Legs straighten waist can not sit straight, may be the lack of muscle strength or ligament flexibility and other physiological reasons, may also be caused by soft tissue injuries and so on. 1. Physiological reasons: if the usual lack of exercise or malnutrition, may lead to a lack of abdominal muscle strength, or due to a lack of stretching exercise ligaments lack of flexibility, the phenomenon of leg straightening waist can not sit straight. This situation does not need to worry too much, it is recommended to participate in physical exercise, to strengthen their physical fitness. 2. Soft tissue injuries: If you can normally sit upright with your waist straight and straighten your legs at the same time, but now you can’t, it may be caused by lumbar muscle or ligament injuries, which may be caused by excessive exercise, improper exercise, ligament strains or muscle strain. The appearance of leg straightening waist can not sit straight, it is recommended to go to the regular hospital in a timely manner to identify the cause, timely treatment.