The second session identified the extent of activation with a variety of stable and unstable lower body exercises. Dyna Discs were used to create an unstable base for the tested leg in an attempt to identify activation using electromyography. The subjects were positioned on the Dyna Disc to ensure the orientation of trunk musculature and angle of hips and knees were similar to their stable counterparts. The following exercises were chosen as a representative sample of common instability exercises.
Static Forward Lunge (Figure 6)
Assuming a long lunge position, the participants positioned their back knee 1 cm above the floor while keeping the front knee (90°, measured before and during testing using a goniometer) over the ankle. Subjects were instructed to keep the head and chest up and to position the hands behind their head to maintain back posture while lowering their hips. The knee of the back leg was slightly flexed. For unstable testing, the Dyna Disc was placed under the midfoot of the front leg (right foot).
Static Side Lunge (Figure 7)
Subjects were instructed to stand with feet roughly 1.2 m apart and told to sit to their right side keeping the weight on the right heel as they sat to a 75° (measured before and during testing using a goniometer) knee angle. Subjects were instructed to keep the head and chest up and position the hands behind their head to maintain back posture. For unstable testing, the Dyna Disc was placed under the midfoot of the bent leg (right foot).
One-Leg Hip Extension (Figure 8)
Subjects were instructed to lie supine with their left leg extended toward the ceiling at 90° (measured before and during testing using a goniometer) from the floor. The right foot was placed flat on the floor or Dyna Disc. The subjects were then instructed to lift their hips while evenly distributing the force over their foot holding this position for 15 seconds. For unstable testing, the Dyna Disc was placed under the midfoot of the active leg (right foot).
Subjects were instructed to stand with their right foot on the floor or Dyna Disc, then reach with their left hand and touch a point 20 cm from the front of their right foot. Subjects were instructed to bend at both knees to maintain balance and to achieve both hip and knee flexion.
Subjects were instructed to balance on their right foot either on the floor or the Dyna Disc without holding onto any supports. They were then told to plantar flex until fully extended.
The third protocol identified the rate of fatigue while performing a wall sit under stable and unstable conditions. A BOSU ball was used to create an unstable base. Subjects assumed a sitting position against a wall with a knee angle of 90° (measured before testing using a goniometer) and a hip angle of 90° and feet spaced 30 cm apart. For unstable tests, subjects placed their feet 30 cm apart on the flat side of the BOSU ball (convex side on floor). The testing was completed when subjects could no longer hold the specified exercise posture. Subjects were instructed to relax when visual inspection indicated a significant deviation of ≥5° from the initial 90° knee angle.
Subjects were analyzed by comparing the rate of fatigue under each condition (stable and unstable) using time as well as an EMG comparison during the protocol.
In the initial investigation (standing and squatting on a variety of instability devices), statistics were performed separately on each muscle group. Data were analyzed with separate 1-way analyses of variance (ANOVAs) with repeated measures for standing and squatting. The 6 platforms to be compared were the Swiss ball, Dyna Disc, BOSU ball up, BOSU ball down, wobble board, and stable floor.
In the second investigation (a variety of exercises performed on a Dyna Disc and the floor), data were analyzed with a 2-way ANOVA with repeated measures on both levels. The 2 levels (2 × 5) were state of stability (stable or unstable) and exercise (front lunge, side lunge, hip extension, reach, calf raise).
The fatigue investigation used a 2-way ANOVA repeated measures (2 × 4) to determine whether significant differences occurred with the EMG activity between the stability condition and fatigue duration (first contraction, contraction at first third of fatigue duration, contraction at two-thirds of fatigue duration, and final contraction). A 1-way ANOVA repeated measures was used to distinguish significant differences in fatigue duration between stable and unstable conditions.
For all protocols, where significant differences were detected (p ≤ 0.05), a Bonferroni (Dunn) procedure was used to identify the individual differences among the exercises. Effect sizes (ESs) are shown in parentheses within the results (25). Reliability was assessed with a Cronbach model intraclass correlation coefficient (21) for all subjects (Table 3). Repeated tests were conducted within a single testing session.
The 1-way repeated-measures ANOVA for the device protocol indicated that the wobble board produced 51%, 44%, 43%, and 38%, respectively, greater soleus EMG activity than standing on a stable floor, Dyna Disc, BOSU down, and BOSU up (p < 0.004, ES = 0.65, 0.57, 0.56, 0.49, respectively). Concurrently, there was 34%, 26%, 24%, and 17%, respectively, greater soleus EMG activity with the Swiss ball than when standing on a stable floor, Dyna Disc, BOSU down, and BOSU up (p < 0.004, ES = 0.41, 0.30, 0.28, 0.20, respectively) (Table 4).
