1. The movement is not standard: This may be because when you do dumbbell bending, your wrist stays still and is in a straight line with your forearm, and the stretching of your biceps drives your joints to bend, thus achieving the strength of exercising muscles.
2. The upper arm is not close to the trunk: when doing dumbbell bending, the upper arm shakes at will, is not fixed, or the elbow moves forward, resulting in a reduction in the force on the biceps brachii.
In order to change this situation, you can put your forearm on your thigh, or fix your arm with a priest's chair and then make a bend. At the same time, pay attention to keep the upper arm close to the trunk and don't shake it at will.
Please note that fitness is not an overnight thing. It takes time and patience to adjust and improve slowly. In addition, if you feel any pain or discomfort during the exercise, it is recommended to stop immediately and consult a professional.