3 repetitions using the 6RM for each exercise. Only 3 repetitions were used to eliminated a fatiguing effect, allow easier control of the weight for less experienced lifters, and reduce the potential for injury. Order of testing for the barbell bench, dumbbell bench, and dumbbell flys was randomized, and rest intervals between lifts were selected by the subjects but usually fell in the 3- to 5-minute range (10). All testing was completed on the same day for each subject.

Subjects positioned themselves on a horizontal bench and performed each lift in a controlled manner according to the lifting guidelines established during the familiarization period. The bar was not allowed to bounce off the chest when lowered to the nipple level. Position of the hands on the barbell was individually selected and usually ranged from 10–30 cm outside the shoulder joint (9). The dumbbells were not allowed to touch at the top of either the bench press or the flys. In addition, the vertebral column was not allowed to hyperextend during any of the lifts. Two spotters ensured subject safety during each lift and stabilized the straight bar and dumbbells before and after lift completion. The participant was instructed to perform each lift at approximately the same velocity, and the principal investigator visually detected completion of the barbell and dumbbell presses at full elbow extension and completion of the dumbbell fly at the top of the arcing movement over the chest. Subjects were admonished to keep arms slightly flexed at the elbows (approximately 160°) throughout the entire dumbbell fly lift.

Data Collection
Before the lifting exercise, the skin was prepared for placement of surface electrodes by shaving predetermined areas to remove dead skin and hair. All sites were then abraded with sandpaper and cleaned with alcohol to decrease skin resistance. Electrode gel was applied to the electrodes to reduce skin impedance between the electrodes and skin, and electrodes were adhered to the skin.

Two surface electrodes were placed over the motor points of both the pectoralis major and anterior deltoid, parallel to the muscle's fiber direction. Raw EMG data were overlaid during testing with markers set by the investigator that identified the concentric phase of contraction. The concentric phase was defined as the interval between the bottom of the barbell or dumbbell movement path (the point where barbell or dumbbell velocity changed from a negative to a positive value) and the top of the movement path (point of full elbow extension).

The EMG signals were sampled at a rate of 2000 Hz and measured by a bioamplifier (Coulbourn Instruments, Lehigh, PA). Signals from each trial were low-pass filtered (maximum cutoff, 8 Hz) and high-pass filtered (minimum cutoff, 150 Hz) and transmitted to an analog-to-digital converter installed in a desktop computer. Following data collection, the signals from each trial were stored on the hard drive and later rectified and integrated using the Datapac 2000 software program.

Data Reduction
The computer software was used to perform IEMG by true integration of the full-wave rectified signal. The IEMG was measured in millivolt seconds for the concentric phase of each lift and averaged for the 3 repetitions of each lift. The highest activity levels in the IEMG during a 100-millisecond interval reflected the peak EMG of each muscle. Relative time of activation was defined as the average time the muscle was active with respect to the total time of the concentric phase. This approach helped control for variations in the range of motion and time of the movements (across and within subjects) and variations in activity level within the range of motion.

Statistical Analysis
Differences in electrical activity and activation time for the pectoralis major and anterior deltoid among the barbell bench press, dumbbell bench press, and the dumbbell fly were examined using a repeated-measures analysis of variance. A Tukey post hoc test was used to identify significant differences, using a probability level of p 0.05.

Results
Although the peak activation levels were slightly higher for the barbell bench press, the IEMG activity was not significantly different for the pectoralis major or anterior deltoid among the 3 lifts (Table 2 ). The relative times of muscle activation were significantly different among all 3 lifts for both the pectoralis major and the anterior deltoid. Tukey post hoc testing indicated that the dumbbell flys had the least relative time of muscle activation, whereas the barbell bench press had the greatest (Table 3 ).

Discussion
The pectoralis major and anterior deltoid appeared to reach approximately the same peak activation level during the concentric phase of a barbell bench press, dumbbell bench press, and dumbbell fly performed on a flat bench under the conditions imposed during this study. Greater electrical activity was not imposed by the use of dumbbells as has been suggested elsewhere (6).

The fact that the electrical activity was recorded during nonfatiguing contractions may have contributed to the similarity in IEMG activity. Had the number of repetitions been continued to a maximal level, the IEMG pattern may have been altered due to fatigue (4). McCaw and Friday (6) suggested that shoulder-stabilizing muscles were generally more active during a free-weight bench press compared with a machine bench press, which could have been caused by the greater fatigue in the ancillary muscles attempting to control the free-weight bar. It is possible that heavier loads than used in this study could alter the IEMG pattern and produce greater motor activity in both prime movers and stabilizing muscles for dumbbell lifts. It was interesting to note that participants used dumbbell loads that totaled approximately 63% of the barbell load for bench presses and 51% for flys.

