The impact of instability on force development and the effect of strength training under unstable conditions
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Objective: The efficacy of maximal strength training on unstable surfaces remains unclear. The aims of this study were therefore to 1) quantify maximal dynamic force during a push-up movement with and without instability, 2) investigate whether training under unstable conditions would decrease the deficit between unstable and stable maximal force and 3) compare strength adaptations in the shoulder complex between traditional strength training and unstable strength training. Methods: 29 physically active university students (23 males, 6 females) performed maximal dynamic force tests under stable (explosive push-up on force platform, MDFstable) and unilaterally unstable (explosive push-up in instrumented slings, MDFunstable) as well as stable (PUstable) and unstable (PUunstable) push-up to failure tests and 1RM bench press. 19 of the subject were randomized to an unstable training group or a stable training group. The remaining 10 was recruited as non-training controls. The two training groups trained 2 days a week for 8 weeks using identical periodization of sets and repetitions. Testing was repeated after 8 weeks. Results: For all subjects, MDFunstable was 26 ±15% lower than MDFstable during preliminary testing (MDFunstable/MDFstable, ratio = 0.75±0.21). Training under unstable conditions did not decrease the deficit between MDFunstable and MDFstable because both improved similarly, but a significant decrease in deficit between PUunstable and PUstable was observed (pre: PUunstable = 51±17% of MDFstable, post: PUunstable = 78±16% of MDFstable, p ≤ 0.01). Both stable and unstable training induced similar improvement in MDFstable (unstable: 25±20%, stable: 27±21%), 1 RM bench press (unstable: 10±7%, stable: 13±6%) and PUstable (unstable: 27±26%, stable: 30±31%). However, the unstable training group increased significantly more than the stable group in MDFunstable and PUunstable (p ≤ 0.05). Conclusion: Unilateral instability directly applied to the shoulder complex results in substantial (~25%), but individually variable loss of maximal dynamic force during a dynamic, maximally explosive push-up movement. Strength training under unstable conditions did not reduce the deficit in force development between stable and unstable conditions. Strength training in unilaterally unstable slings stimulates large improvements in strength and maximal dynamic force development under both unstable and stable conditions, while stable strength training improvements is more limited to stable conditions.
Masteroppgave i idrettsvitenskap Universitetet i Agder 2010