You're asking and I'm answering.
"Hey Fit. In a lot of your weight lifting posts you talk about picking a weight that leads to 'muscle fatigue or failure' but I'm not really sure I know what that means or how that feels. Can you explain it for a newbie?"
– Muscley Marianna
I'm guessing you're not alone in not really knowing what muscle fatigue is, so this is a great question. To see my answer just read more.
The important thing to remember when it comes to strength training is that you must challenge your muscles with more weight than they can handle to make them stronger. This is where muscle fatigue and failure come into play. Have you ever felt some discomfort, a slight burning sensation, or even pain in the muscle you're working when doing resistance exercises? If so, then it sounds like you're probably experiencing muscle fatigue. When resistance training, you should try to reach a minor level of muscle fatigue with at least one set per exercise.
To really give your muscles a workout then you want them to reach failure (or momentary muscle failure). This is the point during your set where the targeted muscles reach complete exhaustion. When you experience muscle failure, you could not possibly do anymore repetitions without compromising your form and/or using other muscles — possibly the only thing in your life you ever wanted to fail at.
When I am picking a weight to start with for a new workout, I choose a weight I can only lift 15 or so times before really feeling fatigued. This means that after two or three sets, my muscles are going to be pretty well worked and I will have most likely worked through fatigue and achieved failure.