Flexibility is the range of motion/movement at your joints and refers to your body’s ability to move freely. It is closely related to the length of the muscles that extend across a joint, such as the hamstrings at the hip and knee. but is is also affected by the joint capsule itself. Flexibility is joint and muscle/muscle group specific. For example, having good range of motion at your hip does not mean you have a good range of motion at your shoulder. Even at the hip, a good range of anterior motion does not guarantee a good posterior or lateral range of motion.
Flexibility helps to prevent injury, improve posture, decrease back pain, maintain healthy joints and improve balance during movement. It is the last of these that particularly helps movement efficiency as it allows the body to perform better, with better technique while moving. Flexibility also helps to improve technique and skill execution by allowing a more fluid movement that flows through the appropriate planes of motion. Better technique then allows for greater force development and more efficient transfer of that force into motion.
Measuring flexibility can be very useful and can help to predict performance in sports that rely heavily on flexibility, such as dance and gymnastics. For example, an athlete who can do the splits will be more likely to perform a better straddle jump. However, this does not guarantee a great gymnastics performance. The degree to which flexibility testing then predicts performance once again varies according to the degree of specificity of the test and the performance. For example, flexibility testing is not helpful in predicting the performance of an ice-hockey player, but will predict with some accuracy the performance of figure skating, though never perfectly.