Effects of eight weeks of bench press and squat power training on stable and unstable surfaces on 1RM and peak power in different testing conditions

1. Introduction

Resistance training exercises under unstable conditions, such as with unstable devices (Swiss ball, BOSU ball, hemispheric and inflatable discs) have gained popularity in the past decade, and additionally, numerous studies have evaluated the role of unstable surfaces in resistance training programs. The use of unstable training environments has been proposed to enhance the specific effects of movement through an increased activation of stabilizers and core muscles [1, 2] and thus has been advocated as more beneficial than machines [3]. According to the concept of training specificity, a training under unstable or unbalanced conditions may provide the instability that can occur with the activities of daily living, work, and athletic environments, providing a more effective transfer of training adaptations [2].

The use of unstable platforms in resistance training should result in the development of higher levels of muscle activation via increased reliance on their stabilizing functions. Intervention studies have indicated that a short-term training program using a Swiss ball is more effective compared with floor exercises in improving the trunk stability [4, 5]. A greater instability should challenge the neuromuscular system to a greater extent than stable conditions, possibly enhancing strength gains attributed to neural adaptations [6]. However, training activities on unstable surfaces and with unstable devices are not recommended as the primary exercise for hypertrophy, absolute strength, or power [6]. As muscle power is an essential component in many athletic activities, it is not unusual that some organizations like The Canadian Society for Exercise Physiology do not fully endorse training on unstable surfaces for athletic or sport performance training. Their position stand [6] warns that “From a performance standpoint, unstable devices should not be utilized when hypertrophy, absolute strength, or power is the primary training goal, because force generation, power output, and movement velocity are impaired and may be insufficient to stimulate the desired adaptations, especially in trained athletes”.

However, the majority of studies involving unstable surfaces, focusing on the training program effects, have used very high loads (above 70% of 1RM) [7, 8, 9]. Similarly, training interventions in previously healthy, well trained subjects [10], have also used high loads (5–15RM), implying moderate movement velocities.

In the literature, loads between 30% and 60% have been suggested as the loads that produce the highest values of muscular power in bench press output [11, 12, 13, 14] and 40–65% of 1RM in half squats [11]. The load of 50% of 1RM should be considered reasonable for power enhancement on unstable surfaces, allowing proper techniques to be employed during the exercises, without any excessive anxiety about falling.

The only study examining explosive movement on an unstable surface, to the best of our knowledge, has been conducted by Koshida et al. [15]. They observed a loss of peak outputs in unstable conditions during dynamic bench press exercises performed with the load of 50% of 1RM. However, the reduction rates remained relatively low, approximately 6% for force, and 10% for power and velocity outputs.

According to the principle of training specifi- city [16], a training activity must attempt to mimic closely the desired physical activity. Similarly, any success in sports and other physical activities that require dynamic action and certain level of muscle power should be best achieved by the training performed with similar movement velocity. Moreover, any testing of power demands maximum movement velocity and it is therefore reasonable that training should be performed with maximum movement velocity allowed by the unstable surface in question.

The purpose of the study was to determine the effects in muscular strength and power outputs after eight weeks of stable and unstable resistance training in physically active subjects with limited resistance training experience in organized free weight training. It was hypothesized that unstable resistance training performed with maximal possible velocity of movement during the concentric phase of movement would provide significantly greater training gains.

2. Method

4. Discussion

The results of this study found the training-induced improvement between the training groups in pre- to post-test in squat 1RM, showing the transfer of specific unstable training exercises on 1RM squat. However, this was not the case in 1RM bench press. No significant difference in the main effect was found with the training-induced improvement between the training groups in pre- to post-test on 1RM bench press. Both modalities (Stable and Unstable) produced statistically significant ( 0.05) training effects, with very small increases ranging from 3–7%. One of the possible explanations for these differences between exercises could be found in a higher degree of instability during squat exercise. During squats, the subjects had to stand on a BOSU ball (hemispherical dome), while during bench press on a gymnastic ball some level of stability was provided with feet on the ground. Studies [19] have demonstrated that performing squats on unstable surfaces could increase core muscle activation compared to performing squats under stable condition. Anderson and Behm [20] reported that higher degrees of instability (inflatable discs $>$ Olympic squat $>$ Smith machine) when performing a squat resulted in approximately 20%–30% greater activation of the trunk stabilizers and postural muscles. Same authors [20], also found a significant increase in the duration abdominal stabilizer activation from the eccentric to concentric phase of the lifts in the unstable protocols compared to the more stable movements (0.66 s for unstable squat, 0.54 s for Smith squat, 0.51 s for free squat). Additionally, increased velocity of movement during exercise on unstable surface without any stability provided by other parts of the body during squat exercises on BOSU ball could amplify the magnitude of stabilizers as well as prime movers activation.

