Instability devices have become popular in resistance training. Instability can be achieved by using, for example, free weights instead of machines (22,24,28) or unstable surfaces instead of stable surfaces (2,4,16,19). Several reasons to employ instability devices have been suggested. For sports performance these include (a) sport specificity; in this case, that means becoming good at exerting force during unstable tasks (e.g., alpine skiing); and (b) to increase the stress on the neuromuscular system to a greater extent than traditional resistance exercises. However, this does not usually seem to be the case when the same relative load is used (5). In rehabilitation, instability devices have been considered to be beneficial as training with instability can induce similar levels of muscle activation while using less external resistance (2,4,20,22).
Instability in strength training may not provide optimal overload for gaining strength (2,4,17,20). Behm et al. (4) reported a 70% decrease in force output performing leg extensions on an unstable surface compared with a stable surface. In addition, Anderson et al. (2) reported a 60% decrease in force output in chest press under unstable conditions, although the electromyographic (EMG) activity was similar. In contrast, Goodman et al. (13) reported no difference in 1 repetition maximum (1RM) strength or EMG activity for bench press on a stable and unstable surface. This demonstrates that there are large differences between exercises, muscles and instability devices.
Speed, power, and strength of the knee and hip extensors are vital for success in sports and necessary to perform tasks of daily living. Many strength coaches think that the squat exercise is essential for athletic success (9). Therefore, the squat exercise warrants extra investigation. Wahl and Behm (27) compared EMG activity in the squat with a variety of instability devices to a stable surface. Wobble board and Swiss ball induced greater EMG activity in rectus abdominis and soleus than Dyna disc, BOSU ball (up and down) and stable floor, whereas similar activities were observed for erector spinae, rectus femoris, and biceps femoris. However, the squat exercises were performed isometrically without any external resistance; therefore, these results do not appear to be relevant for those aiming to improve strength related parameters. Anderson and Behm (1) reported greater EMG activity in the vastus lateralis, soleus, and superficial trunk muscles but not in biceps femoris when comparing free weight squat on a stable vs. unstable surface. The external resistance was 60% of the body weight of the subject. Further, Willardson et al. (30) compared EMG activity in the trunk performing 50 and 75% of 1RM in squat on a stable surface, to 50% of 1RM in squat on a BOSU ball. Conversely, these authors observed no differences in trunk muscle activity among the conditions. McBride et al. (20) tested isometric squats in stable and unstable surfaces and reported 46% lower force output. The EMG activity was about one-third lower in the vastus lateralis and medialis on the unstable surface. However, the study was limited because no familiarization session was included. Therefore, the inferior effect of unstable surface could simply be because of unfamiliarity with the exercise. Further, only one unstable surface was employed, and no EMG recordings of trunk muscles were made (20).
To our knowledge, no study has matched relative resistance during heavy or maximal effort in squats using several surfaces with increasing instability while measuring neuromuscular activity of lower-limb and trunk muscles. Therefore, the aim of the study was to examine the effect of different levels of instability on the lower-limb and trunk muscle activities and maximal force output in isometric squats. We hypothesized greater EMG activity in the stabilizing trunk and soleus muscles with increasing instability (1,27) but similar activity in quadriceps and biceps femoris. Further, we expected that force output would decrease with greater instability.
Experimental Approach to the Problem
A within-subject crossover design was used to asses force output and neuromuscular activity with EMG in squats on (a) stable surface (floor), (b) power board, (c) BOSU ball, and (d) balance cone (Figure 1). Three to 7 days before the experimental test, the subjects participated in 1 familiarization session involving several submaximal and three maximal repetitions on each surface. All the squats were performed isometrically with 90° flexion in the knees. The surfaces had different instability requirements based on their numbers of unstable dimensions and the magnitude of contact with the floor. The power board (Figure 2A) was only unstable in 1 dimension (right-left position). The BOSU ball was used with the stable platform side up. The BOSU ball (Figure 2B) and the balance cone (Figure 2C) were unstable in 2 dimensions. The BOSU ball had a larger base of support than the balance cone and was therefore considered less unstable than the balance cone made of iron. The order of the exercises was randomized and counterbalanced.
Fifteen healthy men (23.3 ± 2.7 years, mass: 80.5 ± 8.5 kg, height: 1.81 ± 0.09 m) volunteered for the study. All the subjects were informed of possible risks involved and testing procedures before the testing. Ethics approval was obtained from the local research ethics committee and conformed to the Helsinki declaration. All the subjects gave written consent to participate. None of the subjects were power lifters but had 4.5 (±1.8 years) of resistance training experience. All the subjects were familiar with the free weight squat exercise. The subjects were excluded if musculoskeletal pain or illness reduced their maximal performance. The subjects were instructed to refrain from any additional resistance exercise 72 hours before testing.
