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Chapter 6
Measurement of Work, Power, and Energy Expenditure
EXERCISE PHYSIOLOGYTheory and Application to Fitness and Performance,
6th edition
Scott K. Powers & Edward T. Howley
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Units of Measure
• Metric system is the standard system of measurement for scientists– Used to express mass, length, and volume
• System International units or SI units – For standardizing units of measurement
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Important SI Units
Units for Quantifying
Human Exercise SI Unit
Mass kilogram (kg)
Distance meter (m)
Time second (s)
Force Newton (N)
Work joule (J)
Energy joule (J)
Power Watt (W)
Velocity meters per second (m . s-1)
Torque newton-meter (N . m)
Table 6.2
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Work and Power
• Work = force x distance– Example:
• Lifting a 5 kg (5 kp*) weight up a distance of 2 m– * 5 kp is the force acting on a 5 kg mass
5 kp x 2 m = 10 kpm
• Power = work ÷ time– Example:
• Performing 2,000 kgm of work in 60 sec
2,000 kgm ÷ 60s = 33.33 kgm•s-1 • Expressed in SI Units:
1 Watt (W) = 0.102 kpm•s-1, so 326.8 W
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Measurement of Work and Power
• Ergometry– Measurement of work output
• Ergometer– Device used to measure work– Bench step ergometer– Cycle ergometer– Treadmill
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Ergometer
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Bench Step
• Subject steps up and down at specified rate– Example:
• 70 kg subject, 0.5 m step, 30 steps•min-1 for 10 min
• Total work
70 kg x 0.5 m•step-1 x 30 steps•min-1 x 10 min = 10,500 kpm
• Power
10,500 kpm ÷ 10 min = 1,050 kpm•min-1 or 171.6 W
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Cycle Ergometer
• Stationary cycle that allows accurate measurement of work performed– Example:
• 1.5 kp resistance, 6 m•rev-1, 60 rev•min-1 for 10 min
– Total work1.5 kp x 6 m•rev-1 x 60 rev•min-1 x 10 min = 5,400 kpm
– Power5,400 kpm ÷ 10 min = 540 kpm•min-1 or 88.2 W
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Treadmill
• Calculation of work performed while a subject runs or walks on a treadmill is not generally possible when the treadmill is horizontal– Even though running horizontal on a treadmill
requires energy• Quantifiable work is being performed when walking
or running up a slope• Incline of the treadmill is expressed in percent
grade– Amount of vertical rise per 100 units of belt travel
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Determination of Percent Grade on a Treadmill
Figure 6.2
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Treadmill
• Example• 70 kg subject, treadmill speed 200 m•min-1, 7.5%
grade for 10 min– Vertical displacement = % grade x distance
0.75 x 200 m•min-1 = 15 m– Total vertical distance = vertical displacement x time
15 m x 10 min = 150 m– Work = body weight x total vertical distance
70 kg x 150 m = 10,500 kpm– Power = work ÷ time
10,500 kpm ÷ 10 min = 1,050 kpm•min-1
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• Direct calorimetry– Measurement of heat production as an indication
of metabolic rate
– Indirect calorimetry– Measurement of oxygen consumption as an
estimate of resting metabolic rate
– Open-circuit spirometry
Measurement of Energy Expenditure
Foodstuffs + O2 ATP + heatcell work
Heat
Foodstuffs + O2 Heat + CO2 + H2O
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Diagram of a Simple Calorimeter
Figure 6.3
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Open-Circuit Spirometry
Figure 6.4
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Estimation of Energy Expenditure
• Energy cost of horizontal treadmill walking or running– O2 requirement increases as a linear function of
speed
• Expression of energy cost in METs– 1 MET = energy cost at rest– 1 MET = 3.5 ml•kg-1•min-1
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The Relationship Between Walking or Running Speed and VO2
Figure 6.5
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Relationship Between Work Rate and VO2 for Cycling
Figure 6.6
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• Net efficiency– Ratio of work output divided by energy
expended above rest
• Net efficiency of cycle ergometry– 15-27%
Calculation of Exercise Efficiency
Work output
Energy expended above rest
% net efficiency = x 100
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Factors That Influence Exercise Efficiency
• Exercise work rate– Efficiency decreases as work rate increases
• Speed of movement– There is an optimum speed of movement and
any deviation reduces efficiency
• Muscle fiber type– Higher efficiency in muscles with greater
percentage of slow fibers
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Net Efficiency During Arm Crank Ergometery
Figure 6.8
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Relationship Between Energy Expenditure and Work Rate
Figure 6.9
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Effect of Speed of Movement of Net Efficiency
Figure 6.10
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Running Economy
• Not possible to calculate net efficiency of horizontal running
• Running Economy– Oxygen cost of running at given speed
– Lower VO2 (ml•kg-1•min-1) indicates better running economy
• Gender difference– No difference at slow speeds– At “race pace” speeds, males may be more
economical that females
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Comparison of Running Economy Between Males and Females
Figure 6.11