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Original Research

An Evaluation of Upper-Body Muscle Activation During Coupled and Uncoupled Instability Resistance Training

Campbell, Brian M.; Kutz, Matt R.; Morgan, Amy L.; Fullenkamp, Adam M.; Ballenger, Ryan

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Journal of Strength and Conditioning Research: July 2014 - Volume 28 - Issue 7 - p 1833-1838
doi: 10.1519/JSC.0000000000000350
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Resistance training has been used to induce adaptations in strength, endurance, and power with a variety of equipment and training methods, including the use of stable and unstable loads (23). A more recent development in resistance training is the replacement of a stable surface with an unstable surface. To this end, the Swiss ball is an unstable training device designed to increase the difficulty of body weight and free-weight resistance training exercises (14,16). The Swiss ball is a popular device used to supplement conventional resistance training surfaces for upper-body exercises such as the chest press and has been used for years in sport training and therapeutic rehabilitation (4). The objective in integrating the Swiss ball into a training regimen is to provide variety and an increased demand on the neuromuscular system (1,2,8,14). Furthermore, using the Swiss ball to induce balance improvements with resistance training may be another valuable tool for improving performance (10).

Numerous research efforts have examined resistance training protocols involving instability devices such as balls, discs, or boards. One such study compared the muscle activation differences in 2 standard upper-body exercises (chest press and shoulder press) on a Swiss ball and on a stable bench surface (21). Although it was anticipated that the unstable surface condition would promote increased muscle activation, no differences in electromyography (EMG) amplitude were observed presumably because of the heavy load (80% of 1 repetition maximum [RM]). Consequently, the authors suggested that the mass of the participants and the associated heavy relative loads caused the Swiss ball to deform, resulting in a more stabilized surface. In a recent review, Behm et al. (5) proposed that performing the chest press exercise while supported by a Swiss ball can be an effective component of a resistance program even when the load used is <70% 1RM. Supporting this hypothesis, Marshall and Murphy (15) found an increase in triceps and deltoid muscle activity during a chest press at 60% 1RM using a Swiss ball when compared with a conventional stable bench surface. There have also been training studies where the long-term benefits of surface instability have been observed (8,11). As an example, Cowley et al. (8) investigated the benefits of instability training in a group of young women over a 3-week period. Post-training assessment demonstrated that there were no group differences between those who used a stable surface for resistance training and those who trained with an unstable surface. Because surface instability is believed to foster increased core stabilizer activation, unstable surface training may serve the dual purpose of promoting hypertrophy and coordination in support musculature and prime movers (6).

It is important to note that findings in the literature do not unanimously support the use of unstable surfaces for resistance training. Anderson and Behm (2) examined muscle activation associated with a stable and unstable surface chest press at 75% 1RM and found no significant differences in overall muscle activity between the 2 exercise protocols. Furthermore, the authors observed a significant decrease in force output associated with unstable surface use. Koshida et al. (13) also observed a reduction in peak force (6%) output while performing a bench press on an unstable surface, but it was unclear whether this reduction would be sufficient to negatively impact potential strength gains. Most recently, Colado et al. (7) examined muscle activation in the paraspinal muscles during both callisthenic and resistance exercises with varying surface stability. In this case, it was observed that the deadlift performed on the stable surface resulted in the greatest amount of muscular activation.

Although the benefits of unstable surface training have been addressed extensively in the literature, there are few studies that have examined the combined effect of both surface instability and load instability (i.e., coupled/barbell [BB] vs. uncoupled/dumbbell [DB]) resistance training on prime mover and stabilizer muscle activation. Behm et al. (6) investigated the interaction between surface and load stability for a group of participants during both shoulder and chest press exercises. In particular, the unstable load was created by having participants perform the exercises unilaterally vs. bilaterally. It was observed that instability elicited greater muscle activation during the chest press. However, it is also important to note that this study only measured EMG from the trunk musculature and not from the prime movers (e.g., pectoralis major [PM]) during the chest press exercise. Kohler et al. (12) studied the combined effects of both surface and load stability on agonist and stabilizer muscle activations. Here, the authors created an unstable surface with a Swiss ball, and they examined load stability by having participants use either a BB or DBs. As with previous studies, it was observed that increasing instability during a maximal effort resulted in reduced force output, and there were actual increases in EMG amplitude during the stable conditions. It is possible that the use of higher loads in this study (10RM) served to compress the Swiss ball resulting in a more stable surface than was intended. This outcome highlights a key gap in the literature. To date, there have been no studies that have examined the benefits of both surface and load instability training with moderate (50% 1RM) and low (25% 1RM) relative resistance loads. It is possible that the use of a lower submaximal load will limit unstable surface compression and produce the muscular activation increases anticipated for instability training. Therefore, the purpose of this study was to assess the impact of both relative load and resistance coupling on the level of muscle activation during 3 unstable chest press conditions. The hypotheses of this study were that (a) mean muscle activation levels would be greater when performing an uncoupled (DB) chest press compared with a coupled (BB) chest press and (b) muscle activation when performing the DB chest press at 50% 1RM would be comparable to the DB chest press at 25% 1RM because the 25% 1RM condition will result in reduced Swiss ball compression yielding a more unstable surface and eliciting comparable muscle activity between conditions.


