There are many reasons for the pain in the hand flexion, and the common reasons are localized strain, tenosynovitis and so on. 1. Localized strain: If the physical exercise of the hand load is too large, long-term manual labor or overloaded workload, may cause acute and chronic strain of the wrist joints, metacarpophalangeal joints and the surrounding tissues, ligaments, hand bending activities can occur when the pain manifestation. 2. Tenosynovitis: the disease is a chronic aseptic inflammation, is also a common cause of pain in hand bending activities. It is characterized by localized pain and limitation of joint mobility, and can be painful when pressed. In the early stages, the pain may be limited to soreness and a slight restriction of movement, but as the disease progresses, the pain may spread distally, and the restriction of movement and pain may worsen. Pain in the flexor of the hand may also be related to other causes, and it is recommended to consult a doctor to confirm the diagnosis.