 There everybody, Dr. O here and this video will talk about why our muscles get tired. Why do muscles fatigue? The exact causes of muscle fatigue are not actually known but I think this is a good solid list of the probable reasons why muscles get tired and why we can't just go and go and go. So the first thing that muscle fatigue is associated with is the depletion of your metabolic reserves, the ATP we need and the fuels we need to make that ATP. But rest your skeletal muscles, they primarily metabolize fatty acids so they store your glucose as glycogen. During light physical activity your body should be able to keep up with generating those metabolic reserves or using these metabolic reserves to generate ATP for quite a long time. Your muscles can break down carbs, lipids, even amino acids but then at peak levels of activity when most of our muscles do fatigue you're going to not be able to generate a lot of fuel because you're going to go from aerobic respiration to anaerobic glycolysis so you're only going to make a few ATP. I cover all that in the metabolism section. Then you're also going to generate lactic acid which we'll come back to in a moment. So we have a finite amount of fuel. If you're doing really light physical activity your muscles shouldn't fatigue for a very very long time. If you're doing very intense activity especially anaerobic activities then you're going to your muscles are going to fatigue more quickly so I think that makes sense. The second one would just be muscle exhaustion and pain. I think we have to understand that sometimes our muscles get so sore that we give out, we give out not them specifically. So muscle exhaustion and pain are going to be causes for muscle fatigue and stopping physical activity. Number three would be a low pH so I've already mentioned that as your body is undergoing anaerobic glycolysis it is going to generate lactic acid. That's going to lower the pH which can affect the enzymes needed for muscle contraction. So at some point if you're doing things that are very anaerobic the pH is going to drop to such a low point where your muscles are just going to give out. I used to watch those World Strongest Man competitions on TV before CrossFit got big and ESPN started doing those instead and these are very powerful individuals but they can only lift the car or throw stones for so long and the drop in pH is definitely a part of why their muscles would give out and fatigue. Then we just have damage so sustained muscle contraction can damage muscle fibers, muscle cells can damage the sarcolemma which is the fancy term for the cell membrane of a muscle cell or muscle fiber and it can damage the sarcoplasmic reticulum which is the term we use for the endoplasmic reticulum in muscle cells. And if that happens then you can see a disruption in calcium flow or the inability to maintain calcium levels and calcium is needed for muscle contraction. So you can see enough physical damage to some muscle fibers that it will actually make it impossible for them to have the calcium they need and the muscles are going to give out. So I think those are the main causes of muscle fatigue. You have it running out of fuel, running out of these metabolic reserves, muscle exhaustion and pain, a drop in pH because of lactic acid and then this physical damage to the muscle cells. Alright that's why our muscles give out. I hope this helps. Have a wonderful day. Be blessed.