Can you improve arm strength?

Throwing a football recruits fast twitch muscle fibers (vs. slow twitch, which are recruited for endurance type activities such as long distance running).

As you age, you lose a higher ratio of fast twitch fibers as opposed to slow twitch.

This may play a part in the reduction of "zip" a QB may have when throwing.

Injury, loss of mechanics/technique, and muscle atrophy probably also factor into this.