ENERGY PATHWAYS:  use during exercise
       Because the body can not easily store ATP (and what is stored gets used up within a few
       seconds), it is necessary to continually create ATP during exercise, either anaerobically or
       aerobically. These pathways can be subdivided in 3 energy systems working in concert.
  
1.  ATP-CP -   of the anaerobic energy pathway:  (the phosphate system)
  
     ATP - Adenosine Triphosphate: a complex chemical compound stored in all cells, particularly muscles.
     Cells perform work from the energy released by the breakdown of this compound. Thebreakdown of ATP
     produces energy and ADP.
    
     CP - Creatine Phosphate: a chemical compound also stored in muscle, which when broken down aids in the
      manufacture of ATP. The combination of ADP and CP produces ATP. ATP) stores in the muscle last only about
      2-3 seconds and the re-synthesis of ATP from creatine phosphate* will continue until CP stores are depleted,
      in approximately 4 to 6 seconds. This gives us around 5 to 8 seconds of total ATP production for high intensity
      work (exercise) at near peak  velocity. 
  
      The result of muscle contraction produces ADP which when coupled with CP regenerates ATP. CP 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 requires the complete oxidation of
      carbohydrates or free fatty acids in the mitochondria. The carbohydrate store will last approximately 90
      minutes and the free fatty store will last several days.
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2.  Aneaerobic Metabolism - Glycolysis & Lactic Acid Production
     Glycolyis makes ATP via SLP and LA - Lactic acid: Once the CP stores are depleted the body resorts to stored
     glucose to make ATP. The breakdown of glucose or glycogen in anaerobic conditions results in the production
     of lactate and hydrogen ions. Anaerobic glycolysis makes energy for short (less than 2 minutes) high intensity
     activity, or until lactic acid build-up reaches a threshold with muscle pain and fatigue occur. the lactate threshold
     or anaerobic threshold is the point where lactic acid accumulates in blood faster than it can be removed. It
     occurs at about 95% max heart rate or a [4mM]. It signifies a shift from predominantly aerobic metabolism to
     anaerobic metabolism. Athletic Training is often to increase an athlete's lactate tolerance. Although excessive
     lactate production is part of the extreme fatigue process, it is the protons produced at the same time that restrict
     further performance, thus the accumulation of hydrogen ions is the limiting factor causing fatigue "sprint events".
 

3. Aerobic Metabolism:  Endurance exercise energy production
    uses O2 via Krebs and the ETC to manufacture ATP from food mainly sugar and fat. This system produces ATP
    copiously and is the prime energy source during endurance activities, but is slower than anaerobic energy
    because it relies on the circulatory system to transport oxygen to muscle cells to generate ATP.  The aerobic
    energy system is used primarily in endurance exercise (longer than 3 minutes).

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Exercise and these energy pathways are time duration* restricted.

Exercise Duration Energy Classification Energy Supplied By
1 to 4 seconds Anaerobic    ATP (in muscles)
4 to 10 seconds Anaerobic    ATP + CP
10 to 45 seconds Anaerobic    ATP + CP + 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 + fatty acids
Sprint events Endurance events

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Contribution of each system to ATP utilization during exercise:

    All three 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 thresholds times.

energy utilization

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Energy System recruitment: 
    Although all energy systems turn on at the same time 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-CP and LA % LA-O2 O2
Golf swing 95 5  
Wind Sprints 90 10  
Field events 90 10  
Fencing 90 10  
Gymnastics 80 15 5
Volleyball 80 5 15
Tennis 70 20 10
Basketball 60 20 20
Hockey 50 20 30
Soccer 50 20 30
Skiing 33 33 33
Rowing 20 30 50
Swimming 1.5km 10 20 70
Distance running 10 20 70
Sprint Events Endurance Events

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