Energy use by muscles, actual work done by muscles and more

Energy use by muscles, actual work done by muscles and more

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Lately, I've started exercising in the gym and outside. I've also started to look at the details of food I eat.

Food usually has a label saying the amount of energy is inside it. For example, some chocolate says it has 400 kilocalorie for 100 grams.

I have some questions about that:

  1. Does the label say the amount of total chemical energy exists inside the food, which will be released if burned, or the energy available to the human body after digested, and because digesting requires energy, the food contains actually MORE energy then written?

  2. When I'm riding on a static bike, it has a screen which counts the kcal I burn. It has a thing connected to my arm, measuring my heart beat. After about an hour of riding, it said I used about 700 kcal. Is it the amount of work I did, i.e if there was a battery connected to the bicycle, I'd generate 700 kcal of electricity, OR, the amount of energy my body LOST, because muscle efficiency is far from 100% (so, I lost 700 kcal, but generated only 150 kcal of electricity, for example)

  3. When doing a physical activity, the heart beat and breathing rate increases. Lets say I'm riding a bicycle or running. Whats the ratio between energy consumed by the muscle to energy consumed by the lungs or heart?

  1. I speak only for the U.S. regulations: the calorie labels on wrappers refer to the energy released when burned. Sometimes these are inaccurate. Many dieticians recommend calculating the calories based on weights of protein, carbohydrates and fats in the serving: 4 kcal in each gram of protein and carbohydrate and 9 kcal in each gram of fat in your food. Multiply carbs, proteins and fats by the appropriate value and add them up.

  2. The reading on the bike does not take into account your pulse. Just the raw work done turning the pedals. Some very smart bikes do take pulse into account, but the often inaccurate heart monitors lead to inaccurate calculations of total energy.

  3. The heart and diaphragm (muscle primarily responsible for breathing) do use more energy when exercising than when you are sitting still. But that increase is small relative to the increase in the primary muscle sets involved in the exercise. Your resting heart rate is probably around 70 bpm, your exercising heart rate is probably around 160-180 bpm. So it has increased its rate and energy consumption by about 3 times. Your skeletal muscles maintain a basal metabolic rate during inactivity (one purpose of this is your body needs the heat from slightly tensed muscles to thermoregulate itself) - but during exertion, their metabolic rate increases enormously. Exercise increases their energetic demands by whole orders of magnitude.

The actual value of the (energy used by heart) / (energy used by skeletal muscles during exercise) depends on what exercise you do and how efficiently you do it. The change in the ratio between inactivity and activity depends on how much muscle mass you have (more muscle uses more energy during inactivity) and how strenuously you exercise. But the point is that the increased energetic demands by skeletal muscles far outpace the 3x increase in energetic demands by the heart and diaphragm - so the latter accounts for pretty much all total energy consumption at even moderate activity levels.

1) It depends on each country and your minister. In Finland, the thing is done so that it says the energy stored in food, which will be released if burned.

2) It is the energy pushed to the bike when the bike does not take into account your pulse. If the bike is smart one, it takes into account your pulse rate, then it tries to estimate the amount of energy released by your body. This is done differently among different manufacturers of those watches.

3) By the way heart is a bunch of muscles, a myogenic muscular organ! So consider instead heart and some part of lungs. Very broad problem otherwise.

Muscle Mitochondria May Form Energy Power Grid

Skeletal muscles are made of long, thin cells that are packed with highly organized proteins and organelles. During strenuous exercise, the rate of energy use in skeletal muscles can increase by more than 100-fold almost instantly. To meet this energy demand, muscle cells contain mitochondria. These organelles, commonly referred to as the cell’s “power plants,” convert nutrients into the molecule ATP, which stores energy. In this process—known as cellular respiration or oxidative phosphorylation—the mitochondria act like small cellular batteries, using an electrical voltage across their membranes as an intermediate energy source to produce ATP.

Scientists have long believed that the energy produced by mitochondria is distributed through muscle cells by some type of diffusion mechanism. However, studies have shown that these diffusion pathways alone are not sufficient to support normal muscle contraction.

