Sports Medicine: Just the Facts

CHAPTER 8 • BASICS IN EXERCISE PHYSIOLOGY 39

L/min and represents a limit for comparison with

maximal VE during exercise (VEmax).

BREATHING RESERVE

• The difference between MVV and VEmax: breathing
  reserve (BR) is sometimes expressed as VEmax/MVV.
  Theoretically, BR is the additional VE available
  during maximal exercise. If a person achieves maxi-
  mal work capacity prior to attaining MVV, the person
  has a normal BR, whereas if MVV =VE, the person
  may have compromised pulmonary function. A typi-
  cal BR is 11 L/min, with normal VEmax/MVV ranging
  between 0.6 and 0.75 (60 to 75%).

OXYGEN UPTAKE (VO 2 )

•Oxygen uptake (VO 2 ) is based on the Fick equation
and can be expressed as: VO 2 =CO ×(a-VO 2 ) =HR ×
SV ×(a-VO 2 ), where CO =cardiac output, a-VO 2 is
the arteriovenous O 2 content difference, SV is stroke
volume, and HR is heart rate. For an average size
adult (70 kg), CO at rest would be about 5 L/min and
during strenuous exercise, 20 to 30 L/min.
•VO 2 , determined during exercise by measuring respi-
ratory gases, is related to the fractional percent of O 2
in inspired and expired air and VE. Inspired air con-
tains 20.93% O 2 and expired air around 17.0%.
•VO 2 is usually expressed in absolute units (L/min) or
relative to body weight (mL/kg/min). Resting VO 2
usually ranges between 0.25 and 0.4 L/min and max-
imal exercise VO 2 values can exceed 5 L/min (15–70
mL/kg/min) (Fig. 8-3). Higher values indicate higher
aerobic fitness.

CARBON DIOXIDE PRODUCTION (VCO 2 )

• Carbon dioxide (CO 2 ) is produced metabolically in
  the tissues, transported in blood by venous return to
  the lung, and eliminated from the lung by breathing
  (VE). Inspired air contains 0.03% CO 2 and expired air
  about 5% CO 2. Like VO 2 , VCO 2 is usually expressed
  as L/min or mL/kg/min. Resting and strenuous exer-
  cise values depend on metabolism and pulmonary
  function, but resting values are less than VO 2.

LACTIC ACID

• Lactate, a product of glycolysis, is formed from pyru-
  vate in the recycling of NAD (Fig. 8-2), or when

insufficient O 2 is available for pyruvate to enter the

TCA cycle. The extent of lactate formation depends

on the availability of both pyruvate and NADH. Blood

lactate at rest is about 0.8 to 1.5 mM, but during

intense exercise can be in excess of 18 mM.

• During exercise, muscle cells dispose of lactate by
  releasing it into the circulation where it becomes an
  important fuel for the heart and nonexercising mus-
  cles. Lactate released from the muscle can also be
  converted to glucose in the liver by the Cori cycle.

OXYGEN PULSE

• The ratio of VO 2 (mL/min) to HR (bpm), when both
  measures are obtained simultaneously, is O 2 pulse
  (mL/beat). It is an important measure since it reflects
  the product of stroke volume and a-VO 2 difference
  (see equation under VO 2 ).
  •O 2 pulse increases with increasing work effort (range
  4 to 30 mL/beat) and is affected by a variety of fac-
  tors, including anemia and heart disease. A low value
  during exercise indicates HR is too high for VO 2 and
  can be an indication of heart disease.

RESPIRATORY QUOTIENT/RESPIRATORY

EXCHANGE RATIO

• The respiratory quotient (RQ) is the ratio of CO 2 pro-
  duced by cellular metabolism to O 2 used by tissues.
  This metabolic marker is used to quantify the relative
  amounts of carbohydrate and fatty acids being oxi-
  dized for energy. The RQ cannot exceed 1.0: an RQ of
  0.7 implies dependence on free fatty acids and a value
  of 1.0 dependence on carbohydrate.


• The respiratory exchange ratio (RER) reflects pul-
  monary exchange of CO 2 and O 2 at rest and during
  exercise. The RER also ranges between 0.7 and 1.0
  during steady state exercise and can reflect substrate
  preference; however, it exceeds 1.0 during strenuous
  exercise because of increasing metabolic activity not
  matched by VO 2 and additional CO 2 derived from
  bicarbonate buffering of lactic acid. The terms RQ
  and RER are often used interchangeably, but their dis-
  tinction is important.

VENTILATORY EQUIVALENTS

•Ventilatory equivalents (VE/VO 2 and VE/VCO 2 ) are unit-

less numbers derived from the ratio of VE to VO 2 and

VCO 2. VE/VO 2 indicates the volume (L) of air required

to use 1 L of O 2 and VE/VCO 2 indicates the appropriate-

ness of ventilation (L of air required to remove L of CO 2 ).