By Dr. Stefano Casali
Time course of oxygen consumption
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The Steady State and the Oxygen Debt
The delay with which oxygen consumption reaches the steady state depends on the relative slowness with which oxidative reactions adapt to an increased energy demand. For as long as the oxygen consumption remains lower than the steady state value, the energy is supplied by an anaerobic system; in a certain sense it is as if the aerobic system contracted a debt because the energy is supplied by another exergonic system. In steady state conditions there are no differences between a trained and an untrained subject. The difference lies in the speed of adaptation of the VO2 to the steady state (VO2S), which is clearly higher in the trained subject.
Maximum oxygen consumption
The VO2S increases monotonically with the intensity of the work up to a maximum, reached which, any increase in intensity is no longer accompanied by any further increase in VO2S. The VO2S level corresponding to this maximum is defined as "maximum oxygen consumption (VO2max)".
Oxygen consumption trends during work and recovery:
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Metabolism in recovery
The concept of debt was proposed by Hill in 1923 and subsequently taken up by other authors including Margaria; all identified 2 components: one called alattacid and the other lactic. This model lasted for about 65 years. At present the term of oxygen debt has been replaced by phase of oxygen consumption in recovery (O2 recovery) or global oxygen consumption in excess of baseline (EPOC, by the Anglo-Saxon authors, acronym of Excess Postexercise Oxygen Consumption). EPOC reflects not only the payment quota for the lactic acid debt but also the condition of increased energy demand of the various organs and systems that were involved in the course of muscular work.
Causes of the EPOC
- Resynthesis of ATP and CP;
- Glycogen resynthesis starting from lactate (Cori cycle);
- Lactate oxidation;
- Blood oxygenation;
- Thermogenic effect linked to the increase in body temperature;
- Thermogenic effect due to the action of hormones, especially the catecholamines;
- Maintaining a heart rate and elevated lung ventilation.
Maximum oxygen consumption
The relationship between the duration of work at exhaustion and the intensity of work between 65-90% of VO2max, in trained subjects is described by:
t (min) = 940-1000 VO2S / VO2max. This relationship is not valid for exercises of intensity greater than 90% of VO2max (the time would in fact be negative for VO2S ›0.94 VO2max) and is independent of the absolute value of VO2max, provided that the subject is in good training conditions.
Conversion factors
| 1 N | 0.1019 kgp | ||
| 1 KJ | 101.9 kgpm | 0.239 kcal | |
| 1 kcal | 426.7 kgpm | 4, 186 KJ | |
| 1 kgp | 9.81 N | ||
| 1kgpm | 9.81 J | 2.34 kcal |
Definition of some physical quantities and the corresponding SI Units
- Strength: ability to give acceleration to a mass. The unit of force is the newton (N) which gives an acceleration of 1 m * s-2 to the mass of 1kg.
- Pressure: force per unit area.
- Work: the joule, unit of work, is the work done when the point of application of the force of 1 N is displaced by 1 m along the direction of the force.
- Power: work per unit of time. 1W is the power equal to 1joule per second.
Widely used until recently was the so-called metric system, in which the unit of force is the kilogram weight (kgp): the force capable of giving an acceleration equal to that of the earth's gravity to 1. kg (9.81 m * s-1). Consequently, the unit of work and power in the technical system are the kgpm (kilogramm) and the kgpm * s-1 (kilogram per second) equal respectively to 9.81 J and 9.81 W. Note that on Earth the acceleration of gravity is constant: each body undergoes the same acceleration g = 9.81 m * s-1, independent of its mass. Another unit of energy and work still widely used is the calorie (cal), equivalent to the amount of energy stored in 1 g of water, following the temperature increase of 1 ° C (from 14.5 to 15.5) ; 1000 cal = 1kcal.
