In addition to the acute training variables associated with resistance training, surface stability has emerged as a potential consideration for the enhancement of human performance (1,18,19). Training on unstable surfaces, often referred to as “core stability training,” utilizes Swiss balls, balance discs, and wobble boards (12) with the intent of stimulating neuromuscular and tissue adaptations beneficial to spine stabilization (10). Specifically, advocates suggest core stability training has been implemented to improve the contraction and recruitment pattern of the muscles of the lumbo-pelvic-hip complex to stabilize the spine during dynamic and static movements (8,9,13,17). However, a consensus regarding the efficacy of training with unstable surfaces has not been reached and the importance of spine stabilization during dynamic and static movements highlights the need for additional research. Further, the effects of surface stability on musculature outside of the lumbo-pelvic-hip complex, such as the prime movers of the upper body, are not completely understood.
Prior studies on the effect of unstable surfaces on muscle activation have yielded conflicting results (2-6,9,11-16). Anderson and Behm (2) discovered that force exerted during a chest press maximal voluntary contraction was significantly greater on the stable surface than the unstable surface (Thera-Band exercise ball; Thera-Band, Akron, OH, USA) despite similar muscle activation. Additionally, force output during dynamic chest press at 75% 1RM decreased nearly 60% on the unstable surface compared to the stable surface; mean electromyography (EMG) in the concentric action significantly exceeded eccentric EMG for the pectoralis major and deltoid muscles. However, Marshall and Murphy (15) found that performing the chest press at 60% 1RM on an unstable surface (Swiss ball) significantly increased mean muscle activation of the rectus abdominus, transverse abdominus/internal oblique, and the anterior deltoid more than when performing the chest press on the stable surface. Additionally, concentric action of the anterior deltoid, triceps brachii, and pectoralis major exhibited significantly greater mean muscle activation than corresponding eccentric actions. Marshall and Murphy (15) concluded unstable surfaces increase muscle activation of the shoulder and abdominals more than a stable surface when performing the chest press. Muscle activation differences between studies may have resulted in part from increased compression, and thus stabilization, of the Swiss ball with heavier loads.
Although previous research analyzed performance using resistance as high as 75% 1RM, no one to our knowledge has conducted similar research using a load of 80% 1RM, which is more typical of the resistance used to develop maximal strength. Therefore, the purpose of this study was to compare muscle activation of the pectoralis major, anterior deltoid, and rectus abdominus when performing the dumbbell chest press and dumbbell shoulder press on stable and unstable surfaces using a load of 80% 1RM. We hypothesized that activation of the rectus abdominus, anterior deltoid, and pectoralis major would not be significantly different between surfaces during both the concentric and eccentric phases of the chest press and shoulder press, in part because of the increased stabilization of the Swiss ball with the heavier load.
Experimental Approach to the Problem
To measure the effects of stable and unstable surface types on muscle activation, 16 men performed the dumbbell chest press and shoulder press on stable (bench) and unstable (Swiss ball) surfaces. All subjects had at least 6 months of resistance training experience performing the dumbbell chest press and shoulder press exercises on stable surfaces. Muscle activation was measured using surface EMG. Whereas other studies have examined the effects of different surface types on muscle activation (2-6,11-16), to our knowledge none have used the heavier loads that are typical of heavy resistance training programs. Therefore, identical loads of 80% 1RM were used in the present study for both surface types. Force production was not directly measured because the purported benefit of using unstable surfaces is to increase muscle activation in a stabilizing role, rather than to increase vertical force production.
Sixteen healthy, recreationally trained men (mean ± SD: age 24.19 ± 2.17 years; height 178.00 ± 6.39 cm; mass 82.03 ± 11.03 kg) participated in this study. Only men participated because of confounding issues in locating the pectoral muscle beneath adipose and breast tissue in women. To be eligible to participate in this study, subjects had to have at least 6 months of resistance training experience and be free of any limitation that would prevent performing the testing protocols. Subjects were asked to refrain from upper body resistance training during the duration of the study. All participants signed an informed consent prior to participation in the study, which was approved by the university Institutional Review Board.
