Proponents of instability in resistance training suggest that instability enhances the stress on the neuromuscular system compared with traditional resistance training methods, but the scientific reports are inconclusive (5). In fact, the muscle activity in unstable versus more stable resistance exercises is reported to be higher, similar, or even lower depending on the exercises and muscles that are examined (2,10,18,19,23,27).
Several methods have been used to induce instability in resistance training, such as unstable instead of stable surfaces (4,6,16,23), free weights instead of machines (27,29,30), dumbbells instead of barbell (27,34), unilateral instead of bilateral exercises (6,24), and standing instead of supine exercises (28). Although unstable surfaces such as wobble boards, swiss balls and BOSU® balls and slings have received much attention in the scientific literature and commercial media, most daily life activities and sports are performed on stable surfaces while moving an unstable external resistance. Examples include a tennis racket, volleyball, or lifting a baby. According to the training principle of specificity (22), it may therefore in many cases be more relevant to include instability components by other means than changing the surface.
Using dumbbells instead of barbells forces those individuals who engage in resistance training to control and balance the weights independently and thereby could increase and/or reduce the involvement of agonists, synergists, stabilizers, and antagonists (27,34). For example, both Welsch et al. (34) and Saeterbakken et al. (27) reported similar EMG activity in pectoralis major and anterior deltoid in dumbbell chest presses compared with barbell despite the fact that dumbbell loads were only 63–83% of the barbell loads (27,34). Moreover, Kohler et al. (16) compared barbell and dumbbell shoulder presses and reported similar EMG activity in anterior and medial deltoid despite that the dumbbell load was only ∼86% of the barbell load.
In contrast to the growing body of literature concerning barbells versus dumbbells (16,27,34), we are not aware of studies investigating standing instead of seated position as an instability factor for agonists, synergists, and antagonists. Free weight resistance exercises performed standing should, in theory, increase the stabilizing requirements compared with seated exercises (3,9). We recently demonstrated this for the core muscles (24), but the effect of standing versus seated position on upper extremity muscles is unclear. It is also unclear how these different instability approaches (body position + loading modality) interact with each other with respect to muscle activity and strength.
The purpose of this study was to compare the 1-RM strength and EMG activity in barbell and dumbbell shoulder presses performed seated and standing. It was hypothesized that increased stability requirement (dumbbells instead of barbell and standing instead of seated) would result in similar EMG activity of the deltoid muscles but decrease the 1-RM strength.
Experimental Approach to the Problem
To examine effects of body position (seated and standing) and loading modality (barbell and dumbbells) on 1-RM strength and neuromuscular activity in shoulder presses, a within-subjects repeated-measures design was used. The shoulder press was chosen because it is frequently used and requires considerable inter- and intramuscular coordination of the deltoid and arm muscles, which makes it suitable as an experimental model in the study of instability in resistance training. The participants attended 4 sessions, each separated by 3–5 days. The first session determined 1-RM in seated presses, while the second session tested standing presses. A 4-minute rest period was given between each 1-RM attempt (10,30). In the third session, the participants performed 5 repetitions at 80% of 1-RM on each of the shoulder press exercises in randomized order (12,21). The experimental (fourth) session was identical to the third but included EMG measurements. EMG activity was recorded from deltoid muscles (anterior, medial, and posterior), and arm muscles (biceps and triceps brachii).
Fifteen healthy men (age, 22 ± 2 years, body mass = 79 ± 14 kg, stature = 1.79 ± 0.06 m) participated in the study. The participants had 5.0 ± 2.6 years of strength-training experience but were not competitive power lifters or Olympic weightlifters. All participants were accustomed to the shoulder press exercises and performed them as part of their regular training program. All participants were informed of testing procedures, possible risks, and gave written consent to participate prior the testing. The participants' stated that they typically performed resistance training 2–4 times per week. The 1-RM results of seated barbell press represented ∼71% of the participants' body weight. Participants were excluded from the study if they had musculoskeletal pain, illness, injuries, or were not familiar with shoulder press exercises using free weights. Before the study, ethics approval was obtained from the local research ethics committee. The study conformed to the latest revision of the Declaration of Helsinki. The participants were instructed to refrain from any additional resistance training that targeted the upper body 72 hours before testing.
