Instructions
Back up and find cover when under attack. If you successfully take cover before the enemy completely depletes one of the four health bars, it will recharge within a few seconds. Note that you can not recharge a fully depleted health bar.
Look for "Sym-Bac Serum" containers on the ground. The Sym-Bac serum is in a bright green container and restores a full block of health when you pick it up.
Allow your character to die and return to the previous checkpoint. Each checkpoint returns four full bars of health to your character.