ENERGY PATHWAYS

Energy production is both time and intensity related. Running at a very high intensity, as in sprinting, means that an athlete can operate effectively for only a very short period of time. Running at a low intensity, as in gentle jogging, means that an athlete can sustain activity for a long period of time. Training introduces another variable, and the sprinter who uses sound training principles is able to run at a high intensity for longer periods of time. Similarly, the endurance athlete who uses sound training methods can sustain higher intensities during a set period of time. There is a relationships between the exercise intensity and the energy source.

D. Matthews and E. Fox, in their revolutionary book, The physiological Basis of Physical Education and Athletes, divided the running requirements of various sports into the following “energy pathways”. ATP-PC and LA, LA-O2, and O2

•  ATP - Adenosine Triphosphate : a complex chemical compound formed with the energy released from food and stored in all cell, particularly muscles. Only from the energy released by the breakdown of this compound can the cells perform work. The breakdown of ATP produces energy and ADP.
•  PC – Phosphate-Creatine : a chemical compound stored in muscle, which when broken down aids in the manufacture of ATP. The combination of ADP and PC produces ATP.
•  LA – lactic Acid : a fatiguing metabolite of the lactic acid system resulting from the incomplete breakdown of glucose. However Noakes in South Africa has discovered that although excessive lactate production is part of the extreme fatigue process, it is the protons produced at the same time that restrict further performance.
•  O2 means aerobic running in which ATP is manufactured from food, mainly sugar and fat. This system produces ATP copiously and the prime energy source during endurance activities.

These energy pathways are time durations restricted. In other words, once a certain time elapses thet specific pathway is no longer used. There is some controversy about these limitations but the general consensus is :

Duration	Classification	Energy Supplied By
1 to 4 seconds	Anaerobic	ATP (in muscles)
4 to 20 seconds	Anaerobic	ATP + PC
20 to 45 seconds	Anaerobic	ATP + PC + Muscle Glycogen
45 to 120 seconds	Anaerobic, Lactic	Muscle Glycogen
120 to 240 seconds	Aerobic + Anaerobic	Muscle Glycogen + Lactic Acid
240 to 600 seconds	Aerobic	Muscle Glycogen + Fitty Acids

The result of muscle contraction produces ADP which when coupled with PC regenerates ATP. PC is stored in the muscles. Actively contracting muscles obtain ATP from glucose stored in the blood stream and the breakdown of glycogen stored in the muscles. Exercise for longer periods of time requires the complete oxidation of carbohydrates or free fatty acids in the metochondria. The carbohydrate store will last approx. 90 minutes and the free fatty store will last several days.

All time energy systems contribute at the start of exercise but the contribution depends upon the individual, the effort applied or on the rate at which energy is used. The following graph depicts how the energy systems contribute to the manufacture of ATP over time when exercising at 100% effort. The thresholds (T) indicate the point at which the energy system is exhausted-training will improve the rhresholds times.

Adenosine Triphosphate (ATP) stores in the muscle last for approximately 2 secounds and the resynthesis of ATP from Creatine/Phosphate (CP) will continue until CP stores are depleted, approximately 4 to 5 seconds. This gives us around 5 to 7 seconds of ATP production.

To develop this energy system, sessions of 4 to 7 seconds of high intensity work at near peak velocity are required e.g.

3 X 10 X 30 meters with recovery of 30 seconds/repetition and 5 minutes/set

15 X 60 meters with 60 seconds recovery

20 X 20 meters shuttle runs with 45 seconds recovery

The Anaerobic Lactate (Glycolytic) System

Once the CP stores are depleted the body resorts glucose for ATP. The breakdown of glucose or glycogen in anaerobic conditions result in the production of lactate and hydrogen ions. The accumulation of hydrogen ions is the limiting factor causing fatigue in runs of 300 meters to 800 meters.

