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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Keywords:Copyright © 2013 by the National Strength & Conditioning Association.
EMG; resistance exercise; instability; free weights; 1-RM; shoulder press