Being able to move all of your joints through their full range of motion is important for good joint function as well as being able to walk, lift, and step normally. For example, if your knees cannot extend all the way, walking with bent knees puts extra stress on the hip and low back. The ability to move a joint through its normal range of motion is affected by the condition of the joint itself (for example, if you have arthritis) and the muscles and connective tissues surrounding the joint. A short (tight) muscle limits the joint’s ability to move normally. If the hamstrings (muscles on the back side of the upper leg) are too short, they limit the ability of the pelvis to tilt, which directly affects the lower (lumbar) spine and can lead to low back pain.
One of the most common fitness tests used to measure flexibility is the Sit-and-Reach test. This test, while not perfect, provides some information about the hamstring muscle group. The more the hamstrings allow one to reach forward, the less it restricts movement of the pelvis.