If a patient has pain in the left side of the lumbar region when bending, it may be caused by the following reasons: 1. It may be caused by a mild injury to the transverse process of the left lumbar vertebrae, for example, some patients may have a slight fracture of the transverse process of the left lumbar vertebrae due to strenuous lumbar activities or impact on the lumbar region, at which time the patient may only have pain in a special position, such as when bending. The pain of the left side of the lumbar region may occur when the patient is in a special position, such as bending over, but not when lying on the back or standing upright; 2, it may be caused by the strain of the left side of the lumbar muscles, such as the patient in the process of lifting heavy objects, which may cause strain of the left side of the lumbar muscles. Spasm, such as patients in the process of running, due to the sudden acceleration of breathing, may cause the left side of the lumbar and quarter rib muscle spasm, then the patient will have the above symptoms.