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

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

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

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

Measurement of Work and Power

• Ergometry– Measurement of work output

• Ergometer– Device used to measure work– Bench step ergometer– Cycle ergometer– Treadmill

Ergometer

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

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

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

Determination of Percent Grade on a Treadmill

Figure 6.2

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

• 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

Diagram of a Simple Calorimeter

Figure 6.3

Open-Circuit Spirometry

Figure 6.4

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

The Relationship Between Walking or Running Speed and VO2

Figure 6.5

Relationship Between Work Rate and VO2 for Cycling

Figure 6.6

• 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

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

Net Efficiency During Arm Crank Ergometery

Figure 6.8

Relationship Between Energy Expenditure and Work Rate

Figure 6.9

Effect of Speed of Movement of Net Efficiency

Figure 6.10

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

Comparison of Running Economy Between Males and Females

Figure 6.11