Flexibility is defined as the range of motion (ROM) around a joint and can be altered with passive and active static, dynamic, and proprioceptive neuromuscular facilitation (PNF) stretching as well as other techniques. These stretching techniques can provide elastic (acute) or plastic (semi-permanent) changes to joint flexibility. Most studies show that static and PNF stretching provide greater increases in ROM compared with dynamic stretching. Normative data for joint ROM use measurement techniques, such as goniometers, fleximeters, inclinometers, photography, sit (stand) and reach tests, display high reliability but may not provide similar device values. Women tend to have greater joint ROM than men due to anatomical differences, less tissue stiffness, and endocrine differences. Inactivity plays a very substantial contributory role in the decrease in flexibility that is recorded with ageing (also related to increased collagen proportions and protein cross-linkages). Young people tend to have higher flexibility levels, but the lower levels of flexibility with puberty compared with childhood can also be related to inactivity. With regard to the effect of genetics on flexibility, activity again plays a more important role than DNA because studies have shown weak-to-strong correlations.