EXERCISE by James Duffin Changes During Exercise The stress of exercise reveals the respiratory and cardiovascular control systems performing at their best! Active muscles use more oxygen and produce more carbon dioxide. So, gas transport must increase. Both cardiac output and pulmonary ventilation increase to provide the extra gas transport needed. What changes occur? Respiratory • • •
Tidal Volume Breathing Frequency Ventilation 6
0.5 12 ⇒
⇒ ⇒ 100
4/5 L 30 Breaths/min. L/min.
75 80 6
⇒ ⇒ ⇒
150 170 30
Cardiac • • •
Stroke Volume Heart Rate Cardiac Output
ml beats/min. L/min.
Circulation • •
Muscle blood flow Non-essential Tissue Blood Flow
⇑ ⇓
Let’s look at the cardiovascular changes in more detail: 1) In the heart both the stroke volume and heart rate increase. (Stroke volume does not increase if supine because the enhanced venous return has already increased filling of the atrium.) These changes require an increased strength of contraction for cardiac muscle. 2) To increase muscle blood flow, not only is blood diverted from non-essential tissues but blood pressure is also augmented. So that the baroreceptor reflex does not attempt to decrease blood pressure, the baroreceptor reflex sensitivity is reduced. Venous return to the heart is enhanced by constriction of the great veins so that they empty faster.
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The time course of cardiovascular and respiratory changes in exercise The respiratory and cardiovascular changes occur in two stages: 1) Initial 2) Delayed
Rapid Slow
Neurally mediated Programmed Response Chemoreflex mediated Feedback Control
The graph of ventilation vs. time shown below illustrates this point.
Changes (e.g. Ventilation)
The Time Course of Changes During Moderate Exercise
0 rest
5 exercise
time (min.) rest
As the graph shows, there is an initial rapid increase followed by a slower one to a final steady state. Although physiologists are still investigating the mechanisms underlying these changes, there is some general agreement that the fast changes are due to “neural” mechanisms, and the slower changes are due to “humoral” mechanisms. Neural mechanisms include feedback from the working muscles, an ascending drive. There is also evidence for a spill-over of the central command that drives the working muscles from the motor cortex and elsewhere into the medullary control centres, a descending drive. Humoral mechanisms include feedback from the central and peripheral chemoreflexes reflecting changes in PCO2 and PO2 as well as [H+]. In heavy exercise lactic acidosis may contribute as well as catecholamines and increased temperature.
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Classes of Exercise There are two classes of exercise: • aerobic producing carbonic acid • anaerobic producing lactic acid The diagram below shows how ventilation and blood gases are related to the exercise work rate.
Respiratory Variables
Aerobic
Anaerobic
Pco2
V
ila ent
tion
Po2 , [H+] & lactate
Exercise Work Rate As you can see, for aerobic exercise ventilation is linearly related to the increase in work rate, but above what some call the anaerobic threshold ventilation increases out of proportion to the increase in work rate. This latter increase means that ventilation exceeds that required for gas exchange and so PCO2 falls and PO2 rises. Aerobic Exercise: Ventilation increases with workload but PCO2 and PO2 remain roughly constant. Most of the drive to breathe is from neural sources, with the chemoreflexes acting to fine-tune the system. Anaerobic Exercise: Ventilation is driven by factors such as: • increased central command and muscle afferent feedback as the muscles tire. • lactic acidosis via the peripheral chemoreflex (the central chemoreflex is behind the bloodbrain barrier to polar solutes). • increased body temperature. • increased catecholamines.
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Fitness How do we rate the system performance? We define physical fitness to be the “ability to transport oxygen”. This ability can be measured as the maximum oxygen uptake and is equal to the maximum aerobic work that an individual can achieve. The higher an individual’s max O2 uptake, the less stress on the system at the normal work rates of everyday life. NOTE: The limiting factor for oxygen uptake is the cardiovascular system not the respiratory system. How do we measure the maximum oxygen uptake for an individual? If we exercise to our maximum ability and measure the maximum oxygen uptake directly, the test puts a great strain on the system. All right for young healthy individuals but not for others. For this reason Dr. Per Olaf Astrand developed a sub-maximum test. The Astrand Fitness Test This test measures the maximum oxygen uptake indirectly, using simple measurements and a moderate work rate. However, its accuracy is dependent on a set of assumptions as follows: Assumption 1: Aerobic work ∝ oxygen uptake, independent of the type of work. While the first part of the assumption is correct, the latter part is probably not true, but can be assumed to be “good enough”. We can measure the work done on a cycle ergometer or treadmill or stairs etc. by calculation. Work = force X distance. But how can we measure the oxygen uptake? Directly, collecting and analysing expired air, or indirectly…
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Assumption 2: Oxygen uptake � cardiac output � heart rate.
Heart Rate
Heart rate is an indirect measure of the oxygen uptake. If we know the work rate and its oxygen cost as well as the heart rate we can make the graph on the following page:
Work Rate or Oxygen Cost
But how do we find the maximum oxygen uptake? Assumption 3: Maximum heart rate is the same for all individuals of the same age and sex. Maximum heart rate declines with age according to the formula 220-age So we can now measure the maximum oxygen uptake indirectly by exercising at two or three know work rates and measuring the heart rates that ensue. Then plot the graph and extrapolate as shown below. max Heart Rate
X X X
max Work Rate or Oxygen Cost
The procedure is still complicated, but can be simplified further by making another assumption.
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Assumption 4: The slope of the heart rate vs. work rate graph is the same for all individuals. Now we need only measure one point on the graph as shown below.
Heart Rate
max
X
max Work Rate or Oxygen Cost
Here’s how it’s done: 1. Exercise at a known work rate for 5-6 minutes to reach steady-state. 2. Measure heart rate in the last minute (take pulse). 3. Use tables to look-up maximum oxygen uptake (correcting for age and sex).
Normal values: 20 - 30 yrs males 3.0 - 3.5 litres/min. 20 - 30 yrs. Females 2.0 - 2.5 litrres/min. Usually these are normalised by dividing by weight. 20 - 30 yrs females 20 - 30 yrs males
35 - 45 ml/kg•min 45 - 50 ml/kg•min
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Fitness Message In the graph below there are results for three individuals A, B and C. Notice that C is the fittest, with the highest maximum oxygen uptake. In everyday life, the stress on C’s systems is less than the others. Even at rest, C will have a lower heart rate. Miguel Indurain, 5 time winner of the Tour de France had a resting heart rate of 34 beats/min. Lance Armstrong, 4 time winner of the Tour de France has a resting heart rate of 32 beats/min. What’s yours?
Heart Rate
max
A increasing fitness
B C
A’s max B’s max C’s max
Work Rate or Oxygen Cost
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