# There Is No Cross Effect of Unstable Resistance Training on Power Produced during Stable Conditions
###### Abstract  
This study evaluates the effect of 8 weeks of the stable and unstable resistance training on muscle power. Thirty-three healthy men recreationally trained in resistance exercises, randomly assigned into two groups, performed resistance exercises either under stable or unstable conditions for 8 weeks (three sessions per week). Before and after 4 and 8 weeks of the training, they underwent squats and chest presses on either a stable surface or on a BOSU ball and a Swiss ball respectively with increasing weights up to at least 85% 1 RM. Results showed significant improvements of mean power during chest presses on a Swiss ball at weights up to 60.7% 1 RM after 4 and 8 weeks of the instability resistance training. Mean power increased significantly also during squats on a BOSU ball at weights up to 48.1% 1 RM after 4 but not 8 weeks of instability resistance training. However, there were no significant changes in mean power during bench presses and squats on a stable support surface after the same training. These findings indicate that there is no cross effect of instability resistance training on power produced under stable conditions. This confirms and complies with the principle for specificity of training. Swiss ball; squats; muscle power; chest presses; BOSU ball +
Footnote †: journal: Applied  
operates in an open-chain fashion in the majority of sporting movements. Conversely, similar interventions could prove to negatively affect performance in the lower extremities, which typically operate in a closed-chain fashion in most athletic activities. Similarly, more recent studies have reported limited effectiveness of instability resistance training on proxies of athlete performance (e.g., straight-line sprint speed, planned and reactive agility, countermovement jump, drop jump, trunk muscle strength/activation, balance, aerobic capacity, the ball kicking speed) over exercises performed under stable conditions [20, 21, 22]. The main argument is that isometric force (about 30-70%), rate of force development (40.5%) and power (59%) are lower during exercises, such as dumbbell chest presses [23], deadlifts [24], leg extensions, plantar flexions [25] and squats [26, 27] performed on an unstable than a stable surface. Values of force, velocity and power are also lower during dynamic chest presses (6-10%) [28, 29] and squats (16%) [29] under unstable conditions. Because of this, resistance exercises on a stable support base have been recommended for improvement of muscle strength and athlete performance [26, 30]. The requirement of 80% of the maximum muscle strength for its improvement in trained subjects [31] is not achieved during resistance exercises on unstable surfaces [26, 30]. Nonetheless, resistance exercises on unstable surfaces are a great tool for functional training. Unlike conventional strength training, instability resistance training addresses both postural and core stability [32, 33, 34]. Its main feature is a more pronounced activation of stabilizing muscles. This may be demonstrated by increased EMG activity of trunk-stabilizing and other muscles involved in a given exercise under unstable conditions, such as curl-ups [35], chest presses [36, 37], push-ups [38], seated overhead shoulder presses [39], side-bridges [40], single-leg holds and press-ups [33] and squats [26, 41]. It is therefore very likely that a greater challenge to the neuromuscular system is induced by unstable than stable resistance exercises. They may potentiate the neural adaptation of stabilizing trunk muscles and thereby improve core stability [42, 43, 44]. However, the question remains as to whether these beneficial effects of instability resistance exercises can be transferred in traditional resistance training on a stable support base and thereby enhance neuromuscular performance. Specifically, the extent to which unstable resistance training contribute to power enhancement during traditional resistance exercises needs to be investigated.