Unfortunately, your elbow's arc isn't quite timed right, and your arm pivots backwards and connects only with the air. Your knee, though, strikes true, and the creature cries out in pain as your strike connects.
Snarling, the creature turns on you and brandishes his spear, thrusting wildly. For a second, his wild thrust looks like it will connect... and then your arm snaps up to block, and the rotted wood that makes up his spear's shaft snaps in two, leaving him unarmed. ((The monster rolled a natural 1 on his attack, which is considered a catastrophic / critical failure.))
His companion wastes no time, though, and charges in with his spear waving wildly. Luckily for you, though, his fear and surprise at seeing you come charging from the trees seems to have made his attacks more wild than effective, and his strike hits only air.