Increasing popularity of instability training devices, such as stability balls and labile surfaces, has led to greater interest from researchers on different related issues, such as resistance training, core training, and athletic performance (4,7). It is generally accepted that training with instability devices does not increase strength due to hypertrophy (4), or increase force (1) and power outputs (30). However, the overall benefits of training with instability devices often include increased muscle activations with decreased force output (1,5) and increased core activation during the same exercise (9,14), which could be beneficial for rehabilitative purposes (6,7,15,29). Instability devices also play a role in the principle of training specificity. A sport is typically not performed under static conditions; therefore, the training should attempt to represent the requirements of the specific sport (3,7).
Regarding devices, often the instability is placed at the trunk/upper body using Swiss balls (1,12,22,26) or under the feet using BOSU balls (Ashland, OH, USA) (26). Other devices focused on unstable resistance have also been researched. For example, Kornecki and Zschorlich (19) found decreased force and power exertion in the upper extremities when pushing on a pendulum device, whereas Colado et al. (11) found strength improvements in physically active women during resistance training programs using elastic tubing (Thera-Band; The Hygenic Corporation, Akron, OH, USA). Specific to the bench press, Santana et al. (28) compared a traditional supine bench press to a standing cable press and found increased activation of the anterior deltoid, erector spinae (at T9), and pectoralis major in the bench press. These findings support the use of a stabile bench press to develop hypertrophy and using the less stable cable press to place a greater demand on stability and neuromuscular control of the core muscles (28). In addition, Langford et al. (21) placed the instability at the hands using a water-filled log tube as part of a short-term strength training program and found similar strength improvements across the log-training, machine-training, and barbell-training regimens. Free weights (dumbbells) have been functionally defined as unstable loads (1,18); however, this may be a misrepresentation as the weights themselves are stable. It is important to consider that a dumbbell is a rigid mass, and this differs from the instability that is created by changes in weight distribution from water movement within a tube.
Muscle activation patterns during the bench press show varying results based on the type of instability device used and the function of the muscle (1,12,22,26). With the core muscles, instability devices tend to increase the activation of internal obliques (22,26) and erector spinae (26) while having minimal effects on rectus abdominis (1,26). In the upper extremities, compared with a stable press, a greater increase in anterior deltoid activity has been shown during a simulated bench press against a medicine ball (12), and during a bench press on a stability ball (22), with a stable press showing the greatest activation of pectoralis major (12). Conversely, Anderson and Behm (1) compared both dynamic (with dumbbells) and an isometric contraction on a bench and ball surface and found no differences in muscle activation within and between muscle groups across the types of stability for pectoralis major, anterior deltoid, triceps brachii, and latissimus dorsi. While electromyography (EMG) is typically reported, kinematic data from both the participant and the device are often ignored. Knowledge of kinematic measures, such as elbow angles, bar trajectories, and velocity, could help further explain the EMG patterns associated with the different types of instability.
For this study, a water-filled device known as the Attitube (BalanceModus Inc., Windsor, Canada), which is similar to the one used by Langford et al. (21), was used to create an unstable load at the hands. Briefly, the Attitube is a clear cylinder filled with water, which flows freely within the tube to provide an unstable load. Therefore, the purpose of this study was to analyze the effects of different devices, and consequently the location of instability, on kinematic and EMG patterns of a bench press exercise. The different conditions tested were an unstable load (Attitube) on a stable surface, an unstable surface (stability ball) with a stable bar, or no instability at either location (stable surface and bar). It was hypothesized that the unstable load (Attitube) would show the greatest EMG activation within all of the muscles tested, followed by the unstable surface (stability ball), then no instability. Furthermore, because of the fluid dynamic component of the Attitube, it was hypothesized that an increase in the 3-dimensional motion of the tube would be found along with a slower lifting/lower velocity.
Experimental Approach to the Problem
Subjects performed 1 set of 3 repetitions for 3 different bench press exercise conditions: (a) Standard Olympic bar on a stable bench (BENCH), (b) Standard Olympic bar on a stability ball (BALL), and (c) the Attitube on a stable bench (TUBE). There were no specific instructions on how to perform the tasks (e.g., body positioning, timing); subjects were only told to perform the exercise, as they normally would during their regular workout. The variables analyzed were the muscle activations from 24 electrode sites, elbow flexion angle, and 3-dimensional Bar/Attitube movement trajectories.
