Mobility training in the form of stretching and flexibility training of our skeletal muscle is a common exercise. However, the evidence are scare and there are only a few studies comparing the effect of prolonged static stretching (> 90 sec) and dynamic flexibility training as to which of these methods has the best length-enhancing effect over time. Further, in clinical practice concerning low back pain, the possibility of a thigh hamstrings muscles to affect movement control of the spine is often mentioned, a link not fully explored in a controlled condition. The purpose of this study is to compare the effect of eight weeks of dynamic flexibility training versus prolonged static stretching on mobility in the hamstring muscles in adults with reduced mobility in the muscles at the back of the thigh. The aim is also to compare the sustained effect of flexibility training between the two groups, eight weeks after completion of the intervention. Furthermore, the aim is to explore how different forms of flexibility training affects movement patterns of the lumbar spine in adult persons with reduced mobility in the hamstrings.