A team led by Drs. Robert S. Balaban and Sriram Subramaniam from NIH’s National Heart, Lung, and Blood Institute (NHLBI) and National Cancer Institute (NCI), respectively, hypothesized that some other faster, more efficient energy pathway must spread energy throughout muscle cells. Their research was conducted on the NIH campus in Bethesda, Maryland. Results were published on July 30, 2015, in Nature.

The group analyzed high-resolution 3-D images of mouse skeletal muscle. They used an imaging technique known as focused-ion beam scanning electron microscopy (FIB-SEM). Dr. Subramaniam and his team have used this approach, which they developed, to determine the structure of many key proteins, including brain receptors and the virus that causes AIDS.

The researchers traced the paths of mitochondria and found that they formed a network, or reticulum. They hypothesized that these organelles might form a vast, interconnected network in a way that resembles electrical transmission lines in a municipal power grid.

To test the idea, they used specially designed optical probes to examine the electrical connections among mitochondria. They found that the mitochondrial “wires” were electrically conductive and that most of the mitochondria were in direct electrical communication through the interconnecting network. The mitochondria were electrically coupled and able to rapidly distribute the mitochondrial membrane voltage—the primary energy for ATP production—throughout the cell.

“The discovery of this mechanism for rapid distribution of energy throughout the muscle cell will change the way scientists think about muscle function and will open up a whole new area to explore in health and disease,” Balaban says.

What Does This System Do?

The big purpose of the muscles found in your body is movement. We could be talking about the movement of your legs while you walk. We could be talking about the beating of your heart. We could also be talking about the contraction of a very small blood vessel in your brain.

You have no control over most of the muscular system. You do control the voluntary muscle in your arms, legs, neck, and torso. You have little or no control over the heart or smooth muscle. Those other muscles are under the control of the autonomic nervous system (ANS).

What are the Energy Sources for Muscle Contraction?

ATP is the immediate source of energy for muscle contraction. However, the ATP stores in the muscle can sustain muscle contraction for up to 3 seconds. In about 3 sec, all the ATP is depleted from the muscle cell. Thereafter, ATP is regenerated using the energy released by the dephosphorylation of creatine phosphate reserves of the muscle fiber.

The creatine phosphate reserves (20 mM/L) of the muscle fiber can sustain contraction for about 5 more seconds, i.e., up to 8 seconds. After depletion of creatine phosphate reserves, further supply of energy for regeneration of ATP comes from glycol sis, which can sustain muscle contraction up to 1 minute. The end product of glycol sis is pyruvate and ATP molecules. When O, is available, the acetyl-CoA is metabolized completely through the Kreb’s cycle. In absence of oxygen however, the acetyl CoA molecules condense to form lactic acid.

The lactic acid is released into blood from where it is taken up by the liver and kidney, reconverted into glucose and released back into circulation. This recycling of lactic acid is called Cori cycle.

During anaerobic glycol sis, conversion of 1 molecule of glucose- 6-phosphate to 2 molecules of pyruvate releases 3 molecules of ATP. If the muscle utilizes glucose taken up from blood, the net ATP generation drops to 2 molecules per molecule of glucose. This is because 1 ATP molecule is used for phosphorylating glucose to glucose-6-phosphate. Glycogenolysis of muscle glycogen however yields glucose-6-phophate without requiring ATP consumption and therefore, the net ATP yield remains 3 molecules of ATP per molecule of glucose.

Glycol sis alone can sustain contraction for about 1 VI minutes only since lactic acid accumulation makes prolonged contraction difficult. In presence of oxygen, the terminal product of glycol sis viz., pyruvate is converted into acetyl CoA and fed into the Kreb’s cycle in which 38 molecules of ATP are generated. Kreb’s cycle can sustain muscle contraction for several hours.

1)i) Force = mass X acceleration

2 j) Energy Gravitational = mass X acceleration X height (distance)

W (total) = W J/lift x n lifts

W (total) = W J/lift x n lifts

5) n) power during stage 1 = 1,312.283J/ 115 seconds

0) power during stage 2 = 3,374.4438 J/ 303 seconds

6) p) moles of ATP used during exercise (stage 1 and 2)

= 13390.65 J x (1kJ/1000J) x (1 mol of ATP/2870 KJ)

7) q) moles of glucose used

= 4.46 x 10^-3 mol of ATP x 1 mol glucose/30 mol ATP

8) r) 1.4866 x 10^-4 mol of glucose used x (1 mol sucrose/ 2 mol glucose)