There was 34%, 26%, 33%, and 33%, respectively, greater lower abdominals EMG activity with the wobble board than when standing on a stable floor, Dyna Disc, BOSU down and BOSU up (p = 0.03, ES = 0.48, 0.36, 0.46, 0.49, respectively). Similarly, there was 31%, 22%, 30%, and 32%, respectively, greater lower abdominals EMG activity with the Swiss ball than in the stable, Dyna Disc, BOSU down, and BOSU up standing conditions (p = 0.03, ES = 0.46, 0.33, 0.45, 0.48, respectively) (Table 4).
When standing, there was the had 88%, 61%, 64% and 64%, respectively, greater rectus femoris EMG activity during the Swiss ball condition than in the stable, Dyna Disc, BOSU down, and BOSU up standing conditions (p < 0.0001, ES = 1.08, 0.77, 0.41, 0.80, respectively) (Table 4).
During wobble board standing, there was 70%, 65%, 56% and 53%, respectively, greater biceps femoris EMG activity than in the stable, Dyna Disc, BOSU down, and BOSU up standing conditions (p < 0.0001, ES = 1.21, 1.13, 0.98, 0.95, respectively). Correspondingly, there was 57%, 49%, 36% and 33%, respectively, greater biceps femoris activity during Swiss ball standing than in the stable, Dyna Disc, BOSU down, and BOSU up standing conditions (p < 0.0001, ES = 1.21, 1.06, 0.81, 0.75, respectively) (Table 4).
During standing postures, there was 52% greater LSES EMG activity during the Dyna Disc condition than the stable condition (p < 0.0001, ES = 0.73). There was also 68%, 44%, and 42%, respectively, less LSES EMG activity in the stable, BOSU up, and BOSU down conditions compared to the wobble board condition (p < 0.0001, ES = 1.36, 0.89, 0.84, respectively). In tandem, there was, respectively, 70%, 47%, and 46% greater LSES EMG activity during the Swiss ball conditions compared to the stable, BOSU up, and BOSU down (p < 0.0001, ES = 1.97, 1,32, 1.26, respectively) (Table 4). There were no significant differences in LSES EMG activity between the Dyna Disc and the wobble board and Swiss ball.
During squatting, there was 69%, 43%, 57%, 49%, respectively, more EMG activity in the soleus muscle with the wobble board than in the stable, Dyna Disc, BOSU up, and BOSU down conditions (p < 0.0001, ES = 0.91, 0.58, 0.76, 0.64, respectively). In addition, during the Swiss ball condition, the soleus demonstrated 54%, 18%, 40%, and 25%, respectively, more EMG activity than in the stable, Dyna Disc, BOSU up, and BOSU down conditions (p < 0.0001, ES = 0.77, 0.24, 0.53, 0.35, respectively). The lower abdominal muscles showed 39%, 57%, 48%, and 63%, respectively (p = 0.0002, ES = 0.49, 0.64, 0.71, 0.54, respectively) more activity with the wobble board than in the stable, Dyna Disc, BOSU down, and BOSU up conditions. Likewise, there was 38%, 56%, 47% and 62%, respectively, more EMG activity in the lower abdominals with the Swiss ball than in the stable, Dyna Disc, BOSU down, and BOSU up conditions (p = 0.0002, ES = 0.58, 0.79, 0.86, 0.67, respectively) (Table 5). There were no significant differences in EMG activity among the conditions in the LSES, rectus femoris, and biceps femoris.
There were no significant differences detected between any of the exercises performed on a stable floor and the unstable Dyna Disc.
Fatigue-related Electromyographic Activity
Main effects were discovered for instability conditions and time during the fatigue testing with the soleus. With data collapsed over time, the stable soleus had 51.2% greater EMG activity than the unstable soleus (p = 0.03, ES = 1.01). There were no other significant muscle activity differences between stable and unstable conditions. Overall, with data collapsed over instability conditions, the last contraction had 36.1% greater soleus EMG activity than the first contraction (p = 0.0008, ES = 0.33). The interactions illustrated that under stable conditions, the last contraction had 46.4% and 34.5% more soleus EMG activity than the first and second contractions, respectively (p = 0.001, ES = 0.39 and 0.84, respectively).
