Personal Training 101: How you get energy; how you use it

We usually talk of energy in general terms, as in “I don’t have a lot of energy today” or “You can feel the energy in the room.” But what really is energy? Where do we get the energy to move? How do we use it? How do we get more of it? Ultimately, what controls our movements? The three metabolic energy pathways are the phosphagen system, glycolysis and the aerobic system.How do they work, and what is their effect?

Albert Einstein, in his infinite wisdom, discovered that the total energy of an object is equal to the mass of the object multiplied by the square of the speed of light. His formula for atomic energy, E = mc², has become the most recognized mathematical formula in the world. According to his equation, any change in the energy of an object causes a change in the mass of that object. The change in energy can come in many forms, including mechanical, thermal, electromagnetic, chemical, electrical or nuclear. Energy is all around us. The lights in your home, a microwave, a telephone, the sun; all transmit energy. Even though the solar energy that heats the earth is quite different from the energy used to run up a hill, energy, as the first law of thermodynamics tells us, can be neither created nor destroyed. It is simply changed from one form to another.

ATP Resynthesis

The energy for all physical activity comes from the conversion of high-energy phosphates (adenosinetriphosphate—ATP) to lower-energy phosphates (adenosine diphosphate—ADP; adenosine monophos- phate—AMP; and inorganic phosphate, P_i). During this breakdown (hydrolysis) of ATP, which is a water-requiring process, a proton, energy and heat are produced: ATP + H₂O —^© ADP + P_i + H⁺ + energy + heat. Since our muscles don’t store much ATP, we must constantly resynthesize it. The hydrolysis and resynthesis of ATP is thus a circular process—ATP is hydrolyzed into ADP and P_i, and then ADP and P_icombine to resynthesize ATP. Alternatively, two ADP molecules can combine to produce ATP and AMP: ADP + ADP —^© ATP + AMP.

Like many other animals, humans produce ATP through three metabolic pathways that consist of many enzyme-catalyzed chemical reactions: the phosphagen system, glycolysis and the aerobic system. Which pathway your clients use for the primary production of ATP depends on how quickly they need it and how much of it they need. Lifting heavy weights, for instance, requires energy much more quickly than jogging on the treadmill, necessitating the reliance on different energy systems. However, the production of ATP is never achieved by the exclusive use of one energy system, but rather by the coordinated response of all energy systems contributing to different degrees.

1. Phosphagen System

During short-term, intense activities, a large amount of power needs to be produced by the muscles, creating a high demand for ATP. The phosphagen system (also called the ATP-CP system) is the quickest way to resynthesize ATP (Robergs & Roberts 1997). Creatine phosphate (CP), which is stored in skeletal muscles, donates a phosphate to ADP to produce ATP: ADP + CP —^© ATP + C. No carbohydrate or fat is used in this process; the regeneration of ATP comes solely from stored CP. Since this process does not need oxygen to resynthesize ATP, it is anaerobic, or oxygen-independent. As the fastest way to resynthesize ATP, the phosphagen system is the predominant energy system used for all-out exercise lasting up to about 10 seconds. However, since there is a limited amount of stored CP and ATP in skeletal muscles, fatigue occurs rapidly.

2. Glycolysis

Glycolysis is the predominant energy system used for all-out exercise lasting from 30 seconds to about 2 minutes and is the second-fastest way to resynthesize ATP. During glycolysis, carbohydrate—in the form of either blood glucose (sugar) or muscle glycogen (the stored form of glucose)—is broken down through a series of chemical reactions to form pyruvate (glycogen is first broken down into glucose through a process called glycogenolysis). For every molecule of glucose broken down to pyruvate through glycolysis, two molecules of usable ATP are produced (Brooks et al. 2000). Thus, very little energy is produced through this pathway, but the trade-off is that you get the energy quickly. Once pyruvate is formed, it has two fates: conversion to lactate or conversion to a metabolic intermediary molecule called acetyl coenzyme A (acetyl-CoA), which enters the mitochondria for oxidation and the production of more ATP (Robergs & Roberts 1997). Conversion to lactate occurs when the demand for oxygen is greater than the supply (i.e., during anaerobic exercise). Conversely, when there is enough oxygen available to meet the muscles’ needs (i.e., during aerobic exercise), pyruvate (via acetyl-CoA) enters the mitochondria and goes through aerobic metabolism.