Participants were instructed to continue with their normal diet throughout the study and to prepare for the sessions as they would for their usual training sessions. The participants came to the laboratory at a time that was as similar as possible to their usual training time. All tests were executed in February.
The participants were instructed to use moderate and controlled lifting tempo in the third session, but the tempo of 1-RM testing (sessions 1 and 2) was self-selected. A digital watch provided feedback to the participants so that they should use approximately 2 seconds in the eccentric phase and 2 seconds in the concentric phase. The use of 80% 1-RM is typical for many training regimens and corresponds to a load between 8 and 12 RM (3,13). Five repetitions were used to avoid confounding effects of fatigue on the neuromuscular activation during the exercises (34). Before a new exercise, 5 repetitions of 60% of 1-RM were used as a warm-up to the exercise. The sets were separated by 2–4 minute rest periods in the third and fourth session.
Before a testing session, the participants performed a standardized warm-up protocol including 10 minutes on a cycle ergometer followed by 6–8 warm-up sets (60–85% of 1-RM, 6–12 repetitions) seated and standing using barbell and dumbbells. After the warm-up, the load was increased to what the participants perceived as their 1-RM. Depending on whether the lift was successful or not, the load was increased or decreased with 2.5 kg for barbells and 1.25 kg for each dumbbell. The participants achieved 1-RM for the exercises within 2–4 attempts. A 4-minute rest period was given between each 1-RM attempt (10,30). Participants were vigorously encouraged in the 1-RM testing but not during sessions 3 and 4.
Two investigators acted as spotters and assisted the participants to ensure safety during each lift and to stabilize the weights before and after the lifts. The eccentric phase started at fully extended elbows. On audio command, the participants lowered the weights so that the center of the barbell or dumbbells was at the level of the acromion. The dumbbells were held with the thumb-side toward the ears at all times. The barbell was lifted in front of the face. The elbows were maintained in the frontal plane at all time. Excessive postural sway was not allowed. Grip width of the barbell or dumbbells was self-selected. The concentric phase ended when the elbows were fully extended (11). Further details of the exercise technique are described by Graham (11). The barbell or dumbbells were pressed straight up and the lifting speed, horizontal position of the barbell, and dumbbells and the body position were controlled by the 2 investigators. A standard bench (75° upward angle) was used in the seated tests with the gluteus and upper torso in contact with the bench. The legs were placed shoulder wide with a 90° angle at the knees (11). In the standing exercises, a string placed in a square about 1 cm from the skin, and 3 cm above the iliac crest was used to prevent postural sway. The feet were placed shoulder width apart with extended knees.
The skin was prepared (shaved, washed with alcohol, and abraded) for placement of gel-coated surface EMG electrodes. Self-adhesive electrodes (Dri-Stick Silver circular sEMG Electrodes AE-131, NeuroDyne Medical, Cambridge, MA, USA) were placed in the presumed direction of the underlying muscle fibers with a center-to-center distance of 2.0 cm and 11 mm contact diameter. Each triode electrode configuration included an electrode pair measuring the electrical signal and also a reference electrode placed on the same muscle. The electrode configuration had a fixed and identical inter-electrode distance. The surface electrodes were positioned at biceps brachii, triceps brachii, anterior deltoid, medial deltoid, and posterior deltoid according to the recommendations by SENIAM (14). Electrodes were only placed on the side of the preferred arm (6).
