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Muscle Fatigue
Normally, muscles use oxygen through a process known as cellular/aerobic respiration to make energy
(or ATP) from sugar (glucose). This process is very efficient and produces 36 ATPs for each molecule of glucose.
Carbon dioxide and water are the results of this reaction. When muscles undergo rigorous exercise they
require more oxygen to make ATP than the blood can supply. At this point the muscle is forced to produce ATP
without oxygen. This is known as anaerobic respiration. Anaerobic respiration produces only 2 ATPs for each
molecule of glucose. The result of this reaction is lactic acid.
The advantages of anaerobic respiration are that the muscle cell can make ATP without oxygen and it
can make ATP very quickly. This a particular advantage when lifting heavy objects. The big disadvantage of
anaerobic respiration is that it produces lactic acid which gives muscles a temporary burning sensation.
Muscle fatigue (or tiredness) results when the demand for ATP is greater than the rate at which ATP
can be produced in the muscle fibers. As a result, ATP levels are too low for muscle fibers to produce their
maximum contraction. True muscle fatigue results in muscles becoming incapable of contracting or relaxing.
1. Obtain a clothespin.
2. Use your non-writing hand to open the clothespin as wide as possible.
3. Continue to open the clothespin for a total of 100 seconds, recording the number of times you open
the clothespin in 10 second intervals.
4. Do not stop squeezing between intervals!
5. Squeeze as quickly and consistently as possible making sure to open the clothespin all the way each
time. **If your number of squeezes starts to decrease, please do not try to speed up to compensate!
6. Repeat using your writing hand.
7. You will need a partner to watch time and record squeezes.
Remember: Time will continue, but you will restart counting every 10 seconds. Record number for each 10
second interval. Do not stop the time and do not stop counting! Squeeze as quickly and consistently as
possible during each 10 second interval, so that your results are precise.
*** (Please write these before you begin!!)***
1. Write a hypothesis regarding what you expect to happen to the number of squeezes per unit of time
throughout the experiment.
2. Write a hypothesis regarding the change in number of squeezes per unit of time when comparing writing
hand with non-writing hand.
Trial Number # of squeezes in 10 seconds
(non-dominant hand)
1 (1st 10
2 (2nd 10
# of squeezes in 10 seconds
**Graph results!
1. Accept or reject your hypotheses. Explain.
2. What relationship exists between the number of squeezes and the increasing time interval?
3. Was there a difference between your dominant and non-dominant hand?
4. How did your squeezing hand and arm feel towards the end of your squeezing time
5. By looking at your results, pinpoint when you first had a lot of lactic acid buildup.
How did you know?
What happened to your “strength” as you progressed through each trial? How does your graph show this?
What physiological (body) factors might cause one to get more squeezes, in other words, to have less fatigue?(Hint: consider
how the reactants of aerobic respiration get to your muscle cells)
10. Your muscles would probably recover enough after 10 minutes to operate at the original efficiency. Explain why.
11. Explain how the products of anaerobic respiration cause your cells to be less efficient.
12. What is the relationship between the strength of your heart and aerobic respiration/exercise?