1. Energy Systems

2. Some basic truisms:
1. All energy systems are “on” at all times, relative contributions of each one will change:
2. As intensity , energy supplied by CHO 
3. As duration , energy supplied by fat .
4. As glycogen , energy supplied by protein .
5. Training  fat use at any given intensity.
6. Heat, cold, altitude, other stress  CHO use.
7. Effects of diet  as exercise intensity 

3. SPRINT (10 seconds of maximal exercise)
HUMAN ENERGY SYSTEMS
ADENOSINE
TRIPHOSPHATE
(ATP)
PHOSPHO-
CREATINE
(PCr)
GLYCOLYSIS
(ANAEROBIC;
CHO BREAKDOWN)
KREBS
CYCLE
(AEROBIC;
CHO, FAT, AND
PROTEIN
BREAKDOWN)
SPRINT (10 seconds of maximal exercise)

4. HUMAN ENERGY SYSTEMS 800 METER RUN (50 seconds) ADENOSINE TRIPHOSPHATE
(ATP)
PHOSPHO-
CREATINE
(PCr)
GLYCOLYSIS
(ANAEROBIC;
CHO BREAKDOWN)
KREBS
CYCLE
(AEROBIC;
CHO, FAT, AND
PROTEIN
BREAKDOWN)
800 METER RUN (50 seconds)

5. MARATHON 2.5 hours of exercise
HUMAN ENERGY SYSTEMS
ADENOSINE
TRIPHOSPHATE
(ATP)
PHOSPHO-
CREATINE
(PCr)
GLYCOLYSIS
(ANAEROBIC;
CHO BREAKDOWN)
KREBS
CYCLE
(AEROBIC;
CHO, FAT, AND
PROTEIN
BREAKDOWN)
MARATHON 2.5 hours of exercise

6. Some basic truisms:
1. All energy systems are “on” at all times, relative contributions of each one will change:
2. As intensity , energy supplied by CHO 
3. As duration , energy supplied by fat .
4. As glycogen , energy supplied by protein .
5. Training  fat use at any given intensity.
6. Heat, cold, altitude, other stress  CHO use.
7. Effects of diet  as exercise intensity 

7. As exercise gets harder, the % energy from
% energy from CHO
% energy from FAT
increasing intensity of exercise
As exercise gets harder, the % energy from
oxidation of CHO goes UP sharply while the
% energy from fat goes DOWN

8. Exercise Intensity & Fuel Source
100 80 60
FAT
(% Total Kcal)
CHO 40 20
25% MAX
85% MAX

9. Misinterpretation
The graph on the previous slide indicates that if you
exercise at very low intensity levels (25% MAX) you
will burn a great deal more fat for energy
However, at the end of the day, body weight is
determined by how many TOTAL calories were
ingested vs. how many TOTAL calories were expended
Consider the next slide which is the same exercise
scenario now expressed in absolute (calorie) terms

10. Exercise Intensity & Fuel Source
600
500
400
FAT
(kcal)
300
CHO
200
100
25% MAX
85% MAX

11. Some basic truisms:
1. All energy systems are “on” at all times, relative contributions of each one will change:
2. As intensity , energy supplied by CHO 
3. As duration , energy supplied by fat .
4. As glycogen , energy supplied by protein .
5. Training  fat use at any given intensity.
6. Heat, cold, altitude, other stress  CHO use.
7. Effects of diet  as exercise intensity 

12. %CHO & FAT Use in Exercise90
% FAT being used 80 70 60 50
%CHO & FAT Use in Exercise 40 30 20
% CHO being used 10 30 60 90
120
Exercise Time (min)

13. WHY does the % of energy from CHO go
down as the exercise duration gets longer?
Glycogen supplies getting low: remember
that use of glycogen
is related
to how much
you have.
rate of glycogen use
amount of glycogen remaining

14. So, as exercise progresses, the use of
glycogen lessens and use of fat goes up.
What happens to intensity?
When glycogen supplies become very low, it
is no longer possible to provide enough
glycogen to sustain high intensity exercise.
How does that make you feel?

15. Inability to maintain
a high exercise
intensity when
muscle glycogen
supplies get low is
called “bonking”,
“hitting the wall”,
“going down in
flames” and other
colorful expressions.

16. So, there is always a trade-off between
exercise intensity and duration:
High intensity (fast, hard) requires a lot of
glycogen so duration has to be short
Low-intensity (slow, easy) requires little
glycogen so duration can be long
What can you do to improve ability to maintain
a higher intensity for a longer period of time?

17. Some basic truisms:
1. All energy systems are “on” at all times, relative contributions of each one will change:
2. As intensity , energy supplied by CHO 
3. As duration , energy supplied by fat .
4. As glycogen , energy supplied by protein .
5. Training  fat use at any given intensity.
6. Heat, cold, altitude, other stress  CHO use.
7. Effects of diet  as exercise intensity 