The bicycle is a human invention for increased locomotion, and has only been around for a spec of time in the evolutionary time frame of homo sapiens. Meaning, we are not born knowing how to ride a bike and our bodies are not intrinsically adapted to doing so.
Putting a person on a machine and inviting them to perform repetitive motions for a sustained time period is an invitation to injury, and yet somehow provides an abundance of enjoyment and pleasure for many. But the invitation to injury is still present.
One of the keys to preventing injury and promoting enjoyment is to ensure a safe and functional bio-mechanical relationship between the bike, and the body that is riding it. Bike sizing and fitting are about this relationship. Sizing and Fitting are related, but are not the same thing, and are sometimes used inaccurately. Both are important.
Sizing precedes fitting. Sizing is about initial matchmaking and compatibility. Will this bike and body get along together? Read More on Sizing here.
Fitting is about strengthening the relationship and working out the wrinkles and niggles.
Read more on Fitting here.