Category Archives: aerobic endurance

The Body’s Fuel Sources

Our ability to run, bicycle, ski, swim, and row hinges on the capacity of the body to extract energy from ingested food. As potential fuel sources, the carbohydrate, fat, and protein in the foods that you eat follow different metabolic paths in the body, but they all ultimately yield water, carbon dioxide, and a chemical energy called adenosine triphosphate (ATP). Think of ATP molecules as high-energy compounds or batteries that store energy. Anytime you need energy—to breathe, to tie your shoes, or to cycle 100 miles (160 km)—your body uses ATP molecules. ATP, in fact, is the only molecule able to provide energy to muscle fibers to power muscle contractions. Creatine phosphate (CP), like ATP, is also stored in small amounts within cells. It’s another high-energy compound that can be rapidly mobilized to help fuel short, explosive efforts. To sustain physical activity, however, cells must constantly replenish both CP and ATP.

Our daily food choices resupply the potential energy, or fuel, that the body requires to continue to function normally. This energy takes three forms: carbohydrate, fat, and protein. (See table 2.1, Estimated Energy Stores in Humans.) The body can store some of these fuels in a form that offers muscles an immediate source of energy. Carbohydrates, such as sugar and starch, for example, are readily broken down into glucose, the body’s principal energy source. Glucose can be used immediately as fuel, or can be sent to the liver and muscles and stored as glycogen. During exercise, muscle glycogen is converted back into glucose, which only the muscle fibers can use as fuel. The liver converts its glycogen back into glucose, too; however, it’s released directly into the bloodstream to maintain your blood sugar (blood glucose) level. During exercise, your muscles pick up some of this glucose and use it in addition to their own private glycogen stores. Blood glucose also serves as the most significant source of energy for the brain, both at rest and during exercise. The body constantly uses and replenishes its glycogen stores. The carbohydrate content of your diet and the type and amount of training that you undertake influence the size of your glycogen stores.

The capacity of your body to store muscle and liver glycogen, however, is limited to approximately 1,800 to 2,000 calories worth of energy, or enough fuel for 90 to 120 minutes of continuous, vigorous activity. If you’ve ever hit the wall while exercising, you know what muscle glycogen depletion feels like. As we exercise, our muscle glycogen reserves continually decease, and blood glucose plays an increasingly greater role in meeting the body’s energy demands. To keep up with this greatly elevated demand for glucose, liver glycogen stores become rapidly depleted. When the liver is out of glycogen, you’ll “bonk” as your blood glucose level dips too low, and the resulting hypoglycemia (low blood sugar) will further slow you down. Foods that you eat or drink during exercise that supply carbohydrate can help delay the depletion of muscle glycogen and prevent hypoglycemia.

Fat is the body’s most concentrated source of energy, providing more than twice as much potential energy as carbohydrate or protein (9 calories per gram versus 4 calories each per gram). During exercise, stored fat in the body (in the form of triglycerides in adipose or fat tissue) is broken down into fatty acids. These fatty acids are transported through the blood to muscles for fuel. This process occurs relatively slowly as compared with the mobilization of carbohydrate for fuel. Fat is also stored within muscle fibers, where it can be more easily accessed during exercise. Unlike your glycogen stores, which are limited, body fat is a virtually unlimited source of energy for athletes. Even those who are lean and mean have enough fat stored in muscle fibers and fat cells to supply up to 100,000 calories—enough for over 100 hours of marathon running!

Fat is a more efficient fuel per unit of weight than carbohydrate. Carbohydrate must be stored along with water. Our weight would double if we stored the same amount of energy as glycogen (plus the water that glycogen holds) that we store as body fat. Most of us have sufficient energy stores of fat (adipose tissue or body fat), plus the body readily converts and stores excess calories from any source (fat, carbohydrate, or protein) as body fat. In order for fat to fuel exercise, however, sufficient oxygen must be simultaneously consumed. The second part of this chapter briefly explains how pace or intensity, as well as the length of time that you exercise, affects the body’s ability to use fat as fuel.

As for protein, our bodies don’t maintain official reserves for use as fuel. Rather, protein is used to build, maintain, and repair body tissues, as well as to synthesize important enzymes and hormones. Under ordinary circumstances, protein meets only 5 percent of the body’s energy needs. In some situations, however, such as when we eat too few calories daily or not enough carbohydrate, as well as during latter stages of endurance exercise, when glycogen reserves are depleted, skeletal muscle is broken down and used as fuel. This sacrifice is necessary to access certain amino acids (the building blocks of protein) that can be converted into glucose. Remember, your brain also needs a constant, steady supply of glucose to function optimally.