Experimental Approach to the Problem

The unstable surface instrument used for this effort was an exercise Swiss ball. A 1RM on a traditional stable flat bench was established using the National Strength and Conditioning Association (NSCA) protocol (3) to determine the intensity for each participant's 3 trials on the Swiss ball. The 1RM was then used as the normalized baseline to determine the appropriate load for the chest press exercises.

Under randomized conditions, participants performed chest press exercises on the Swiss ball using a weighted BB (coupled) at 50% of the 1RM (50% BB), DBs (uncoupled) at 50% 1RM (50% DB), and DBs (uncoupled) at 25% 1RM (25% DB). Mean EMG activity of the PM, triceps brachii (TB), anterior deltoid (AD), and rectus abdominis (RA) was recorded and compared to determine differences elicited between chest press conditions.


Ten healthy, recreationally active male subjects volunteered to participate in this study (age, 23.9 ± 2.6 years; body weight, 82.8 ± 10.2 kg). Each participant was required to have been engaged in regular chest press exercises on a stable surface for at least 1 year. Additionally, all participants were required to be free of any upper or lower extremity injuries, and none of the participants recruited for this study had any experience performing the resistance chest press using a Swiss ball. Finally, participants were instructed to refrain from any upper-body resistance training until completion of the testing sessions. Before participation, participants were asked to read and sign an informed consent document. All procedures and documents were approved by the local University Human Subjects Review Board.


An 8-channel telemetered EMG system (Noraxon USA, Inc., Scottsdale, AZ, USA) was used to collect and analyze muscle activity during the 3 chest press conditions. Also, an electronic parallel beeper (Bigger, Faster, Stronger, South Salt Lake City, UT, USA) was strapped around each participant's nondominant arm during testing to provide an auditory stimulus signaling that 90° of elbow flexion had been achieved and to begin the concentric (i.e., upward) phase of the movement. Chest press trials were conducted using a 65-cm Gold's Gym (ICON Health & Fitness, Inc., Logan, UT, USA) Swiss ball as the unstable surface and participants were instructed to lie supine with the head, shoulders, and thoracic spine supported by the surface. Each participant's trunk was parallel to the floor, knees flexed at 90° and both feet placed flat on the floor, shoulder width apart. At the beginning of each unstable chest press trial, participants started with the load in full elbow extension until given the cue to start repetitions. Before each subject began the Swiss ball chest press trials, the ball was inflated or deflated to match the height of a traditional stable bench to provide consistent conditions for each subject.


Testing took place over 2 visits, each lasting less than an hour. The first of the 2 test sessions consisted of determining each participant's 1RM on a traditional stable flat bench. The 1RM testing followed the NSCA protocol (3) requiring participants to progressively increase resistance across attempts until the 1RM was successfully achieved. Each participant's 1RM was used to determine the appropriate lifting intensities for the 25 and 50% 1RM conditions collected during the second visit.

During the second visit, 3–7 days after the first session, participants performed the 3 predetermined chest press exercises on the Swiss ball. On arriving for the second session, body weight was collected from each participant. Next, the participant's EMG electrode sites were shaved, cleaned, and allowed to dry before electrode application. Bipolar Noraxon (Noraxon USA, Inc.) 2.0 cm disposable, self-adhesive dual electrodes were used to measure muscle signals from the PM, TB, AD, and RA muscles. Electrodes were placed on the dominant side of each subject's body; dominant side was determined by the subject's writing hand and followed guidelines set by Perotto (17). In each case, the EMG ground electrode was placed on the ulnar styloid process of the dominant arm. Before the chest press trials, a dynamic 5-minute warm-up consisting of jumping jacks, forward arm circles, backward arm circles, arm flies across mid-line, and push-ups was performed. Participants were given a 2-minute rest period between the warm-up and between each of the 3 chest press trials. The order of chest press conditions was counterbalanced per subject by rotating each trial according to the order completed by the previous subject to control for biasing of order. A completed repetition was determined by full elbow flexion to 90° during the eccentric phase of the movement (signaled by the parallel electronic beeper), and by full elbow extension at the end of the concentric phase of the movement. A failed repetition was determined by the inability to accomplish 90° of elbow flexion or full elbow extension during a trial. Participants performed 5 repetitions of each exercise condition on the Swiss ball: 50% BB, 50% DB, and 25% DB.

