PT on the Net Research

Caloric Expenditure on Road Bike


Question:

How would I figure out caloric expenditure for someone who did outdoor cycling on a road bike if they only gave me miles per hour average and distance in miles? The leg ergometry metabolic equations usually need a power output in watts, and I do not believe I can get that from just this information.

Answer:

Caloric or energy expenditure is commonly measured in exercise physiology laboratories using the method of indirect calorimetry. Indirect calorimetry is a measurement of oxygen uptake, which can determine energy expenditure. There is a direct relationship between calories burned and oxygen consumed; therefore, it is an easy measurement to complete in a lab environment.

However, conducting indirect calorimetry tests using a metabolic cart with clients is not always the most convenient method for determining energy expenditure. Metabolic equations published in the American College of Sports Medicine Guidelines for Exercise Testing and Prescription require a power measurement to determine energy expenditure; therefore, these equations cannot be used based on the information provided by the client. However, there is another method for determining energy expenditure when power measurements are not known.

By obtaining the metabolic equivalent (MET) value for a given intensity of exercise, it is possible to further calculate energy expenditure. The Compendium of Physical Activities, an update of activity codes and MET intensities provides a detailed list of various activities and their corresponding MET value. Specifically, there are five bicycling examples presented that would apply to this client:

  1. 10-11.9 mph, leisure, slow effort, 6 METs
  2. 12-13.9 mph, leisure, moderate effort, 8 METs
  3. 14-15.9 mph, racing or leisure, fast, vigorous effort, 10 METs
  4. 16-19 mph, racing/not drafting or <19 mph drafting, very fast, racing, 12 METs
  5. >20 mph, racing, not drafting, 16 METs

Evaluation steps include the following:

  1. Determine MET value for client
  2. Calculate VO2 in mL/kg/min
      • MET x 3.5 ml/kg/min = VO2 ml/kg/min
  3. Express VO2 in L/min
      • VO2 ml/kg/min x kg (body weight)/1000 = VO2 L/min
  4. Calculate energy expenditure
      • VO2 L/min x 5 = calories expended per minute
      • Multiply this number by 60 to determine calories expended per hour.

Example

A client weighs 80 kilograms and bicycles at an average speed of 19 miles per hour.

  1. Determine MET value for client
      • 12 METs
  2. Calculate VO2 in mL/kg/min
      • MET x 3.5 ml/kg/min = VO2 ml/kg/min
      • 12 x 3.5 ml/kg/min = 42 ml/kg/min
  3. Express VO2 in L/min
      • VO2 ml/kg/min x kg (body weight)/1000 = VO2 L/min
      • 42 x 80/1000 = 3.4 L/min
  4. Calculate energy expenditure
      • VO2 L/min x 5 = calories expended per minute
      • 3.4 L/min x 5 = 17 calories expended per minute
      • Multiply this number by 60 to determine calories expended per hour: 17 x 60 minutes = 1020 calories expended per hour.

Remember, equations used to predict energy expenditure are most appropriate for steady state submaximal aerobic exercise. Variables that can change the mechanical efficiency of the person exercising (wind, terrain) can result in a loss of accuracy. Obtaining power measurement data would provide a more accurate determination of energy expenditure as the ACSM equations could then be utilized, but the aforementioned method of calculating energy expenditure from speed (miles per hour) can be used as a guide.

References:

  1. Ainsworth, B et al. (2000). Compendium of Physical Activities: an update of activity codes and MET intensities. Medicine and Science in Sports and Exercise, Vol. 32, No. 9, Suppl., pp S498-S516.
  2. Lippincott, Williams and Wilkins. ACSM’s Guidelines for Exercise Testing and Prescription, Sixth Edition. 2000.