Fuel Metabolism and Endurance Exercise

Carbohydrate, protein, and fat each play distinct roles in fueling exercise.


  • Provides a highly efficient source of fuel—Because the body requires less oxygen to burn carbohydrate as compared to protein or fat, carbohydrate is considered the body’s most efficient fuel source. Carbohydrate is increasingly vital during high-intensity exercise when the body cannot process enough oxygen to meet its needs.
  • Keeps the brain and nervous system functioning—When blood glucose runs low, you become irritable, disoriented, and lethargic, and you may be incapable of concentrating or performing even simple tasks.
  • Aids the metabolism of fat—To burn fat effectively, your body must break down a certain amount of carbohydrate. Because carbohydrate stores are limited compared to the body’s fat reserves, consuming a diet inadequate in carbohydrate essentially limits fat metabolism.
  • Preserves lean protein (muscle) mass—Consuming adequate carbohydrate spares the body from using protein (from muscles, internal organs, or one’s diet) as an energy source. Dietary protein is much better utilized to build, maintain, and repair body tissues, as well as to synthesize hormones, enzymes, and neurotransmitters.


  • Provides a concentrated source of energy—Fat provides more than twice the potential energy that protein and carbohydrate do (9 calories per gram of fat versus 4 calories per gram of carbohydrate or protein).
  • Helps fuel low- to moderate-intensity activity—At rest and during exercise performed at or below 65 percent of aerobic capacity, fat contributes 50 percent or more of the fuel that muscles need.
  • Aids endurance by sparing glycogen reserves—Generally, as the duration or time spent exercising increases, intensity decreases (and more oxygen is available to cells), and fat is the more important fuel source. Stored carbohydrate (muscle and liver glycogen) are subsequently used at a slower rate, thereby delaying the onset of fatigue and prolonging the activity.


  • Provides energy in late stages of prolonged exercise—When muscle glycogen stores fall, as commonly occurs in the latter stages of endurance activities, the body breaks down amino acids found in skeletal muscle protein into glucose to supply up to 15 percent of the energy needed.
  • Provides energy when daily diet is inadequate in total calories or carbohydrate—In this situation, the body is forced to rely on protein to meet its energy needs, leading to the breakdown of lean muscle mass.

This excerpt is from the book, Endurance Sports Nutrition-3rd Edition. It’s published with permission of Human Kinetics. Please purchase this book from Human Kinetics.

Use training zones to achieve your best workout

Training Zones

Training zones are used to quantify and track intensity. Remember that workload is the product of volume (duration and frequency) and intensity. The volume component of your workload is easily tracked; all you need is a watch and a calendar. Intensity, however, is a whole different ball game. This is the toughest part of your training to get right.

As mentioned previously, every time you train you should have a goal for the workout. To reach that goal, you’ll need to be aware of how hard you’re riding (i.e., your intensity).

The purpose of a good training program is to work different aspects of your physiology. You’ll be training your aerobic and anaerobic systems, your strength, and your mental fortitude. Some workouts may be for base training, building up the vascular machinery that will allow you to go hard later on. Other workouts may focus on training your maximal speed, allowing you to blow past a friend as you race for a city limit sign.

Each training zone represents a different level of effort, ranging from easy to hard. An overwhelming amount of information is available on training zones. Different coaches and books use different nomenclature, and this can make it confusing. This book is designed to give you a solid foundation in the world of training; the goal is to simplify things so that you’ll have a good understanding that’s adaptable to whatever terminology you encounter along the way. Continue reading

Establishing training goals on the bike

Training Goals

If you want to train more seriously, you need to have a plan. Every time you get on your bike, you are essentially training. The question is whether you’re training effectively or just gaining some conditioning through random episodes of exercise. If you are brand new to the sport, you will see great gains in your riding fitness, skill, and comfort simply by getting out on rides. Your body will respond to the stress of riding and will adapt accordingly. But, you can achieve much more progression if you take the time to establish a plan of action.

Effective training is what this book is all about. Most of us have other commitments—family, work, friends, and so on. That’s why cyclists need to make the most of the time they spend on the bike.

As a performance cyclist, you should always be striving to improve, and you should focus your attention on your cycling goals. If you want to hit the target, you first have to define that target.

What are your goals? Why are you riding your bike? Are you riding in order to stay healthy, to beat a friend up a local climb, or to complete your first century? Every person has a different goal, and that’s the point. You own your goals and all the training that you complete—every pedal stroke, every climb, every Saturday you drag yourself out of bed and onto the road.

