Purpose: The aim of the study was to assess the task-specificity (greater improvements in trained compared to non-trained tasks), transferability and time-course adaptations of resistance-training programs with varying instability requirements.
Method: Thirty-six resistance-trained men were randomized to train chest press 2 days week-1 for 10 week (6 repetitions × 4 series) using a Swiss ball, Smith machine or dumbbells. A six-repetition maximum-strength test with the aforementioned exercises and traditional barbell chest press were performed by all participants at the first, 7th, 14th and final training session in addition to electromyographic activities of the prime movers measured during isometric bench press.
Results: The groups training with the unstable Swiss-ball and dumbbells, but not the stable Smith-machine, demonstrated task-specificity, which became apparent in the early phase and remained throughout the study. The improvements in the trained exercise tended to increase more with instability (dumbbells vs. Smith machine, p = 0.061). The group training with Smith machine had similar improvements in the non-trained exercises. Greater improvements were observed in the early phase of the strength-training program (first-7th session) for all groups in all three exercises, but most notably for the unstable exercises. No differences were observed between the groups or testing times for EMG activity.
Conclusion: These findings suggest that among resistance-trained individuals, the concept of task-specificity could be most relevant in resistance training with greater stability requirements, particularly due to rapid strength improvements for unstable resistance exercises.
Keywords: Bench press; Electromyography; Instability; Strength training.