text: Physiology
  4-44. The amount of blood pumped (in liters) per beat is the stroke volume (SV) and the cardiac output (Q)
  is the liters of blood pumped per minute. Q = heart rate (100 bpm) x SV (0.1 liters per beat). Q can be as
  much as 25 to 30 liters of blood per minute in healthy, young Soldiers. As Soldiers become better trained,
  exercise heart rate goes down and stroke volume goes up, which means Soldiers can work at a higher rate or
  work longer at a constant rate with less effort. The Soldiers are fitter. Their relative workload is less and
  therefore, the work is easier for them to perform. An example would be, when a Soldier finds it much easier
  to dig a foxhole after a period of training.
  Physiology
  Balanced training has, at its core, an appreciation for how the human body works—its
  anatomy and physiology. Knowledge of these sciences supports training that is
  rational, scientific, and most effectively enhances physical performance while
  minimizing injury risk.