Similarly, with data collapsed over instability conditions, the lower abdominals exhibited a 44% increase in EMG activity during the final contraction compared to the second contraction (p = 0.003, ES = 0.50). The biceps femoris also exhibited a 35% increase in activity during the final contraction compared to the first contraction (p = 0.001, ES = 0.55).
As for the fatigue time, there was a trend (p = 0.09, ES = 1.1) for longer wall sit times under stable conditions (Figure 10).
Intraclass correlation coefficients illustrated the very good to excellent reliability (0.72-0.99) of the procedures (Table 3).
The most unique finding of this study was the lack of increase in muscle activation (EMG) of experienced resistance-trained individuals with activities performed on the unstable bases provided by Dyna Disc and BOSU balls. This finding applied in the first protocol (devices) to the soleus, rectus femoris, biceps femoris, and lower abdominals when standing on a Dyna Disc or a BOSU ball. It applied to the all muscles tested when squatting on a Dyna Disc and BOSU ball. It also applied to all muscles tested in the second protocol (exercises) for the exercises performed on a Dyna Disc. Finally, the lack of muscle activation differences for the rectus femoris, biceps femoris, LSES, and lower abdominals was also applicable to the wall sit fatigue test performed on a BOSU ball. This is the first study to use experienced resistance-trained individuals to demonstrate a lack of significant difference in muscle activation when comparing moderately unstable balance devices to a stable base. Similar to previously published research, the apparently greater instability of the Swiss ball and wobble board did result in greater muscle activation than found with a stable surface and, specific to this study, generally greater muscle activation than Dyna Discs and BOSU balls.
Current research both complements and challenges the findings of this study. Several studies have investigated the neuromuscular responses to training under stable and unstable bases using different exercises, tools, and populations (2,3,12,18,29). Cosio-Lima et al. (10) showed a significant increase in trunk muscle EMG activity and balance scores with an unstable versus stable trunk training program in previously untrained women. Vera-Garcia et al. (31) demonstrated that a curl-up performed under unstable conditions significantly increased rectus abdominus and external oblique activation over curl-ups performed on a stable base. Behm et al. (7) found similar results, indicating that unilateral upper body exercises as well as lower abdominals- and LSES-targeted callisthenic exercises performed under unstable conditions exhibited greater EMG activity than their stable counterparts. Anderson and Behm (2) reported greater soleus and LSES EMG activity when squats were performed on a Dyna Disc than on a stable floor. Interestingly, the present study did not show any significant difference in activation between stable and moderately unstable (Dyna Discs and BOSU balls) exercises. However, the aforementioned studies did not use experienced resistance-trained individuals whose balance may have been augmented by years of training (mean, 8.2 ± 7.4 years) with free weights.
According to Schmidt and Lee (27), even 2 very similar tasks, such as throwing a football and throwing a javelin, will correlate nearly zero with each other. Conversely, our study found very similar EMG values between exercises performed under moderately unstable (Dyna Disc and BOSU ball) and stable conditions. It could be speculated that resistance training with free weights provides an environment of low to moderate instability where learned motor programs may be transferred to other moderately unstable platforms. In accordance with the concept of training specificity, training with moderately unstable free weights transfers to other similar moderately unstable exercises. This may indicate why no significant differences were found between 2 apparently different environments. De Luca and Mambrito (12) showed that EMG activity decreased with the uncertainty of movement and increased with task awareness. Highly resistance-trained individuals who have performed years of resistance training with moderately unstable free weights have become accustomed to specific exercises and therefore have a strong familiarity with the movements resulting in augmented EMG activity. This experience could reduce the unpredictability of an exercise performed on a moderately unstable tool (Dyna Discs and BOSU balls) due to the ingrained motor program of the exercise. Regardless of the cause, the current study shows that not all instability devices are effective for increasing muscle activation in highly resistance-trained individuals. If greater balance skills are sought, then devices with greater instability (i.e., smaller point of contact with body or base, greater distances of body from base of support, greater malleability of the device) should be used.
Not all studies have reported increased muscle activation with instability devices when the subjects were not highly trained. Behm et al. (7) found that there was no significant difference in activation of the trunk musculature during bilateral upper body exercises (chest press, shoulder press) performed on both unstable and stable bases. Anderson and Behm (3) also showed no significant increase in activation of the trunk musculature during bench presses performed on the Swiss ball. Both studies used bilateral contractions of the upper limbs, which may not generate similar disruptive moments seen in unilateral exercises because both limbs are involved in the movement, allowing the resistance to be maintained directly above the torso and center of gravity. As well, the increased load associated with these specific resistance training exercises may serve to distort the instability device and actually provide a more stable platform by flattening at the bottom (3).