Statistical Analyses

Mean EMG data were gathered using the MyoResearch XP (Noraxon USA, Inc., Scottsdale, AZ, USA) software during each of the 3 chest press variations. The raw EMG signals were rectified and then smoothed using a 50 ms RMS averaging technique. The first and fifth repetitions of each chest press trial were excluded to eliminate variations in muscle activity due to a participant starting a set with an improper repetition, or preparation to rack the weight before the repetition was fully completed. Next, the mean EMG amplitude values for the 3 middle repetitions within a trial were averaged. The mean EMG amplitude results for the 3 conditions were then compared as a percent change to the 50% DB condition. For example, the percent change in mean amplitude EMG from the 50% BB chest press to the 50% DB chest press trial.


Figures 1–4 illustrate the group mean EMG value for each of the muscles tested across the 3 chest press conditions. Each figure also depicts the percent difference in muscle activation between 50% DB and the other 2 conditions. Specifically, the uncoupling of the hands at 50% 1RM (50% DB) resulted in 15% increases in EMG activation compared with the coupled 50% BB condition for both the RA and PM muscles. However, muscle activation for the AD and TB muscles was relatively unchanged by the uncoupling of the hands at 50% 1RM (−3 and −2%, respectively). The effect of increasing the resistance load for the uncoupled conditions from 25% 1RM to 50% 1RM was substantially increased activation for all muscles tested (RA +43%, PM +54%, AD +54%, and TB +40%).

Figure 1:
Mean rectus abdominis activation during the 3 chest press conditions.
Figure 2:
Mean pectoralis major activation during the 3 chest press conditions.
Figure 3:
Mean anterior deltoid activation during the 3 chest press conditions.
Figure 4:
Mean triceps brachii activation during the 3 chest press conditions.


Existing evidence indicates that higher percent loads combined with subject body weight influences compression of an unstable surface such as a Swiss ball (1,8,9,21). Compression of a Swiss ball might increase the surface area, creating a more stable surface than was initially intended. For this reason, a 25% 1RM load condition was evaluated in this study under the assumption that using a lighter relative load may, in fact, decrease the stability of the Swiss ball, thereby influencing the amount of muscle activity necessary to stabilize the chest press movement. The results of this study do not support the hypothesis that performing the 50% DB chest press would require a similar amount of muscle activity when compared with the 25% DB chest press condition. One possible reason that the 50% DB and 25% DB muscle activation levels were not comparable is that the experienced lifters recruited for this study produced 1RM values in the 300 lb range such that the 25% DB load was substantial enough along with the lifters body weight to compress the ball, creating a more stable surface. As a result, all muscle groups demonstrated greater mean activation during the 50% DB condition compared with the 25% DB condition. Particularly, the second greatest percent difference in muscle activation between the 50% DB condition and the other 2 chest press conditions occurred for RA. These results highlight the increased demand on the core musculature to provide stability under increased load conditions and during uncoupled movements.

Previous studies have examined the influence of instability training in resistance-trained participants (12,14,15,18). The 1RM's performed by the experienced lifters in this study approached 300 lbs which resulted in 150 lb lifts for the 50% 1RM trials. This heavy load may have increased the stability of the Swiss ball, perhaps limiting the unstable surface sought in this investigation. Another possible explanation for the clear activation distinction between the 25% DB and 50% DB conditions could pertain to improved muscle coordination as a function of resistance training experience. Wahl and Behm (22) examined both lower extremity and core muscular activation for a variety of exercises during both stable and unstable conditions. While stability condition differences were observed, the authors suggested that highly trained individuals may require enhanced instability conditions compared with sedentary or untrained individuals because of an improved stability resulting from regular free-weight use. There is some evidence to support the notion that trained individuals may present with improved muscular stability from previous free-weight training. Kohler et al. (12) investigated the impact of platform stability on muscle activation during an overhead press for a group of resistance-trained participants. It was observed that the greatest amount of EMG activation was elicited by a stable condition vs. an unstable condition. This finding lead the authors to suggest that the reduced 10RM load and associated decreases in muscle activation observed during the unstable condition may indicate a limited hypertrophy benefit for resistance-trained individuals.