The raw EMG signal was amplified and filtered using a preamplifier located as near the pickup point as possible to minimize noise induced from external sources. The raw EMG signals were converted to root mean square (RMS) signals using a hardware circuit network (frequency response 450 kHz, averaging constant 12 ms, total error ± 0.5%). The common mode rejection rate was 106 dB and the impedance between each electrode pair was <1012 Ω. The EMG signals were sampled at a rate of 1000 Hz. Signals were band-pass filtered with a cutoff frequency of 8 and 600 Hz, rectified and integrated. The RMS converted signals were resampled at a rate of 100 Hz using a 16-bit A/D converter with a common mode rejection rate of 106 dB. The stored data was analyzed using commercial software Musclelab V8.13 (Ergotest Technology AS, Langesund, Norway).
To identify the descending and ascending motion, a linear encoder (Ergotest Technology AS) was attached to the barbell and dumbbells. The linear encoder was synchronized with the EMG recordings using Musclelab 4020e (Ergotest Technology AS). Position and EMG data were overlaid and marked to precisely identify the beginning of the end of each repetition. The RMS EMG was calculated as the mean activity from the start of the second to the end of the fifth repetition (24).
To assess differences in 1-RM strength and neuromuscular activity of the seated, standing, barbell, and dumbbell shoulder presses, we used a 2-way analysis of variance (ANOVA). The independent variables were body position (seated and standing) and loading modality (barbell and dumbbells). The dependent variables were the EMG activity for each of the muscles and 1-RM strength. When a significant interaction was detected, paired t-tests with Bonferroni post hoc corrections were applied to determine the location of the differences. To compare the lifting time of the exercises, we used a separate one-way ANOVA. All results are presented as mean ± SD and Cohen d effect size (ES). Effect size of 0.2 was considered small, 0.5 as medium, and 0.8 as large. Statistical significance was accepted at p ≤ 0.05. SPSS (version 19.0; Inc., Chicago, IL, USA) was used for statistical analyses.
Deltoid Muscle Activation
Representative filtered deltoid EMG signals of the exercises are presented in Figure 1A–D. For anterior deltoid (Figure 2A), the position × exercise interaction was not significantly different (F = 1.611, p = 0.225). However, the main effects were significantly different for position (F = 5.053, p = 0.041) and exercise (F = 27.109, p < 0.001). After post hoc analysis, ∼11% lower EMG activity in seated barbell versus dumbbell shoulder presses was observed (1.016 ± 0.34 vs. 1.136 ± 0.36, p = 0.038, ES = 0.33). There was ∼15% lower EMG activity in standing barbell versus standing dumbbells (1.048 ± 0.34 vs. 1.237 ± 0.45, p < 0.001, ES = 0.47). Furthermore, there was similar EMG activity in seated barbell versus standing shoulder presses (1.016 ± 0.34 vs. 1.048 ± 0.34, p = 0.760, ES = 0.33). There was a trend for (∼8%) lower EMG activity in seated dumbbells versus standing dumbbells shoulder presses (1.136 ± 0.36 vs. 1.237 ± 0.45, p = 0.070, ES = 0.25).
For medial deltoid (Figure 2B), the position × exercise interaction was significantly different (F = 10.698, p = 0.006). After post hoc analyses, similar EMG activity in seated barbell versus dumbbells shoulder presses was observed (0.559 ± 0.19 vs. 0.549 ± 0.14, p = 0.653, ES = 0.06). There was ∼7% lower EMG activity in standing barbell versus dumbbell shoulder presses (0.601 ± 0.18 vs. 0.647 ± 0.22, p = 0.050, ES = 0.23). Furthermore, there was a trend for (∼7%) lower EMG activity in seated barbell versus standing barbell shoulder presses (0.559 ± 0.19 vs. 0.601 ± 0.18, p = 0.062, ES = 0.23). Approximately 15% lower EMG activity in seated dumbbell versus standing dumbbell shoulder presses was observed (0.549 ± 0.14 vs. 0.647 ± 0.22, p = 0.008, ES = 0.53).
