Mechanical Efficiency Lab



Mechanical Efficiency Lab

Name __________________________________ Date _____________

Introduction

The law of conservation of energy tells us that energy cannot be either created or destroyed but can only change forms. We see this during exercise when, as a result of the required energy expenditure, the muscle temperature increases. The increase in muscle temperature is due in large part to the conversion of chemical energy in the phosphate bonds of ATP to heat energy. Therefore we know that all of the energy in the chemical bonds is not being used to perform external work.

Mechanical efficiency is a description of the percentage of energy expended that goes to performing mechanical work. When we perform work the energy expended is usually much higher than the energy required because of the aforementioned loss of heat energy and also friction and other mechanically inefficient movements.

We describe mechanical efficiency in three different ways; gross efficiency, net efficiency, and delta efficiency. Gross efficiency meets the same definition as mechanical efficiency, the proportion of energy expended that contributes to the performance of mechanical work. Net efficiency considers only that portion of the energy expenditure that contributed to external work. Therefore with net efficiency we subtract out the resting energy expenditure since, in theory, that resting energy expenditure would occur whether the individual was exercising or not. Delta efficiency considers the efficiency as we change workloads. So delta efficiency tells us about the efficiency as we transition form one workload to the next.

Purpose

The purpose of today’s lab is to determine the efficiency measures during two cycle ergometry workloads.

Equipment/Personnel

Metabolic cart, Monark leg ergometer, one subject, three assistants

Definitions

External work – Measurable work performed

Law of Conservation of Energy – Newton’s First Law of Thermodynamics which states that energy is neither created nor destroyed but changes forms

Work output – The power output on the cycle ergometer times the period of time the subject is cycling. This value is converted to a common unit of energy expenditure such as kilocalories.

Energy expended – The kilocalorie expenditure as determined by the oxygen consumption during the exercise session. For this lab we will use the estimation of 5 kcals per liter of oxygen consumed.

Procedures

The subject will first lie supine on a table for the purpose of measuring resting energy expenditure. To get a more accurate resting energy expenditure we would take steps such as having the subject fast and measure resting energy expenditure immediately after 8 to 10 hours of sleep. In this case we will have the subject relax on a table and measure 5 minutes of expired gases in an effort to get an idea of the resting energy expenditure.

After resting measures the subject will mount the cycle ergometer and complete two 10-minute workloads. The first workload will be 300 kp-m/min and the second workload will be 600 kp-m/min. The pedal rate will be 50 rpm. With each revolution of the pedals the flywheel moves 6 meters. Therefore the resistance on the flywheel would be 1 kp for the first workload and 2 kp for the second workload.

50 revs/min x 6 meters/rev x 1 kp = 300 kp-m/min

50 revs/min x 6 meters/rev x 2 kp = 600 kp-m/min

The oxygen consumption for each minute will be recorded. For each workload the ten minutes of oxygen consumption will be averaged to reflect the total energy expenditure for the workload. The average oxygen consumption for the workload will be multiplied by 5 kcals/LO2. This will provide actual energy expenditure for each workload.

The work output will be measured as the kp-m/min load times the duration of the bout in minutes. Therefore at 300 kp-m/min a 10-minute bout would yield a work output of:

300 kp-m/min x 10 minutes = 3000 kp-m

This value will be converted to it’s equivalent in kilocalories by using the value 1 kilocalorie per

426.8 kp-m.

3000 kp-m ÷ 426.8 kp-m/kcal = 7.03 kcals

Using the formulae in your textbook on page 295 you can then calculate the gross efficiency, net efficiency, and delta efficiency. For net efficiency you will use the last minute of resting data and assume that this value will be the minute-to-minute resting expenditure if the subject were not exercising. Therefore, you will multiply the kilocalories expended in one minute of rest by 10 minutes to get the assumed resting energy expenditure. For example, a person has a resting oxygen consumption during the last minute of rest of 0.55 L/min. We convert this to kcals by multiplying the oxygen consumption by 5 kcals/L.

0.55 L/min x 5 kcals/L = 2.75 kcals/min

Since the stages last 10 minutes each we multiply this value by 10 minutes

2.75 kcals/min x 10 minutes = 27.5 kcals

Therefore, 27.5 kcals represents the resting energy expenditure for a 10-minute period. This value would be subtracted from the energy expended during each 10-minute exercise bout.

To calculate delta efficiency we will use the differences between the two ergometer workloads. Therefore the same data used when calculating gross efficiency will be used in this calculation.

Subject Name _________________________________ Sex ____ Age ________

Ht. ____________in ______________cm Wt. ______________lbs. __________________kg

Resting VO2 in 5th minute ________L/min Calculated resting energy expenditure __________kcals

Workload 300 kp-m/min Minute VO2 (LO2/min)

1. __________

2. __________

3. __________

4. __________

5. __________

6. __________

7. __________

8. __________

9. __________

10. __________

Average __________

Workload 600 kp-m/min Minute VO2 (LO2/min)

1. __________

2. __________

3. __________

4. __________

5. __________

6. __________

7. __________

8. __________

9. __________

10. __________

Average __________

Workload

Efficiency measure 300 kp-m/min 600 kp-m/min

Gross ___________ ___________

Net ___________ ___________

Delta ______________

1. Using the data presented in your text how did the efficiency percentages of your subject rate?

2. If the subject were of the opposite sex would you expect the efficiency measures to be any different? Why or why not?

3. How would wind resistance or a tailwind affect measured efficiencies in outdoor activities?

4. How can efficiency be optimized?

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