Few studies have looked at phase effects (eccentric lowering and concentric lifting) of the bench press on muscle activation patterns (1,17,22,30), with each phase analyzed as a either a single peak (1,30) or average (17,22,30) value. It is possible that differences in activation could occur within each of the phases, meaning that analyzing the entire concentric and eccentric phases as a single value could potentially mask any effects of the instability devices. Therefore, the eccentric and concentric phases were analyzed in 10% intervals each for a total of 20 intervals.
Ten healthy male subjects were recruited from the university population (mean ± SD, age: 21 ± 3 years; age range: 17–28 years; height: 1.86 ± 0.09 m; mass: 86.04 ± 14.4 kg). All subjects resistance-trained regularly and had exposure to both instability devices used for the bench press exercise. The institution's Research Ethics Board approved this study and all subjects signed informed consent documents.
The Attitube device is cylindrical in shape, measuring 1.58 m long and 0.16 m in diameter. The tube was half-filled with water, resulting in a static weight of 22.68 kg, which was matched for the bar. Although no direct comparison of the 22.68 kg load to a 1 repetition maximum (RM) was available, anecdotally, each participant claimed the load to be light. The bar measured 2.19 m in length and had a diameter of 0.05 m at each end, whereas the stability ball was 0.75 m in diameter. Before the start of the study, subjects were given a familiarization session with an earlier prototype version of the Attitube outside of the laboratory. This prototype had a smaller diameter and no handles; however, it did allow participants to gain experience with the device. Also, before beginning the study, each subject was given a chance to practice with the current device until they stated that they were comfortable using it. The different conditions of instability in different phases of the bench press exercise are illustrated in Figure 1.
Seventy-five passive-reflective markers were used to track the 3-dimensional motion of the subject and the Bar/Attitube using a seven camera VICON MX40 motion capture system sampled at 50 Hz (Vicon Systems Ltd., Oxford, United Kingdom). The markers were placed over various landmarks on the body and used to define and track the head, trunk, left and right upper arms, forearms, thighs, shanks, and feet. Five markers were also attached to the Bar/Attitube: 2 at either end and 1 near the center.
Electromyography signals were recorded using pairs of bipolar silver/silver chloride electrodes (Ambu Blue Sensor N; Ambu A/S, Ballerup, Denmark) with a center-center spacing of 2.5 cm from 12 muscles bilaterally that were grouped according to function. The prime movers included pectoralis major (PEC) (10), triceps brachii (TRIB) (13), and anterior deltoid (ADLT) (31), whereas the secondary/stabilizers were latissimus dorsi (LD) (22), biceps brachii (BIB), middle deltoid (MDLT), and upper trapezius (TRAP) (31). Core muscles made up the third group and included upper (UES) and lower erector spinae (LES), rectus abdominis (RA) (23), and internal (IO) and external oblique (EO) (24). The EMG signals were differentially amplified (frequency response 10–1000 Hz, common mode rejection 115 dB at 60 Hz, input impedance 10 GΩ; 3 of model AMT-8; Bortec, Calgary, Canada) and converted from an analog to digital signal at a rate of 2500 Hz (Vicon Systems Ltd.).
After consent and EMG instrumentation, a 5-minute rest trial was collected with the subject lying supine on a therapy table, followed by a series of manually resisted maximum voluntary contractions (MVCs) to obtain the largest EMG recording from each muscle site. A modified sit-up was used for the abdominals (RA, EO, IO) and a back extension off a therapy table was used for the UES and LES. For MDLT and ADLT, a lateral and anterior arm raise was used, respectively. Manually resisted arm curls and arm extensions were performed for BIB and TRIB, respectively, whereas a lateral pull-down for LD and a resisted shoulder shrug for TRAP were also used. The PEC MVC consisted of the subject pressing against an unloaded Olympic bar against resistance from the researchers. A rest period (3–5 minutes) was provided between MVC trials to minimize the effects of fatigue.
After the MVC protocol, the exercise conditions (BENCH, BALL, and TUBE) were presented in a random order, and the 3 repetitions were repeated consecutively. Each subject started and ended with their elbows fully extended, with the first movement being downward during the eccentric phase.
