Disclosed herein are exemplary methods and systems for selecting a shaft flex for a golfer based on characteristics of the golfer's swing. Some exemplary methods comprise using measuring in-swing-plane shaft deflection values of a golfer's golf club during the golfer's swing, determining a phase of the golfer's swing when maximum in-swing-plane shaft deflection occurs; determining a kick velocity of the golfer's swing at impact with a golf ball; and selecting a shaft flex for the golfer based on the determined phase of the golfer's swing when maximum in-swing-plane shaft deflection occurs and based on the determined kick velocity of the golfer's swing at impact with a golf ball.