Transcript Provide anatomy and physiology advice to clients

Provide anatomy and
physiology advice to clients
Muscle action part 2
Muscle action
• In our previous session we looked at how
muscles actually contract and we saw that
contraction was generated by a nerve impulse,
which in turn liberated acetyl choline. That had
the effect of liberating calcium throughout the
muscle cell, which in turn caused a change in
the conformation of proteins that made up the
filaments of muscle. Finally the muscle
contracted when energy from ATP was supplied.
This slide will remind you about energy sources
that supply ATP for muscle contraction.
Muscular contraction
• Each muscle has at least one nerve attached to it and
where the nerve enters the muscle it branches into a
number of axon terminals, which all join to a single fibre.
We saw this in our last session. A motor neuron and all
the muscle fibres that it innervates is called a motor unit.
When a motor neuron fires, all the fibres that innervates
respond by contracting. Large weight bearing muscles
have large motor units. This means that there may be
several hundred muscle fibres per motor unit. Muscles
that exert fine control, such as moving the fingers have
small motor units with only a few fibres. The muscle
fibres in a single motor unit are spread throughout the
muscle, so that a single motor unit stimulation causes a
weak contraction of the entire muscle. You can see a
motor unit in this slide.
The muscle twitch
• The response to a brief neural stimulus is called a muscle twitch.
The muscle contracts briefly and then relaxes. Of course, the twitch
can be weak or strong, and this depends on the number of motor
units that are activated. Our muscles can contract with different
frequencies and by changing the strength of each stimulus.
Successive contractions in a muscle can add to the contraction size
by reducing the relaxation time between twitches. So this adds to
the strength of the contraction. But this cannot continue indefinitely
and muscle fatigue soon sets in. In this case glycolysis stops
because NADH builds up. So, pyruvate is converted to lactate and
NAD is restored. Lactate then diffuses out of the cells and goes
back to the liver, where it is converted back to glucose for more
contractions. Muscle contractions are also made stronger by the
involvement of several motor units in contraction.
Muscle fibres
• Muscle fibres can have hundreds of nuclei and during
foetal growth progenitor cells form myoblasts that
become myofibrils. Genes control the production of
isoforms of these fibres which give rise to subtypes of
fibres such as slow twitch and fast twitch.
• It’s actually variations in the myosin protein that gives
muscle its different contraction times. Fast twitch
muscles are sub divided into fast twitch oxidative, which
means that their primary energy source is aerobic and
fast twitch glycolytic. Here their primary energy source is
anaerobic glycolysis. Slow twitch fibres primarily use
aerobic energy sources. This table gives information
about different isoforms of muscle fibres. Some people
believe that different types of training can stimulate
changes in muscle types but that has yet to be proven.
Slow (Type I Fibres)
• Slow twitch fibres have rich vascularisation (very
good blood supply) and abundant myoglobin.
They use aerobic energy for ATP production.
You can see in this slide the dark spots that
represent NADH. The energy from oxidation of
glucose now resides on NADH prior to it being
shunted into the oxidative phosphorylation
chain. These muscle fibres are slow to contract
and slow to fatigue. We use them in endurance
exercise and in simply maintaining our upright
position.
Fast Fibres (Type II)
• This slide shows abundance of glycogen which
is associated with anaerobic or fast twitch. The
red fibres are Type I, others are Type IIA and
Type IIB. Fast twitch fibres Type IIA are quick to
contract and have reasonable fatigue resistance.
They also use aerobic energy supplies. We use
these in walking and sprinting. On the other
hand fast twitch Type IIB muscles use stored
glycogen for their energy supplies. They are fast
to twitch and fast to fatigue. They are the
muscles that we use in power movements such
as lifting weights.
Energy used in exercise
• We use all our muscles fibres in exercise or sports.
Generally, the stored ATP and CP energy lasts about 15
seconds and is used in power. It can be rebuilt in about
60 seconds. The other anaerobic system system can be
used in power for about 60 seconds and then it needs
about 60 minutes to be replenished. We can actually
train our muscles to store more glycogen for both power
and aerobic energy. This is done by eating/drinking
carbohydrate such as a sports drink for up to 30 minutes
after exercise. This trains our muscles to store more
glycogen. Good marathon runners have about 2 hours
worth of glycogen stored.
Muscle Adaptation
• Any regular exercise changes our muscles. We build up
a greater capillary network within the muscle and the
number of mitochondria increase in number. Remember
that mitochondria provide us with aerobic ATP. These
changes are more dramatic in slow twitch fibres.
• When we engage in high intensity resistance exercise,
such as weight lifting, then muscles increase in bulk and
generally the Type IIB increase their fibre diameter. It is
thought that the fibres add more glycogen, water and
connective tissue when high intensity resistance training
is used. There also seems to be more connective tissue
laid down between the fibres. Note that the number of
fibres probably do not increase, only their diameter.
Muscle Strength
• We all know that some people are stronger than
others. What gives them their strength is a
combination of muscle force, contraction velocity
and duration of contraction. The force of
contraction is related to the diameter of the
muscle fibres and the time it takes to make the
muscle contract. The velocity of contraction is
also related to the contraction time and the type
of fibre present. We’ve already seen the
difference between fast twitch and slow twitch
muscles. And we’ve looked at the duration of
contractions.
Muscle fatigue
• Muscles have glycogen stores that allow them to
contract for up to several hours and after that they can
call on fat supplies to provide ATP. Now during
anaerobic contraction oxygen is not required, but an
oxygen debt results because oxygen still must be used
to deal with the by products of anaerobic metabolism.
So if you run 100 metres very quickly then you will need
oxygen when you’ve finished. The high levels of lactic
acid in your blood trigger the respiratory centre of your
brain to make you breathe very deeply and gulp air. As
you train and become fitter your oxygen carrying
capacity improves and you therefore “puff” less after
anaerobic exercise.
Activity
• Prepare a presentation that describes to clients
the physiology behind fast and slow twitch
muscle fibres.
• As a group discuss answers to the following
questions:
• Can a person retain the strength that they had
as a youth?
• Why is it said that you should never stretch
before you are warmed up?
• Why is it that some people are stronger than
others yet seem the same size and weight?