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 cost- linear 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 = work output X 100efficiency energy expended

above rest

•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