VO2 Max and METS Output

VO2 Max

Energy (Adenosine Triphosphate) required for muscle contraction or other forms of biologic work is produced by anaerobic and aerobic energy pathways.  The anaerobic process only produces approximately 5% of the potential ATP that can be produced when the glucose molecule is completely degraded to CO2 and H2O.  The aerobic system, which uses glycogen, fats, and protein as energy substrates, provides large amounts of ATP for muscular energy.  Carbohydrate and fat are the primary fuels for energy production.  Protein contributes less than 5%.  However, oxygen delivery to the cell is critical, and the capacity to deliver it to the tissue usually determines the level of activity an individual can perform

The critical measure of metabolism and energy expenditure is to evaluate oxygen consumption (VO2) that is derived by a rearrangement of the Fine equation:

VO2=HR X SV X (CaO2-CvO2)

The term MET (metabolic equivalent) denotes the energy requirement for basal homeostasis. 1 MET equals approximately 3.5ml of O2 per Kg of body weight which is the amount of energy required to sleep.  Furthermore, multiples of this value are often used to quantify relative levels of expenditure.  VO2 Max is the maximum energy that a person can produce and is expressed in ml/kg/min or METs.

So, first you have to get oxygen into the lungs, diffuse it from the alveoli to the blood where it binds with hemoglobin the red blood cell.  Then it gets delivered to the muscle by the heart where it enters the muscle cell and is converted to ATP by the mitochondria.  Utilization of that oxygen as energy yields the byproducts of CO2 and H2O.

You have to have glycogen stored in the muscle and you have to be able to deliver oxygen to the muscle to convert it to energy.  You can do a test to see how effective that process works and it’s called an exercise tolerance test/treadmill test/graded exercise test.  The best way to accomplish this is to have an individual exercise while measuring the volume of air inspired and comparing that to the exhaled air that is collected into a large bag and then analyze it for its oxygen and carbon dioxide content.  The oxygen consumption is calculated by multiplying the volume of air breathed by the percent of O2 extracted.

Several investigators have proposed that VO2 Max can be predicted utilizing published formulas or normograms, based on treadmill speed and grade or cycle ergometer workload or power output.  Theoretically there is a linear response of pulse/heart rate to increasing workloads, which allow us to calculate VO2 max without taking a person to exhaustion, and without directly measuring oxygen consumption.

A person can manipulate his/her VO2 max with training.  

What it means:

For years you have done either a treadmill or bicycle exercise test as part of your annual physical exam with the Phoenix Fire Department.  It is done to look for coronary artery disease and to assess your fitness level.  

NFPA 1582, in its most recent incarnation, established a minimum MET level of 12 required for firefighters.  Workload demands of firefighters have been shown to exceed these levels.

In the current tiered system you need to complete at least 2:30min/sec of the running protocol or 10:50min/sec of the walking protocol at a sub maximal heart rate to stay out of tier 4.  To stay out of tier 3 you need to complete at least 3:45min/sec of running or 12:30min/sec of walking at a sub maximal heart rate.

If your heart rate goes up quickly with exercise, then stabilizes, you can elect to do a more maximal test.  To stay out of tier 3 you need to do at least 7:30min/sec of running or 21:30min/sec of walking.  

To stay prepared for firefighting duties and to do well at your annual physical requires an investment in physical fitness, especially as you age.  Basal metabolism declines 1% to 2% per year after the age of 20, so it becomes increasingly important to dedicate to a fitness routine as your career progresses.  Your job is to stay mentally and physically ready to fight fires, and I’ve been told that fire does not care how old you are.

If you establish a consistent exercise program and eat a healthy diet to stay close to your ideal weight, you will perform better on the fire ground and do well at your annual physical.