How We Fuel Movement

How do we fuel movement?

• The body uses macronutrients we eat such as carbohydrates, fats, and protein, to be used as fuel for movement. The intensity of exercise, availability of oxygen, and fuel dictate which system will generate energy.

Metabolism

• Metabolism is the total of all anabolic and catabolic reactions in the body. It maintains our body temperature, breaks down fat, and builds muscle.
• Adenosine Triphosphate (ATP) is our main energy source to break and build. Hydrolysis is how we split apart ATP to release energy and ADP as a byproduct using water. Enzymes speed up this release of energy all over the body by catalyzing.
  • For example, the enzyme Myosin ATPase splits ATP for cross-bridge cycling of actin and myosin during muscle contraction. 

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Phosphagen System

• • • The phosphagen system creates energy for intense activities that are short, usually 0-6 seconds. Creatine phosphate, CP, can be exhausted completely with sustained exertion for 30 seconds. CP requires 8 minutes for complete recovery.
      • Imagine sprinting through the airport to catch your departing flight.
    • The phosphagen system breaks down phosphocreatine (PCr) to rapidly create ATP. CP supplies a phosphate group to ADP→ATP.
      • CP cannot be stored in substantial amounts; the average human has 120 grams.
        • This is why the phosphagen system only supplies energy for the first few seconds of activity.
      • Our body has 100 grams of stored ATP. Only 50-60% can be used in exercise because it is conserved for vital cell processes in the brain. Full ATP recovery requires 3-5 minutes of rest.
        • CP is useful in muscle cells because it allows ATP to be regenerated to preserve body-wide homeostasis.

How can we adapt the phosphagen system?

• • • • Work-to-rest intervals are 1:12 to 1:20 to train the phosphagen system because to produce maximum power you must wait for CP repletion.
      • CP stores are replenished during resting periods by aerobic metabolism. Thus, aerobic endurance training may increase resting phosphagens and decrease the rate of depletion.
      • Total CP can be larger after hypertrophy or sprint training due to increased muscle mass and type II fiber adaptation which carries a higher CP concentration.
      • Oral creatine can increase the saturation in the body and has been shown to be a safe and effective ergogenic aid.
        • Research has demonstrated increased power, agility, improved recovery time, and decreased cognitive decline in adults. Creatine’s impact occurs after daily supplementation of 5-20 grams for 28 days

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Glycolysis System

• Breakdown of carbohydrates to generate energy. The glycolytic system is used during high-intensity exercise from 6 seconds to 2-3 minutes. Glycolysis uses glycogen in the muscle or glucose in the blood. 
• The capacity to generate ATP through glycolysis is more than with phosphagens because the body stores 600 grams of glycogen to use as fuel. 
  • Most glycogen is stored in the muscle, and some is stored in the liver.
  • The result of glycolysis is pyruvate which can be converted to lactate for use in anaerobic glycolysis. ATP synthesis with lactate is fast as NAD+ is regenerated, however, this produces H+ ions as a byproduct
    • This causes an acidic environment which makes anaerobic glycolysis unsustainable.
• Glycolysis regulation
  • The presence of ADP, Phosphorous, ammonia, and mild acidity stimulates glycolysis. 
    • Phosphofructokinase PFK is the rate-limiting step of glycolysis and can be inhibited or activated by the intracellular environment.
• Lactate is not the cause of fatigue
  • • Acidity is accumulated from the splitting of ATP anaerobically causing metabolic acidosis, not the formation of lactic acid.
    • This metabolic acidosis prevents glycolysis from continuing and also prevents muscle contraction due to the inhibition of calcium binding to troponin.
    • Lactate can also be used in gluconeogenesis, the formation of glucose from non-carbohydrates during extended exercise.
• Lactate 
  • • Type 2 fibers (fast twitch) produce lactate twice as fast as type 1 (slow twitch) because type 2 fibers have more enzymes.
    • If concentrations increase to 20-25 mmol/L lactate can cause fatigue with intense dynamic exercise. The metabolic influence of H+ ions can be buffered with Bicarbonate (HCO3-) by disrupting by accepting the proton (H2co3).
      • This is the theory behind ingesting sodium bicarbonate, also known as baking soda to improve an athlete’s resilience to acid buildup.
    • Cori Cycle
      • Blood lactate can be transported to the liver to be converted into glucose. Type 1 slow twitch fibers, used for endurance can also use lactate for fuel.
    • Peak blood lactate occurs approx 5 min after exercise ends
      • Time required to buffer and transport lactate to blood. Peak lactate is higher after intense exercise.
        • Blood lactate returns to normal within an hour post-exercise.
        • Active recovery resulted in faster lactate clearance than passive recovery, and thus light activity is encouraged post-workout for recovery.

• Lactate threshold (LT)
  • Exercise intensity where blood lactate begins rapidly increasing marks increased reliance on anaerobic energy.
  • LT also correlates with ventilatory threshold which is the point where intensity causes ventilation to rapidly increase to keep up with VO2 demand.
  • Marks anaerobic threshold which is the point where oxygen intake can no longer reliably predict energy production due to anaerobic dominance.
  • Begins at 50-60% max oxygen uptake in untrained individuals.
  • This is because the untrained have a low VO2 max and have not aerobically adapted to increased stroke volume, increased enzymes, and increased capillary density.
  • In athletes, LT begins at 70-80% VO2 because they are aerobically adapted to maintain a higher intensity.

