Rustad, O., & Seiler, S. (2009). Maximal dynamic force development in the shoulders under stable and unstable conditions. A paper presented at the 14th Annual Congress of the European College of Sport Science, Oslo, Norway, June 24-27.

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"The use of surface instability has gained popularity in strength training. However, the impact of unstable conditions during training on functional changes in force production is not well established and methods of isolating instability to specific joint complexes are lacking." This study quantified the impact of surface instability on maximal force development with the shoulder complex during a weight bearing (closed kinetic chain) push-up task. Additionally, the test-retest reliability of maximal dynamic force measurements was quantified under both conditions.

Physically active university students (M = 14; F = 6) performed two maximal dynamic force tests under stable and unstable conditions. The stable condition consisted of maximally explosive push-ups performed on a force platform while holding push-up bars. The unstable condition involved the same movement while holding hand grips hanging from two slings attached to the ceiling. Body angle and hand width were identical under both conditions. The slings were equipped with force transducers and the unilateral peak force production was summed. Movement was initiated from an outstretched position, lowering to 90 degrees elbow flexion and explosive extension. Ss performed three series of three maximal efforts for each test, on two separate days separated by 48 hours or more. The results from the test day 2 were used to quantify the impact of instability on maximal force.

Test-retest comparisons showed no systematic bias for either condition from day 1 to day 2 (r = 0.94 and 0.90 for stable and unstable conditions respectively). Unstable conditions resulted in a 30% reduction in peak force compared to stable conditions.

Implication. Unilateral instability directly applied to the shoulder complex results in a loss of maximal force of approximately 30% during a dynamic maximally explosive push-up.

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