As our bodies perform strenuous exercise, we begin to breathe faster as we attempt to shuttle more oxygen to our working muscles. The body prefers to generate most of its energy using aerobic methods, meaning with oxygen. Some circumstances, however—such as evading the historical saber tooth tiger or lifting heavy weights—require energy production faster than our bodies can adequately deliver oxygen. In those cases, the working muscles generate energy anaerobically. This energy comes from glucose through a process called glycolysis, in which glucose is broken down or metabolized into a substance called pyruvate through a series of steps. When the body has plenty of oxygen, pyruvate is shuttled to an aerobic pathway to be further broken down for more energy. But when oxygen is limited, the body temporarily converts pyruvate into a substance called lactate, which allows glucose breakdown—and thus energy production—to continue. The working muscle cells can continue this type of anaerobic energy production at high rates for one to three minutes, during which time lactate can accumulate to high levels. A side effect of high lactate levels is an increase in the acidity of the muscle cells, along with disruptions of other metabolites. The same metabolic pathways that permit the breakdown of glucose to energy perform poorly in this acidic environment. On the surface, it seems counterproductive that a working muscle would produce something that would slow its capacity for more work. In reality, this is a natural defense mechanism for the body; it prevents permanent damage during extreme exertion by slowing the key systems needed to maintain muscle contraction. Once the body slows down, oxygen becomes available and lactate reverts back to pyruvate, allowing continued aerobic metabolism and energy for the body’s recovery from the strenuous event. Contrary to popular opinion, lactate or, as it is often called, lactic acid buildup is not responsible for the muscle soreness felt in the days following strenuous exercise. Rather, the production of lactate and other metabolites during extreme exertion results in the burning sensation often felt in active muscles, though which exact metabolites are involved remains unclear. This often painful sensation also gets us to stop overworking the body, thus forcing a recovery period in which the body clears the lactate and other metabolites. Researchers who have examined lactate levels right after exercise found little correlation with the level of muscle soreness felt a few days later. This delayed-onset muscle soreness, or DOMS as it is called by exercise physiologists, is characterized by sometimes severe muscle tenderness as well as loss of strength and range of motion, usually reaching a peak 24 to 72 hours after the extreme exercise event. Though the precise cause of DOMS is still unknown, most research points to actual muscle cell damage and an elevated release of various metabolites into the tissue surrounding the muscle cells. These responses to extreme exercise result in an inflammatory-repair response, leading to swelling and soreness that peaks a day or two after the event and resolves a few days later, depending on the severity of the damage. In fact, the type of muscle contraction appears to be a key factor in the development of DOMS. When a muscle lengthens against a load—imagine your flexed arms attempting to catch a thousand pound weight—the muscle contraction is said to be eccentric. In other words, the muscle is actively contracting, attempting to shorten its length, but it is failing. These eccentric contractions have been shown to result in more muscle cell damage than is seen with typical concentric contractions, in which a muscle successfully shortens during contraction against a load. Thus, exercises that involve many eccentric contractions, such as downhill running, will result in the most severe DOMS, even without any noticeable burning sensations in the muscles during the event. Given that delayed-onset muscle soreness in response to extreme exercise is so common, exercise physiologists are actively researching the potential role for anti-inflammatory drugs and other supplements in the prevention and treatment of such muscle soreness, but no conclusive recommendations are currently available. Although anti-inflammatory drugs do appear to reduce the muscle soreness—a good thing—they may slow the ability of the muscle to repair the damage, which may have negative consequences for muscle function in the weeks following the strenuous event.
Muscles are supplied with oxygen at 3 times the amount when active as compared to at rest. Other ways in which muscles are supplied with oxygen include:
These are only some of the many ways in which muscles are supplied with oxygen during exercise.
Muscles will stop working without oxygen, especially if you are going to be exercising for more than a couple of minutes. How much oxygen your muscles utilize depends on two processes: first, getting the blood to the muscles; second, extracting oxygen from the blood into the muscle tissue. When muscles are working, they take oxygen out of blood three times as well as when muscles are at rest. The body can increase the flow of oxygen-rich blood to working muscle in several ways:
These five mechanisms will increase the blood flow to your working muscle by nearly five times. So when the muscle is working during exercise, the amount of oxygen the muscle utilizes can be increased by almost 15 times.