When oxygen is not supplied fast enough to meet the muscles’ needs (anaerobic glycolysis), there is an increase in hydrogen ions (which causes the muscle pH to decrease; a condition called acidosis) and other metabolites (ADP, P_i and potassium ions). Acidosis and the accumulation of these other metabolites cause a number of problems inside the muscles, including inhibition of specific enzymes involved in metabolism and muscle contraction, inhibition of the release of calcium (the trigger for muscle contraction) from its storage site in muscles, and interference with the muscles’ electrical charges (Enoka & Stuart 1992; Glaister 2005; McLester 1997). As a result of these changes, muscles lose their ability to contract effectively, and muscle force production and exercise intensity ultimately decrease.

3. Aerobic System

Since humans evolved for aerobic activities (Hochachka, Gunga & Kirsch 1998; Hochachka & Monge 2000), it’s not surprising that the aerobic system, which is dependent on oxygen, is the most complex of the three energy systems. The metabolic reactions that take place in the presence of oxygen are responsible for most of the cellular energy produced by the body. However, aerobic metabolism is the slowest way to resynthesize ATP. Oxygen, as the patriarch of metabolism, knows that it is worth the wait, as it controls the fate of endurance and is the sustenance of life. “I’m oxygen,” it says to the muscle, with more than a hint of superiority. “I can give you a lot of ATP, but you will have to wait for it.”

The aerobic system—which includes the Krebs cycle (also called the citric acid cycle or TCA cycle) and the electron transport chain—uses blood glucose, glycogen and fat as fuels to resynthesize ATP in the mitochondria of muscle cells (see the sidebar “Energy System Characteristics”). Given its location, the aerobic system is also called mitochondrial respiration. When using carbohydrate, glucose and glycogen are first metabolized through glycolysis, with the resulting pyruvate used to form acetyl-CoA, which enters the Krebs cycle. The electrons produced in the Krebs cycle are then transported through the electron transport chain, where ATP and water are produced (a process called oxidative phosphorylation) (Robergs & Roberts 1997). Complete oxidation of glucose via glycolysis, the Krebs cycle and the electron transport chain produces 36 molecules of ATP for every molecule of glucose broken down (Robergs & Roberts 1997). Thus, the aerobic system produces 18 times more ATP than does anaerobic glycolysis from each glucose molecule.

Fat, which is stored as triglyceride in adipose tissue underneath the skin and within skeletal muscles (calledintramuscular triglyceride), is the other major fuel for the aerobic system, and is the largest store of energy in the body. When using fat, triglycerides are first broken down into free fatty acids and glycerol (a process called lipolysis). The free fatty acids, which are composed of a long chain of carbon atoms, are transported to the muscle mitochondria, where the carbon atoms are used to produce acetyl-CoA (a process called beta-oxidation).

Following acetyl-CoA formation, fat metabolism is identical to carbohydrate metabolism, with acetyl-CoA entering the Krebs cycle and the electrons being transported to the electron transport chain to form ATP and water. The oxidation of free fatty acids yields many more ATP molecules than the oxidation of glucose or glycogen. For example, the oxidation of the fatty acid palmitate produces 129 molecules of ATP (Brooks et al. 2000). No wonder clients can sustain an aerobic activity longer than an anaerobic one!

Understanding how energy is produced for physical activity is important when it comes to programming exercise at the proper intensity and duration for your clients. So the next time your clients get done with a workout and think, “I have a lot of energy,” you’ll know exactly where they got it.

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SIDEBAR: Energy System Characteristics

SIDEBAR: Energy System Workouts

Have clients warm up and cool down before and after each workout.

Phosphagen System

An effective workout for this system is short, very fast sprints on the treadmill or bike lasting 5–15 seconds with 3–5 minutes of rest between each. The long rest periods allow for complete replenishment of creatine phosphate in the muscles so it can be reused for the next interval.

