A sprinter propels himself off of blocks, Which body exerts the force that propels the sprinter?

The sprinter tenses their muscles and then unleases this tension. A force is therefore applied to the block - since the block is stationary the block applies the equal and opposite force (well not so much applies as simply returns the favour). The ground applies the same force the block. Therefore the block does not move. It compresses a tiny bit) But the sprinter has only the force from the block applied to him and therefore leaps forward...