Deliberate Muscle Spasm

[steddie]

I was laid on my back with legs raised in an S shape on a chair. With a dumbbell in each hand I held them at arms length above my upper chest and then moved them forwards so they were arms length above my waist. My upper chest area (breast) muscle began to spasm violently as if they moving 100 mile an hour up an down whilst the rest of my body was still. The question is what affect does this have exactly? Does it strain the muscle in a good damaging way!?!