['The study compares the power outputs in concentric phase of chest presses and squats performed without and with countermovement on stable and unstable support surface, respectively.', 'A group of 16 physical education students performed randomly in four different days 3 repetitions of: 1) barbell chest presses on the bench and Swiss ball, respectively, and 2) barbell squats on stable support base and Bosu ball, respectively. Exercises were performed without and with countermovement (CM) using maximal effort in concentric phase of lifting. Initial weight of 20 kg increased by 10 kg or 5 kg (at higher loads) up to at least 85% of previously established 1RM under stable conditions. A PC based system FiTRO Dyne Premium was used to monitor force and velocity and to calculate power. As a parameter of the capability to use elastic energy was considered the difference in mean power in concentric phase of weight exercises (ΔP) performed with and without CM.', 'There were higher power outputs in concentric phase of CM chest presses as compared to those performed from quiet position on the chest under both stable and unstable conditions. This enhancement of power due to CM was rather modest at lower weights and become more pronounced with increasing weights reaching a maximum at 57.1% 1RM on stable and at 47.6% 1RM on unstable support surface. Lifting heavier weights not only failed to increase the enhancing effect but led to its decline. Similar trend was observed during squats with maximal enhancement of power in concentric phase of lifting at about 80% 1RM under both conditions. The ΔP was significantly lower during chest presses on Swiss ball than on the bench, however, only at higher weights lifted (≥60% 1RM). On the other hand, the ΔP during squats performed on Bosu ball and on stable support base did not differ significantly across all weights lifted.'] Does utilization of elastic energy during weight exercises differ under stable and unstable conditions?