A regular flex shaft is generally appropriate for those with an average head speed (80-94 mph), while an A-Flex (or senior shaft) is for players with a slower swing speed (70-79 mph), and the stiffer shafts, such as S-Flex and X-Flex (Stiff and Extra-Stiff shafts) are reserved only for those players with an above average swinging speed, usually above 100 mph .