Kicking arts such as TKD requires the lumbar spine and core to be mobile and strong as a lot of stress is put through it whilst conducting kicking movements as well as when correctly delivering blocking and striking techniques. A number of you might find that you are having a lack of mobility and pain in this area potentially caused by the riggers of what we do as an art. However, it is not always down to a localised lumbar issue, it can be created by a spinal problem below or above it. The spine acts as a unit, if one area gets compromised and becomes injured, generally as a protective response it will have a knock on effect and transfer upwards and/or downwards to other areas making them mobile and painful.
In the below video it shows a method of assisting to free off the thoracic spine if this is the issue to potentially assist with lumber pain (as well as tackling the causitive thoracic issue).