For posterior deltoid (Figure 2C), the position × exercise interaction was not significantly different (F = 0.008, p = 0.930). There was a main effect for position (F = 31.521, p < 0.001) but not exercise (F = 2.080, p = 0.171). After post hoc analyses, ∼25% lower EMG activity in seated barbell versus standing barbell shoulder presses was observed (0.316 ± 0.16 vs. 0.421 ± 0.16, p < 0.001, ES = 0.66). Furthermore, there was ∼24% lower EMG activity in seated dumbbells versus standing dumbbells (0.331 ± 0.13 vs. 0.438 ± 0.20, p = 0.002, ES = 0.63).
Biceps and Triceps Activation
For biceps (Figure 3A), the position × exercise interaction was significantly different (F = 8.158, p = 0.013). After post hoc analyses, ∼33% greater EMG activity in seated barbell versus dumbbell was observed (0.604 ± 0.30 vs. 0.405 ± 0.20, p = 0.002, ES = 0.98). There was a trend for (∼16%) greater EMG activity in standing barbell versus dumbbell shoulder presses (0.623 ± 0.28 vs. 0.522 ± 0.245, p = 0.074, ES = 0.41). Furthermore, there was similar EMG activity in seated barbell versus standing barbell shoulder presses (0.604 ± 0.30 vs. 0.623 ± 0.28, p = 0.904, ES = 0.07). Approximately 23% lower EMG activity in seated dumbbell versus standing dumbbell shoulder presses was observed (0.405 ± 0.20 vs. 0.522 ± 0.245, p < 0.001, ES = 0.52).
For triceps (Figure 3B), the position × exercise interaction was significantly different (F = 6.416, p = 0.024). After post hoc analyses, similar EMG activity in seated barbell versus dumbbell shoulder presses was observed (0.250 ± 0.12 vs. 0.218 ± 0.13, p = 0.620, ES = 0.26). There was ∼39% greater EMG activity in standing barbell versus dumbbell shoulder presses (0.315 ± 0.15 vs. 0.192 ± 0.10, p < 0.001, ES = 1.18). Furthermore, a trend for (∼20%) lower EMG activity in seated barbell versus standing barbell was observed (0.250 ± 0.12 vs. 0.315 ± 0.15, p = 0.094, ES = 0.48). There was similar EMG activity in seated dumbbell versus standing dumbbell shoulder presses (0.218 ± 0.13 vs. 0.192 ± 0.10, p = 0.602, ES = 0.22).
The position × exercise interaction was significantly different for 1-RM strength (F = 14.235, p = 0.002, Figure 4). After post hoc analyses, similar 1-RM strength in seated barbell versus dumbbell shoulder presses was observed (56.3 ± 8.4 kg vs. 56.0 ± 7.6 kg, p = 0.751, ES = 0.04). There was ∼7% lower 1-RM strength in standing dumbbell versus barbell shoulder presses (50.7 ± 5.3 kg vs. 54.7 ± 6.4 kg, p = 0.002, ES = 0.68). Furthermore, there was similar 1-RM strength in seated barbell versus standing barbell shoulder presses (56.0 ± 8.4 kg vs. 54.7 ± 6.4 kg, p = 0.272, ES = 0.22). Approximately 10% lower 1-RM strength in standing dumbbell versus seated dumbbell shoulder presses was observed (50.7 ± 5.3 kg vs. 56.0 ± 7.61 kg, p < 0.001, ES = 0.81).
There were no differences between shoulder press exercises in total lifting time of the 4 repetitions used for analysis: seated barbell 11.2 ± 2.3 seconds, seated dumbbells 11.8 ± 2.5 seconds, standing barbell 12.1 ± 2.6 seconds, and standing dumbbells 12.3 ± 3.5 seconds, (F = 0.438, p = 0.727–1.000).
