There are many reasons for the vague pain inside the left hand and small arm, such as excessive exercise, myofasciitis, soft tissue swelling, nerve entrapment and so on. 1. Excessive exercise: left forearm strenuous exercise, such as lifting dumbbells, playing ball and other sports, may lead to chronic injury or lactic acid buildup in the left hand, small arm muscles, which will result in the phenomenon of vague pain in the left hand, small arm. Generally, it can be relieved after resting. 2. Myofasciitis: aseptic inflammation of the muscles and fascia of the left hand and forearm caused by trauma, cold and other factors, the onset of the disease will also lead to vague pain in the left hand and arm, especially when the elbow joint is flexed and extended, the pain is more obvious. 3. Soft tissue swelling: if there are tendon sheath cysts, fibromas and other swellings in the left hand and arm, it will also lead to vague pain in the left hand and arm, mainly due to the growth of the swelling on the local compression. 4. Nerve compression: cervical spondylosis, elbow tube syndrome and other diseases may lead to the left hand, small arm nerve compression, so it will lead to the left hand, small arm numbness, vague pain and other phenomena. There are many reasons that lead to vague pain in the left hand and small arm, if it is not relieved after resting, it is recommended to go to the hospital in time to improve the examination, clear diagnosis and take standardized treatment according to the doctor’s instructions, and do not ignore the symptoms and take them lightly.