To understand the short-term systemic effects of local PRP injections, we will measure the serum concentrations of six growth factors before and during the four days following PRP injection: human growth hormone (hGH), IGF-1, basic FGF, PDGF-BB, VEGF, and IGFBP-3. These molecules are of particular interest because they may have stimulatory effects that enhance athletic performance, and because they have been banned in competitive athletes by the World Anti-Doping Agency (WADA).