Download CHAPTER 3 – MUSCLES

APPLIED PHYSIOLOGY TO OPTIMISE PERFORMANCE
CHAPTER 3 – MUSCLES
Text between pages 24 and 29, answers to questions on page 30 of the text book.
figure 3.10 – fibre type recruitment
A force exerted
1) a) Skeletal muscle tissue consists of different fibre types. These types are suited to particular intensities of work. Figure 3.10 shows muscle fibre usage for three different work rates. Identify muscle fibre types A, B and C.
3 marks
Answer
• A = fast glycolytic muscle fibres (FG type IIb) or fast twitch fibres type IIb.
• B = fast oxidative glycolytic muscle fibres (type IIa FOG) or fast twitch
fibres type IIa.
• C = slow oxidative muscle fibres (type 1 SO) or slow twitch muscle fibres
(ST).
B
C
light
work
heavy
medium
work load work load
b) Using the information in figure 3.10 describe the order in which fibre types are recruited as the number of motor units increase. number of motor units 3 marks
Answer
• Different types of muscle fibres are recruited during physical activity with increasing resistance.
• When little force is needed in light work SO type 1 fibres are called upon first.
• Before the fast twitch type IIa (FOG) become active during medium work intensity.
• As the load progressively gets harder type IIb (FG) fibres are used until all the energy supply has been used up.
c) How would the motor unit recruitment pattern differ between a weightlifter and an endurance runner? 4 marks
Answer
• The weightlifter would have a higher percentage of fast twitch muscle fibres.
• And therefore a greater number of fast twitch motor units, particularly in arm and leg muscles.
• Advantageous for power lifting.
• Hence an increase in number and recruitment of fast twitch motor units will produce greater maximum explosive force over a very
short period of time.
• And reduced recruitment of slow twitch muscle fibre.
•
•
•
•
•
•
Whereas the endurance runner would have a higher percentage of slow twitch muscle fibres.
And therefore a greater number of slow twitch motor units particularly in the leg muscles.
Advantageous for endurance running.
To show an increased pattern of recruitment of slow twitch motor units.
And reduced recruitment of fast twitch fibres types 11a and b.
Producing less force but over a longer period of time.
2) Figure 3.11 has been created from a slide of skeletal tissue as seen with a light
microscope at a magnification of 800 times. It shows part of two motor units.
a) Use evidence from the drawing to suggest a meaning of the term motor unit. 2 marks
Answer
• A motor unit consists of a motor neurone which connects to a group of muscle fibres.
• The connection is made by specialised structures called motor end-plates.
figure 3.11 – muscle
motor units
motor end
plates
motor neurones
b) Why is it that all the muscle fibres shown will not necessarily contract at the same time?
3 marks
skeletal muscle fibres
Answer
• Motor nerve fibres have different lengths, hence the impulse takes less time down a
shorter one.
• Which will cause its muscle fibre to contract earlier (than others connected to longer nerve fibres).
• It is not necessary for both motor units to be stimulated at the same time = spatial summation.
• Hence if the shorter of the two motor units is stimulated all the muscle fibres connected to that motor unit will contract - this is
the ‘all-or-none law’.
• But at the same time the longer of the two motor units could be resting and so those muscle fibres connected to that motor
neurone would be in a state of relaxation.
3) A muscle can vary the strength of its contractile response through wave summation. Explain the role of motor units in
Questions and answers
11
ANSWERS TO QUESTIONS
controlling this response.
Answer
• Each motor unit controls a number of muscle fibres.
• So that all the fibres attached to the motor unit are activated at the same time.
• This is called the ‘all-or-none-law’.
• Each firing produces a single fibre ‘twitch’ (figure AQ3.1).
• Which lasts a very short time.
• When a fibre is fired repeatedly in this manner.
• The way in which the force builds up is called ‘wave summation’ (figure AQ3.1).
4 marks
figure AQ3.1 – a single fibre
forc e
SECTION A –
CHAPTER 3
single
twitch
wave summation
time
4) The strength of a muscle contraction involves the use of motor units. How
are the motor units used to produce muscle contractions of varying strength
in a weight lifting exercise?
