Using a resistance training program is the most effective way to increase muscular strength in any part of the body (6). Increasing upper-body strength is of utmost importance in any activity or sport that puts a premium on body-to-body contact or where an implement is carried and thrown using the upper body. Measuring upper-body strength is commonly accomplished by using the bench press exercise. The prime movers in the bench press are the pectoralis major, triceps brachii, and the anterior deltoid, with the medial deltoid acting as a stabilizer muscle (9). The bench press can be performed using a free weight barbell or a Smith machine. Although the less stable nature of the former is commonly accepted as the preferred method for maximal upper-body strength gain for the experienced lifter, the more stable motion of the latter offers the less experienced lifter a safer and more controlled means of training the same muscles (2).
The free weight bench press offers instability in all 3 planes of motion forcing the lifter to contract the muscles in a more natural fashion to balance in all 3 planes of motion while exerting force at a velocity that is not constant. This is important to the lifter who wants to increase maximal muscular strength of the prime movers while engaging the stabilizing effect of the prime movers and stabilizer muscles. This often takes practice to master because of the balance and tempo required. A Smith machine guides the bar in a fixed path requiring very little balance by the lifter compared with the free weight bench press. Although the fixed path may hinder the synergist effect of the prime movers, it has been surmised that the lack of required balance may allow for increased attention to force production by the prime movers (10). This effect may be greater in more experienced lifters as the prime movers of the bench press are likely to be more developed (1,10,11,13). This is attractive to the lifter involved in rehabilitation, novice lifters, or very young and old lifters. Also, those experienced lifters looking for maximal fatigue at the end of a workout routine might find the Smith machine useful.
McCaw and Friday (11) were among the first researchers to directly quantify the difference in muscle activation of the anterior and medial deltoid, triceps brachii, and pectoralis major during machine and free weight bench press through electromyography (EMG), a measure of muscle electrical activity resulting from action potentials (3,7). McCaw and Friday found significantly (p < 0.05) greater activation of the medial and anterior deltoid during a free weight bench press compared with a machine bench press and that this is evident at lower (60% 1 repetition maximum [1RM]) but not higher (80% 1RM) intensities. They concluded that the higher muscle activation was because of the stabilizing role of the deltoid on the humerus during the free weight bench press. They also surmised that the results were only evident at the lower intensity because of less joint stiffness.
The McCaw and Friday (11) study, although applicable to certain populations, is limited and requires further examination. The generalizability of the results are limited because of the lack of subjects (5), the use of men only with significant training experience (at least 1 year), and the utilization of relatively light loads (60 and 80% 1RM). Experience may alter muscular recruitment patterns. Therefore, a study is needed that includes subjects of differing experience levels and in which the results are applicable to a larger population. In addition, an examination of higher loads (70-90% 1RM) would be more reflective of the intensities typically used to gain strength (6).
Therefore, the purpose of this study was to compare muscle activation of the anterior deltoid, medial deltoid, and pectoralis major during a Smith machine and free weight bench press at lower (70% 1RM) and higher (90% 1RM) intensities in experienced and inexperienced lifters. It was hypothesized that the free weight bench press would result in significantly greater muscle activation of the anterior deltoid, medial deltoid, and pectoralis major when compared with a Smith machine bench press (2,11) and that the difference in muscle activation for all muscles would be most evident at the lower (70% 1RM) intensity (11). In addition, it was hypothesized that experienced lifters would have greater differences in muscle activation between modes than inexperienced lifters (2,13).
Experimental Approach to the Problem
To investigate the difference in muscle activation between a Smith machine and free weight bench press, we used a repeated measures design consisting of 2 testing sessions where each subject performed Smith machine bench press in one session and free weight bench press in the other session. The order of these conditions was counterbalanced to the first subject, whereas order of loads was randomized during every session. Electromyography was recorded during the concentric phase of each repetition and 1RM test to compare muscle activation levels for the medial deltoid, anterior deltoid, and pectoralis major muscles. Subjects were categorized according to bench press experience to examine the effect of prior bench press activity on muscle activation.
Fourteen experienced (age, 19.9 ± 2.1 years; height, 176.3 ± 7.5 cm; mass, 88.5 ± 19.4 kg) and 12 inexperienced (age, 20.5 ± 2.1 years; height, 179.8 ± 8.0 cm; mass, 75.5 ± 10.4 kg) male subjects volunteered for this study. An analysis of variance (ANOVA) run on height and body mass showed no significant difference between groups, although it was very close for body mass (p = 0.053). Experience was defined as having performed the bench press exercise at least twice per week for at least the last 6 months. Most experienced bench pressers preferred the free weight bench press to the Smith machine bench press for training. In addition to their normal daily water intake, each participant was instructed to drink 1 L of water the night before testing and half a liter on the day of testing to help ensure adequate hydration status. In addition, participants were instructed to refrain from other physical activities for 48 hours before testing. All participants signed an informed consent before participation in the study, which was approved by the Institutional Review Board of the University. A National Strength and Conditioning Association (NSCA)-certified strength and conditioning specialist monitored every session.
