1. Insufficient muscle strength: If you are a novice in fitness, you may not fully exercise the muscles of your left hand, resulting in insufficient strength when doing dumbbells with your left hand. It is suggested to strengthen biceps brachii exercise, such as losing weight slowly, strengthening exercise intensity, or consulting a professional fitness coach for help.

2. The strength of the two arms is different: the strength of the two arms is different, which makes one side boring and the other side can continue to exercise, thus resulting in muscle imbalance between the left and right sides. It is suggested that one-arm dumbbell training should be carried out to strengthen the weaker arm.

3. The influence of round shoulder posture: Round shoulder may lead to different feelings of two-arm training. It is suggested to do posture correction training before dumbbell practice, such as doing some stretching exercises.

4. Standing posture: If the distance between the legs and feet is different, or the pelvic spine angle is inclined, of course, the stress points at different angles are different, so it is necessary to adjust the posture to change the body shape in order to achieve the ideal body shape.

Please note that the above are only the possible reasons why one hand is boring when practicing dumbbells, and the specific reasons need to be judged according to personal circumstances. If the problem persists or there is discomfort such as pain, it is recommended to consult a professional fitness coach or doctor.