Energy Systems

ATP and Energy Transfer

The body needs a constant supply of energy to fuel muscle contraction, whatever the activity, and the intensity/duration of that activity determines which energy system is used to supply it.

Adenosine triphosphate (ATP) is the only usable form of chemical energy in the body. It is made up of one molecule of adenosine bonded to three phosphate groups.

Energy is released when the bond holding the outer phosphate to the molecule is broken, converting ATP into adenosine diphosphate (ADP) and an inorganic phosphate (Pi). This is described as a coupled reaction:

Because only small amounts of ATP can be stored in the muscle at any time, it must be continually resynthesised from ADP and Pi during exercise. This resynthesis is fuelled by food (carbohydrate and fat) or by phosphocreatine stored in the muscle.

There are three energy systems that can resynthesise ATP: the aerobic system, the ATP-PC (phosphocreatine) system, and the anaerobic glycolytic system. Which one predominates depends on the intensity and duration of the activity.

The Aerobic Energy System

The aerobic system is used when exercise intensity is low and oxygen supply is sufficient, and it is the preferred energy pathway at rest and during long-duration, lower-intensity exercise.

It breaks down glucose in the presence of oxygen, producing carbon dioxide and water as by-products, and can also use fats (and, if necessary, protein) as a fuel source.

The aerobic system works in three stages:

1. Glycolysis – takes place in the sarcoplasm of the muscle cell; glucose is broken down anaerobically into pyruvic acid, yielding a net gain of 2 ATP. If enough oxygen is present, pyruvic acid is converted to acetyl coenzyme A and passed into the mitochondria for the next stage
2. The Krebs cycle – occurs in the matrix of the mitochondria; acetyl coenzyme A combines with oxaloacetic acid to form citric acid, which undergoes a series of reactions producing carbon dioxide (removed as a waste product), a small amount of ATP (2 ATP), and hydrogen (carried onward by coenzymes to the next stage)
3. The electron transport chain – takes place in the cristae of the mitochondria; hydrogen carried from the Krebs cycle is split into electrons and hydrogen ions, releasing energy that is used to resynthesise around 34 ATP, with the hydrogen and electrons eventually recombining with oxygen to form water

Beta oxidation: stored fat (triglyceride) is broken down into glycerol and free fatty acids, which are transported in the blood and converted into acetyl coenzyme A so they can enter the Krebs cycle. Fat metabolism yields more ATP per molecule than glucose, but the process is more complex, requires more oxygen, and cannot function without also breaking down some carbohydrate, so it becomes the predominant fuel source only in longer-duration, lower-intensity exercise where a fitter individual has more time to take up and use oxygen.

• Advantages – produces the largest yield of ATP (around 36-38 molecules per glucose molecule); by-products (carbon dioxide and water) are not fatiguing and are easily removed; large stores of muscle glycogen and body fat mean the system can supply energy for a long time
• Disadvantages – it is a complex, multi-stage process, so ATP production is comparatively slow; it can only function while sufficient oxygen is being delivered to and used by the muscle, so it cannot meet the demands of high-intensity exercise; fat as a fuel requires roughly 15% more oxygen to break down than glycogen

The ATP-PC (Phosphocreatine) System

The ATP-PC system is an anaerobic process (it works without oxygen) that provides energy very rapidly for short, maximal bursts of exercise, such as a 100m sprint or a heavy single lift.

Phosphocreatine (PC) is a high-energy compound stored in the sarcoplasm of the muscle. The enzyme creatine kinase detects rising levels of ADP during exercise and breaks PC down into creatine and an inorganic phosphate, releasing energy.

This released energy is used in a coupled reaction to resynthesise ATP from ADP:

• Advantages – ATP can be resynthesised very rapidly, making it ideal for high-intensity, explosive efforts; it does not produce fatiguing by-products; phosphocreatine stores are replenished quickly during recovery (around 50% within 30 seconds and full restoration within roughly 2-4 minutes)
• Disadvantages – only very limited stores of phosphocreatine are held in the muscle, so this system can only fuel maximal exercise for around 8-10 seconds before stores are depleted; PC can only be resynthesised again once oxygen becomes available, i.e. during a period of lower-intensity exercise or rest

The Anaerobic Glycolytic System

The anaerobic glycolytic system is used for high-intensity exercise lasting from around 10 seconds up to roughly 2-3 minutes, once ATP-PC stores are exhausted but demand for energy is still too high to be met aerobically.