Results demonstrating similar activation as exhibited by the current subjects may best be explained by Gage et al. (13). With the body acting as an inverted pendulum (13), the center of gravity vacillates constantly. Activation of postural muscles including trunk musculature acts to maintain equilibrium or balance. Since highly resistance-trained individuals add further resistance above the center of gravity with many exercises (squats, shoulder press, cleans), there is even a greater stress placed on maintaining the equilibrium of the body's inverted pendulum during these exercises. Hamlyn and Behm (14), investigated trunk activation under stable and unstable conditions, demonstrating that squats and dead lifts using 80% of the 1 repetition maximum performed on a stable floor elicited a greater activation of the trunk musculature than unstable trunk callisthenic exercises. This study supports Siff (28) who indicated that the best exercises to stimulate trunk muscles are those that load the trunk with external resistance such as a squat or dead lift. Thus, some of the instability devices now available such as BOSU balls and Dyna Discs may not present sufficient stability challenges to the highly resistance-trained individual. The highly resistance-trained individual may need to increase his or her disruptive torque through a combination of load and moderate instability (i.e., squats, dead lifts, and cleans).
An established shortcoming of instability training is the lesser ability to load under unstable conditions (3,6,12,18). Highly resistance-trained individuals performing exercises under moderately unstable conditions may not exhibit changes in EMG activity with the exercise. As motor programs are ingrained, the load and its positioning in relation to the center of gravity may become the formative variables. An investigation into loading using a variety of instability devices may yield further results as to the training effect of these tools. While instability is inherent with free weights, the current importance placed on instability training devices may be overemphasized with individuals who consistently create moderately unstable environments with free-weight exercises. Greater degrees of instability such as found with Swiss balls and wobble boards may be necessary in this type of population to increase limb and trunk muscle activation.
Similar to the first 2 protocols, a moderate degree of instability (BOSU ball) did not produce significant changes in activation in any tested muscles except the soleus during the fatigue protocol or produce a significant change in the rate of fatigue in highly resistance-trained individuals. However, stable wall sit conditions elicited greater soleus activation. It is speculated that under unstable conditions with the feet placed on the moderately unstable BOSU ball, the plantar flexors would not be able to exert similar forces as under stable conditions (6). This might force the individuals under unstable conditions to use a greater variety of lower limb muscles (i.e., gastrocnemius, peronei) to lock their lower body into place. The tendency for a greater rate of fatigue with unstable conditions may be related to the additional work of the synergists to cope with the moderate instability.
In conclusion, it has been shown that the use of moderately unstable training devices such as Dyna Discs and BOSU balls are not as effective as Swiss balls and wobble boards in increasing activation in the lower body and trunk musculature with highly resistance-trained individuals. A determining variable in this research is that all subjects were highly resistance-trained individuals who had extensive experience in the use of heavy free-weight resistance and load-bearing exercises. The current study tested exercise postures using body weight even though resistance training typically employs the use of greater overload. An investigation into the EMG activity associated with these postures and devices under loaded conditions may provide more definitive answers as to the effectiveness of these tools. Moreover, an investigation into the effectiveness of training with instability tools, such as the wobble board, Swiss ball, Dyna Disc, and BOSU ball in a less highly trained population that may benefit from instability devices, would provide useful insight. This may extend to populations who seek to rehabilitate muscle without harboring external load, which may amplify injury or dysfunction.
The present research does not nullify the benefits of instability training devices to augment trunk muscle activation and stress balance. However, just as highly resistance-trained individuals might not obtain strength training adaptations from low loads, they may also not receive additional muscle activation or balance training effects from moderately unstable devices. Training experience with free weights can provide an environment of moderate instability that stresses and forces training adaptations to an individual's equilibrium. Individuals who have extensive experience with free weights need greater instability challenges. The instability of a device is dependent on or can be manipulated by altering the extent of the base of support (a smaller base results in less stability), the vertical or horizontal distance from the base of support (i.e., large-diameter balls may be less stable to stand on), and the architecture of the device, among other factors.
The National Science and Engineering Research Council of Canada supported this research. TheraBand Inc. provided the instability devices.
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Keywords:© 2008 National Strength and Conditioning Association
stability; balance; electromyography; strength; fatigue