There are, however, reports that demonstrate the benefits of instability training for resistance-trained individuals (15,18). Marshall and Murphy (15) examined the muscle activation effects of a bench press exercise in resistance-trained individuals both on and off a Swiss ball. Consequently, they observed that the use of an unstable platform increased muscle activation for both the deltoids and abdominal muscles. Saeterbakken and Fimland (18) also examined the impact of surface stability on muscle activation with a group of resistance-trained individuals. Here again, the authors observed significant increases in RA activation during the unstable condition, although it was ultimately recommended that using a balance cushion may be preferable to using a Swiss ball.

The observation that core muscular activation increases while performing an upper extremity exercise (e.g., chest press) addresses another key selling point for practitioners who advocate for the use of instability training. It has been argued that, where instability training is sufficient to promote hypertrophy in agonist muscle groups, there may be the added benefit of core stability training. Specifically, Behm et al. (6) observed a significant increase (37.7–54.3%) in the activation of trunk stabilizing musculature during instability training involving shoulder and chest press exercises. Norwood et al. (16) also examined the benefits of upper extremity instability training during a bench press exercise and found that the activation of core stabilizing musculature was increased. These findings are supported by the observation of a 15% increase in RA activation in the uncoupled (50% DB) compared with the coupled (50% BB) trials in this study.

A key distinction in this study was that both the surface and the load were unstable during the 50% DB condition. It is possible that the combination of the 2 unstable configurations further challenged the core musculature during the chest press exercise. However, there are currently few studies that have specifically examined the core muscle activation benefits associated with an uncoupled load in addition to an unstable surface. Welsch et al. (23) studied the impact of using coupled vs. uncoupled loads on muscle activation. However, this study was not conducted using an unstable surface, and the muscles examined were prime movers rather than core stabilizers. Kohler et al. (12) provided the most comprehensive assessment of combined instability by examining the impact of both surface and load stability on core muscle activation during an overhead press. Although not identified for statistical comparison in the study, there was an apparent increase in RA root mean square EMG from the stable load/unstable surface condition to the unstable load/unstable surface condition (21 ± 35 vs. 148 ± 18, respectively).

It is possible that populations not accustomed to resistance training may stand to benefit the most from instability training. Cowley et al. (8) examined the benefits of instability resistance training in a group of young untrained women. They observed that, while workload capacity was initially lower on the unstable surface, there were no differences in strength gains between groups who trained on stable surfaces and those who trained on unstable surfaces. Sparkes and Behm (20) also observed the benefits of an instability resistance training program in a population of participants without recent resistance training experience. The authors observed that participants were able to achieve the same strength and performance benefits regardless of the surface stability condition. Finally, Sekendiz et al. (19) studied the benefits of Swiss ball core strength training in a group of sedentary women. While this study did not specifically involve free-weight resistance training, the authors observed significant increases in core strength following instability training. These findings have begun to highlight the potential benefits of instability training for sedentary or untrained populations as well as those engaged in active rehabilitation (4).

In summary, there is theoretically increased trunk instability while performing an uncoupled (DB) chest press on a Swiss ball compared with a coupled (BB) chest press because the load itself is more unstable. Previous research has shown that abdominal muscle activity increases using the Swiss ball for abdominal exercises, shoulder press, and chest press exercises compared with stable surfaces. In this study, RA activity increased during the 50% DB chest press compared with the 50% BB chest press on the Swiss ball. In this case, the subjects experienced 2 levels of instability (i.e., the Swiss ball and the DBs). It is possible that the instability caused by the uncoupled load increases the demand for the RA to stabilize the spine and maintain the proper spine neutral posture during the chest press. Future research efforts should focus on the singular and combined effects of unstable platform and unstable load training in populations without substantial resistance training experience.

Practical Applications

Though it is likely that the relative loads established in this experiment were of a sufficient magnitude to compress, and increase the stability of the Swiss ball, there was an obvious impact of using DBs to increase the demands on PA as a prime mover and on RA for core stabilization. It appears that the combined effect of an unstable surface and uncoupled loading during submaximal lifting creates a demand for core stabilization beyond that of uncoupling alone. To promote core muscle stabilization benefits, while maintaining prime mover strength development, it is recommended that resistance-trained individuals mix in instability training rather than replacing stable surface and load training entirely. It is possible, however, that the greatest benefits of unstable surface and load resistance training may be achieved by untrained individuals and those engaged in rehabilitation. Persons who fall into these categories stand to benefit from the added neuromuscular challenge and core stability requirements imposed by instability training.


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chest press; EMG; unstable surface; combined

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