First of all, do you mean waist and abdomen strength or back and spine strength? Girls can exercise abdominal muscles well by practicing waist and abdomen strength, sit-ups and flat support, and stick to it. The vest line is not a problem. It is recommended to practice three groups of sit-ups every time, 20-30 in each group, supported by three groups of flat plates every time, one minute in each group, once every other day. If you are practicing back spinal muscles, it is recommended to lower your back. If the spinal muscles are not strong and have no foundation, you can't do too difficult actions rashly. You should do it step by step.