L-Arginine is a conditionally essential amino acid responsible for a variety of physiological functions including immune system defense and hormone secretion and immune system defense. Typically, the human body produces arginine on its own, but in times of extreme stress (i.e. working out), the body simply can't synthesize enough to meet the demand, thus requiring supplementation.
Arginine is required in the body’s formation of creatine, increasing growth hormone (GH) output, removing waste products, and regulating hormone secretion and sodium levels. The most prominent feature of Arginine is as a precursor to Nitric Oxide (NO).
Nitric Oxide is a powerful vasodilator that is used in the treatment of several cardiovascular diseases (including high blood pressure) as well as erectile dysfunction. In regards to exercise, Arginine's vasodilating properties relax and expand blood vessels allowing more blood to flow through them, ultimately resulting in increased circulation and a much bigger and badder pump when lifting!
Lastly, increased circulation sends additional blood, oxygen, and nutrients to your working muscles ultimately allowing them to perform at peak output for longer bouts.