In the present study, we examined effects of body position (seated and standing) and loading modality (barbell and dumbbells) on 1-RM strength and neuromuscular activity in shoulder presses, for the first time. The main finding in this study is that the standing dumbbell press exercise, which was the exercise with the greatest stability requirement (standing + dumbbells), demonstrated the highest neuromuscular activity of the deltoid muscles, although this was the exercise with the lowest 1-RM strength. Furthermore, standing versus seated execution, and to some extent dumbbells versus barbell, both resulted in increased muscle activation of the deltoid muscles. Standing instead of seated presses raises the center of the mass and also provides a smaller base of support as the contact points decreases from 3 to 2, particularly when using a bench with a back-rest. When using a pair of dumbbells instead of a barbell, the main difference is that the dumbbells must be controlled independently of each other. Hence, performing shoulder presses standing and with dumbbells should lead to greater instability.
Contradictory EMG results have been reported from studies examining exercises with various stability requirements (16,23,25–27,33). However, several of the studies observing higher EMG activity in unstable resistance training used the same absolute and not relative resistance (1,6,18,23,31). In those studies, it is not possible to differentiate the contributions of higher relative loads and higher stability requirements on neuromuscular activity because using the same absolute load usually means that a higher relative load is used in the unstable condition. As we matched intensity of exercises, this was not a confounding issue in the present study. Furthermore, previous studies that did match relative resistance have observed that the EMG activation of the prime movers in unstable exercises have either been lower or similar compared with more stable alternatives (2,4,16,25–27,34). Hence, to our knowledge, this is the first study to report higher muscle activity of prime movers of common resistance training exercises with increasing instability.
Another important aspect of this investigation is that we compared realistic alternatives of the same exercise. That is, shoulder presses standing or seated, with a barbell and dumbbells. In many studies, exercises with large differences in stability requirement have been compared (i.e., very unstable vs. very stable) (2,4,18,19). However, comparisons of exercises with major differences in stability requirement cannot be generalized to exercises with more subtle differences in instability requirements. Furthermore, very unstable exercises are generally not recommended for athletes, as the force output may decrease to a suboptimal level for strength or power gains (2,4,19,20).
In the seated position, the legs were placed shoulder wide with a bench supporting the back, creating a solid base of support against movement in the sagittal plane. In the standing position, the increased degrees of freedom of the torso may have provided the participants the opportunity to coordinate the weight lifting in a way that provided higher muscle activation, or the increased stability demands on the deltoids may have increased motor drive. Another possibility is that standing instead of seated position resulted in more remote voluntary activation of leg and trunk muscles, which is known to cause concurrent activation potentiation (7). In addition, one could expect that a more stable base of support in the seated position could create a more favorable position for lifting heavier weights that would also activate the anterior and medial deltoid muscles to a higher extent. However, the results demonstrated that going from seated to standing position negatively affected absolute strength performance only for dumbbell presses.
Greater EMG activity was generally observed for the deltoid prime mover (anterior and medial part) muscles in dumbbell exercises compared with barbell exercises (except for medial deltoid in the seated position). This is not in line with previous studies comparing EMG activity using barbells and dumbbells (16,27,34). Although both barbell and dumbbell presses are free weight exercises, dumbbells are more unstable than barbells. Conversely, dumbbells instead of barbell lifting did not seem to affect posterior deltoid activity. This is probably caused by the stability inducing differences between dumbbells and barbell in overhead presses; both exercises are unstable in the sagittal plane, but only dumbbells are unstable in the coronal (frontal) plane. The posterior deltoid was probably more important as a stabilizer against perturbations in the sagittal plane, explaining why there were no differences between dumbbell and barbell presses for this muscle.
There was elevated muscle activity for posterior deltoids in standing versus seated position. The central nervous system continuously monitors the position of all body segments and must continuously feed the posterior deltoids with neural drive for appropriate coactivation during shoulder presses. During standing presses, postural movements inevitably occur in the sagittal plane, which is not the case for seated presses with a backrest.
