CHAPTER 8 • BASICS IN EXERCISE PHYSIOLOGY 41by a simple verbal expression: a person exercising at
130 bpm should report a RPE of 13. A variant scale
using 0 to 10 as the numeric ratio has been proposed.MEASUREMENTS OF ENERGY,
WORK, AND POWER
DEFINITIONS OF ENERGY, WORK,
AND POWER
•Physical exercise involves both mechanical and chem-
ical work by the muscles, with the degree of muscular
effort dependent on duration, frequency, and intensity.
Together these define energy, work, and power.
- Energy:The capacity to do work, with energy meas-
ured in Joules: 4.186 J =1 kcal and 1 J =2.3889 ×
10 −^4 kcal. - Wo r k :When a force acts against resistance to pro-
duce motion: Force × Distance. Work is often
expressed in kilogram-meters or kgm, where 1 kcal =
426.85 kgm =4.18 kJ Power—Rate at which work is
performed or the rate of energy transfer—Work/Time.
Power is usually expressed as Watts (W), where 1W =
1 J/s; 6.12 kpm/min; and 0.01433 kcal/min.
ENERGY EXPENDITURE
- Energy expenditure (EE) is usually determined in one
of two ways: direct and indirect calorimetry. For
direct calorimetry, both external work and heat output
are measured, and heat production is used as an esti-
mate of metabolic rate. For indirect calorimetry, either
open or closed circuit spirometry can be used.
•With open circuit spirometry, VO 2 and VCO 2 are
measured and the RER is calculated. Simplistically
RER can be added to 4 and then multiplied by L of
O 2 /min to derive kcal/min. - Example: If VO 2 and RER determined from gas analy-
sis were 0.3 L of O 2 /min and 0.75 and 2.5 L/min and
0.95, at rest and during exercise, respectively, then EE
at rest would be 4.75 (4.0 +0.75) ×0.3 =1.4 kcal/min
and during exercise, 4.95 ×2.5 =12.4 kcal/min. Net
EE would be 12.4 – 1.4 or 11.0 kcal/min. - EE can also be estimated from VO 2 by assuming 1 L
of O 2 is ~5 kcal. If resting VO 2 was 0.3 L/min, energy
costs would be (0.3 ×5) 1.5 kcal/min.
CALCULATING WORK AND POWER
•Work and power are calculated based on the particu-
lar activity.
•For example, work for cycle ergometry is measured as:
resistance (kg) ×rev/min ×flywheel distance (m) ×
time. If a 80-kg male cycled at 100 rpm against a 3-kg
load for 20 min (flywheel distance of 6 m), then:
Work =3 kg ×100 rpm ×6 m/rev ×20 min = 36,000
kgm Power =36,000/20 min = 1,800 kgm/min =
1800/6.12 =294 W (4.2 kcal/min).- If a 70-kg man stepped up and down a 0.5-m bench
30 times/min for 10 min then: Work =70 kg ×0.5m ×
30 steps ×10 min =10,500 kgm Power =10,500/10
min = 1,050 kpm/min =1,050/6.12 =171.6 W (2.5
kcal/min).
WORK EFFICIENCY- Gross work efficiency is the ratio of mechanical work
output to energy expended; it typically ranges
between 15 and 30%. - Net work efficiency is the ratio of mechanical work
output to total energy expended minus resting EE. - Example: A woman with a resting VO 2 of 0.3 L/min
rides a cycle ergometer for 30 min at 150 W (1 W =
0.01433 kcal) and uses 2 L O 2 /min. Gross efficiency =
Mechanical work output: 150 W (250×0.01433)
~ 2.2 kcal/min; Total energy expended: 2 L/min
~10 kcal/min or 2.2 × 100/10 = 22%. Net
efficiency = 2.0 – 0.3 = 1.7 L/min ~8.5 kcal/min
or 2.2 ×100/8.5 = 25.9%.
•Factors influencing exercise efficiency include work
rate, speed of movement, and muscle fiber composi-
tion, and a variety of biomechanical factors, such as
equipment and clothing.
EXERCISE ECONOMY- Economy of movement is defined in terms of VO 2
(energy required) for a specific power output or veloc-
ity (mL/min/W or mL/kg/min to km/min or mile/min).
Selected factors affecting economy include shoes,
stride length, and frequency for running; and body
mass, velocity, and aerodynamic positioning for
cycling. Values may range from 200 to 350 mL/kg/km
for running and 10 to 15 mL/W for cycling.
METABOLIC ENERGY EQUIVALENT- Metabolic energy equivalent(MET) is the energy cost
of activities in terms of multiples of resting metabolic
rate. If 1 MET (resting metabolic rate) is taken as
3.5 mL of O 2 /kg/min), then 3 MET would be 10.5
mL/kg/ min and 6 MET would be 21 mL/kg/min.