# There Is No Cross Effect of Unstable Resistance Training on Power Produced during Stable Conditions
## 4 Discussion  
Findings revealed significant increase in power outputs during unstable chest presses at weights 20-40 kg after 4 weeks and at weights 20-50 kg after 8 weeks of the instability resistance training. Power outputs also increased during squats on a BOSU ball at weights 20-60 kg after 4, but not after 8 weeks of the instability resistance training. However, the values did not change during bench presses or squats on the stable support surface after the same training. A between-group comparison revealed that significant stable to unstable differences in power outputs before the training disappear after 8 weeks of chest presses and after 4 weeks of squats. Lower pre-training values of power during resistance exercises on unstable than on stable surfaces are in agreement with previous studies. A recent systematic review by [PERSON] [55] reported that instability decreases muscular power, strength and speed in adults when compared to stable conditions. However, compromised muscle power during instability resistance exercises was only found at higher weights, i.e., \(\geq\)60% 1 RM [29]. The power decline was more evident for unstable chest presses than unstable squats when performed in an interval mode (6 sets of 8 reps with 70% 1 RM) [56]. Reduced power during unstable chest presses may be attributed to the delayed transition phase of the stretch-shortening cycle (SSC). The SSC activation enhances muscle power while lifting a weight [57]. This includes the energy storage of elastic components and the stimulation of the stretch reflex. If the coupling time is too long, elastic energy may be lost as heat and the stretch reflex is not fully activated [58]. Instability resistance exercises may compromise all SSC phases, in particular the transition phase. Around this phase maximum force is produced. To provide the support for contracting muscles, individuals must stabilize their body on an unstable surface. This task may affect the contraction of muscles acting on the barbell. Their less intensive contraction prolongs the transition time and, because of  
Figure 1: Maximal values of mean power during (**a**) chest presses and (**b**) squats before and after 4-week training under stable and unstable conditions. lower peak force, also affects elastic energy accumulation. This results in lower velocity and power produced in the concentric phase of instability resistance exercises [45]. However, significant differences in mean power during chest presses under stable and unstable conditions were also found after 4 weeks of training. The bench press is a sport-specific skill learned by athletes in the autonomous stage of motor learning [59]. It is most likely that resistance trained individuals performed this task largely automatically with minimal demands on cognitive processing. 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 on an unstable surface. On the other hand, chest presses on the Swiss ball were not performed by athletes before and therefore this motor skill is in the cognitive stage of motor learning [59]. This new task was inconsistent and inefficient and required considerable attention in individuals with no experience with instability resistance exercises. Many muscles are contracting during this exercise, though not all of them are involved in the chest press motion, but they may relieve the work of muscles that are involved. Many other muscles also contract to keep the body balanced on an unstable surface. This is a new skill and therefore the power during initial weeks of the training was smaller. As the athlete become more familiar with chest presses on an unstable Swiss ball, the movement also becomes more efficient. Their repetitive performance during training sessions led to an increase in muscle power.