9) (s) 7.43 x 10^-5 mol sucrose used x 342g/mol

10) (t) 2.54 x 10^-2 g sucrose x ( 1 teaspoon sugar/ 4 g sucrose)

= 6.35 x 10^-3 teaspoons of table sugar

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I would like to know what a muscle cell looks like and the size and shape please? I can't find it anywhere and I'm doing a project on it. FitzMaurice January 11, 2011

Muscle cells themselves have an intricate anatomy which enables them to function well on a very small level. The exchange and usage of various chemicals in their system enables them to send and receive signals which govern the overall function of the entire group of cells. These cells receive data which tells them how to behave and function differently from person to person. For instance, some people are genetically endowed with larger muscles than others due to DNA encoding sent to muscle cells. CarrotIsland November 13, 2010

An easy way to look at muscle cells is that they behave like little batteries, with voltages (membrane potentials) in the millivolt range. This enables cells to generate tiny little electrical currents which makes it possible for the muscle cell to twitch, or contract and pump blood.

Heart muscle cells in an adult normally don’t divide. They just increase in size.

Muscles can account for around 40% of your body weight. The longest muscle in the body has muscle cells that are over a foot long. Muscles can only pull. They cannot push.

Ideal protein to help seniors rebuild lost muscle

While exercise buffs have long used protein supplements to gain muscle, new research from McMaster University suggests one protein source in particular, whey protein, is most effective for seniors struggling to rebuild muscle lost from inactivity associated with illness or long hospital stays.

The study, published online in The American Journal of Clinical Nutrition, compared the impact of different forms of protein supplements on older adults, a growing population challenged by the loss of muscle and strength, or sarcopenia, which in turn can affect balance, gait and the ability to perform the simple tasks of everyday life.

Researchers found that protein did not stop lean muscle loss caused by inactivity, however, whey supplements helped to rebuild muscle once the participants activities resumed.

"The important message here is that not all proteins are created equal. Whey is one of the highest quality proteins and is ideal for older persons," says Stuart Phillips, senior author on the paper and a professor of kinesiology at McMaster.

Researchers set out to compare the impact of whey versus collagen protein on muscle loss during periods of inactivity and then recovery.

Whey is considered a high-quality or complete protein, meaning it is rich in all essential amino acids and is higher in leucine, one of the essential amino acids the body cannot make itself and therefore, must derive from food.

Collagen peptides, by comparison, are much lower in their leucine content, lack or are low in essential amino acids.

For the study, researchers recruited men and women who were non-smokers, non-diabetic and between the ages of 65 and 80 years old. One group of subjects consumed whey protein, the other collagen peptides, throughout the study.

For a five-week period their diet was controlled, including a two-week time frame where their daily steps were restricted to 750 per day and their calorie intake reduced by 500 calories per day, conditions that might mimic what older people often experience during a hospital stay.

Participants returned to normal activity levels during a one-week recovery period.

The team had predicted that the collagen peptide group would experience a significantly greater muscle loss than the whey protein group, but that didn't happen. Both groups lost the same amount of muscle.

"While we already know that complete protein sources are more potent for stimulating building processes we were surprised to discover that after two weeks of limiting steps among the participants, there were no apparent differences in muscle loss between the two groups," says Sara Oikawa the lead author and a graduate student in the Department of Kinesiology at McMaster.

While protein was ineffective in mitigating muscle loss, say researchers, when participants returned to normal, muscle-building activity, the whey group recovered more skeletal muscle.

"When we consider measures that can be taken to help seniors as they age, it's clear that whey is an important ingredient. Conversely, we should avoid collagen in formulations targeting older people," says Oikawa.

In future research, Oikawa plans to focus on women specifically, who tend to experience greater difficulties in rebuilding strength.

How muscles produce lactic acid

Throughout most of the day, our body burns energy aerobically — that is, in the presence of oxygen. Part of that energy comes from sugar, which our muscle cells break down in a series of chemical reactions called glycolysis. (We also get energy from fat, but that involves a whole other chemical process). The end product of glycolysis is pyruvate, a chemical that the body uses to produce even more energy. But energy can be harvested from pyruvate only in the presence of oxygen. That changes during hard exercise.