These findings suggest that unstable resistance training may have had a small tendency to be more efficient in increasing muscle strength in some exercises. Some of the earlier studies [7, 8] have also shown a similar tendency for training on unstable surfaces. One training study [8] showed no significant main effect difference after seven weeks of resistance training in stable vs. unstable conditions, but the interaction effects showed that unstable training provide an advantage in some of the test performance measures. It is important to mention that the percentage of 1RM was not the same for both groups (75% for stable and 50% of 1RM for unstable exercises). A study by Sparkes et al. [7] found no training effects in any measure, but they suggested a tendency for unstable training to be more efficient for maximum voluntary isometric contraction in bench press exercises. They used the same relative resistance for both surfaces (10RM), but the repetition maximum on unstable surface was lower compared to stable surface, which implied that the group that trained on unstable surfaces used lower loads during exercises. In our study both experimental groups had the same external resistance (50% of 1RM tested on stable surface), which with additional instability resulted in increased intensity of the exercise for unstable group. This may be the principal reason for better results of unstable training group in strength test expressed as 1RM on stable surface.

Previous studies that examined the impact of platforms on power outputs on bench press have shown conflicting results. Several studies [21, 22, 23] showed a significant reduction of prime movers maximum force in unstable conditions, while some studies [7, 9, 17, 24] focused on the effects of instability revealed no significant difference in muscle activation between surface types for bench press exercises. However, all these studies examined different parameters of muscle output: maximum force in isometric conditions, rate of force development, maximal strength expressed as 1RM in different conditions. Common for the above studies was that all of them used maximal or submaximal resistance with high loads.

The study of maximum power output in dynamic conditions was performed by Koshida et al. [15], who found a 10% loss of peak power output on unstable surface. The results of our study did not show any statistically significant difference in maximal power output in bench press exercise under stable and unstable conditions in the initial testing. In contrast, non-significant higher values of power were obtained in unstable compared to stable conditions. The reason for the difference between maximal muscle power under the same load of 50% of 1RM could be found in different postures adopted during the use of exercise ball. In our study, the participant head and neck regions were supported, as commonly recommended in the literature [17, 25, 26]. Goodman et al. [17] hypothesized that different postures taken during resistance exercises on an exercise ball could be the reason for the observed discrepancy of the results. Moreover, one additional familiarization session compared to the study by Koshida et al. [15] could have resulted in a better learning effect and better use of elastic energy of the ball.

One of the most important findings of this study was that both training programs (eight weeks of training on stable and unstable surfaces, with load of 50% of 1RM) produced greatest power gains on the surface on which the training took place (for both exercises, bench and squat). There was a significant main effect for peak power after 8 weeks of training on different surfaces between groups (UTG, STG & CG) for both bench press and squat exercises. Training on stable surface bench press was effective only on stable surface testing, without transfer on unstable bench press power testing. On the other hand, training on unstable bench press produced significant increases for both surfaces, with larger increases on unstable (7%) compared to stable conditions (5%). Squat training on stable surface showed significant main effects observed for peak power, indicating that there were training advantages of power training with 50% of 1RM in stable surfaces on squat power test in stable and unstable conditions. The stable training group had a larger increase in stable power testing (7%) compared to unstable testing conditions (5%). However, the training resulted on statistically significant increases for both conditions. The experimental group that trained on unstable conditions (BOSU ball) also showed significant main effects for peak power tested on different surfaces. Similarly to the stable training group, the largest increases were observed during surface on which the training occurred. Only, increases during testing on unstable surface were statistically significant (12%), compared to increases obtained in stable conditions (3%). Specificity during testing indicates that gains were in part specific to the type of exercise employed in the training. This specificity of strength gains is well known [16, 27] and has been attributed to neural adaptations resulting in the ability to recruit the muscles to perform a particular type of action.

