Following training, the body adapts by increasing muscle cross sectional area through an increase in the size of actin and myosin filaments. This results in the addition of sarcomeres to existing muscle tissue. Hypertrophic increases occur in a selective manner primarily to fast twitch muscle fibers and may take as long as eight weeks to become evident. There also appears to be some limit to how large a myofibril can become; at some point, they split.