Sparkes, R and Behm, DG. Training adaptations associated with an 8-week instability resistance training program with recreationally active individuals. J Strength Cond Res 24(7): 1931-1941, 2010-Instability devices are popular training modalities; however, their training effectiveness has not been well established. The objective of this study was to determine differences in physiological and performance measures after stable and unstable resistance training. Eighteen subjects (10 men and 8 women) resistance trained 3 d·wk−1 under either stable or unstable conditions for 8 weeks. Pre and posttraining measures included chest press isometric force and electromyographic activity of the triceps brachii and pectoralis major under stable and unstable conditions and 1-legged throwing distance, balance, countermovement jump (CMJ) and drop jump (DJ) heights. There were no significant training group effects found with any measure. However, there was a tendency (p = 0.06) for the unstable training group to improve the stable to unstable chest press force ratio to a greater degree (24.8%) than the stable group (10.8%). There were significant overall pre to posttraining improvements in maximum voluntary isometric contraction (MVIC) force (13.3%: p < 0.0001), unstable/stable force (18.2%: p = 0.0005), bench press (11%: p < 0.0001), squat (14.9%: p < 0.0001), CMJs (11.2%: p = 0.002), and DJs (3.3%: p = 0.001), wobble board contacts (12.4%: p = 0.03), and wobble board on-off ratios (62%: p = 0.005). There was a significant (p < 0.0001) 42.2% greater MVIC force and 43.2 and 33.2% greater triceps (p = 0.003) and pectoral (p = 0.005) neuromuscular efficiency with stable vs. unstable isometric chest press. It appears that instability resistance training, which reportedly uses lower forces, can increase strength and balance in previously untrained young individuals similar to training with more stable machines employing heavier loads.