The point of exhaustion where the muscles are physically no longer capable to do any more repetitions of an exercise is called muscular failure. Always use a spotter when you train to this point. If you are training for endurance then you will probably get to this point because you do many more repetitions than training for power or hypertrophy. Without a spotter it can be dangerous because it is easy to lose control of the weight when you reach muscular failure. In the figure above muscular failure was reached while doing a dumbbell shoulder press. If you do not have a workout buddy that can spot you, make sure you are using a machine or body weight as opposed to free weights for exercises that will lead to muscular failure.