Smith Machine and Free Weight Bench Press Testing
Testing took place in 2 visits, each of which consisted of either Smith machine testing or free weight bench press testing. The first of 2 testing days consisted of determining each participant's 1RM on either the Smith machine (Nautilus NT 1800; Nautilus, Inc., Vancouver, WA, USA) or free weight (Hammer Strength Flat Bench; Hammer Strength, Schiller Park, IL, USA) bench press followed by 2 repetitions at 70% 1RM and 2 repetitions at 90% 1RM on the tested mode. The order of conditions (free weight and Smith machine) was counterbalanced, and the order of loads (70% 1RM and 90% 1RM) was randomized to control for biasing of order for each participant. One week later, the same protocol as the first visit was used for the mode that was not tested. The 1RM testing followed the NSCA protocol (4), which requires that participants progressively increase resistance across attempts until the 1RM is achieved. The technique used for the lifts was also done in accordance with NSCA guidelines (4).
Participants in each condition began by lying horizontally with the feet, gluteus maximus, lower back, upper back, and head firmly planted on the bench with elbows fully extended and gripping the bar. This positioning on the bench was used for both free weight and Smith machine bench presses according to guidelines recommended by NSCA (4). Participants were instructed to lift the bar off of the rack. Participants then isometrically held this position at full elbow extension until instructed to eccentrically lower the bar to the chest in a slow controlled manner where they touched the chest at approximately nipple level. To control for bar bouncing, the concentric pushing phase began after the tester gave a verbal cue to “push.” To maximize external validity, lifting cadence was determined by the tempo that each subject felt was most natural to him. Each subject used a standardized grip width of 165% of biacromial breadth. This was determined by measuring the distance between the acromion processes, multiplying by 1.65, and dividing by 2. This quotient was then measured out from the center of the bar on each side.
In an order to avoid confounding the EMG signal, the participant's skin was prepared before placement of EMG electrodes. Hair at the site of electrode placement was removed by shaving, and skin was abraded with an abrasive pad and cleaned with isopropyl alcohol. Electromyography data were collected and stored on a personal computer (Dell Latitude D610, Dell, Round Rock, TX, USA). Three separate bipolar (3.5 cm center to center) surface electrode (BIOPAC EL500 silver-silver chloride; BIOPAC Systems, Inc., Goleta, CA, USA) arrangements were placed over the longitudinal axes of the medial deltoid, pectoralis major, and anterior deltoid muscles. For the anterior deltoid, 2 electrodes were placed on the anterior aspect of the muscle, 4 cm below the clavicle. The medial deltoid had 2 electrodes placed on the lateral aspect of the deltoid, 3 cm below the acromion process. Electrodes for the pectoralis major were placed 4 cm medial to the axillary fold. All measurements were taken on the right side of the participant's body. The reference electrodes were placed over the iliac crest. The EMG signals were preamplified (gain 1,000×) using a differential amplifier (EMG 100C; BIOPAC Systems, Inc., Santa Barbara, CA, USA; bandwidth = 1-500 Hz). Surface EMG was used to measure muscle activation during the concentric phase of the 1RM tests and repetitions performed at 70 and 90% 1RM. Electromyography data were averaged for the 2 repetitions performed at 70 and 90% 1RM.
The EMG signals were band-pass filtered (fourth-order Butterworth filter) at 10-500 Hz. The amplitude of the signals was expressed as root mean square values. All analyses were performed with custom programs written with LabVIEW software (version 7.1; National Instruments, Austin, TX, USA).
Before statistical analysis, each participant's EMG amplitude values were normalized to their highest recorded value during Smith machine 1RM testing. A 4-way (load [70%, 90% 1RM] × mode [Smith machine, free weight] × muscle [medial deltoid, anterior deltoid, pectoralis major] × experience level [experienced, inexperienced]) mixed factor ANOVA was used to compare muscle activation (normalized EMG amplitude) for the anterior deltoid, medial deltoid, and pectoralis major. An alpha level of p ≤ 0.05 was used for all statistical tests, and SPSS version 16.0 software (SPSS, Inc., Chicago, IL, USA) was used to analyze the data. A 2-way (experience level [experienced, inexperienced] × mode [free weight, Smith machine]) mixed factor ANOVA was used to compare bench press 1RM values. Effect sizes (ES) were also calculated.
There were no significant 4-way or 3-way interactions (Table 1). There was, however, a significant interaction for the mode × muscle 2-way ANOVA. Normalized EMG amplitude from the medial deltoid was significantly (p < 0.05, ES = 0.642) greater for the free weight mode compared with the Smith machine mode (Figure 1). There was no significant difference in normalized EMG amplitude for the pectoralis major or anterior deltoid. There was a significant (p < 0.05) main effect for normalized EMG amplitude across loads (Figure 2). The normalized EMG amplitude for the 90% 1RM condition was significantly (p < 0.05, ES = 0.791) greater than for the 70% 1RM condition.
There was no significant interaction for bench press 1RM values; however, there was a significant main effect for experience level. Experienced bench pressers had significantly (p < 0.05, ES = 0.949) higher 1RMs than inexperienced bench pressers averaged across modes (Table 2).