All kinematic and EMG data were processed using Visual3D v.4 software (C-Motion, Inc., Germantown, MD, USA). Raw EMG signals were high pass filtered at a 30-Hz cutoff to attenuate heart rate contamination (14), full-wave rectified, and then passed through a low-pass, fourth order dual-pass Butterworth filter with a cutoff frequency of 6 Hz to produce the linear envelope. For each EMG channel, the rest bias (calculated as the mean EMG value from the last 30 seconds of the rest trial) was subtracted out, followed by normalization to the respective maximum value obtained from the MVC trials (%MVC). The left and right channels were averaged to provide a representative value of each muscle group, and left side kinematic data were used for analysis for both elbow angle (in degrees) and bar/tube trajectory (in meters). The Bar/Attitube trajectory was measured in terms of mediolateral, anterior-posterior, and superior-inferior movement of the end of the bar with the starting position being subtracted out from each interval, resulting in 0 m being the initial position and the movements being relative to the starting position. The vertical velocity was measured from the center of gravity of the Bar/Attitube and reported as meter per second.
The 10% intervals of the eccentric and concentric phases were determined by the center of gravity velocity of the Bar/Attitube. First, when the velocity crossed zero was what determined the beginning and end of each phase, accounting for any pause that may have occurred at the start/end points of each repetition. After the start and end events were determined for each of the 3 repetitions, each interval was calculated as one-tenth of the time between start/end events. This resulted in 3 intervals of the same name (1 from each repetition), which were averaged over each repetition to provide 1 representation of the bench press from each condition. This was followed by averaging the values between each 10% data point to obtain a 10% average interval (e.g., Start-10% eccentric [ECC]; 10–20% ECC… 90%-End concentric [CON]) for a total of 20 intervals (10 ECC, 10 CON). These intervals were used for analyzing EMG, elbow angle, and movement of Bar/Attitube trajectory data in all 3 planes, and the vertical velocity of the Bar/Attitube.
A 3 × 20 repeated-measures analysis of variance (Condition: BENCH, BALL, TUBE × Interval: Start-10% ECC to 90%-End CON) was run on each of the dependent variables: muscle %MVC, elbow angle, Bar/Attitube trajectory, and vertical velocity (SPSS, version 17.0; SPSS, Inc., Chicago, IL, USA). Differences were considered significant at an alpha level of p ≤ 0.05, and significant F-tests were further analyzed pairwise using a Bonferroni correction to control for type I error. In cases where sphericity was violated, Greenhouse-Geisser corrections were used to determine the degrees of freedom. Intraclass correlations (ICCs) were performed within each dependent variable for the different conditions and intervals using a 2-way model.
Briefly, the prime movers (PEC, ADLT, and TRIB) tended to show a reduction in activity during the TUBE trials compared with both BENCH and BALL conditions. However, PEC initially showed increased activation during the eccentric phase of the TUBE condition. Trunk muscle activity (LES, EO, and IO) increased from BENCH to BALL to TUBE. Stabilizers such as BIB and TRAP showed no differences between conditions, whereas LD and MDLT showed highest values during the BALL condition. Range of elbow flexion angle decreased with the TUBE, and motion of the Attitube itself had reduced downward velocity and increased medial-lateral movement.
A significant interaction between condition and interval on %MVC was found in PEC (Figure 2), ADLT (Figure 3), and TRIB (Figure 4) (F(38,342) > 2.98, p < 0.001). In PEC, the TUBE showed up to 2.0 times higher activation than the BENCH from Start through 40% of the ECC phase (p < 0.044) (Figure 2). At the start of the concentric phase, this trend reversed as the BENCH was 1.4 times greater than the TUBE, at 10–20% CON (p = 0.012) (Figure 2). With ADLT, the BALL showed greater activation during the ECC phase and was 1.5 times greater than the BENCH at 20–30% ECC (p = 0.028) (Figure 3). The TUBE condition showed up to 1.8 times less concentric ADLT activity compared with the BENCH (Start through 20% CON; 40–50% CON, p < 0.047). Yet overall, the BALL showed the greatest activation of ADLT as highlighted by the 1.6 times increase from BENCH to BALL during the last 80%-End CON intervals (p < 0.045) (Figure 3). For TRIB, the TUBE showed up to 1.5 times less activity during the CON phase compared with both BENCH (40% through 80% CON [p < 0.043]) and BALL (50% through 70% CON [p < 0.035]) conditions (Figure 4). The ICCs for the prime movers were r > 0.962.