Each subject visited the laboratory twice. During the first visit, subjects performed dynamic 1 repetition maximum (1RM) tests for both the dumbbell chest press and shoulder press exercises on a stable surface (Hammer Strength Flat Bench, Schiller Park, IL, USA) to determine maximal strength. The 1RM tests and exercise techniques were conducted using the National Strength and Conditioning Association (NSCA) protocol for 1RM testing (7). This protocol required participants to progressively increase resistance across attempts until the 1RM was achieved.
A minimum of 48 hours after the first visit, subjects performed the chest press and shoulder press on stable and unstable surfaces (Figs. 1-4). Surface and exercise order were randomized for each subject. Subjects began each exercise at a preload position with the dumbbells positioned at the shoulder (shoulder press) or chest (chest press). Beginning exercises at the preload position eliminated influence from the stretch shortening cycle. Subjects lifted the dumbbells to full extension of the elbows. Subjects then held this position at full elbow extension for approximately 1 second until instructed to eccentrically lower the dumbbells in a controlled manner to their original preload position. Subjects performed a total of 3 consecutive repetitions; each repetition was performed at a self-selected cadence with cues dictated by the researchers. Each repetition was performed using a load of 80% 1RM as determined on the stable surface for each exercise. A total of 3 repetitions, fewer than subjects could perform with this load, was chosen to avoid the confounding effects of fatigue on muscle activation.
For the chest press exercise on an unstable surface, subjects were positioned so that the scapulae and upper back were supported on a Swiss ball. The head and neck were unsupported from the ball. The Swiss ball was adjusted so that the subject's torso was parallel to the floor, the knees were flexed at 90 degrees, and both feet were flat on the floor. On the stable surface, the bench was adjusted so subjects were similarly positioned. When performing the dumbbell shoulder press on an unstable surface, participants were positioned on the Swiss ball so that the torso was perpendicular to the floor and the knees were flexed at 90 degrees. For the stable surface, the bench was adjusted so that the shoulder press was performed under the same constraints. During each visit, surface EMG electrodes were placed over the anterior deltoid, pectoralis major, and rectus abdominus to measure muscle activation.
Electromyography and Signal Processing
Three separate bipolar (3.5 cm center-to-center) surface electrode (BIOPAC EL500 silver-silver chloride) arrangements were placed over the longitudinal axes of the deltoid, pectoralis major, and rectus abdominus muscles on the right side of the body (Fig. 5). The reference electrodes were placed over the iliac crest (Fig. 5). Interelectrode impedance was minimized by shaving the area and carefully abrading the skin. The EMG signal was preamplified (gain 1,000 ×) using a differential amplifier (EMG 100C, BIOPAC Systems Inc., Santa Barbara, CA, USA; bandwidth = 1-500 Hz). EMG data were stored on a personal computer (Toshiba A105-S4074, Toshiba Inc., Los Angeles, CA, USA).
The sampling frequency was 1,000 Hz. The EMG signals were bandpass filtered (fourth-order Butterworth filter) at 10-500 Hz. The amplitude of the signals was expressed as root mean square (rms) values. All analyses were performed with custom programs written with LabVIEW software (version 7.1, National Instruments, Austin, TX, USA). The intraclass reliability coefficient (ICC) for EMG amplitude measurements was R = 0.93.
Prior to the statistical analysis, each subject's EMG amplitude data were normalized to their highest recorded value during 1RM testing. The data were then analyzed with a 4-way (exercise [chest press, shoulder press] × contraction type [concentric, eccentric] × surface type [stable, unstable]) × muscle [anterior deltoid, pectoralis major, rectus abdominus] analysis of variance (ANOVA). In the event of a significant F-ratio, pairwise comparisons were evaluated with a Bonferroni post hoc. An alpha level of 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.