• The onset of Blood Lactate Accumulation (OBLA)
  • The point where lactate begins to rapidly increase is usually when the largest motor units are recruited at high intensity.
  • Training at a high intensity close to LT or OBLA, athletes can adapt their nervous system to release fewer catecholamines, increase mitochondrial enzymes, and many more adaptations to improve lactate clearance. This training allows athletes to remain aerobic longer and at a higher intensity before OBLA.

How can we adapt glycolysis?

• • Training times from 15 seconds to 3 minutes use the glycolytic system predominately.
  • Work-to-rest intervals are typically 1:3 to 1:5
  • High-Intensity Interval Training HIIT
    • Maximize time spent at or above 90% VO2 max.
      • Yields oxidative muscle fiber adaptation, myocardial hypertrophy, Vo2max, proton buffering, glycogen content, anaerobic threshold, and others.
      • HIIT is effective because six reps of 30-second sprints may be comparable to longer endurance training with less time investment. 
  • The amount of glycogen available in the body is trainable with resistance and cardiovascular training. This is because your body stores more glycogen closer to the active muscle tissue.
  • Recovery of muscle glycogen is dependent on nutrition and carbohydrates.
    • NASCM suggests that (.7-.3 g of carbs per kilo of bodyweight) every 2 hours post-exercise for best muscle-glycogen repletion.
    • Carbohydrate-loading is a practice where marathon runners will take a large amount of carbs the days before a race to increase their glycogen reserve. 
  • Aerobic and anaerobic training can improve lactate clearance efficiency.

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Aerobic System

• We primarily produce energy aerobically, with oxygen, at a lower intensity. This is because we can produce the most volume of atp over time with aerobics but at a vastly slower rate than glycolysis or the phosphagen system. During moderate to low-intensity exercise, we begin relying on aerobic energy production after 2-3 minutes. 
  • As intensity increases, there is a shift from fat burning to carb burning until 100% at max intensity. At submaximal long-duration activities such as a marathon, there is a shift from carbs to fats.
  • At rest, we burn around 70% fat and 30% carbohydrate mostly aerobically.
  • Protein is usually spared from energy production for use as building blocks for our cells, but it can be burned in an emergency.

Glucose and glycogen oxidation

• • In the presence of oxygen, pyruvate is transported to mitochondria to make Acetyl-CoA for use in the Krebs cycle to produce energy.
  • The Krebs cycle harvests ATP by breaking apart glucose.
    • 6 NADH and 2 FADH2 are produced from glucose.
      • The molecules are used to transport hydrogen atoms to the ETC to produce ATP from ADP
    • Hydrogen atoms are passed down electron carriers to form a proton gradient which provides a potential energy slide for ADP to be phosphorylated. Oxygen is the final electron acceptor which forms water as a byproduct.
    • One glucose can create 38 atp if it was blood glucose or 39 atp if it was muscle glucose. 
    • Oxidative phosphorylation accounts for over 90% of total ATP produced.

Fat Oxidation

• • Fats can be made into energy with the use of the electron transport chain (ETC), and enzymes to help break down the giant molecules. This process is so powerful, one triglyceride can produce over 300 ATPs from beta-oxidation.
  • Lipase is an enzyme that splits triglyceride into free fatty acids (FFA), and glycerol which enter the circulation for fuel. Triglyceride can also be stored in muscle with a hormone-sensitive lipase which will release intramuscular FFA.
  • When energy is requested, FFA enters mitochondria and undergoes beta-oxidation. This is where FFA are broken down resulting in acetyl-CoA and H+ for use in the Krebs cycle.

Protein Oxidation

• Protein can be broken down into amino acids and converted into glucose in gluconeogenesis. This process is minimal during short-term exercise, however, in endurance exercise, it may contribute up to 18% of energy. 
• Branched-Chain amino acids or (BCAAS), Leucine, Isoleucine, and Valine, are thought to be the first line to be used as energy before other amino acids. 
• Nitrogen waste is produced and eliminated through the production of ammonia which is excreted. This production is toxic and may be associated with fatigue.

Oxygen Debt

• • Aerobic oxidation may subsidize intensity in the short term.
  • During intense exercise at 80% of max power output, the VO2, or oxygen consumption is temporarily much larger than the individual VO2 max. The anaerobic system will produce energy well above what can be sustained, creating an oxygen deficit.
    • This creates the need for excess post-exercise oxygen consumption or EPOC. As intensity increases, the demand for EPOC does too.

How can we adapt the aerobic system?

• • To stimulate the aerobic system, long workouts with durations of 3 minutes to days will cause adaptations.
  • Rest-to-work ratios are usually 1:1 to 1:3.
    • This could look like running for 4 minutes and resting for 4 minutes.

Combination training

• Concurrent aerobic and anaerobic may introduce an interference effect for specialists. This is because if you are a powerlifter, running a marathon is stimulating slow twitch muscle fibers and may reduce performance. However, cross-training may enhance recovery because aerobics increases recovery from EPOC. Power sports burn anaerobically and recover aerobically
• Upper limits of strength may be limited by endurance training
  • This may be due to less glycogen, lower rapid type 2 activation, or fiber-type transition to slow twitch.
• Runners will benefit from lifting for the increased power, strength, and bone mineral density. It has been shown that runners will increase their specific sport performance with strength training.

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References

• Buford, Thomas W et al. “International Society of Sports Nutrition position stand: creatine supplementation and exercise.” Journal of the International Society of Sports Nutrition vol. 4 6. 30 Aug. 2007, doi:10.1186/1550-2783-4-6
• Haff, G., & Triplett, N. T. (2021). Essentials of strength training and conditioning. Human Kinetics.