Red blood cells carry the oxygen that muscles and other cells need during exercise. The more exercise people do, the more oxygen their cells need, so their blood has to deliver more oxygen to the cells. When people are active, their heart works hard pumping blood and oxygen all around their body. The more active people are, the stronger their heart becomes, and the more oxygen gets to their cells.
Important: This content reflects information from various individuals and organizations and may offer alternative or opposing points of view. It should not be used for medical advice, diagnosis or treatment. As always, you should consult with your healthcare provider about your specific health needs.
Muscle Function Muscles use the stored chemical energy from food we eat and convert that to heat and energy of motion (kinetic energy). Energy is required to enable growth and repair of tissue, to maintain body temperature and to fuel physical activity. Energy comes from foods rich in carbohydrate, protein and fat. The source of energy that is used to power the movement of contraction in working muscles is adenosine triphosphate (ATP), the body’s biochemical way to store and transport energy. ATP is a high-energy nucleotide which acts as an instant source of energy within the cell. When muscles contract, they break down ATP in a reaction that provides energy. However, muscle cells only store enough ATP to fuel a few seconds of maximal contraction. Once muscle contraction starts, the making of ATP must start quickly.
Since ATP production is so important, muscle cells have several different ways to make it. These systems work together in phases. The three biochemical systems for producing ATP are, in order:
Using Creatine Phosphate To continue working, muscle cells must replenish their ATP supply. All muscle cells contain a high-energy compound, creatine phosphate, which is quickly broken down to make ATP. Because stores of creatine phosphate are also limited, this energy system can only sustain maximal muscle output for about 10 seconds. The phosphagen system is the primary energy source during very short, rapid bursts of activity, such as sprints.  Using Glycogen (Anaerobic Glycolysis) To sustain exercise for more than 10 seconds, muscles must break down fuel sources such as carbohydrates and fats to provide the energy to re-synthesize ATP. Carbohydrate metabolism is faster than fat metabolism. Therefore, carbohydrates provide a high percentage of the energy during very high-intensity workouts. Because carbohydrates can be metabolized anaerobically, without oxygen, they become a vital energy source when oxygen supply to muscles cannot keep up with demand. The breakdown of carbohydrates to provide energy without oxygen is called anaerobic glycolysis. This process releases energy very rapidly and will produce enough energy to last about 90 seconds. It is important that oxygen is not required because it takes the heart and lungs some time to get increased oxygen supply to the muscles. Glucose and stored carbohydrates in the form of glycogen in muscle cells are broken down through a series of reactions to form a compound called pyruvate. This process yields two to three molecules of ATP for each molecule of glucose. A by-product of making ATP without oxygen is lactic acid, which can accumulate in your muscles during rapid exercise causing tiredness and soreness.
Using Aerobic Respiration Within two minutes of exercise, the body starts to supply working muscles with oxygen. When oxygen is available, pyruvate can be further broken down aerobically to produce as many as 30 additional molecules of ATP, making aerobic metabolism, although slower, much more efficient than anaerobic metabolism. Fats can be broken down aerobically to produce large quantities of ATP. After vigorous workouts, muscles restock ATP supplies aerobically. Aerobic respiration can supply ATP for several hours or longer as long as a supply of glucose lasts. This glucose can come from several places:
Lactate (Lactic Acid) Production When the body has plenty of oxygen, pyruvate is transferred to an aerobic pathway to be further broken down to ATP (pyruvate is produced by glycolysis from the breakdown of glucose). However, when oxygen is limited, the body temporarily converts pyruvate into lactate, which allows glucose breakdown – and thus energy production – to continue. The working muscle cells can continue this type of anaerobic energy production at high rates for one to three minutes, during which time lactate can accumulate to high levels. A side effect of high lactate levels is an increase in the acidity of the muscle cells. The same metabolic pathways that permit the breakdown of glucose to energy perform poorly in this acidic environment. This is a natural defense mechanism for the body. It prevents permanent damage during extreme exertion by slowing the key systems needed to maintain muscle contraction. Once the body slows down, oxygen becomes available and lactate is converted back into pyruvate, allowing continued aerobic metabolism and energy for the body’s recovery from the strenuous event. Lactate buildup is not responsible for the soreness felt in the days following strenuous exercise. Rather, the production of lactate and other metabolites during extreme exertion is the results in a burning sensation often felt in active muscles. This often-painful sensation also gets us to stop overworking the body, thus forcing a recovery period in which the body clears the lactate. |
When plenty of oxygen is available in the muscles
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