It is estimated that you haven't run for a long time, and sudden running leads to strain of your lumbar muscles, which also shows that your lumbar muscles are weak and your biological performance is reduced. When running, the stability of trunk is required, and the lumbar muscles will be overloaded, so there will be overload and excessive tension of the lumbar muscles, which will lead to low back pain.

When bending over, it has a pulling effect on the lumbar muscles, which will relax the lumbar muscles, so that the symptoms will be alleviated.

Personal suggestion: If it is just a simple strain of the lumbar muscles, it is enough to simply exercise the lumbar muscles. It is best to check whether the lumbar keyboard is prominent and whether the vertebral body is displaced. If you want to run, don't panic, walk quickly and step by step. Running depends on the degree of lumbar muscle injury.