When you break into an all-out sprint your muscles start working overtime. The harder you work, the more energy your muscles need to sustain your pace. Luckily, our muscles have built-in turbo-boosters, called fast-twitch muscle. Unlike slow-twitch muscle, which we use for most of the day, fast-twitch muscle is super-effective at producing lots of energy quickly and does so anaerobically, Gleeson said. Fast-twitch muscle also uses glycolysis to produce energy, but it skips harvesting energy from pyruvate, a process that takes oxygen. Instead, pyruvate gets converted into a waste product, lactic acid, and released into the bloodstream.

It's a common misconception that muscle cells produce lactic acid when they can't get enough oxygen, Gleeson said. "That's not the case. Your muscles are getting plenty of oxygen," he said. But in times of intense energy needs, muscles switch to anaerobic respiration simply because it's a much quicker way to produce energy.

Although they are both running sports the 100-meter sprint and the marathon are two completely different events. The 100-meter sprint is a brief, explosive event (Newsholme, Leech & Duester, 1994 Ross & Leveritt, 2001) the marathon is a prolonged, high-intensity, endurance event (Wagenmakers, 1999).

The essence of the 100-meter sprint is speed, with little oxygen breathed in during its 10-second duration, making the event almost entirely anaerobic (Newsholme, et al., 1994) whereas, although completed by elite marathon runners at a pace between 80-85% of their maximal capacity and the anaerobic system being utilized during sprint efforts in or at the end of the race, the marathon is a primarily aerobic event completed in 2 to 2.5 hours by elite marathon runners (Newsholme, et. al., 1994).

This article will compare and contrast the metabolic demands of the 100-meter sprint and the marathon. It will explain the different energy systems and metabolic pathways that are used by the working muscles to synthesize energy.

It will identify the source of the energy used during these events, discussing situations in which the working muscles use fuel of different types from more than one source. Finally, it will look at limitations of the stores and supply of those fuels and relate it to optimized performance in each event.


For the body to perform exercise for any given intensity or duration it requires energy. Energy is provided chemically in the form of Adenosine Triphosphate (ATP), a high-energy phosphate stored within skeletal muscle.

ATP is the only fuel that can be used directly by the working muscles for contraction. The body has limited stores (

80-100 grams) of ATP, only enough to supply energy for two seconds of maximal sprinting therefore ATP must be continually be resynthesized from other sources through different metabolic pathways.

The 100-meter sprint utilizes both PCr and glycogen as fuel.

The intensity and duration of the event determines the fuel and the energy system that is required to provide energy (Maughan, Gleeson & Greenhaff, 1997 McArdle, Katch & Katch, 2007 Newsholme et. al., 1994).

Energy Systems

There are three different energy systems in the body:

  1. The Immediate System - Fuelled by the intramuscular high-energy phosphates ATP and Phosphocreatine (PCr)
  2. The Short-Term System - The anaerobic glycolysis (glycogenolysis and/or glycolysis) or lactate system fuelled by glycogen, glucose, and the glycerol backbone of triglycerides and
  3. The Long-Term System - The aerobic glycolysis and oxidative phosphorylation of the macronutrients carbohydrate, lipids and protein (Maughan, et. al., 1997 McArdle, et. al., 2007).

The purpose of each energy system "is to regenerate ATP at sufficient rates to prevent a significant fall in the intramuscular ATP concentration" (Maughan, et. al., 1997, p. 16).

The Immediate Energy System

Maximal, high-intensity, short duration exercise, like the 100-meter sprint, uses the immediate energy system. There is 4 to 5 times more PCr in skeletal muscle than ATP and the immediate energy system begins to resynthesize ATP anaerobically via PCr hydrolysis as soon as the working muscles contract.

The rate of PCr hydrolysis is highest within the first two seconds of contraction, but it begins to decline after only a few seconds of maximal contraction due to incomplete PCr resynthesis (Maughan, et. al., 1997 McArdle, et. al., 2007).

The Short-Term Energy System

"Anaerobic energy for ATP resynthesis in glycolysis can be viewed as reserve fuel activated when a person accelerates at the start of exercise" (McArdle, et. al., 2007, p. 166) or performs an all-out 100-m sprint.