The first part of the hypothesis was partially supported by the finding that across load and experience level, activation of the medial deltoid was significantly greater during the free weight bench press than it was during the Smith machine bench press. The instability caused by the free weight bench press necessitates a greater response by the medial deltoid as both a force producer and perhaps more importantly as a stabilizer. This corroborates findings by McCaw and Friday (11) and highlights the important stabilizing role of the medial deltoid in the glenoid cavity during the concentric phase of the bench press.
There were no differences in muscle activation for anterior deltoid and pectoralis major between modes, regardless of load or experience level. This relative constancy in muscle activation of the anterior deltoid and pectoralis major between modes and experience levels may have been due to the increased stability offered by the Smith machine, which tips the balance of muscle activation used for stabilization and force production during the free weight bench press in favor of the latter during the Smith machine bench press (10,11). However, if this assumption was to be validated, the mean Smith machine 1RMs should have been greater than the mean free weight 1RMs. However, this was not the case. It may be that the unnatural bar path of the Smith machine forced the subjects to press in an unnatural linear path instead of the reverse “C” observed during free weight bench press (2), subsequently hindering optimal force production for this exercise.
McCaw and Friday (11) found that the difference in muscle activation of the medial and anterior deltoids between machine and free weight bench press was greater at a lower (60% 1RM) load than it was at the higher (80%1RM) load. In fact, the authors reported no significant differences in muscle activation between bench press modes at the 80% 1RM load. They suggested that while working against lighter loads, the lowered muscle activity decreases joint stiffness and subsequently emphasizes the role of the medial and anterior deltoids as stabilizers of the humeral head in the glenoid cavity. It was hypothesized that the same would be true in the present study. However, the results did not support this. The light load used in the present study was 70% 1RM, which may have been sufficiently heavy to elicit enough muscle activity to increase the joint stiffness and limit the stabilizing role of the medial and anterior deltoids.
Neuromuscular adaptations and strength are developed through experience in a given exercise such as the bench press, improving capability in that exercise (2,13). McCaw and Friday (11) posited that inexperienced lifters would undergo greater differences in muscle activity between machine and free weight bench press because experienced lifters may have developed efficient free weight bench press technique, minimizing the role of stabilizing muscles. However, in the present study, it was expected that experienced lifters would undergo greater differences in muscle activity between modes because their neuromuscular adaptations to the free weight bench press would be significantly hindered by the less natural and more restricted bar path that they are forced to press into with the Smith machine bench press (2). The results of the present study offer little to support this hypothesis. There were no significant differences in muscle activity between experience levels, although experienced bench pressers had greater mean differences in their 1RMs between modes, favoring free weight bench press. One possible explanation for the lack of support for this hypothesis is the definition of experience used in this study. Experience was defined as having performed bench press twice per week for at least the last 6 months. Previous studies (2,11) used subjects who had between 1 and 3 years of bench press experience. Compared with 1 year or more, 6 months of bench press experience may not allow for as much neuromuscular adaptation, strength, and subsequent capability to be developed. Also, the fact that experienced subjects had to have performed the bench press twice per week may have caused some subjects to consider themselves inexperienced despite training with the bench press once per week for years. This may also have affected the results.
Another explanation for the findings has to do with the mechanics of the Smith machine used in this study. The Smith machine that was used in this study was one in which the bar path was angled slightly backward, meaning that at the bottom of the repetition, the lifter started at nipple level and at the top finished with the bar slightly closer to the shoulders. This bar path, although linear, is more reminiscent of the natural reverse “C” path than the bar paths of other Smith machines that have either perfectly vertical or slightly forward angled guided bars. Therefore, in this study, the Smith machine press may have been similar enough to the free weight press so that differences between experience levels were minimized.
In the present study, the 90% 1RM load elicited greater muscle activation than did the 70% 1RM load, regardless of exercise mode (free weight or Smith machine) or experience level. This supports the findings of others (5,8,11,12) and validates the usefulness of EMG to detect differences in muscle activation at varying loads. In conclusion, the results of the present study suggest that the free weight bench press leads to greater activation of the medial deltoid as a stabilizer than the Smith machine bench press. It would be advantageous to study different types of Smith machines such as those that have either forward or perfectly vertical guided bar paths as these bar paths are less similar to a free weight bar path.
The bench press is a common exercise performed by both athletes and recreational lifters and prescribed by strength coaches and trainers. The results of the present study suggest that the free weight bench press may lead to an increased requirement for stabilization about the glenohumeral joint from muscles such as the medial deltoid. Sports such as basketball, baseball, tennis, racquetball, hockey, and volleyball include movements that require deltoid strength and stability about the glenohumeral joint. The strength coach may better serve his/her athletes by choosing the free weight bench over the Smith machine bench press because of its potential for more sports-specific muscular development. The results of the present study also suggest that it may be advantageous for those who have not developed the neuromuscular adaptations necessary for correct stabilization of the glenohumeral joint (patients in rehab, or novice weightlifters) to use the Smith machine bench press over its free weight counterpart because of its reduced requirement for stabilization of the glenohumeral joint. However, future research should examine training-related changes in activation and strength development of these muscles as a result of different modes of bench pressing.
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