1. Overtraining: If you train too much or too often, your body may feel tired and weak. In this case, you need to give your body enough rest time to recover.

2. Malnutrition: If your diet is unbalanced or lacks some nutrients, your body may feel weak. In this case, you need to pay attention to a balanced diet to ensure adequate nutrition intake.

3. Lack of water: If you don't replenish water in time during exercise, you may feel weak. In this case, you need to ensure that you replenish water in time during the fitness process.

4. Lack of sleep: If you don't get enough sleep, you may feel weak. In this case, you need to ensure that you have enough sleep time every night.