Exercise

Energy Sources in Exercise

Anyone ever do a stress test?  They say, “Get on the treadmill and run until you fall down from exhaustion.”  While you are doing that, you are going through different modes of energyThe capacity to do work or cause change. harvesting.  You only need to make ATPThe energy currency of cells used for muscle contraction. to detach all your cross bridges. The source from which you harvest the ATP changes as you exercise. The method you use to harvest it also changes when you exhaust your storehouses of energy.  When you are at rest, the mitochondria in your cellsThe basic structural and functional units of life. are taking oxygen from myoglobin. Myoglobin is just a red pigment that binds oxygen. They also take glucoseA simple sugar that is the main source of energy for cells. from glycogenA storage form of glucose found in animals., which is a polysaccharide, to make ATP.  This process is called cellular respirationThe process of gas exchange, including ventilation, external and internal respiration. and was briefly discussed in a previous topic. 

If you’re on the treadmill of death in a stress test, you will quickly exhaust your supply of ATP. Your muscles will be using more than your muscles can produce.  At some point, your muscles will switch over to something anaerobicprocess that does not use oxygen fermentationprocess of regenerating NAD with either an inorganic or organic compound serving as the final electr.  Anaerobic fermentation is your muscles saying, “OK, you don’t want to give me enough oxygen. I’m going to make ATP for you without oxygen.”  Oh, and, sorry, I’m gonna make lactic acidA substance that releases hydrogen ions (H⁺) in solution. as a byproduct.  And, we know that acid denatureThe process in which a protein loses its shape and function due to heat or pH changes. proteinsLarge molecules made of amino acids with various functions in the body. and muscle fibers are made of proteins.  Not good. 

Your body is switching from aerobic respirationprocess in which organisms convert energy in the presence of oxygen to anaerobic fermentation. During this change, some enzymesProteins that speed up chemical reactions in the body. try their best to make ATP. The action of these enzymes is referred to as the phosphagen systemThe immediate energy system for muscle contraction..  The two enzymes are myokinase and creatine kinaseAn enzyme that helps transfer energy for muscle contraction..  Many companies sell products that claim to provide these two enzymes and the type of moleculesGroups of atoms bonded together. they convert to ATP.  They are recommended as a pre-workout supplement.  Do they work?  You’d have to ask the FDA.  Oh, wait, you can’t because there is no entity investigating if these things do what they claim to do. 

At this point, you’ve only been running on the treadmill of death for about a minute.  You’ve probably exhausted all these forms of available energy. Now, your ability to keep going relies solely on your heart’s ability to pump enough oxygen to your tissues.  This is why a stress test is a test for cardiopulmonary fitness. 

Energy Sources in Exercise

You are a long-distance runner if you manage your pace and heart rate. Doing this avoids even the possibility of exhausting energy sources. Consequently, you will switch back to cellular respiration.  In sprinting, muscle fibers use anything that’s already in the muscle to make you ATP.  But, once those are exhausted, your mitochondria will start to ask for oxygen and glucose from your blood.  Carbohydrates are the first organic energy sources that are exhausted.  The time varies depending on the person. After 20 to 40 minutes of regulated exercise, the mitochondria start to harvest lipidsOrganic molecules including fats, oils, and steroids. from the blood. This process helps make ATP.  If you push beyond that, you will actually be harvesting the muscle proteins you are trying to supply with proteins.

Hitting the Wall

If you keep going at full speed, your oxygen demand will exceed what you can take in with your respirations. Your body will start to harvest stored oxygen from organs such as the liverA large organ that produces bile, detoxifies blood, and stores nutrients.. When you are done with your sprint, you’ll still be breathing heavily. You will feel as though you can’t stop. It may seem like you can’t catch your breath.  Yes.  Your liver and the other organs that gave you their oxygen want pay back.  Your respirations will remain deepAway from the surface of the body. and frequent. They continue until these organs are paid back. This occurs even though you are no longer actively exercising.  This is called oxygen debtThe amount of oxygen needed to restore muscle energy levels after exercise..  It happens when you push yourself through the wall.  This why exercising with a heart-rate monitor is a very useful thing.  By managing your heart rate, you can prevent hitting the wall.  The stress tests on the treadmill of death are designed to make you hit the wall

Muscle Fiber Classification

Sprinters and long distance runners are two different types of people. The fast-acting muscles around your eye, such as the oculomotor cranial nerve, function quickly. They are very different from the large weight-bearing muscles in your lower body.  There are different type of muscle fibers.  These two classifications here are the extremes of a spectrum.  As you can see in the center picture, an entire muscle can contain both types of muscle. 

The purple-y muscle fibers are referred to as fast-twitch muscle fibers.  These are the muscle fibers that are used for sprinting.  These muscle fibers are fast-acting, they use glycolytic energy sources, but they are easily fatigued.  The muscles in your hands are fast-twitch muscles.  If you use them too much, they will fatigue and give you writer’s cramp. 

Slow-twitch fibers, in contrast, use cellular respiration as their primary method of making ATP.  These muscle fibers are great for marathon runners because these take a long long time to fatigue.  However, the time to fatigue depends on your ability to keep yourself out of oxygen debt. You need to keep your heart rate within a reasonable range for your age. Basically, don’t exercise like you are punishing your body.  I personally do not know of another way to exercise. It should come as no surprise to you all that I was a sprinter in my days as an athlete.  If you wanted me to make it around that track more than once, I would say, “Yeah, I don’t do that.”