# There Is No Cross Effect of Unstable Resistance Training on Power Produced during Stable Conditions
## 4 Discussion
Interestingly, this power enhancement from the 4 th to 8 th week of the training was much faster during chest presses on an unstable surface with greater muscle involvement in work when compared to bench presses. This may be demonstrated by no significant differences in muscle power during stable and unstable chest presses after 8 weeks of training. With respect to squats, the power was also significantly higher under stable than unstable conditions, but only prior to the training. Its steep increase was observed during the initial 4 weeks followed by slower improvement within the 8 weeks of the training. The squat on a stable surface is a sport specific skill learned by athletes in the autonomous stage of motor learning [59]. Similar to during bench presses, it is most likely that resistance trained individuals performed this motor task effectively and consistently. Therefore only muscles involved in this specific movement are contracting, thus the power produced by them during initial weeks of training is higher as compared to those performing under unstable conditions. However, the squat on an unstable surface is also not a new exercise because it is performed by athletes during movements on natural unstable surfaces (grass, sand, etc.). Therefore the dynamic stereotype of squats on the BOSU ball is learned much better than chest presses on the Swiss ball. This can explain a lack of significant differences in power produced during stable and unstable squats. This confirms and complies with the principle for specificity of training [60; 61; 62]. However, these instability resistance exercises are very similar to those performed on a stable support base. Therefore training using unstable surfaces could lessen the proper exercise performance on the stable surface due to impaired dynamic stereotype of this movement under stable conditions. This could contribute to no significant changes in muscle power during squats and chest presses on the stable support base in the group that underwent instability resistance training. Similarly, [PERSON] and [PERSON] [5] reported no significant differences after 8-week training on unstable and stable surfaces, though there was a trend for the UG to enhance the stable-to-unstable chest press force ratio to a higher level than the SG. This may be ascribed to greater stress on the neuromuscular systems under unstable conditions and thus greater training adaptations [63; 64]. Contrary to this, the lower force, power and movement velocity [1; 2; 23; 34; 65; 66; 67; 68] associated with instability resistance exercises can result in less rigorous power and strength adaptations. However, there are also studies that did not demonstrate the force reduction under unstable conditions [3; 8; 28]. Besides the type of exercise (i.e., chest press versus squat) and the instability device used (i.e., the more compliant Swiss ball versus the stiffer BOSU ball providing different levels of task difficulty), also number of sets and repetitions, the weight used and rate of loading may induce load- and velocity-specific training adaptations under unstable conditions [69]. For instance, lower loads applied during training on unstable surfaces provides sufficient stimuli to ensure similar power or strength training gains as compared to training on stable surfaces using higher loads [5]. It is most likely that the group who trained bench presses and squats on a stable surface was able to exert themselves at higher contraction velocities throughout training sessions and thereby received more specific training stimuli for power development. However, the group that performed resistance exercises on unstable surfaces trained very probably at lower velocities due to the requirements for balance control. Therefore if the one is aiming to specifically improve muscle power then unstable exercise balls or wobble boards should not be included in resistance training. A review by [PERSON] et al.