There were no differences in the angles of the knee during maximal contractions between stable surface (82 ± 8), power board (77 ± 13), BOSU (84 ± 10), and balance cone (79 ± 9; p = 0.240–0.996).
The main finding of this investigation is that force output was reduced with increasing instability (i.e., stable surface and power board < BOSU and balance cone), but EMG activity in lower-limb and superficial trunk muscles was similar (with the exception of rectus femoris activity, which was highest in the stable surface).
In this study, the same relative resistance was employed on all surfaces (i.e., maximal isometric effort). Several studies observing higher EMG activity in unstable resistance training used the same absolute and not relative resistance (1,6,21,26). Employing the same absolute load usually means that a higher relative load was used in the unstable condition. Thus, in those studies, it is not possible to differentiate the contributions of higher relative loads and higher stability requirements on neuromuscular activity. Anderson and Behm (1) reported similar EMG activity for vastus lateralis and biceps femoris but greater for soleus when using free weight squats on balance discs compared with a stable surface. The study was limited by using the same absolute as opposed to relative load in each condition and the highest load used was only 60% of the body weight (1). Wahl and Behm (27) examined the EMG activity during stable surface, Dyno discs, BOSU ball, wobble board, and Swiss ball in isometric squat position (60° flexion in the knee). Similar EMG activities were reported in the rectus femoris, biceps femoris, and erector spinae but greater in soleus and lower abdominal using wobble board and Swiss ball compared with in the other conditions. However, that study was limited by testing squats with only body weight. Again different relative resistances were employed; and in addition, results obtained with only body weight are not necessarily relevant for strength training. Considering that training intensity usually is prescribed as a RM load (e.g., 10RM) or percent of 1RM, comparisons of different relative loads are also of little relevance for athletes and recreational trainers. Because we used maximal isometric contractions, we did not suffer from problems with matching loads in the different conditions.
We acknowledge that isometric contractions are usually not used in strength training, and thus, these results may lack ecological validity. However, results obtained under isometric conditions have been reported to be strongly correlated with dynamic lifting performance (25). Therefore, we believe that the results obtained in this study also have relevance for heavy dynamic strength training on unstable surfaces.
A strength of our experimental model is that isometric testing permitted us to test EMG activity on unstable surfaces during heavy contractions, which is in contrast to most previous lower-limb studies who used light loads (27,30). One study reported that they had to use low loads for safety reasons (30). Further, there are substantially greater methodological concerns with dynamic than isometric EMG measurements (12).
We are aware of only 1 study investigating maximal force output in squats on a stable and unstable surface (20). These investigators used an inflatable balance disk and reported an approximately 46% decrease in maximal force. This study supports the findings of McBride et al. (20), albeit there was a larger force decrement in that study, which could be attributed to the lack of a familiarization session. The strength trained subjects in our study performed 1 familiarization session, which is a strength of our results compared with the study by McBride et al. (20). Still, as can be observed in Figure 5, there were considerable intersubject variation, which may have been reduced with more familiarization sessions.
Behm et al. (4) proposed a hierarchy of force outputs with decreasing force output with increasing instability. This study partly supports those findings. Based on this study, greater instability requirement (i.e., stable vs. power board or BOSU vs. balance cone) did not always significantly reduce force output, although a trend can be observed in Figure 4. However, because of the isometric testing mode, the stability requirements may have been more similar than we initially anticipated. The subjects gradually built up the force while stabilizing and maintaining balance on the different surfaces. During the 3 seconds of maximal effort, the subjects may have been able to stabilize the limbs and trunk and thus be able to exert a considerable amount of force in the unstable conditions.
The EMG activities were similar on all surfaces for all lower-limb muscles except rectus femoris in which greater EMG activity was obtained on the stable surface. There was also a trend for higher activation of the stabilizing soleus muscle on BOSU compared with the stable surface. However, as can be observed in Figure 5, also the vastii muscles tended to be less activated on unstable surfaces; however, considerable interindividual differences for the medial- and smaller magnitude of the difference for the lateral quadriceps, could explain the lack of statistically significant reductions for the whole quadriceps. Indeed, McBride et al. (20) reported lower EMG activity in both vastis for isometric squats on an unstable balance disk.
We observed similar EMG activities of superficial trunk muscles for different surfaces. However, the squats were performed isometrically in our study, while studies reporting greater EMG activity in the trunk, tested dynamically (1,6,16,18,19,21). The trunk muscles stabilize the core (3,10,11) and core stability is a “dynamic concept that continually changes to meet postural adjustments or external loads accepted by the body” (p. 980) (29). The perturbations applied to the body during dynamic resistance exercises causes the center of gravity to change, leading to increased trunk activation to avoid losing balance. Similar adjustment and activation of the trunk may not be necessary during isometric testing. However, although several studies have reported greater trunk EMG activity in unstable resistance exercises than under stable conditions (1,6,16,18,19,21), several of these did not match relative resistance (1,6,18,19,21), and some have reported similar activities or mixed results (13,26,27). Willardson et al. (30) compared dynamic squats with 50% of 1RM obtained on a stable surface with the same absolute load on the flat side of a BOSU trainer. However, although probably higher relative loads were used in the unstable condition, the muscle activities of the trunk muscles were similar. Nonetheless, to our knowledge, we are the first to report muscle activity of the trunk muscles during heavy squats on unstable surfaces. Further research should investigate the neuromuscular pattern of heavy dynamic squats with different stability requirements employing the same relative resistance.
In athletic training, the goal of instability resistance training is often to exert higher forces on unstable surfaces (i.e., sport specificity), such as in alpine skiing, ice hockey, or soccer. Therefore, the efficacy of training on unstable surfaces should be determined in sport specific tasks. In a cross-sectional study, Behm et al. (7) demonstrated a significant correlation between a balance test and skating speed for young but not older ice hockey players. Further, Yaggie and Campbell (31) concluded that balance training improved some performance tasks, but it was unclear whether this could be transferred to general functional improvements. After 6 weeks of instability training using slings and focusing mainly on the core muscles, Saeterbakken et al. (23) demonstrated significantly increased throwing velocity. However, Kibele and Behm (15) reported no differences in the training responses between unstable and stable resistance training on various functional tests. More research should address whether unstable resistance training is more, less, or equally effective compared with traditional resistance training in enhancing various functional performance tasks.
In conclusion, increasing the instability of the surface during maximum effort isometric squats usually maintains the muscle activity of lower-limb and superficial trunk muscles although the force output is reduced.
Here, we generally observed similar EMG activity in the lower-limbs and trunk during isometric squats with 4 different surfaces. The force output was reduced for the 2 surfaces with highest stability requirement (floor and power board > BOSU ball and balance cone). To gain optimal overload to increase maximum strength and power, stable surface or power board may be the best options. However, in, for example, knee or back rehabilitation, Bosu ball and balance cone may be better choices, because force production is lower while muscle activity usually is maintained. For the same reasons, these devices may also be useful for athletes as part of periodized training programs.
The authors thank Jarand Tilland and Erik Frastad for assistance in participant recruitment and data collection. They thank the participants for their enthusiastic participation.
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Keywords:© 2013 National Strength and Conditioning Association
EMG; resistance exercise; unstable surface; trunk; core