The results of the 4-way ANOVA revealed no significant interaction or main effect for any exercise, muscle, muscle action type, or surface type, with the exception of a significant muscle × exercise interaction. Tables 1 and 2 show the comparisons of normalized EMG amplitude values (mean ± standard deviation) values from concentric and eccentric muscle actions within the same exercise and between surface types.
The results of this investigation support our hypothesis that for chest press and shoulder press, the rectus abdominus, anterior deltoid, and pectoralis major exhibit similar muscle activations on stable and unstable surfaces. Consequently, this study suggests neither an advantage nor disadvantage for using an unstable surface with the chest press and shoulder press exercise at 80% 1RM.
Similar studies have previously examined the effects of performing a chest press and a dynamic chest press on a stable and unstable surface with loads up to 75% 1RM (2,15). Researchers who found a significant increase in anterior deltoid muscle activation during a dynamic chest press on a Swiss ball attributed this response to a greater level of stabilization required on the unstable surface compared to the stable surface (15). Subjects in this study were positioned on the Swiss ball such that the thoracic and lumbar spine were supported, while vertebrae C7 and above were unsupported, thereby limiting contact between shoulder and ball. The present study, however, found no significant differences in mean muscle activation of the deltoid between surface types, potentially because subjects were positioned on the Swiss ball such that the ball adequately supported the deltoid despite the ball's inherent instability.
The location of the subject on the unstable surface and the body parts in contact with the unstable surface might influence muscle activation (11,12). Lehman et al. (12) analyzed the effects of performing push-up variations on muscle activation, destabilizing the hands and/or feet with a Swiss ball. These variations included either one's hands or feet placed on an unstable surface (Swiss ball) or a stable surface (bench). They found significant differences in EMG amplitude between push-up variations, suggesting that muscle activation varies as a function of body position. Variations in contact between subject and surface might explain why the present study found no difference in muscle activation within the same muscle between surface types, while Marshall and Murphy (15) did find significant differences for the rectus abdominus and the anterior deltoid.
Deformity induced by the combined mass of the subject and the load also merits consideration as a contributor to the lack of significant differences. Anderson and Behm (2) suggested that greater loads (>75% 1RM) may deform roundness of a surface such as a Swiss ball to reflect a more horizontal platform. Consequently, the mass of a subject and the resistance on a Swiss ball might increase the surface area of the unstable platform and its direct contact with the surface beneath, creating a more stable surface. No attempts were made in this study to control for the deformity of the Swiss ball because doing so would limit external validity. However, visual observation of the subjects lifting on the Swiss ball with the 80% 1RM loads indicated that the surface became stable to a degree comparable to the bench surface.
Research also suggests that subject training status may influence the degree of muscle activation (15). Muscle activation may differ among individuals based on their experience and understanding of the philosophy behind core stability training. Individuals with greater experience and understanding of the philosophy behind core stability training might consciously increase muscle activation. Although subjects in this study were not homogenous in resistance training experience, most (n = 14) categorized themselves as possessing little experience performing resistance exercise on unstable surfaces. Thus, there were insufficient numbers of subjects with experience to make between groups (experienced vs. inexperienced) comparisons.
Coaches and practitioners often combine traditional resistance training exercises such as the dumbbell chest press and dumbbell shoulder press with an unstable surface as a means of placing greater demand on musculature; however, objective measures need to be taken to confirm the Swiss ball's efficacy in training. The results of this study showed no significant differences between surface types in mean concentric and eccentric muscle activation when performing the dumbbell chest press and shoulder press. As a result, coaches and practitioners should consider the efficacy of using the Swiss ball in exchange for a bench as a means of altering the levels of muscle activation when having recreationally-trained men perform the dumbbell chest press or shoulder press with a load of 80% 1RM.
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Keywords:© 2010 National Strength and Conditioning Association
electromyography; stable surface; unstable surface; muscle activation