Anaerobic glycolysis, like PCr hydrolysis, is initiated at the onset of contraction of high intensity exercise, but, unlike PCr hydrolysis, its highest rate of reaction occurs after five seconds of contraction.

During intense exercise, if hydrogen production exceeds its oxidation, pyruvate temporarily binds to it to create lactate. Lactate then either 1) accumulates, because its production exceeds its oxidation (lactate accumulation, as will be discussed later, can be a cause of fatigue in the 100-meter sprint) 2) is oxidized to pyruvate or 3) is diffused into the blood, where it is transported to the liver and enters the Cori cycle to undergo gluconeogenesis (McArdle, et. al., 2007 Newsholme, et. al., 1994).

The Long-Term Energy System

During prolonged exercise like the marathon, as long as there is oxygen produced to the working muscles ATP can be resynthesized through aerobic metabolism (Brooks, 1999 McArdle, et. al., 2007).

During aerobic glycolysis steady-state oxygen consumption matches energy demands and pyruvate converts to Acetyl-CoA, which then enters the Krebs cycle and subsequently the electron transport chain in the mitochondria, where electrons are transferred from NADH and FADH2 to oxygen to form water and carbon dioxide in a process called oxidative phosphorylation, to regenerate ATP (Maughan, et. al., 1997 McArdle, et. al., 2007).

Energy System Continuum

"While one energy system will predominate over the other, both aerobic and anaerobic energy systems are always working, regardless of intensity or type of activity" (Brooks, 1999, p. 103). The relative contributions of each energy system are dependent upon both the exercise intensity and duration (McArdle, et. al., 2007 Newsholme, et. al., 1994).

The contribution of energy from each energy system falls along a continuum.

At one extreme, during high-intensity, short duration (

10 seconds) exercise such as the 100-m sprint, where energy expenditure of the working muscles exceeds their resting value by 120 times or more, the intramuscular high-energy phosphates supply almost all of the energy required.

At the other extreme, during less-intense, prolonged exercise such as the marathon, where energy expenditure of the working muscles only exceeds resting levels by 20 to 30 times, anaerobic processes are not needed and aerobic processes provide almost all of the energy required (McArdle, et. al., 2007 Newsholme, et. al., 1994).

Fuels In The Body

There are various fuels in the body that can maintain as-needed ATP resynthesis: The intramuscular high-energy phosphate PCr, as well as the macronutrients carbohydrates, lipids, and protein.

Metabolic Pathways

There are many different metabolic pathways that these fuels utilize in the regeneration of ATP.

PCr is stored only in skeletal muscle, in amounts four to five times that of ATP. PCr rephosphorylates Adenosine Diphosphate (ADP) in skeletal muscle to form ATP anaerobically. The equation for PCr hydrolysis is ADP + PCr Creatine Kinase ATP + Cr (McArdle, et. al, 2007). PCr is very important for events such as the 100-meter sprint, when energy is required immediately and without oxygen (McArdle, et. al., 2007 Newsholme, et. al., 1994).

Carbohydrate Metabolism

Carbohydrates are stored in the body as either glycogen or glucose, predominantly in the liver and muscle, but also in adipose tissue, the bloodstream, and the brain. Carbohydrates are metabolized through glycolysis, a process that can be both anaerobic (conversion to pyruvate or lactate) and aerobic (continuation through metabolic pathways as pyruvate converts to Acetyl-CoA and enters the Krebs cycle and electron transport chain for oxidative phosphorylation).

During the 100-meter sprint, anaerobic glycolysis of glucose uses two ATP during the reaction, thus yielding a net two ATP if glycogen undergoes glycogenolysis before anaerobic glycolysis the net yield is three ATP.

During the marathon a further two ATP is provided through substrate-level phosphorylation in the Krebs cycle, and 32 ATP molecules are resynthesized during oxidative phosphorylation, regenerating a total of 36 ATP through the aerobic metabolism of carbohydrate.

Muscle glycogen is the most important fuel in the 100-meter sprint, as it is immediately available when contraction occurs (Newsholme, et. al., 1994).

Lipid Metabolism

Lipids are stored in the body as triglycerides, mostly in adipose tissue, with some intramuscular stores, as well as in the liver and the bloodstream. Lipids are hydrolyzed down to glycerol and three fatty acids in a process called lipolysis. Whilst the fatty acids must undergo the aerobic process beta-oxidation in order to regenerate ATP, glycerol can enter the metabolic pathways through glycolysis.

