A good starting point for gaining strength is to feel that you are tiring your muscles by the 15th time you complete an exercise movement (repetition). Tiring a muscle often feels like a burning sensation at the muscle that is working.
For example, when squatting you may "feel a burn" at the top part of your thighs or hips. Some fatigue (feeling tired or sensation of muscle burn) is needed to build muscle strength – don’t try to avoid this! The number of repetitions you can complete with good form throughout the full movement of an exercise can tell you if you need to increase difficulty. A good rule to follow is that you can make an exercise more challenging when you can do more 20 repetitions of an exercise without tiring your muscles (experiencing fatigue) while using good form.
An important point: Maintaining good form is essential to prevent injury while exercising. Good form includes completing the exercise through its full range of motion and maintaining the same speed throughout the activity. Losing good form can be a sign that you are beginning to tire, or that you advanced in difficulty too quickly. To check your form, take a look in the mirror and compare your movements with the exercise animation. You can also ask a friend, partner, or caregiver to compare your form to the animation.