It works by breaking down muscle glycogen into glucose (via the enzyme glycogen phosphorylase) and then breaking that glucose down anaerobically to pyruvic acid through glycolysis, producing a net gain of 2 ATP per glucose molecule.

Because oxygen is not available in sufficient amounts, the enzyme lactate dehydrogenase converts the pyruvic acid produced into lactic acid, rather than the pyruvic acid being taken on into the aerobic pathway.

• Advantages – ATP can be resynthesised quickly, allowing high-intensity efforts of up to roughly 2-3 minutes; it does not depend on the availability of oxygen
• Disadvantages – the build-up of lactic acid as a by-product denatures enzymes and interferes with further muscle contraction, contributing to fatigue; it produces a much smaller yield of ATP from each glycogen molecule than the aerobic system

The Energy Continuum

The energy continuum describes how the contribution of the three energy systems changes according to the intensity and duration of an activity. All three systems operate simultaneously during exercise, but one will dominate depending on the exercise demands at that moment.

As a general rule: very short, maximal efforts (under 10 seconds, e.g. 100m sprint) are predominantly fuelled by the ATP-PC system; efforts of around 10 seconds to 2-3 minutes (e.g. 400m) rely predominantly on the anaerobic glycolytic system; and longer, lower-intensity efforts (over roughly 2-3 minutes, e.g. a marathon) are predominantly fuelled by the aerobic system.

• The ATP-PC/anaerobic glycolytic threshold is the point at which PC stores are exhausted and the anaerobic glycolytic system becomes the dominant energy provider, typically around 10 seconds into a maximal effort
• The anaerobic glycolytic/aerobic threshold is the point at which the anaerobic glycolytic system is exhausted (or lactic acid build-up becomes limiting) and the aerobic system takes over as the dominant energy provider, typically around 3 minutes into continuous exercise

In activities with continuously changing intensity (e.g. team games), the balance of energy systems used shifts constantly, moving back and forth along the continuum depending on the demands of each phase of play.

Energy System Use and Muscle Fibre Type

The energy system predominantly used by a muscle fibre relates closely to its fibre type.

• Slow twitch (type I) fibres rely mainly on the aerobic system: they have a slower pathway for ATP production but can sustain it for a long time, with the maximum amount of ATP available per glucose molecule (up to 36-38 ATP)
• Fast twitch (type II) fibres rely mainly on anaerobic energy production (ATP-PC and anaerobic glycolytic systems): ATP production is much quicker, but the process is less efficient and cannot be sustained for long, producing far less ATP per glucose molecule

Oxygen Consumption During Exercise

Oxygen consumption is the amount of oxygen used to produce ATP, usually measured as VO2 (volume of oxygen consumed per minute).

At the start of exercise, oxygen consumption cannot rise instantly to meet demand because it takes time for the cardiovascular and respiratory systems to respond and deliver more oxygen to the working muscles. This shortfall is called the oxygen deficit.

During this period the muscles rely on anaerobic sources of ATP (stored ATP, PC, and anaerobic glycolysis) to make up the shortfall while oxygen delivery catches up to demand, at which point a steady state of oxygen consumption is reached (in submaximal, aerobic exercise).

VO2 max is the maximum volume of oxygen that the body can take up and use per minute, and represents the ceiling of aerobic capacity.

In maximal exercise, oxygen demand can exceed VO2 max for the whole duration of the effort, meaning the exercise cannot be fully fuelled aerobically; this results in a maximal accumulated oxygen deficit (MAOD), which is used as an indicator of anaerobic capacity.