Lipids are an important energy source during long-distance events such as the marathon, as they are more energy dense than carbohydrates (Maughan, et. al., 1997 McArdle, et. al., 2007 Newsholme, et. al., 1994). Compared to the 36 ATP regenerated by a glucose molecule, each triglyceride molecule regenerates a net 460 ATP: 441 ATP through beta-oxidation, the Krebs cycle and oxidative phosphorylation, and a further 19 ATP through the glycolysis of glycerol.

Lipids are not the ideal fuel for exercise because fatty acids must be transported to skeletal muscle from adipose tissue via albumin in the blood, before they can be oxidized in the metabolic pathways.

However, this makes them the ideal fuel in the later stages of endurance events, such as the marathon, once glycogen stores have been depleted and the marathon runner&aposs pace has dropped to approximately 50% of their maximal capacity (Maughan, et. al., 1997 McArdle, et. al., 2007 Newsholme, et. al., 1994).

Protein Metabolism

Protein is stored predominantly in muscle in the body, but also in the liver and minutely in adipose tissue. When glycogen stores have become depleted protein can become an important fuel, and during endurance events such as the marathon, protein can provide up to 10% of the energy needed for ATP resynthesis (Maughan, et. al., 1997 Wardlaw & Hampl, 2007).

Proteins have their nitrogen removed through deanimation in both the liver and skeletal muscle, leaving the amino acids to enter the metabolic pathways through several ways: some amino acids are glucogenic and yield intermediates for gluconeogenesis other amino acids are ketogenic and yield intermediates that are synthesized to triglycerol or catabolize aerobically for energy in the Krebs cycle.

The total ATP regenerated by protein is dependent upon where it enters the metabolic pathways (McArdle, et. al., 2007 Newsholme, et. al., 1994).

Fuel Use In The 100-Meter Sprint And The Marathon

The fuel used to regenerate ATP during exercise depends upon a variety of factors, including the intensity and duration of the exercise, substrate availability, and muscle fiber type composition, amongst others (Maughan, et. al., 1997).

The 100-Meter Sprint

The 100-meter sprint utilizes both PCr and glycogen as fuel. There is enough PCr in the leg muscles to provide half the ATP needed for the race. Glycogen therefore provides the other half of the energy required.

In the 100-meter sprint it takes 1-2 seconds for the rate of glycolysis to increase at the required factor of at least 1000 (Newsholme, et. al., 1994). Thus PCr hydrolysis contributes to the majority of ATP production during the first two seconds of the race, while glycolysis reaches its maximal rate. Research has shown that maximal sprinting performance is achieved through the simultaneous use of PCr and glycogen (Newsholme, et. al., 1994).

The Marathon

During a race the marathon runner uses approximately 75 kilograms of ATP, and as this amount cannot be stored in the body ATP is resynthesized from different fuels (i.e. PCr, carbohydrates, lipids and protein), with the catabolism of the fuels providing the energy required.

Based on simulated tests elite marathoners utilize 5 grams of glycogen per minute (Newsholme, et. al., 1994) therefore it can be calculated that the elite marathon runner would be exhausted after only 90 minutes of running if they only used carbohydrates as fuel (Maughan, 1997).

Glycogen and glucose cannot be stored in amounts sufficient to entirely fuel the marathon thus other fuels also contribute to ATP resynthesis.

Lipid stores in the body are almost 35 times that of carbohydrate stores. They are also stored &aposdry&apos whereas carbohydrates are stored &aposwet&apos (i.e. carbohydrates store 3 grams of water for every gram of glycogen lipids are hydrophobic). Therefore lipids (i.e. fats) become the primary energy fuel during the marathon once carbohydrate (i.e. glycogen) stores are depleted (Hausswirth & Lehenaff, 2001 Maughan, et. al., 1997 McArdle, et. al., 2007 Newsholme, 1994).

Limitations Of Fuel Stores And Supply And Fatigue

Fatigue is inevitable during high-intensity exercise. Fatigue is defined as "the inability to maintain a given or expected output or force" (Maughan, et. al., 1999, p. 155), and it is likely to be a multifactoral process during both the 100-meter sprint and the marathon (Juel & Pilegaard, 1999 Maughan, et, al., 1997).