Goals can be intimidating because they come with an inherent chance of failure. A goal that is easy to achieve and includes no chance of failure would be ineffective because it goes against the very premise of this book—getting the most out of your riding. The possibility of success or failure is the crux of a good goal. You need to struggle to improve, and the only way to truly struggle is to know that there is a risk of failure. It is the risk, the chance of failure, that drives you toward success.

To help ensure that you establish attainable goals, you should apply the Four Ps of goal setting: personalized, positive, perceivable, and possible.

Personalized means that the goals are your own. Only you can determine what is important, what will motivate you to keep your commitment, and what will give you a sense of accomplishment.

All your goals should be positive. Negative energy sucks! At Disneyland, they live by this philosophy. If you ask the workers when the park closes, they will respond, “The park stays open until 8 o’clock.” You should set a goal to accomplish a desired result rather than to avoid failure. Word your goals so that the outcome is positive.

You need to set goals that have a tangible outcome. Your goals must be perceivable to yourself or to others. This aspect of goal setting is all about accountability.

Finally, your goals need to be realistic but challenging. When you think about your goal, you should have a strong sense that the desired outcome is possible, but by no means assured. You need to believe even with the possibility of failure. This will help you suffer a little longer, struggle just a bit more, and get the most out of your training plan.

Don’t think that goals are only for professionals or racers. EVERY RIDER NEEDS GOALS. Think of goals the same way you think of the rest of the training program. Training is all about progression, and goals should follow suit. They start with more obtainable outcomes. But with each accomplishment, the task becomes more difficult. Each goal builds on the last in a stepwise fashion (figure 1.1), until you find yourself faced with your ultimate accomplishment.

Be sure to write down your goals. For each time frame—short, medium, and long—fill in your primary and secondary goals (figure 1.1). Again, these goals can be anything. They should be whatever motivates you to train when you might feel like flicking on the TV instead. There is something about actually writing down your goals. This brings them outside your brain and into the real world—an accountable world.

Training is all about commitment, discipline, and perseverance. It is a slow grind, and sometimes you feel as though you’re going backward instead of forward. But if you stick to your program, you WILL get better. Writing down your goals is the first barrier to overcome.

Goals will perpetually be included in your training program. Every time you reach a goal, you can have a little celebration, even if it is internal. Treat yourself to a double half-caf, mocha chai latte if that’s your thing. As soon as you are finished basking in the glory of the accomplishment, write down a new set of goals. Stay on target!

This excerpt is from the book, Fitness Cycling. It’s published with permission of Human Kinetics.

Improve your endurance by knowing what affects your heart rate

This excerpt is from the author of Heart Rate Training. It’s published with permission of Human Kinetics

One of the most valuable long-term pieces of information you can gather is resting heart rate. When you wake up each morning, take a minute to get an accurate resting heart rate and keep a log. You’ll find this an invaluable tool, providing feedback on injury, illness, overtraining, stress, incomplete recovery, and so on. It is also a very simple gauge of improvements in fitness. We know athletes who have gathered resting heart rate data for years and in a day or two can identify a 1 or 2 bpm elevation that precedes an illness or a bonk session. Some newer heart rate monitors have the capacity for 24-hour monitoring.

Several factors affect heart rate at rest and during exercise. In general, the main factors affecting heart rate at rest are fitness and state of recovery. Gender also is suggested to play a role, albeit inconsistently (more about this later). In general, fitter people tend to have lower resting heart rates. Some great athletes of the past have recorded remarkably low resting heart rates. For example, Miguel Indurain, five-time winner of the Tour de France, reported a resting heart rate of only 28 bpm. The reason for this is that, with appropriate training, the heart muscle increases in both size and strength. The stronger heart moves more blood with each beat (this is called stroke volume) and therefore can do the same amount of work with fewer beats. As you get fitter, your resting heart rate should get lower.

The second main factor affecting resting heart rate is state of recovery. After exercise, particularly after a long run or bike ride, several things happen in the body. Fuel sources are depleted, temperature increases, and muscles are damaged. All of these factors must be addressed and corrected. The body has to work harder, and this increased work results in a higher heart rate. Even though you might feel okay at rest, your body is working harder to repair itself, and you’ll notice an elevated heart rate. Monitoring your resting heart rate and your exercise heart rate will allow you to make appropriate adjustments such as eating more or taking a day off when your rate is elevated.

These same factors of recovery and injury also affect heart rate during exercise. The factors that elevate resting heart rate also elevate exercise heart rate. If you’re not fully recovered from a previous workout, you might notice, for example, at your usual steady-state pace, an exercise heart rate that is 5 to 10 bpm higher than normal. This is usually accompanied by a rapidly increasing heart rate throughout the exercise session.