3 marks
Answer
• The strength of contraction will depend on the number and type of motor units involved.
• Since motor units are either fast twitch or slow twitch.
• The size of the motor units, for example, fast twitch motor units have larger diameter and shorter length neurones when compared
with slow twitch motor units.
• Giving increased conduction of the neural impulse.
• Control is achieved by increasing or decreasing the number and type of motor units in operation (spatial summation).
• And increasing the frequency of stimulus (wave summation).
• This results from the fine-tuning mechanism of the cerebellum to produce muscle contractions of varying strength in a lift.
5) Figure 3.12 shows the thick and thin filaments of a relaxed sarcomere within
a skeletal muscle fibre.
Draw and label the sarcomere in a state of contraction. Explain how the
change from the relaxed to the contracted state is thought to occur. In your
answer describe the roles of the nervous system, myosin-ATPase, troponin
and tropomyosin molecules.
14 marks
Answer
4 marks for diagram of contracted sarcomere (figure AQ3.2), 10 marks for
explanation:
• Neural action potential (electrical impulse from the brain) creates a muscle
action potential over muscle fibre’s sarcoplasm.
• Which triggers the release of Ca++ ions (from ‘T’ vesicles) into the
sarcoplasm.
• Troponin is the globular protein component of the actin filament.
• And has a high affinity for Ca++ ions.
• Ca++ ions bind to troponin molecule on an actin filament.
• And neutralises tropomyosin also on the actin filament.
• Tropomyosin is a thread-like protein that winds around the surface of actin.
• Exposing the actin’s active sites.
• Energised myosin cross-bridges (from ATP released by enzyme myosinATPase).
• Bind to actin’s active sites.
• Thus the actin filaments are drawn in between the thicker myosin filaments.
• As cross-bridges attach, detach and reattach – called the ratchet mechanism.
• Causing the H zone within the sarcomere to disappear.
• And the Z zones to move closer together.
• The greater the overlap of filaments the stronger the contraction.
• Called Huxley’s sliding filament theory of muscle contraction.
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figure 3.12 – relaxed sarcomere
figure AQ3.2 – contracted sarcomere
Z line
H zone I band
myosin
filament
actin
filament
APPLIED PHYSIOLOGY TO OPTIMISE PERFORMANCE
6) Skeletal muscle contains both slow and fast twitch muscle fibres but the proportion of each depends upon the function of a
muscle as a whole. Table 3.4 lists some of the differences between slow and fast twitch muscle fibres.
Table 3.4 – muscle fibre type characteristics
characteristic
contractile time / ms
mitochondrial density
glycogen store
phosphocreatine stores
capillary density
sarcoplasmic reticulum
oxidative enzyme activity
a) slow twitch type
110
high
low
low
high
poorly developed
high
fast twitch type
40
low
high
high
low
well developed
low
Suggest why the muscles concerned in maintaining the trunk posture of the body of the sprinter might be expected to have a larger percentage of slow twitch muscle fibres.
Using table 3.4 explain why fast twitch muscle fibres may build up an oxygen debt during a 400m sprint. 5 marks
Answer
Postural muscles in the trunk would have more slow twitch fibres because:
• They maintain posture for long periods of time.
• It would be important not to get fatigued easily.
• No need for fast or powerful contraction.
Oxygen debt during a 400m sprint:
• Low capillary density.
• Few mitochondria.
• Therefore less O2 supplied because they contain fewer oxidative enzymes.
b) Account for the difference in the speed of contraction between slow and fast twitch muscle fibre types.
Fast twitch muscle fibres are divided into two types, IIa and IIb. Identify the major functional characteristic between these sub groups.
In what sporting activities would the adaptation of fast twitch type IIb to type IIa fibres be relevant to a sportsperson?
6 marks
Answer
3 marks for the two types of fibre contract at different speeds because of:
• High glycogen content of fast twitch muscle fibres (low in slow twitch muscle fibres).
• High levels of phosphocreatine (PC) stores in fast twitch muscle fibres, (low levels in slow twitch muscle fibres).
• Fast twitch muscle fibres have well-developed sarcoplasmic reticulum, which means more Ca++ available and improved
transportation system for nutrients (e.g. glucose) compared to slow twitch fibres.