The 100-Meter Sprint

During the 100-meter sprint the greater the oxygen deficit, the greater the ATP and PCr stores are being depleted, and the greater the accumulation of lactate (Maughan, et. al., 1997 Newsholme, et. al., 1999). Lactate decreases the pH of the tissues in which it accumulates and studies with isolated muscle preparations have shown that decreasing skeletal muscle pH either 1) impairs muscle contraction by disrupting calcium release, or 2) inhibits ATP resynthesis (Allen, Balnave, Chin & Westerblad, 1999 Jule & Pilegaard, 1999 Maughan, et, al., 1997 Ross & Leveritt, 2001).

Studies involving biomechanical analysis of muscle biopsy samples taken from the vastus lateralis at rest and after 10 seconds of maximal sprint exercise i.e. (the 100-meter sprint) show that "ATP is supplied initially by maximal rates of ATP degradation and glycogenolysis, particularly in type II fibers" (Greenhaff, Casey, Constantin-Teodosiu & Tzintzas, 1999, p. 283) and that it is the depletion of PCr in those fibers that are primarily responsible for fatigue (Greenhaff, et. al., 1999).

The Marathon

Further studies show that during endurance exercise like the marathon, fatigue is a response of the brain as a consequence of glycogen depletion in the type I muscle fibers (Greenhaff, et. al., 1999 McArdle, et. al, 2007 Newsholme, et. al., 1999).

There has been suggestion of a &aposcentral governor&apos in the brain, that subconsciously paces the working muscles so that they do not reach complete exhaustion, and that the brain creates sensations interpreted as fatigue to limit exhaustive exercise and prevent maximum lactate accumulation (Noakes, Peltonen & Rusko, 2001).


The 100-meter sprint is an explosive, maximal race lasting about 10 seconds. The immediate and short-term energy systems are utilized to anaerobically chemically regenerate ATP from intramuscular stores of PCr and glycogen respectively during the race.

Aerobic metabolism via the long-term energy system contributes primarily to the resynthesis of ATP during the marathon, utilizing carbohydrates, lipids, and some protein as fuel (Maughan, et. al., 1997 McArdle, et. al., 2007 Newsholme, et. al., 1994).

Both carbohydrates and lipids are used as intramuscular fuels at rest. Studies using the respiratory exchange ratio have established that both carbohydrates and lipids are used to provide fuel for skeletal muscles during exercise and that their relative contribution to ATP regeneration changes based on the exercise intensity and duration (Spriet & Odland, 1999).

Short, high-intensity exercise, such as the 100-meter sprint has a greater reliance upon carbohydrates as a fuel source than the still-high-but-lower-intensity marathon, which relies more upon the oxidation of lipids to regenerate ATP (Maughan, et. al., 1997 McArdle, et. al., 2007 Newsholme, et. al., 1994 Spriet & Odland, 1999).

Metabolic changes such as lactic acidosis, fuel depletion, impaired excitation-contraction coupling and product inhibition, that occur during the 100-meter sprint and the marathon, inevitably cause fatigue, thereby reducing the pace that the sprinter or marathon runner can maintain for the race, which can result in reduced performance in each respective event (Maughan, et. al., 1997 McArdle, et. al., 2007 Newsholme, et. al., 1994).

Anaerobic Muscle Contraction

During high-intensity exercises, blood flow to your muscles is drastically reduced therefore, inadequate oxygen will be delivered to the muscles. In the absence of oxygen, muscle cells use a process termed glycolysis to produce ATP. Glycolysis generates ATP much faster than oxidative phosphorylation however, glycolysis can only yield two ATP per carbohydrate molecule available. Because glycolysis generates ATP much faster than oxidative phosphorylation, some muscles will resort to glycolysis even in the presence of oxygen to meet their energy needs at a faster rate. Although faster, one major consequence of glycolysis is the production of lactic acid which accumulates inside of your muscles to cause soreness and fatigue.

Ashley Petrone is currently pursuing a PhD in neuroscience at West Virginia University. Petrone graduated from Gannon University with a Bachelor of Science in biology and was also a member of the softball team.