A 10-minute warm-up was performed on a treadmill or cycle ergometer at an intensity corresponding to a rating of perceived exertion between 8 and 10 on the Borg scale (range: 6–20) (8). Before the testing, the subjects performed submaximal isometric contractions as specific warm-up in each of the different exercises.
In the familiarization session, the subjects placed their feet with preferred distance between their legs. The distance was measured, used, and controlled before each repetition and exercise. A board (60 × 90 cm) was placed on top of the balance cone to increase the area were the feet were placed. The force output was measured using 2 force cells (Ergotest Technology AS, Langesund, Norway) attached to the floor (2). Two nonelastic straps were attached to the Olympic bar and the force cells. The lengths of the straps were adjusted to each subject and exercise so that all the exercises were performed with a 90° flexion in the knees (Figure 3). Two minutes' rest periods were given between each isometric contraction with a 4 minutes' rest period between each exercise. Between contractions, the subjects sat on an adjustable chair, with the barbell on their shoulders and with the shoulders, knees and feet in the same vertical position. The subjects had a natural sway in their low back and were instructed to maintain the position during the repetitions (Figure 3). A protractor was used to control the knee angle before each exercise. A goniometer was placed on the knee (femur-fibula direction) to control the knee angles during testing.
During testing, 2 test leaders acted as spotters if a subject lost balance. In the squat position, the subjects maintained the balance and gradually increased the force to maximal, which was maintained for 3 seconds. If a subject lost balance or the balance cone touched the floor, the test was interrupted and excluded from further analyses. A new attempt was given.
The 2 force cells (Ergotest Technology AS, Langesund, Norway) were synchronized with the EMG recordings using a Musclelab 4020e (Ergotest Technology AS) and analyzed by commercial software V8.13 (Ergotest Technology AS). Before the experimental test, the skin was prepared (shaved, washed with alcohol, abraded) for placement of gel coated surface EMG electrodes. Electrodes were placed according to the recommendations by SENIAM (14). The electrodes (11-mm contact diameter) had a center-to-center distance of 2.0 cm. Self-adhesive electrodes (Dri-Stick Silver circular sEMG Electrodes AE-131, NeuroDyne Medical, Cambridge, MA, USA) were positioned on the rectus femoris, vastus medialis, vastus lateralis, biceps femoris, soleus, rectus abdominis, oblique external, and erector spinae. The electrodes were placed on the preferred leg but contra lateral side in the trunk. The raw EMG signal was amplified and filtered using a preamplifier located as near the pickup point as possible to minimize noise induced from external sources. The signals were high- and low-pass filtered (maximum cut-off frequency of 8–600 Hz). The root-mean square (RMS) signals were converted using a hardware circuit network (mean constant of 12 milliseconds, frequency response 450 kHz, total error ±0.5%) from the raw EMG signal. The RMS-converted signal was sampled at a rate of 100 Hz using a 16-bit A/D converter with a common mode rejection rate of 106 dB. The stored force and EMG data were overlaid and marked to identify the beginning and end of isometric force output, which were analyzed. The mean RMS EMG was calculated for the 3 last seconds of tests. The greatest force output within the three attempts for each exercises and the concomitant neuromuscular activity were used in further analyses.
To assess differences in EMG activity in the squat testing, a 2-way (4 surfaces × 7 muscles) analysis of variance (ANOVA) with repeated measures was used. When differences were detected by ANOVA, paired t-tests with Bonferroni post hoc corrections were applied to determine where the differences lay. To assess differences in force output and knee angle, a repeated measures 1-way ANOVA was used with a Bonferroni post hoc. All the results are presented as means ± SD and Cohen's d effect size (ES), unless otherwise noted. An ES of 0.2 was considered small, 0.5 medium and 0.8 large. SPSS (v 19.0, Chicago, IL, USA) was used. Statistical significance was accepted at p ≤ 0.05.
The force output relative to the stable squat (749 ± 222 N) was approximately 93% using the power board (694 ± 220 N, p = 0.320), approximately 81% using the BOSU (603 ± 208 N, p = 0.003, ES = 0.66) and approximately 76% (570 ± 257 N, p ≤ 0.001, ES = 0.73) using the balance cone (Figure 4). There was greater force output in power board compared with BOSU (p = 0.037, ES = 0.41) and balance cone (p = 0.001, ES = 0.50). Similar force outputs were observed for BOSU and balance cone (p = 0.852).
For the EMG activity, the surface × muscle interaction was significantly different (F = 1.969, p = 0.008). After post hoc analyses, the EMG activity revealed no significant differences between the different surfaces except for rectus femoris (Table 1, Figure 5). For the rectus femoris, stable squat provided greater EMG activity than the other exercises (p = 0.004–0.030, ES = 0.56–0.93). Further, EMG activity in rectus femoris was lower using balance cone than BOSU (p = 0.030, ES = 0.50). In the soleus, a tendency for greater EMG activity in BOSU than stable surface was observed (p = 0.056).
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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