The Stanford BioMotion Lab is conducting a study to examine how a blood protein product, platelet-rich plasma (PRP) may be associated with the way people walk. We are recruiting people who meet the following: • Between 18 -- 30, or 45 -- 60 years of age • Willing to give one small sample of blood • Willing to come in for three appointments for gait analysis • No history of knee joint disease, injury, or surgery Participants will be compensated for their time. This research is being conducted under the direction of Dr. Constance Chu. To see if you are eligible for this study, please contact us.