Physiology of High Intensity Training

Energy Supply

In order to supply the body with sufficient amounts of energy for performance at maximal efforts, large amounts of ATP are required to be available within a relatively short space of time. In order to provide this, the body will have to use the anaerobic energy systems to provide energy. The 2 systems include:

1. The Creatine Phosphate Energy System

This system uses stored Creatine Phosphate stores to regenerate ATP from ADP stores:

Creatine Phosphate (CP) + ADP => ATP + Creatine

This system is able to produce energy from ATP rapidly to allow for short, powerful bursts of energy, however, the CP reserves usually deplete within 10 seconds during maximal efforts. Regeneration of CP is also rapid, only taking 30 seconds for 70% regeneration, and 3 – 5 minutes for 100% regeneration.

CP and ATP stores can be increase by up to 50% following 7 months of endurance training (cardiopulmonary and high intensity).

The CP system is therefore most effective for short bursts involving maximal efforts, no longer than 10 seconds.

The most effective form of training to bring about adaptation of the CP system will include maximal efforts (90% +) between 10 – 30 seconds with 30 seconds to 3 minute rest intervals.

2. The Glycolytic / Lactate Energy System

This system is used when activity exceeds 10 seconds in duration and the CP system is exhausted. The aerobic energy system, which utilises oxygen as a fuel source, would usually take over at this stage, however, with the energy demand being very high during high intensity, the aerobic system is unable to produce energy quickly enough. At this point, large volumes of lactic acid begin to accumulate, increasing the acidity levels within working muscles. This increase in acidity will eventually result in muscle fatigue due to a disruption of the energy reactions.

In order to adapt to these lactic acid increases to body undergoes to mechanisms:

  • Lactic Acid Shuttling
    • The body takes the lactic acid accumulating in working muscles and shuttles it out into the liver and in active muscles
    • Does require an effective circulatory system with good cardiovascular adaptations to physical activity
  • Acidic Buffering
    • The body adapts by improving its capacity to withstand a more acidic environment
    • The muscle is able to continue functioning for a longer period, delaying fatigue

The lactate energy system is most effective during high intensity activity (80 – 95%) up until 7 minutes.

The most effective form of training to bring about adaptation of the lactate system will include multiple high intensity efforts (80 – 95%) for sustained periods (1 – 3 minutes) with active recovery periods of between 30 seconds to 3 minutes

Muscle Fiber Types

Red Muscle Fibers

  • Primarily supplied energy from the aerobic energy system
  • High blood supply (therefore red description)
  • Do not fatigue easily, but produce energy slowly
  • Used during endurance activities

White Muscle Fibers

  • Primarily supplied energy from the anaerobic energy systems
  • Low blood supply
  • Fatigue quickly
  • Produce high force efforts upon contraction
  • Used during high intensity and explosive efforts

Fiber Type Ratio

The ratio of the red and white fiber distributions in each muscle will determine the ability of the muscle to sustain low intensity efforts or perform explosive efforts more effectively. It is primarily genetically determined, but can be conditioned to an extent through focused training efforts.