Excess Post-Exercise Oxygen Consumption (EPOC)

After exercise, oxygen consumption remains elevated above resting levels for a period during recovery. This extra oxygen used during recovery is called excess post-exercise oxygen consumption (EPOC), sometimes referred to as oxygen debt. EPOC has two components:

• Fast (alactacid) component – the rapid initial stage of recovery, in which oxygen is used to restore ATP and phosphocreatine stores in the muscle and to resaturate myoglobin with oxygen. This is largely complete within around 2-4 minutes, with roughly 50% of PC stores restored within about 30 seconds
• Slow (lactacid) component – the longer, more gradual stage of recovery, in which oxygen is used to help remove lactic acid from the muscles and blood, restore glycogen stores, maintain the elevated breathing and heart rate needed to deliver this oxygen, and deal with the effects of raised body temperature

The removal of lactic acid during the slow component happens via several routes: oxidation into carbon dioxide and water for use as an energy source; conversion back to glucose and glycogen in the liver (the Cori cycle); conversion into protein; or excretion in sweat and urine.

Glycogen Replenishment

Glycogen is the main fuel source for both the aerobic and anaerobic glycolytic systems, so replenishing glycogen stores after exercise is an important part of recovery.

How long this takes depends on the type and intensity of exercise performed and how much carbohydrate is consumed afterwards; restoring glycogen after prolonged exercise such as a marathon may take several days, whereas after shorter, higher-intensity exercise a significant amount can be restored more quickly.

There are two key windows for optimal glycogen replenishment through diet: eating a high-carbohydrate meal within around 30 minutes of finishing exercise (ideally with protein in roughly a 3:1 carbohydrate to protein ratio) speeds up glycogen resynthesis; a second window of around 1-3 hours after exercise also benefits from a high-carbohydrate, moderate-protein, low-fat meal.

Lactate Threshold and OBLA

As exercise intensity increases, the body moves from working mostly aerobically towards working more anaerobically. The lactate threshold is the point at which lactic acid begins to accumulate in the blood at a faster rate than it can be removed.

The onset of blood lactate accumulation (OBLA) is the point at which blood lactate levels rise sharply above resting levels (typically defined as around 4 millimoles per litre).

Lactate threshold is usually expressed as a percentage of VO2 max. Fitter, well-trained individuals typically have a higher lactate threshold (as high as 70-90% of VO2 max in elite endurance performers) than untrained individuals (around 50-60% of VO2 max), meaning they can work at a higher relative intensity before lactic acid begins to accumulate rapidly and cause fatigue.

Factors affecting the rate of lactate accumulation include: exercise intensity (the higher the intensity, the greater the reliance on fast twitch fibres and anaerobic glycolysis, so the faster lactate accumulates); muscle fibre type recruited (fast twitch fibres produce more lactate than slow twitch fibres); the rate of lactate removal relative to production; and the fitness of the performer, since trained individuals have more mitochondria, greater capillary density, and an improved capacity to use lactic acid as a fuel, all of which delay accumulation.

Elite sprint and power athletes tend to have a greater anaerobic capacity and tolerance of high lactate levels than endurance athletes, partly through training adaptations such as buffering (which helps neutralise the acidity of lactic acid), allowing them to sustain higher lactate levels and work at higher intensities for longer before fatigue sets in.

VO2 Max and Aerobic Power

VO2 max (maximal oxygen uptake, or aerobic power) is the maximum volume of oxygen the body can take up and use per minute, usually expressed relative to body mass (ml/kg/min).

Average VO2 max values for an untrained young adult are typically around 35-45 ml/kg/min for males and 27-35 ml/kg/min for females; elite endurance athletes can reach values in excess of 80-90 ml/kg/min.

