There is nothing painful about muscle fatigue. You try to lift the weight one more time, but your muscles are too tired to do it. There is nothing painful about it just tiresome. What this interpretation is talking about is muscle soreness. This too is incorrect.
Muscle soreness is the result of microscopic tears in the muscle fibers.
This is usually caused by long periods of inactivity followed by the desire to lift a very heavy weight. In other words, when people first begin their weight training program they have the tendency to overdo it. This causes muscle strains and tears. When you begin your weight training program start out lifting light weights. You need to get your body used to the idea of pumping iron.