• Fast twitch muscle fibres are attached to large motor neurones with fast conductive velocity (small contractile time), compared to
smaller motor neurones for slow twitch muscle fibres.
2 marks for the two types, IIa and IIb, major functional characteristics between these subgroups:
• Type IIb fibres have a high capacity for glycolytic release of energy (known as FG).
• Type IIa fibres utilise both aerobic and anaerobic energy sources (known as FOG).
1 mark for conversion of type IIb to type IIa:
• Endurance-based events, such as middle distance running.
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SECTION A –
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ANSWERS TO QUESTIONS
7) Table 3.5 shows the percentage of slow twitch muscle fibres in two muscle groups of elite male (M) and female (F) athletes.
The percentage of fast twitch muscle fibre is calculated as the difference between 100% and the percentage of slow twitch
fibres.
(Data from research literature – source – ‘Essentials of Exercise Physiology’ 2e, McArdle, Katch and Katch)
Table 3.5 – percentage of slow twitch muscle fibres
athletic group
long distance runners
triathletes
sprinters
shot putters
shoulder
(deltoid)
60% (M)
calf
(gastrocnemius)
79% (M) 69% (F)
59% (M)
24% (M) 27% (F)
38% (M)
a) Compare and account for the differences in percentage distribution of slow twitch muscle fibres with respect to long distance runners and sprinters.
3 marks
Answer • Male long distance 79% to male sprinter 24% in calf muscle = difference of 55%.
• Female long distance 69% to male sprinter 27% in calf muscle= difference of 42%.
• Expected trend, could be the effects of aerobic and anaerobic training.
• And/or genetic potential.
b) Calculate the percentage of fast twitch muscle fibres for the long distance runners and sprinters.
Answer
• Long distance runner: males 21%, females 31%.
• Sprinters: males 76%, females 73%.
2 marks
c) Data collected for male triathletes shows a fairly even distribution of slow twitch muscle fibres across both muscle groups. Discuss three possible reasons for this trend.
3 marks
Answer
• Male triathletes would be expected to have a fairly high even distribution of ST fibres in both muscle groups.
• This trend could be explained due to aerobically stressing all muscle groups during the swimming, cycling and running parts of this
event, both in training and competition.
• On the other hand, it is not fully proved whether fibre type changes in response to training or whether these fibre type distributions
are genetically determined.
• And so the performances of these elite athletes may be a consequence of abnormal fibre type distribution.
d) For shot putters only the calf muscle is given a value in table 3.5. What percentage distribution of slow twitch muscle fibres would you expect in the deltoid muscle for shot putters? Give a reason to support your answer.2 marks
Answer
Expected percentage:
• Less than 30% of ST fibres.
Reason:
• Because shot putting is an anaerobic power event it relies on flat-out explosive action.
• Stressing the predominant use of FT muscle fibres in the deltoid muscle.
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APPLIED PHYSIOLOGY TO OPTIMISE PERFORMANCE
8) Identify the physiological adaptations that occur in muscle cells as a result of anaerobic training.
Answer
• Hypertrophy of the muscle or increased muscle mass.
• Increased stores of ATP.
• Increased PC stores.
• Greater levels of enzymes present:
• For example, lactate dehydrogenase (LDH).
• Increased rate of glycolysis.
• Extended energy production via the alactic energy pathway.
• Delayed OBLA or increased lactate threshold or increased anaerobic capacity.
• Greater tolerance of lactic acid through buffering.
• Reduced tendency to experience DOMS.
• Increased strength of skeletal muscle or connective tissue.
• Type IIa fibres adopting type IIb characteristics. 3 marks
9) Describe three changes that occur in muscle cells as a result of endurance training.
Answer
• Increase in muscle size or hypertrophy.
• Increase in myoglobin content.
• Increases number and size of mitochondria.
• Increases in oxidative enzymes.
• Therefore increase in activity of Kreb’s cycle.
• Increase in a-v-O2diff .
• Increase in glycogen or triglyceride stores.
• Fat stores more mobilised = glycogen sparing.
3 marks
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