No interaction between condition and interval was found for any of the core muscles tested (UES, LES, RA, EO, IO) (F(38,342) < 0.905, p > 0.634). There were main effects of condition in LES, IO, and EO (F(2,18) > 4.24, p < 0.031), of interval on UES, EO, and IO (F(19,171) > 11.41, p < 0.001), but no significant differences detected in RA (condition: F(1.07,9.66) = 4.05, p = 0.071; interval: F(1.47,13.25) = 2.01, p = 0.178). The effect of condition showed that overall, EO (Figure 5) and IO were up to 5.0 times greater in the TUBE than the BENCH (p < 0.003). A similar nonsignificant trend was found in LES with the TUBE being 2.7 times greater than the BENCH (p = 0.069). The ICCs for the core muscles were r > 0.959.
A significant interaction between condition and interval was found in MDLT and LD (F(38,342) > 2.92, p < 0.001), and a significant main effect of interval was detected in TRAP (F(2.92,26.27) = 11.31, p < 0.001) and BIB (F(19,171) = 12.30, p < 0.001). In MDLT, the BALL had up to 1.5 times greater activation than the TUBE from Start through 20% ECC (p < 0.034) and up to 2.0 times more activation at the end of the trial, from 60% through End CON (p < 0.01) (Figure 6). Also, at the very end of the trial, the BALL had up to 1.6 times higher activation than the BENCH (80% through End CON, p < 0.046) (Figure 6). With respect to LD, the BALL showed activation up to 1.9 times greater than the BENCH at all intervals except the first (p < 0.040); and the TUBE was up to 2.6 times greater than the BENCH at the beginning of the trial (Start through 20% ECC), the end of the ECC phase (80–90% ECC through Start-10% CON), and midway through the CON phase (40% through 70% CON, p < 0.046). The ICCs for the secondary muscles were r > 0.986.
A significant interaction between condition and interval on elbow angle (in degrees) was revealed (F(38,342) = 28.77, p < 0.001). The TUBE condition showed significantly greater elbow flexion compared with both BENCH and BALL at the beginning (Start through 20% ECC) and end (70% through End CON) of the trials by up to 16.14° (p < 0.004) (Figure 7). The angle during the TUBE was also less than BENCH at the 20–30% ECC interval (p = 0.005) and less than the BALL at the 60–70% CON interval (p = 0.036). At the end of the eccentric phase and start of the concentric phase, the elbow angle of the TUBE switched trends and showed up to 11.84° less elbow flexion compared with both BALL (from 60 to 70% ECC through 30–40% CON, p < 0.030) and BENCH (from Start through 30% CON; p < 0.049) conditions (Figure 7). The ICC for elbow angle was r = 0.980.
A significant interaction between condition and interval was found for all directions of the Bar/Attitube motion (F(38,342) > 2.39, p < 0.001). Mediolateral (right-left) shifting was greater in the TUBE condition compared with both the BALL and BENCH trials at each interval from Start-10% ECC through 70–80% CON (p < 0.036), whereas the superior-inferior (head to toe) position showed that the TUBE condition was further forward (toward the feet) than the BALL at 20–30% ECC (p = 0.049). The anterior-posterior position (up-down height) showed that the TUBE did not move as far down as the BENCH and BALL trials during the later stages of the eccentric phase and the earlier stages of the concentric phase (from 60 to 70% ECC through 30–40% CON, p < 0.023). The ICCs for the Bar/Attitube trajectories were r > 0.902.
A significant condition by interval interaction was also found for the lifting/lowering velocity of the Bar/Attitube (F(38,342) = 33.64, p < 0.001). The TUBE condition was performed up to 2.3 times slower than both BAR and BALL conditions during the eccentric phase (p < 0.015), and up to 2.7 times slower during the concentric phase (p < 0.015) (Figure 8). The ICC for the velocity was r = 0.478.
