Viagra works by increasing the effects of nitric oxide (NO), a substance that serves many key functions in biological processes throughout the body. One of the most well known and important functions of NO is the dilation of blood vessels. This allows greater blood flow to the muscles, which of course can be valuable to an athlete during competition.

What is more interesting to me is the role of NO on muscles during resistance training. JE Anderson found that NO appears to be a vital signal in the activation of muscle satellite cells in response to damage.

Satellite cell activation is the key first step in the repair and hypertrophy of muscle cells after heavy training. Viagra may therefore enhance the hypertrophy response to exercise, working at the most basic and primary level of the process.

In addition to this, there is evidence that suggests that Viagra may work to amplify the "pump" response during training. The pump is thought to happen when contracting muscle fibers signal local vascular relaxation (increasing the blood flow to the working muscles). According to KS Lau and coworkers, NO generated by neuronal NO synthase in contracting skeletal muscle fibers may regulate vascular relaxation via a cGMP-mediated pathway.

Since the mechanism of action for Viagra is amplification of the cGMP pathway, there is ample reason to believe that the drug may indeed affect the blood flow and pump to the muscle, and therefore indirectly aid in the hypertrophy response.