Stable to unstable differences in force-velocity-power profiling during chest presses and squats

Despite several studies investigating the effect of instability resistance exercises on neuromuscular performance, the force-velocity-power characteristics of muscles involved in lifting tasks and the underlying mechanisms have not been fully explored. This study investigates power-velocity and force-velocity relationships during resistance exercises performed on stable and unstable surfaces with different weights. A group of 63 physically active young men performed chest presses on the bench and Swiss ball, and squats on the firm surface and BOSU ball with weights from 20 kg to at least 85% of one-repetition maximum. Peak and/or mean values of power, velocity and force were analyzed. Results showed significantly lower peak power and force during chest presses on the Swiss ball as opposed to the bench at lower velocities (147.6 W and 176.0 N at 1.1 m·s^-1, 108.7 W and 126.4 N at 1.3 m·s^-1, 112.0 W and 72.7 N at 1.5 m·s^-1; all at p < 0.01). Their values produced at lower velocities were also significantly lower during squats on the BOSU ball when compared to the firm surface (232.2 W and 257.1 N at 1.1 m·s^-1, 228.2 W and 173.3 N at 1.2 m·s^-1, 245.1 W and 156.8 N at 1.3 m·s^-1, 254.5 W and 113.5 N at 1.4 m·s^-1; all at p < 0.05). These significant differences between power produced during stable and unstable resistance exercises at lower velocities (or at higher loads) have to be taken into account in sports that require production of a high force in a short time. Because of the variable loading patterns under unstable conditions, it is necessary to quantify the optimal exercise load for each individual athlete.

Instability resistance training for health and performance

Recently, resistance exercises performed on an unstable surface have become part of athletic training and rehabilitation. Accordingly, their role in performance and health-oriented strength training has increasingly emerged as a matter of interest to researchers and conditioning specialists. A more pronounced activation of stabilizing muscles is assumed to be the main feature of instability resistance exercises. This assumption has been proven by EMG studies, which have highlighted significantly greater electromyographic activity of trunk-stabilizing muscles during exercises under unstable as compared to stable conditions. Intervention studies also demonstrated an enhanced improvement of trunk stability after training programs utilizing unstable devices as compared to floor exercises. Findings indicate that instability resistance training may facilitate the neural adaptation of trunk-stabilizing muscles, resulting in an improvement in trunk stability. However, both acute and long-term responses of primarily activated muscles to exercises performed on an unstable surface remain a matter of debate. It has been established that there is a significantly lower peak isometric force and rate of force development during resistance exercises under unstable as compared to stable conditions. In addition, the power output was compromised when exercises were performed on unstable surfaces. However, we have demonstrated that this effect depends on the type of exercise, instability device used, weight lifted, subject's training background, and so forth. Our findings on muscular power in the concentric phase of resistance exercises with different weights under stable and unstable conditions complement this review. Applications of instability resistance exercises for the improvement of neuromuscular functions in the physically active, plus for those following anterior cruciate ligament reconstructions, are also presented.