The EMG results for biceps and triceps partly supported the hypotheses. Greater EMG activity in standing versus seated dumbbell presses in biceps was observed but similar when using barbell. These results are in line with Saeterbakken et al. (27) and Lehman et al. (17), as these authors reported greater antagonist coactivation with greater instability requirements. For triceps, a strong trend was observed for greater EMG activity in standing versus seated barbell presses but similar using dumbbells. Higher triceps activation during standing barbell presses compared with dumbbells is in line with chest press results reported by Saeterbakken et al. (27). Extending the arms using the triceps alone in dumbbell presses would only result in moving the dumbbell load further away from the shoulders in a horizontal position. However, in barbell press, both arms are locked to the barbell making it possible to achieve greater triceps activation without moving the load further sideways from the shoulders (32).
In the stable seated condition, dumbbells as an instability-inducing factor did not negatively influence 1-RM strength. In contrast, Welsch et al (34) and Saeterbakken et al. (27) reported dumbbell loads of 63% (34) and 83% (27) of the barbell loads in chest presses. Furthermore, we observed similar 1-RM strength when comparing seated versus standing barbell presses. In the present study, when the 2 instability inducing factors were included (dumbbells and standing), a 7% reduction in strength was observed compared with standing barbell and a 10% reduction compared to seated dumbbells were observed.
The present study was limited by not testing EMG in the 1-RM testing and not performing the exercises (80% of 1-RM) to exhaustion. Fatigue has been shown to increase the EMG activity (15). Eighty percentage of 1-RM was selected because it is a typical load of many resistance-training programs (∼8–12 repetitions). Performing the exercises to fatigue would have impaired the performance of the subsequent exercise, which would have influenced the results. Alternatively, more sessions could have been used, but as it is difficult or impossible to place EMG electrodes on the exact same location, we believe that the present approach is a good compromise. Furthermore, there are inherent technical limitations with the surface EMG, and the electrodes can only provide an estimate of neuromuscular activity (8). The risks of cross talk from neighboring muscles are present even if a small interelectrode distance was used.
Resistance trained young males were recruited for this study, and these results cannot necessarily be generalized to other populations. Further research should investigate the use of these exercises for sedentary individuals or patients and investigate the neuromuscular pattern of exercises with both greater and lower stability requirements and intensity levels.
In conclusion, for the deltoids, the use of standing instead of seated body position increased the neuromuscular activity, as did dumbbells instead of barbell as the loading modality, although not consistently for the latter. Combining the 2 instability approaches (standing and dumbbells) demonstrated the highest neuromuscular activity of the deltoid muscles, but this was also the exercise with the largest 1-RM strength reduction (7–10%) compared with the other exercises.
We compared 4 shoulder press exercises: seated military press, standing military press, seated dumbbell press, and standing dumbbell press. This study demonstrated that combining 2 common instability-inducing strategies in shoulder presses (standing + dumbbells) increased deltoid muscle activation. This was despite the fact that the resistance-trained volunteers lifted the least weight in this exercise. Our findings suggest that standing dumbbell presses may be more beneficial for muscular development of the deltoid muscles than more stable alternatives, however, if mechanical power output is a higher priority, more stable alternatives may be preferable. Furthermore, both standing instead of seated body position and dumbbells instead of barbell as the loading modality increased the stability requirement compared with seated and barbell execution, and we suggest to include these factors in periodized resistance training programs, which may reduce the risk of injuries and provide variation for athletes and others engaging in resistance training. Furthermore, as many sports require standing position and independent arm movements (pitching, smashing, and serving), we suggest, according the principle of training specificity, that these instability-inducing variables may be beneficial. Finally, in shoulder rehabilitation, standing shoulder presses may be a viable choice as higher muscle activation can be attained with lower external load.
The authors thank Espen Krohn-Hansen and Mats Smaamo for assistance in participant recruitment and data collection and the participants for their enthusiastic participation. This study was conducted without any funding from companies or manufacturers or outside organizations.
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