Based on the current EMG results, it seems that barbell and dumbbell bench presses may be used interchangeably to introduce variation to weight-training programs without reduction of muscle activation or negative transfer effects (1). Conversely, dumbbell flys might not be a lift that should be viewed as a primary exercise to target either the pectoralis major or anterior deltoid muscles, since this lift produced the shortest relative time of activation. Since time under tension is assumed to be a primary factor in the development of strength, it would appear more advantageous to rely on the barbell or dumbbell bench press for chest development. Furthermore, dumbbell flys may not provide the chest with as great a stretch at the bottom of the movement or as great a contraction at the top of the movement as noted with barbell or bench press machines (1). This is not to say that the dumbbell fly is an ineffective lift, but rather that it may be better categorized as a supplementary lift for training pectoral and anterior deltoid muscles.

As the neuromuscular system noticeably adapts to specific movement patterns (5), lifters are likely to experience, at some point, slowed progress or plateaus in their strength training programs. Variations to chest workouts, such as substituting barbell bench presses for dumbbell bench presses or vice versa, could yield beneficial results, since muscles are challenged to a similar degree but with a different movement. This study did not examine the range of motion achieved at the shoulder during the concentric phase of the barbell or dumbbell lifts; thus, no noteworthy conclusions could be drawn regarding the effects of the increased range of movement during dumbbell lifts (8). Discrepancies noted between the bench presses and dumbbell fly may have resulted from variability of lifting mechanics between lifters, because elbow bend may have deviated from the prescribed angle. Dumbbell position may or may not have been influential in the observed differences between relative times of muscle activation between lifting modes, but it is definitively a factor worth considering. Experienced lifters may have developed efficient techniques to control the free-weight barbell and dumbbells, thereby reducing the muscle activity in stabilizing muscles (6). Further research that examines muscle activity in the prime movers and stabilizing muscles of the 3 lifting techniques among lifters of varying experience might answer that question.

Practical Applications
Many strength training enthusiasts use various lifts that involve the same muscle groups in an attempt to alter their routine under the hope that it may stimulate greater growth and/or strength in those specific muscles. Since the chest musculature is a major focus of most strength training programs, several variations of lifts exist to target those muscles. In this study, the 3 widely used lifts that concentrate on development of the pectoralis major appear to offer equivalent peak activation levels of the muscle, but the shorter activation time of the dumbbell flys may regulate that exercise to a supplemental role rather than a primary lift. The significantly greater activation time and slightly greater peak neural activity could suggest that the barbell bench press is the best exercise for pectoral development.

References
1. Aaberg, E. Biomechanically Correct. Dallas: Realistic Individualized Professional Training Services, 1996.

2. Barnett, C., V. Kippers, and P. Turner. Effects of variations of the bench press exercise on the EMG activity of five shoulder muscles. J. Strength Cond. Res. 9:222–227. 1995.

3. Egger, G., N. Champion, and G. Hurst. The Fitness Leaders Exercise Bible (2nd ed). Kenthurst, New South Wales: Kangaroo Press, 1989.

4. Glass, S., and T. Armstrong. Electromyographical activity of the pectoralis muscle during incline and decline bench presses. J. Strength Cond. Res. 11:163–167. 1997.

5. Judge, L.W., C. Moreau, and J.R. Burke. Neural adaptations with sport-specific resistance training in highly skilled athletes. J. Sports Sci. 21:419–427.

6. Lander, J., B. Bates, J. Sawhill, and J. Hamill. A comparison between free-weight and isokinetic bench pressing. Med. Sci. Sports Exerc. 17:344–353. 1985.

7. McCaw, S., and J. Friday. A comparison of muscle activity between a free weight and machine bench press. J. Strength Cond. Res. 8:259–264. 1994.

8. McDonagh, M., and C. Davies. Adaptive responses of mammalian skeletal muscle to exercise with high loads. Eur. J. Appl. Physiol. 52:139–155. 1984.

9. Wagner, L.L., S.A. Evans, J.P. Weir, T.J. Housh, and G.O. Johnson. The effect of grip width on bench press performance. Intl. J. Sport Biomechanics. 8:1–10. 1992.

10. Weir, J.P., L.L. Wagner, and T.J. Housh. The effect of rest interval length on repeated maximal bench presses. J. Strength Cond. Res. 8:58–60. 1994.


Tables

TABLE 1. Physical characteristics of the participants (n = 12)
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TABLE 2. Average peak activation levels in muscles according to lift.*
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TABLE 3. Average relative time of activation in muscles ac cording to lift.*
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