A comparison of the effects of different stability devices during the bench press exercise found that overall, the instability condition (no instability [BAR], instability at the trunk [BALL], or instability at the hands [TUBE]) played a significant role in both muscle activation and motion patterns during the bench press exercise. Muscle activations showed varying results depending on their role in the movement (prime mover, core muscle, secondary/stabilizing muscles) and on the instability condition. These activation patterns did not follow an increasing linear trend from BENCH to BALL to TUBE as was originally predicted. Generally, the TUBE increased activation of the core muscles while decreasing activation of the prime movers. Interestingly, during the eccentric phase of the bench press, the PEC muscle activation was greater in the TUBE condition than the BENCH condition. In addition, the BALL condition tended to increase the activation of the shoulder muscles (ADLT, MDLT). The findings also showed that the 3-dimensional movement patterns of the Bar/Attitube itself did not necessarily increase with instability device as the TUBE showed minimal differences in forward-backward movement and less range in terms of height position.
Several researchers have suggested that resistance training using unstable platforms (e.g., instability ball) may be more beneficial in terms of core strengthening rather than upper body/prime mover development (4,22,26). Results of this study lend evidence to support this, as both instability devices resulted in different core muscle activation patterns, particularly in LES, EO (Figure 5), and IO. Similar to previous research looking at the bench press on a stability ball and flat bench (22,26), there were increases in muscle activation in LES and IO during these same conditions in this study. Likewise, Behm et al. (9) found increased activity when moving from a stable to unstable surface, and from bilateral to unilateral exercises, in the upper and lower ES as well as a lower abdominal stabilizer (similar to IO in this study) during a unilateral chest press on a bench and a stability ball. The increase in trunk muscle activation was partially attributed to the destabilizing moments generated during the unilateral chest press (9) and perhaps a similar mechanism is at play in this study in the TUBE condition. The TUBE showed more medial-lateral movement, and it is possible this added movement acted as a destabilizing moment, resulting in the increased activity of the obliques and ES muscles.
Conversely, the prime movers in this study (PEC, ADLT, and TRIB) showed distinct patterns across instability devices. With the BALL condition, PEC and TRIB did not increase activity, whereas activation of ADLT was increased, similar to the results of Marshall and Murphy (22). In the TUBE condition, there was an unexpected tendency for these muscle groups (PEC, ADLT, and TRIB) to show decreased activation compared with the BENCH trial. Of particular note was the pattern of the PEC muscle (Figure 2), which seemed to have taken on more of a stabilizing role during the TUBE trial. During the initial stages of the eccentric phase, when prime mover activation should be minimal, the PEC TUBE trial showed twice the activation during the first 40% of the lowering phase compared with the BENCH trial. This indicated that the TUBE condition was more challenging during the eccentric phase than the concentric phase of the bench press. Furthermore, this stabilizing role was also supported by the activation, or lack thereof, during the initial stages of the concentric phase. Figure 2 shows that during the early stages of the concentric contraction (Start-10% CON through 10–20% CON), the PEC was lacking a notable burst of activity when compared with the BENCH and BALL trials, with TUBE and BENCH being significant at about a 10.5% MVC difference. Similar trends were present in both ADLT and TRIB (Figures 4 and 5, respectively), where the TUBE showed less of an initial burst and an overall decrease in activity during the concentric intervals in which activity is expected to be highest. This showed that with the unstable load (TUBE), the lower activations in these muscles may reflect a need for more control at the initiation of the concentric phase and, consequently, a less forceful exertion would be applied. The highest deltoid activation during the BALL may be a function of shoulder support during the trial. On the BENCH, the shoulders were supported, whereas with the BALL further muscle activation in the shoulders may be required to achieve the same degree of stability, as highlighted in both ADLT (Figure 3) and MDLT (Figure 6).
In terms of kinematic measures, less range of motion found at the elbow (Figure 7) and the corresponding decrease in Bar/Attitube height change, coupled with the increase in medial-lateral movement and the decrease in the vertical velocity within the TUBE trial, could be indicative of an attempt to keep the Attitube stable during the movement. Duffey and Challis (16) recently looked at fatigue effects on bar kinematics during a stable chest press and suggested that learning the proper kinematics of the movement may play a role in improving lifting performance. Perhaps using the TUBE to train the stabilizers (or decrease the prime movers) and practice control of the bar would eventually lead to increases in lifting performance. Similar to Goodman et al. (17), no differences in elbow range of motion were found between BENCH and BALL, further illustrating the differences the novel (TUBE) task evokes.
