As the hand enters the water, the hand must isometricly flex to prevent excessive movement. This isometric contraction needs to hold the wrist, fingers and thumb in their optimal position.
If the shoulder has poor proximal stability, then the shoulder undergoes slight movements traveling through the entire arm. Just like a building with poor foundation, the top of the building will shake more than the bottom during an earthquake. This can increase the swimmers risk for injury and increase drag. Cue them to hold their shoulder blade down and back to stabilize this proximal joint to prevent distal movement.
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