Factors affecting VO2 max include:

• Physiological factors – e.g. maximum cardiac output, stroke volume, greater heart range, oxygen-carrying capacity of the blood (haemoglobin/red blood cell count), number and size of mitochondria, capillarisation around the muscles, myoglobin content, and lactate tolerance
• Training – aerobic endurance training (continuous, fartlek, interval) can improve VO2 max by roughly 10-20% over a training period
• Genetics – an individual's inherited physiology sets a limit on how much VO2 max can ultimately be improved through training
• Gender – males generally have a higher VO2 max than females of similar training status, partly due to differences in body composition and haemoglobin levels
• Age – VO2 max tends to decline as the body's systems become less efficient with age
• Body composition – a higher percentage of body fat is associated with a lower VO2 max
• Lifestyle – factors such as smoking, a sedentary lifestyle, and poor diet can all reduce VO2 max

Measuring Energy Expenditure

Several methods are used to measure how much energy an athlete is using, to help inform training and dietary requirements.

• Indirect calorimetry – measures energy expenditure by analysing the gases exchanged during exercise (oxygen consumed and carbon dioxide produced) while the performer wears a mask connected to a gas analyser, typically during a treadmill or cycle ergometer test. It gives an accurate estimate of energy expenditure and can also identify which fuel (fat or carbohydrate) is predominantly being used
• Lactate sampling – a small blood sample is taken (often from the ear lobe or fingertip) and analysed using a handheld device to measure blood lactate concentration. It is used to assess exercise intensity and training zones, monitor fitness, and track improvements in lactate threshold over time through repeated testing
• Respiratory exchange ratio (RER) – the ratio of the volume of carbon dioxide produced to the volume of oxygen consumed (VCO2/VO2), calculated from the gases measured during indirect calorimetry. RER indicates which fuel source the body is predominantly using:
  • An RER value close to 0.7 indicates the performer is using predominantly fat as a fuel source
  • An RER value close to 1.0 indicates the performer is using predominantly carbohydrate as a fuel source
  • An RER value greater than 1.0 indicates anaerobic respiration is occurring, with more carbon dioxide being produced than oxygen consumed

Impact of Specialist Training Methods on Energy Systems

Altitude training: training at high altitude (typically above around 2,500m), where the partial pressure of oxygen is lower, causing a reduction in the diffusion gradient of oxygen between the alveoli and the blood, meaning less oxygen is saturated onto haemoglobin. This stimulates the body to produce more red blood cells, increasing the oxygen-carrying capacity of the blood.

• Benefits – an increase in red blood cell count/haemoglobin level, improved capillarisation, and enhanced lactate tolerance, which can improve aerobic performance and VO2 max when the athlete returns to sea level
• Disadvantages – difficulty training at the same intensity at altitude due to reduced oxygen availability, a detraining effect on other fitness components, possible altitude sickness, benefits from altitude training being lost relatively quickly on return to sea level, and possible psychological problems such as homesickness from being away from home

High intensity interval training (HIIT): a form of training involving periods of high-intensity exercise interspersed with periods of lower-intensity exercise or rest. Key variables that can be manipulated include the duration of the work interval, the intensity/speed of the work interval, the duration of the recovery interval, and the number of work/recovery intervals performed.

HIIT trains both the aerobic and anaerobic glycolytic systems, since short bursts of maximal or near-maximal exercise are followed by lower-intensity recovery periods that allow partial replenishment of PC stores and removal of some lactic acid before the next work interval.

Plyometrics: a method of training designed to increase muscular power, involving explosive, high-intensity activities such as bounding, hopping, depth jumps, and use of a medicine ball, which rely on fast twitch fibres and predominantly anaerobic energy provision. Plyometric exercises use the stretch-shortening cycle and consist of three phases:

1. Eccentric (pre-loading/pre-stretching) phase – the muscle lengthens under tension as it prepares to produce force, e.g. on landing from a jump
2. Amortisation phase – the brief transition between the eccentric and concentric phases; this needs to be as short as possible so that the elastic energy stored during the eccentric phase is not lost as heat before it can be used
3. Concentric (muscle contraction) phase – the stored elastic energy is used to produce a forceful, explosive contraction, e.g. the upward jump or throw

Speed, agility and quickness (SAQ) training: a form of training aimed at improving a performer's ability to move quickly, change direction efficiently, and react to a stimulus, combining speed, coordination, balance and flexibility. It typically uses drills such as zig-zag runs and agility ladder work, and because it involves maximal-effort movements it is fuelled predominantly by the anaerobic energy systems.