In a comparison of a bench press on a flat bench to a stability ball, Koshida et al. (20) found decreases in velocity, force, and power outputs during the ball condition. In this study, no differences in velocity were found across the BENCH and BALL conditions, along with minimal differences in muscle activation between these conditions. It is presumed that an increase in velocity will lead to greater effort in an attempt to accelerate the bar (27) and has been shown to increase the EMG demands on the prime movers (25). The unique result of increased eccentric phase activation of the PEC muscle with the slower velocity of the Attitube could be a direct result of the fluid instability, further highlighting the functional challenges associated with lifting an unstable load. Although slower velocities are often recommended for strength gains (as opposed to higher velocity for power increases) (2,25), the increase in PEC activation at lower eccentric velocities could also have benefits during the neuromuscular training portion of a strength training program due to the greater demands of a smaller load over a longer period of time.
One of the limitations of this study comes from a methodological basis when these results are compared with other published results. In this study, EMG was analyzed as a %MVC while the load being pressed was fixed as an absolute value. Other researchers have analyzed EMG in terms of average root mean square value (1,18,22,26) or a normalization to task procedure (18,26). Although direct comparisons across other studies is difficult, normalizing EMG to a %MVC provides a viable option to compare different types of training devices and intervals between individuals. The %MVC not only accounted for increases in muscle activation but also provided a relative indication as to the level of which an individual was performing. Regarding loading, relative values have been commonly used, including 1RM (17), 75% of 1RM (1), 60% of 1RM (22), and 10RM intensity (18). In this study, an absolute value of 22.68 kg was used due to limitations of the equipment and for safety purposes. Trying to accurately add and remove water for each subject was not feasible, and considering the novelty of the exercise, an excessive amount of weight of an unknown device may have placed the subjects at risk of injury. In addition, more or less water would change the dynamics of the unstable loading and would not be consistent across participants. Thus, the Attitube was only filled halfway with water to the aforementioned the mass. Another limitation of using the Attitube was the grip requirements. As this device was a prototype without handles of similar size to the Olympic bar, the subjects were required to hold the tube itself.
In summary, the effects of instability devices were analyzed in terms of muscle activations, elbow joint angles, and 3-dimensional Bar/Attitube trajectory. A novel device called the Attitube provided an unstable load at the hands, whereas a stability ball invoked instability at the trunk. Both conditions of instability were compared with each other and against a stable trial. The eccentric and concentric phases were analyzed in 10% intervals over the trials to help further identify phase effects. Generally, the TUBE tended to increase EMG activity of the trunk muscles while decreasing activity of the prime movers. These results further support the notion put forward by Norwood et al. (26) that using unstable training devices during a bench press may be more beneficial for trunk muscles rather than prime movers. Analyzing the phases by 10% intervals provided additional detail regarding the bench press as a whole, possibly uncovering previously masked trends. For example, the PEC muscle showed considerable changes during the TUBE trials as the trial progressed, revealing differences at the start of both eccentric and concentric. No differences in bar trajectory were found between the BENCH and BALL trials, however, the TUBE showed a decrease in eccentric/concentric range of motion along with an increase in medial-lateral movement and slower lifting/lower velocity.
Assuming the focus of the bench press is to work the prime movers (PEC, TRIB, and ADLT), then different conditions of instability will change the muscle activation patterns. It has been stated that strength gains come not only from increase in muscle cross-sectional area but also with improvements of neuromuscular control (3). Use of instability training devices for an exercise such as the bench press may not elicit increases in cross-sectional area; however, there may be improvements in neuromuscular control, which could be used in cycle as part of a strength training program. In addition, using the Attitube may help with learning proper technique and the associated decrease in muscle activation may be beneficial to a rehabilitation program of an athlete returning from injury, as instability resistance training has been previously suggested for such rehabilitative purposes (6,8). Likewise, performing a bench press with an unstable load may also be beneficial as part of an in-season exercise program, as lower activations of the prime movers could indicate reduction of joint stress and could help with challenging the neuromuscular system due to the slower velocity.
The authors thank BalanceModus, Inc. for providing the Attitube.
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