# There Is No Cross Effect of Unstable Resistance Training on Power Produced during Stable Conditions
## 2 Materials and Methods
### Experimental Protocol  
Participants underwent resistance training for 8 weeks (3 sessions a week). The SG performed barbell bench presses and barbell squats on a stable surface, whereas the UG performed barbell chest presses on a Swiss ball and barbell squats on a BOSU ball (1 st week: day 1--chest presses, day 2--squats, day 3--chest presses, 2 nd week: day 1--squats, day 2--chest presses, day 3--squats, etc.). The training program was designed to improve muscle power. Participants performed exercises with countermovement using maximum effort in the concentric phase. They were instructed to perform each exercise with increasing weights up to the one at which maximal values of power were achieved during testing sessions before and after four weeks of the training. Maximal values of mean power in the concentric phase of resistance exercises are usually achieved at 50-60% 1 RM under stable and 40-50% 1 RM under unstable conditions [45; 46]. Using higher weights during unstable chest presses and particularly during unstable squats may increase fear of falling and thereby compromise the proper technique of movement. However, instability resistance exercises that uses lower forces, can increase strength in untrained and recreationally active young subjects similar to those with stable machines using heavier loads [5]. Numbers of repetitions were also set before and after four weeks of the training (8-10 repetitions at lower weights and 3 to 6 repetitions at higher weights, with 2 min of rest between sets; 5 to 4 sets at lower weights and 3 to 2 sets at higher weights). Preliminary findings revealed that power during chest presses decreases below 90% of its maximum after four repetitions when performed on the bench and after two repetitions on a Swiss ball [47]. Furthermore, the power falls below this limit after seven repetitions of squats on a stable surface and after four repetitions on a BOSU ball [47]. In order to increase maximal power, 2 to 3 repetitions are recommended for training in the Pmax zone (45-60% 1 RM maximizes power during the jump squat and bench throw) and 3 to 5 or 6 repetitions in the general and ballistic power zone (30-45% 1 RM) [48]. Higher repetitions (e.g., 8-10) with lighter resistances should be performed for learning technique [48]. Within a familiarization session, the training design was explained and trial exercises were performed. Emphasis was given to proper exercise techniques on unstable surfaces. All exercise sessions were supervised by members of the research team. Adherence to the exercise program was registered and regularly encouraged. An assessment of muscle power was carried out before and after 4 and 8 weeks of the training. Prior to testing, a standardized warm up protocol was undertaken. Afterwards, participants performed in random order a) chest presses on a bench, b) chest presses on a Swiss ball, c) squats on a stable surface, and d) squats on a BOSU ball. They were instructed to perform all exercises with maximum effort in the concentric phase. The weight lifted of 20 kg was increased by 10 kg or 5 kg (at higher loads) up to 85% of their previously determined 1 RM in stable conditions. As shown previously, muscle EMG activity and 1 RM strength do not differ significantly during chest presses on a stable surface and an unstable exercise ball [8]. Two minutes of rest between repetitions were allowed for each weight lifted. The highest value of power obtained during 3 or 2 (at higher loads) trials was taken for evaluation. When chest presses were performed, participants lowered the barbell to the chest, without contacting it in the transition phase, and then pressed it up. Repetitions that failed to come within 5 cm of the chest or contacted the chest were ignored and repeated after 1 min rest. The barbell movement distance was monitored by means of the FiTRO DynePremium. Participants were asked to keep the same width of the grip for the whole test protocol and to maintain the contact between the bench and their back and hips.