Sessions to develop this energy system:

5 to 8 X 300 meters fast – 45 seconds recovery – until pace significantly slows

150 meter intervals at 400 meter pace – 20 second recovery – until pace significantly slows.

8 X 300 meters – 3 minutes recovery (lactate recovery training)

There are three different working units within this energy system: Speed Endurance, Special endurance 1 and Special Endurance 2. Each of these units can be developed at follows:

	Speed Endurance	Special Endurance 1	Special Endurance 2
Intensity	95 to 100%	90 to 100%	90 to 100%
Distance	80 to 150 meters	150 to 300 meters	300 to 600 meters
No of Repetitions/Set	2 to 5	1 to 5	1 to 4
No of Sets	2 to 3	1	1
Total distance/session	300 to 1200 meters	300 to 1200 meters	300 to 1200 meters
Example	3 X (60,80,100)	2 X 150m + 2 X 200m	3 X 500 meters

The Aerobic Energy System

The aerobic energy system utilizes protein, fats and carbohydrate (glycogen) for resynthesising ATP. The energy system can be developed with various intensity (tempo) runs. The types of tempo runs are:

Continuous Tempo – long slow runs at 50 to 70% of maximum heart rate. This places demands on muscle and liver glycogen. The normal response by the system is to enhance muscle and liver glycogen storage capacities and glycolytic activity associated with these processes.

• Extensive Tempo – continuous runs at 60 to 80% of maximum heart rate. This places demands on the system to cope with lactate production. Running at this level assists the removal and turnover of lactate and body’s ability to tolerate greater levels of lactate.
• Intensive Tempo – Continuous runs at 80 to 90% of maximum heart rate. Lactate levels become high as these run border on speed endurance and special endurance. Intensive tempo training lays the base for the development of anaerobic energy system.

Session to develop this energy system:

4 to 6 X 2 to 5 minutes runs – 2 to 5 minutes recovery.

20 X 200m – 30 seconds recovery

10 X400m – 60 to 90 seconds recovery

5 to 10 km

Energy System Recruitment

Although all energy systems basically turn on at the same the recruitment of an alternative system occurs when the current energy system is almost depleted. The following table provides an approximation of the percentage contribution of the energy pathways in certain sports. (Fox et al 1993).

Sport	ATP-PC and LA	LA O2	O2
Basketball	60	20	20
Fencing	90	10
Field Events	90	10
Golf Swing	95	5
Gymnastics	80	15	5
Hockey	50	20	30
Distance Running	10	20	70
Rowing	20	30	50
Skiing	33	33	33
Soccer	50	20	30
Sprints	90	10
Swimming	10	20	70
1.5km	70	20	10
Tennis	70	20	10
Volleyball	80	5	15

Table adapted from Fox E. L. et al, The Physiological Basis for Exercise and Sport, 1993

ATP-PC System (Phosphagen System)/Anaerobic Alactic

The resynthesis of ATP through ATP-PC system do not require the percence of oxygen (anaerobic). When ATP is broken down during mascular contraction, it is continuously reformed from ADP (adenosine diphosphate) and Pi (inorganic phosphate) by the energy liberated during the breakdown of the stored PC (phosphocreatine)

ATP Lactic System/ Anaerobic Glycolysis

The ATP lactic system is the second anaerobic pathway for the regeneration of ATP. Glucose and glucose from glycogen is chemiccaly broken down into lactic acid by a series of reactions. During the breakdown, energy is releases and through coupled reactions, is used to resynthesise ATP. This process is called Anaerobic Glycolyisis.

ATP Aerobic System

The ATP Aerobic System comprise of three main series namely the Aerobic Glycolysis, Kreb Cycles and the Electron Transport System. Together these series release enough energy to resynthesize ATP. This system is by far, yields the largest production of ATP.

Sports and Energy Requirements

Sports can be categorized along a continuum going from mostly aerobic (long distance), through a maxture of aerobic and anaerobic (middle distance), and on to mostly anaerobic (short sprint) activities.