text: Chapter 4
  Physical Readiness Requirements and a Standardized, Baseline Physical Readiness Test. The United States
  Army Research Institute of Environmental Medicine (USARIEM) conducted extensive physical demands
  studies to determine the reliability of simulated physical soldiering tasks relevant to combat arms Soldiers.
  The studies demonstrated that muscular strength and power drove 60 percent of the variability in physical
  demands. In other words, endurance-centric approaches to training and testing did not support all of the
  baseline requirements of being a Soldier.
  4-4. As part of the analysis, USARIEM identified five domains of combat physical fitness: muscular
  strength, muscular endurance, aerobic endurance, explosive power, and anaerobic endurance. These five
  domains were independently confirmed in the physical fitness assessment studies conducted by the
  USACIMT. The Army Physical Fitness Test assessed only two of the five domains of combat physical
  fitness, aerobic endurance and muscular endurance.
  4-5. The second objective of the physical fitness assessment studies was to determine common field-
  expedient physical fitness test events that predicted a Soldier’s success on the high physical demand warrior
  tasks and battle drills (WTBD). After surveying MOS schools and several thousand combat veterans on the
  physical demands of Soldering, 1,000 Soldiers performed WTBD simulations. Their performance validated
  the selection of the six events that became the ACFT.
  ENERGY PATHWAY
  4-6. Energy to move the human skeleton derives from a series of chemical and biological reactions that
  involve the respiratory system (lungs), cardiovascular system (heart, blood vessels, and blood), the
  neuromuscular system (nerve and muscle system), and the neuroendocrine system (nerve and hormone
  system). Energy is produced through three pathways—phosphagen, glycolytic, and oxidative
  phosphorylation—that convert calories into energy. The body can use the resulting energy later in
  physiological processes, including but not limited to movement. The onset of exercise activates all three
  pathways, with each one predominating after different durations and levels of effort. Energy is produced in
  the form of a chemical called adenosine triphosphate. Adenosine triphosphate is a large molecule comprised
  of adenosine and three phosphate groups. When this molecule is broken down by chemical reactions in the
  muscle, the energy released by this reaction is used for movement.
  PHOSPHAGEN PATHWAY
  4-7. In the phosphagen pathway, adenosine triphosphate stored in the muscle and another molecule called
  phosphocreatine provide the energy for very short duration (5-10 seconds), high intensity or high power
  activities. These might include three to five second sprints, the Seated or Standing Power Throw, or maximum
  repetition lifts. This type of training rapidly depletes energy available in the muscle cells that require long
  rest periods to re-energize. If Soldiers do not rest enough or are forced to keep going at this level of intensity,
  the loss of adenosine triphosphate causes degraded performance. Depletion of the phosphagen pathway will
  force the use of other energy pathways and those pathways will begin to predominate. Rest intervals between
  bouts of high-intensity exercises should be 2 to 5 minutes to support molecule restoration.
  GLYCOLYTIC PATHWAY
  4-8. The glycolytic pathway is the predominant source of energy for high-intensity exercise lasting up to
  90 seconds. As its name suggests, this pathway breaks down glucose (sugar) in the blood and glycogen (sugar
  deposit) stored in the muscle cells. The chemical reactions in this system happen without the need for oxygen
  (anaerobic reactions). These reactions produce pyruvate or lactate depending on exercise intensity. Pyruvate
  is a component in the release of stored energy; lactate is a source of stored energy that can be converted back
  into glucose or glycogen during exercise. The lactate threshold is the point at which lactate production
  exceeds its clearance from, or utilization by, the muscle. Soldiers who are more fit have higher lactate
  thresholds. Across the training period, multiple sets of heavy lifts, Climbing Drills, the Strength Training
  Circuit, and sprint intervals result in greater overload of the glycolytic pathway and a higher lactate threshold.
  Precise measurement of this threshold occurs with an exercise stress test in a laboratory.