Measurement of Work, Power
and Energy
Definitions
• Force = mass x acceleration
• Work = force x distance
– units - kpm, kgm, kcal, J, ft-lbs
• Power = work / time
– units - W, hp, kpm/min
Work and Power Calculations
• 5-kg weight (11 lbs) lifted 2 m vertical distance
– W= 5 kp x 2 m = 10 kpm (kp=kilopon unit of force,
same as a kilogram; but kilogram is the force gravity
exerts on an object)
• Cyclist does 2000 kpm of work in 60 seconds
• P=2000 kpm/60s = 33.3 kpm/s
Modes of exercise testing
•
•
•
•
Ergometry/ergometer
Ergo = work
Ergometry is the measurement of work
Ergometer is a device used to measure work
Bench step
• 70 kg subject steps up 0.5 m bench at 30
steps / min for 10 min
– W = 70 kg x 150 m = 10,500 kpm
Cycle ergometer
• 1-min exercise @ 50 rpm (1 rev = 6 m) with
1 – kg resistance:
– W = 300m x 1 kg = 300 kpm
Determination of percent grade
Treadmill
• *vertical displacement = % grade x distance
• 70 kg subject running for 10 min at 200
m/min up a 7.5 % grade
– W = 70 kp x 15 m (= 1050 kpm)
Direct calorimetry
Direct Calorimetry
• Measurement of heat production as estimate
of metabolic expenditure
• Food + O2 ATP + Heat  Heat
cell work
Measurement of heat production tells us the
amount of metabolic work that has been
performed
Open circuit spirometry/indirect calorimetry
Indirect Calorimetry
• Measurement of Oxygen consumption as
estimate of metabolic rate
• Food + O2  Heat + CO2 + H2O
• By determining O2 consumed, can estimate
amount of metabolic work performed
Estimation of Energy Expenditure
• Subject runs 1 mile and consumes 20 l of
O2.
– Estimated expenditure = 20 l O2 x 5 kcal/l O2=
100 kcal
Relationship between speed and VO2 costlinear for walking and running
Estimation of Energy Expenditure
• Energy cost (O2 consumed) is linear in
relation to speed while walking or running
• At rest, O2 cost expressed as 1 MET
• 1 MET = 3.5 ml/kg/min
% Net efficiency
% net
=
efficiency
work output
energy expended
above rest
X 100
•Net efficiency of cycling ~ 15-25 % (20 %)
Efficiency as Predictor of
Performance
• Compare runners on horizontal treadmill
• The runner who consumes less oxygen at
given speed is more efficient
• If runners have comparable VO2 max, the
more efficient will be more successful
• Efficiency can make up for lower VO2 max
– Efficiency can make up for poor genetics
Change in net efficiency during arm crank
ergo vs work rate
Relationship between energy expenditure and
work rate
Muscle Fiber Type and
Efficiency
• Muscles with high percentage of Type I
fibers (slow twitch) are more efficient than
muscles with high percentage of Type Iib
(fast twitch)
So….
• Athletes with higher percentage of Type I
(slow twitch) fibers will be more efficient
than athletes with higher percentage of Type
II b (fast twitch)
And….
• Athletes with high percentage of Type I
excell at endurance versus athletes with
high percentage of Type IIb