The largest peak power increases occurred after squat training on unstable surface, which suggests that unstable surface will not necessary have negative effects on performance. Previous research [10] suggests that unstable interventions could prove to negatively affect performance in the lower extremities, which typically operate in a closed-chain fashion in most athletes. By training slowly and tentatively, the athlete may be conditioned to perform in the same slow manner when faced with athletic challenges. The antagonist activity is heightened during unstable training to maintain joint stability [22], stating that it is not unreasonable to conjecture that such a training effect could be detrimental to optimal rate and magnitude of force production when applied for an extended training period [10]. In the current study the participants were instructed do perform the movement with the maximal possible speed of movement in current conditions, which lead to larger increases in unstable surfaces compared to stable conditions.

However, compared to other similar studies, the training-related increases in our study on both surfaces were relatively small (up to 12% in testing specific to training conditions), with maximal effect size of 0.6, indicating small to moderate changes. Some previous studies [7, 8, 9] similar in duration (five to eight weeks) and test types (bench and squat), have demonstrated that resistance training on unstable surfaces may be considered as effective as traditional stable resistance training. Moreover, all of the studies have produced larger overall pre- to post-training improvements.

In the study by Cowley et al. [9], 1RM bench press increases between 15 and 19% on stable and unstable surfaces, with no intergroup differences, have been found after only five weeks. A larger increase after shorter study could be explained by the subject status (young untrained women, with initial 1RM bench of 32 kg and 33 kg). An experimental study by Behm and colleagues [8] employed the identical squat exercises as we did (50% of 1RM on BOSU ball), but resulted in an increase of over 30% compared to maximum 12% found in our research. However, their training program consisted of several additional exercises. Another study by Behm and colleagues [7] used the same resistance for both surfaces (10RM). Their 8-weeks of unstable resistance training program resulted in the improvement in 3RM bench press exercises (11%) and squats (14.9%). The smaller observed increase of muscle strength (3–7%) and power (3–12%) in our study could be explained by the fact that we used recreatively active students as subjects. Even though none of the subjects participating in this study was not involved in organized and programmed resistance training in the period of six months before the study, they still had average of 6 hours of practical classes per week involving different exercises and sport drills within their faculty curricula in the previous three semesters. Also, the training program with only two exercises had much less impact than the previously mentioned experimental program. Finally, the lack of overload and variation in the design of experimental program may explain why the magnitude of the changes in both the bench press and the squat are smaller than expected. These could be considered as one of the limitations of this study.

Another problem in the current power assessment is fact that there are some limitations for lower body movements [28], suggesting that barbell kinematics should not be used for power assessment applied to the barbell and body system center of mass during a lower body resistance exercise. Participants accelerated the barbell as much as possible during the concentric phase not throwing the bar during the bench press or jump during the squat because of unstable conditions, which lead to the deceleration phase during the concentric phase. Otherwise the bench press would be a bench throw and the squat a squat jump.

Finally, the use of additional performance evaluation would give more light on the effect of the program on performance test. The limitation of this study is the lack of performance test that would be an appropriate evaluator of training program effects. The use of a performance test (e.g. medicine ball throw or standing high jump) would give more insights into the effectiveness of the experimental program in stable and unstable surface with 50% of 1RM on performance test that require explosiveness and fast movement.

Although unstable training produced significant increases when tested on unstable surfaces and also showed a positive transfer in 1RM squats, additional research is warranted to examine if this increase is only the result of adaptation to the training task or the effects of unstable training could be transferred to different physical performance tasks, especially the ones that demand a certain level of instability, as well as the tasks that include fast explosive movement.

5. Conclusion

The current study suggests that a 8-week resistance training program under unstable conditions with load of 50% of 1RM focused on the maximal velocity of movement could be effective for the increase of power in bench press and squat. The greatest power gains occurred on the surface on which the training took place (for both exercises, bench and squat) confirming the principle of specificity. The largest peak power increases occurred after squat training on unstable surface, which suggests that unstable surface will not necessarily have negative effects on lower body performance. However, there was no clear evidence that training under unstable conditions is more effective in increasing the values of muscular power outputs in relation to traditional exercise under stable conditions for every exercise and every condition.

The results of this study also found training-induced improvement between the training groups in pre- to post-test in squat 1RM, showing the transfer of specific unstable training exercises on 1RM squat. However, this was not the case in 1RM bench press. The increase of 1RM in previously resistance untrained subjects is lesser than expected, and it is therefore necessary to include additional exercises or additional loads in the training program on unstable surface if the increase of maximal strength is the training goal.