EXERCISE PHYSIOLOGY LABORATORY #5
Ergometry and Measurement of Efficiency
Purpose: The purpose of this lab is to introduce you to the definitions, equipment, and techniques of ergometry as it applies to exercise and to introduce you to the methods by which oxygen consumption is determined during exercise. In addition, there will be an introduction to the concept of mechanical efficiency and how it applies to the human body during exercise.
Introduction: The Law of Conservation of Energy states that energy can be neither created nor destroyed, but can only be converted from one form to another. In exercise physiology, we are concerned with the conversion of the chemical energy of stored fuels into useful work. The techniques for the measurement of this work are referred to as ergometry. An ergometer is any device that permits the measurement of external work production. In converting chemical energy from food stuffs into mechanical energy, the muscles of your body use oxygen. The oxygen consumed by the body during work is a measurement of the calories expended to do work. Measurement of oxygen consumption during exercise is an indirect estimate of energy expended to perform the work. In estimating the body's energy expenditure, approximately 5 kilocalories (kcal) are expended for every one liter of oxygen that is consumed.
Work (W) is defined as the product of the force (F) applied and the distance (D) over which it is applied. W = F x D. For example, a 150 LB man stepping on a 3 foot high bench will have done 450 ft-lbs of work. The international standard unit for force is the Newton while the distance is measured in meters. Work is then measured in Newton-meters which is also termed a joule. Work accomplished is independent of the time required to perform a given task. It requires the same amount of work to climb a flight of stairs slowly as it does to run up the same stairway.
Power (P) is the amount of work done per unit time (t). P = W/t or P = (F x D)/t. Power output is referred to throughout this lab manual as "work rate". Using our first example, if a 150 LB man steps up on a 3 ft high bench, and does so every second, he is producing power at a rate of 450 ft-lbs/sec. If he steps up on the bench every two seconds, he is producing power at a rate of (150 lbs x 3 ft)/2 secs = 225 ft-lbs/sec. The same amount of work can be performed at different rates (or different power outputs). The international standard unit for power is the joule/ sec which is also termed a watt.
The muscles of your body convert potential or chemical energy derived from foodstuffs into mechanical energy, or useful work. However, just like any other device or machine for conversion of energy from one form into another, this process is not without waste. Efficiency is defined as the amount of work performed divided by the amount of energy utilized to do so and is expressed as a percentage. Efficiency = work output/ energy expended. The total amount of energy expended to perform a given amount work is determined by measuring the oxygen consumption (5 kcal expended Å 1 liter of O2 consumed). The more efficient a person is, the more work they can perform for a given energy expenditure, and vise-a-versa, the more efficient a person is, the less energy it takes to perform a given amount of work.
The most common ergometer is a Monark cycle ergometer. It has the advantage of simplicity, portability and cost over other types of ergometers. Cycling is a task that most people are familiar with and can comfortably perform. In addition, the fact that the person is relatively stationary during cycling exercise makes collection of physiological measurements easier. Tension on the Monark ergometer is created by a belt that passes around the flywheel and is the frictional force that must be overcome to move the wheel. This tension, which is adjustable using a knob, is measured in kilograms. One revolution of the pedals moves a point on the flywheel rim 6 meters in distance. If the resistance is set to one kilogram, every revolution of the pedals results in 6 kilogram-meters (kg-m) of work. If a person were pedaling at 60 revolutions per minutes (rpm) at a resistance of one kg, they would be producing power at a rate of 360 kg-m/min (1 kg x 6 meters/rev x 60 rev/min).
Many ergometers express work rate in kg-m/min because of the influence of the Monark ergometer. Another common, and scientifically more correct means of expressing power is in units of watts (also known as joule/sec or N-m/sec). The conversion factor from kg-m/min to watts is: 1 watt = 6.1 kg-m/min or 100 kg-m/min = 16.35 watts.
People have very similar efficiencies when cycling. Therefore, the oxygen cost of cycling at any given work rate is the same from person to person. The approximate oxygen consumption on a cycle ergometer at different work rates (powers) is as follows:
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The energy expended during walking and running is a function of body weight. When body weight is accounted for by expressing the oxygen cost of exercise in ml/kg/min, most individuals consume approximately the same amount (±10%) of oxygen at a given velocity. The oxygen cost of walking/running at the following velocities is as follows:
Velocity (mph) Velocity (kph) O2 Uptake (ml/kg/min) 3.0 4.8 14 4.0 6.4 18 5.0 8.0 25 6.0 9.6 32 7.0 11.2 38 8.0 12.8 43 9.0 14.4 46 10.0 16.0 53 11.0 17.6 58 12.0 19.2 65
Exercise intensity can be increased by increasing the grade. A rough rule of thumb is that a 1% increase in grade increases the oxygen cost by 4%.
Procedures: A subject will exercise on a Monark ergometer at a moderate (300 kg-m/min) and a heavy work rate (900-1200 kg-m/min) for 5 min each. The subject will cycle at the rpm of their choice and the resistance on the flywheel will be set to achieve the desired work rate. The subject will breathe through a two-way breathing valve so that their expired air can be collected and analyzed for CO2 and O2 content.
Lab Report Requirements
1) Calculate the subject's efficiency for each work rate. In order to do so, the work produced and the energy expended must both be expressed in the same units (in our case, kg-m/min). The average work rate for one minute measured from the ergometer will represent the work output (expressed as kg-m/min). The average oxygen uptake (l/min) is converted to the kcal expended per minute according to the relation: 5 kcal expended Å 1 liter of O2 consumed. Kcals expended/min are then converted to the energy expended per minute according to the relation: 1 kcal expended/min = 426.4 kg-m/min (3 points).
2) Present your data in a table in the following format (1 point):
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3) Answer the questions (6 points).
Questions
1) During the moderate exercise level, was our subject more efficient, less efficient or the same efficiency as the average person exercising at the same work rate? To make this determination, you must know what the oxygen consumption is for the average person at the same work rate. Consult the table on the previous page for that information.
2) If your purpose is to lose weight through exercise and your time to exercise were limited to only 30 minutes a day, would you want to be more or less efficient in regards to energy expenditure? Please explain why.
3) If your purpose is be a competitive runner and race as fast as possible, would you want to be more or less efficient? Please explain why.
Text References
"Definitions of Common SI Units" Appendix A p. 703.