• 2 sets of 8 x 5 seconds at close to top speed with 3:00 passive rest and 5:00 rest between sets
• 5 x 10 seconds at close to top speed with 3:00–4:00 passive rest

Glycolysis

This system can be trained using fast intervals lasting 30 seconds to 2 minutes with an active-recovery period twice as long as the work period (1:2 work-to-rest ratio).

• 8–10 x 30 seconds fast with 1:00 active recovery
• 4 x 1:30 fast with 3:00 active recovery

Aerobic System

While the phosphagen system and glycolysis are best trained with intervals, because those metabolic systems are emphasized only during high-intensity activities, the aerobic system can be trained with both continuous exercise and intervals.

• 60 minutes at 70%–75% maximum heart rate
• 15- to 20-minute tempo workout at lactate threshold intensity (about 80%–85% maximum heart rate)
• 5 x 3:00 at 95%–100% maximum heart rate with 3:00 active recovery

Tabata Intervals: A Simple and Effective Protocol For Cyclists and Endurance Athletes

The recent interest in high-intensity interval training has seen the term Tabata applied to many different exercise regimens, but did you know that one of the original studies was performed on a bicycle? The training protocol of repeated twenty-second work periods coupled with ten-second rest periods was first introduced by Kouichi Irisawa. Irisawa was a head coach of the Japanese National Speed Skating Team. What would be the perfect workout? Short and effective? Well Tabata interval training might be just what you're after. The name "Tabata" gets thrown round an awful lot when talking about HIIT, but few people actually know what the original study consisted of. It has often been adapted to make their own less effective versions. In 1994, Izumi Tabata reported on training experiments that used this protocol on a cycle ergometer. Over a six-week period, his athletes underwent one of the following: 1. Moderate intensity endurance training at 70% of VO2 max for sixty minutes, five days per week. 2. Intermittent training intervals at 170% of VO2 max for 20 seconds with 10 seconds rest, repeated for seven to eight sets, five days per week. The endurance-training group showed increased VO2 max from 53 ml/kg/min to 58 ml/kg/min - a rise of 5 ml/kg/min or 10%. The endurance training made no significant difference to anaerobic capacity, which would be important in short bursts of activity (as you might expect given that this mode of training did not stress anaerobic energy production). The interval training increased VO2 max by 7 ml/kg/min, slightly more than the endurance protocol, and also increased anaerobic capacity by 28%. For each participant in the study a linear relationship between power and oxygen demand had been established, so this means that the above percentages can be used to approximate power gains, too. Breathing more oxygen might be interesting, but it’s of little use unless it can be converted into useful work. It is not necessary to go to the expense of having a full VO2 max test to work out the levels of effort required to undertake this type of intermittent training. An approximation to the power output at VO2 max can be established by a simpler functional threshold power (FTP) test. The FTP test estimates the average power that could be sustained over sixty minutes. A typical way to do this test is to use a power meter on a turbo trainer and work flat out over twenty minutes. You record the average power and multiply this by 0.95 to estimate the power for sixty minutes. (Or you can, of course, do the test for sixty minutes - if you can do that!) The intensity for the Tabata-type intervals can then be established by taking this FTP figure and multiplying it by 1.7. If you have a power meter or turbo trainer that can measure or estimate power you are all set. As this is a high-intensity exercise regime, you do need to make sure that it is suitable for you and that your bike and turbo are securely set up. A gym towel falling in the spokes during a twenty-second high intensity interval will certainly spoil your day. Based upon this study, an exercise program would by comprised of: 1. Warm up for ten minutes. 2. Perform seven or more sets of intervals of 20secs (at 170% FTP) and 10secs rest. 3. Cool down. Perform this five days per week. On the sixth day, perform thirty minutes at 70% and then do four Tabata-style intervals. Rest on the seventh day. Repeat for six weeks. If you measure your FTP before and after this program, you should see some useful gains. So, if you are emerging from your winter training and wish to put in some finishing touches before some early events or have only just started to think about training for the year ahead, this might be a simple and effective protocol for you. References: 1. Tabata, Izumi et al., "Effects of moderate-intensity endurance and high-intensity intermittent training on anaerobic capacity and VO2max." Medicine & Science in Sports & Exercise Issue: Volume 28(10), October 1996, pp 1327-1330.