An extremely important factor affecting exercise heart rate is temperature. Warmer temperatures cause the heart to beat faster and place considerable strain on the body. Simply put, when it is hot, the body must move more blood to the skin to cool it while also maintaining blood flow to the muscles. The only way to do both of these things is to increase overall blood flow, which means that the heart must beat faster. Depending on how fit you are and how hot it is, this might mean a heart rate that is 20 to 40 bpm higher than normal. Fluid intake is very important under these conditions. Sweating changes blood volume, which eventually can cause cardiac problems. The simplest and most effective intervention to address high temperature and heart rate is regular fluid intake. This helps to preserve the blood volume and prevent the heart from beating faster and faster.

Another important factor affecting exercise heart rate is age. In general, MHR will decline by about 1 beat per year starting at around 20 years old. Interestingly, resting heart rate is not affected. This is why the basic prediction equation of 220 – age has an age correction factor. As a side note, this decrease in MHR often is used to explain decreases in .VO2max and endurance performance with increasing age, because the number of times the heart beats in a minute affects how much blood is moved and available to the muscles. We have coached and tested thousands of athletes, and the general trend is that athletes of the same age who produce higher heart rates often have higher fitness scores. However, your MHR is what it is, and you cannot change it. Don’t obsess over it.

A final factor is gender. Recent studies have suggested a variation in MHR between males and females. However, the data are inconclusive with the calculations resulting in lower MHRs for males versus females of the same age, while anecdotal reports suggest that the MHRs are actually higher in males. In general, females have smaller hearts and smaller muscles overall than males. Both of these factors would support the conclusion of a higher MHR in females, certainly at the same workload. We have to conclude that the jury is still out on the gender effect.

Zone Adjustments for triathlon

If you’re planning a triathlon this year in the summer, or in a different location than you are used to, please read this excerpt to plan for the adjustment your body will make. It’s an excellent excerpt from John Mora‘s Triathlon Workout Planner. This excerpt is reprinted with permission.

“Once you know your target zones, you must still do a little tweaking of the range numbers in order to further individualize your training for improved accuracy and efficiency. All training has to be individualized, and these adjustments take into account the different characteristics of each sport, outside conditions, and any illness or overtraining symptoms that may be happening within your body.

Some adjustments are sport-specific. It’s become obvious to me through the years that my heart rate while running is at least 10 beats higher than a similar perceived effort while riding my bike. That’s not uncommon, since running puts a pretty good wallop on the legs and causes a greater degree of stress on major muscle groups from the impact. Cycling is less stressful on joints, often resulting in a lower heart rate, and swimming is even less taxing.

To adjust for the unique demands of all three sports, you may want to adjust your training zones for cycling to be 10 fewer beats than what you would use for running. For example, if you’ve field-tested the target zone numbers you derived with the given formulas on a few runs at various intensities, then subtract 10 beats from your lower and upper limits in each zone to determine your cycling zones. For swimming, adjust your target zones down 5 beats from your adjusted cycling target zones.

You would also be wise to make a number of other adjustments to your heart rate training, depending on altitude, weather, and illness.

Altitude adjustments. If you are training or traveling to a race above an altitude of 6,000 feet (1,830 meters) for the first time, or if you do so infrequently, your heart rate will naturally be higher, even at rest. Above 10,000 feet (3,050 meters), you may find your heart rate is a full 50 percent higher. This increase is due to the lower concentration of oxygen in the air at higher altitudes. Of course, the more time you spend at higher altitudes, the greater your body’s ability to adapt, and you’ll probably see a return to your normal heart rate levels after 14 to 21 days. In fact, you can track your acclimatization with your heart rate monitor, noting how your rate decreases and finally gets back to normal within a few weeks. During this time of acclimatization, don’t push beyond your ability, and stay in your target zones. This means that you may have to slow down or lower the intensity of your training in the interim. Be patient—your body will adjust.

Hot-weather adjustments. Exercising in hot weather causes your body to work harder to keep itself cool. Increased blood flow to the skin and sweating cause an elevated heart rate response. The good news is that consistent training in heat brings about acclimatization in much the same way altitude training does. The body becomes much more efficient in dealing with the heat, resulting in a normal blood flow, decreased salt content in sweat, and a return to your normal heart rate. This adjustment usually takes about 10 days of consistent training or about half a dozen workouts in hot conditions. Always remember to hydrate properly (in hot or cold weather, but it’s usually more critical in heat). Dehydration can decrease your total blood volume, making the heart work harder and elevating your heart rate.

Illness. If you find that your resting heart rate has spiked unusually or that it is more difficult than normal to reach your target zones, it may well be that you are courting an illness such as a cold or flu. If you experience either or both of these conditions, back off and take a rest day or a few easy recovery workouts.”