1. Insufficient preparation activities. Muscle activity can't reach the state required for exercise.

2. Weak physique, low training level, insufficient muscle elasticity, extensibility and strength or overload.

3. Incorrect posture, uncoordinated movements, and excessive exertion exceed the endurance range of muscle strength.

4. Site, equipment and other factors.

The treatment of muscle strain depends on the specific situation. As you said, most of them are tendon tears, and it is enough for small muscle groups to recuperate slowly. Large muscle groups will recover quickly if they have the best operation. Go to the hospital for a check-up and make a definite diagnosis. I wish you a speedy recovery.