Exercising when your muscles are fatigued is not beneficial. What happens when you push yourself to fatigue is that your form falls apart. Most of us, myself included, just like to push through to finish. Or we are so driven we think pushing ourselves even when we’re tired is good for us (“It will make us stronger and push our limits so that we can do more in the future”). Pushing into fatigue a few minutes is good if your goal is aerobic training. However, the longer you work in the realm of fatigue the more damage you are doing.
Here are a few examples.
A runner pushes him/herself to increase their aerobic capacity. They get tired near the end of the run and decides, “Well, I want to push myself so this is not hard in the future so I’m going to just keep going.” But what is happening once they are fatigued? The eccentric action of the quadriceps and gastrocnemius muscles have been maxed so you start doing damage to the muscle tissue. The muscles in your hip that maintain your running form are exhausted so you start to get aberrant movements at the knee, which starts to overload the IT (iliotibial) band and the ligamentous structures around the knee. At the end of the run you feel exhausted and good about yourself for pushing yourself past your body’s limits. Yet, what you have done is just make it harder to run in the future and started laying the groundwork for chronic injuries that will stop or significantly limit your future running.
Exercising at the gym: when a person lifting weights lifts beyond the point when they begin to feel fatigued, they are doing more harm than good. They keep lifting telling themselves they can push themselves and do a couple more sets. They don’t decrease the weight and they don’t rest to give the muscle time to recuperate because they want to “challenge” the muscle so they get stronger faster. They are exposing themselves to injury because they don’t have the strength to control the motion. They end up with compensations (aberrant movements), and instead of strengthening the muscles, they are creating poor form habits and damaging the supportive tissue. You might argue that your bicep can take it and you want to cause damage so it will rebuild. I’m asking you not to look at the bicep but rather the supportive muscles that allow you to do the motion correctly. Once the supportive structures are fatigued you are not doing yourself any good. You could be putting added and unnecessary strain to ligaments, joints and tendons.
Last example: athletic training. You practice and practice and then because you are driven and won’t quit, you practice more even when your body is exhausted. A gymnast, for example. When you practice your roundoff back tuck when you are fatigued, your form gets sloppy. Not only are you more prone to injury but you are creating muscle memory of poor sloppy form and creating bad habits that will affect your future performance.
You want to get better? Practice until fatigue and then stop. When you are no longer fatigued practice more. If you fatigue sooner, then you need to work on improving your aerobic capacity. Do endurance training for the muscles that keep fatiguing first. This is how you will progress the fastest with long term strength and prevention of injury.
I address the same concept in Exercise Form is Important, Why We Don’t Heal on Our Own, Appropriate Exercise Resistance, and Training Bias. If we do not address the specific weakness and rather push through, focused only on the performance outcome, we are laying the foundation for injury.