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Learning Objective:
To be able to explain how muscle fibres
are recruited in groups according to
the movement required.
MUSCLE TENSION
Partial relaxation
TIME
Another stimulus is applied before the motor unit (muscle)
relaxes completely, resulting in more tension.
When stimuli are at a high frequency there is no time for relaxation
between impulses so muscle tension is maintained.
• A motor unit consists of a motor neurone and all of the fibres that are supplied by
that motor unit.
• A neurone does not activate a single muscle fibre, but a group of muscle fibres (the
‘all or none law’).
• All the muscle fibres in a motor unit will be the same type (e.g Type IIa).
• Therefore it is more accurate to talk of fast and slow twitch motor units rather than
muscle fibres.
• The number of muscle fibres supplied by a neurone in a motor unit is dependent on
the degree of control required by the muscle.
• In muscles that require a high degree of fine control there may be as few as 5
muscle fibres supplied by the neurone e.g. eyelids.
• In large muscles that do not require a high degree of control there may be 1000
muscle fibres supplied by a neurone in a motor unit e.g. the hamstrings
Motor units vary in size according to how many muscle fibres are served by the
neurone in that unit. A larger motor unit, with more fibres, will create a greater
force than a smaller motor unit, with less fibres.
• The response of a motor unit follows the ‘all or nothing law’.
• However, the strength of the response of the whole muscle is
determined by the number of motor units involved.
• For greater force the brain recruits more motor units.
• This is called multiple unit summation.
The strength of contraction performed by a muscle is dependent on how many of its motor
units it recruits. The diagram below demonstrates a muscle in latent state as no motor units
are being recruited, building up to all of the motor units within the muscle being recruited to
create maximal force of contraction.
The recruitment of motor units within a muscle is rotated to avoid fatigue.
MUSCLE TENSION
Some motor units are used
(BLUE), while the other
remain unused (GREEN &
RED).
Motor units are rotated so different
units (GREEN) take over the work.
Those previously working (BLUE) can
rest while others remain unused
(RED).
Motor units are rotated again
(RED). Those previously working
(GREEN & BLUE) rest.
TIME
A whole muscle (e.g anterior deltoid) with many motor units.
Motor units vary in size according to the number of fibers attached to the motor
neurone.
(The different colours shown are only to illustrate “special summation” below)
All of the fibres within a motor unit are of the same type (e.g type 1, 2a or 2x).
A fast and slow motor unit of the same size will produce different contractions.
Greater force shown by higher peak, and faster
speed of contraction shown by a more rapid
rise following impulse.
FORCE
FORCE
Less force shown by lower peak, and slower
speed of contraction shown by a more gradual
rise following impulse.
TIME
A slow twitch motor unit produces less force
than a fast twitch motor unit the same size.
The slow twitch is also slower to produce the
force.
TIME
• How do you know how much force is required for a 30m pass in football?
Long term memory – you might have got it wrong first time (and many more times)
but you store that experience and each time you perform the skill again you refer
to past experience.
• What about during a contraction? Where do we get feedback about the
contraction being performed?
Muscle spindles detect change in muscle length over time and, therefore, speed of
contraction. This information is passed to the brain for it to alter the signals being
sent if necessary.
An example….
1. Muscle Spindles detect
length of muscle changing
over time, realise it isn’t
produce enough force.
2. This message is relayed to
the brain.
3. The brain alters the
message being sent to
increase force by either:
a. Increase the number of
units being recruited.
b. Recruit larger motor units.
c.
Recruit motor units of a
different (fibre) type.
• O. Size of motor units recruited/larger for greater contraction/smaller for less powerful
• P. Multiple unit summation
• Q. Number of units recruited/more or less
• R. Type of muscle fibre size determines force of contraction/fast twitch fibres rather than slow
twitch fibres for more powerful contractions
• S. All or none law/All or nothing law/or explanation/action potential or threshold is reached /
contraction occurs in all of the fibres in the motor unit
•
•
•
•
•
•
•
T. Wave summation
U. frequency of impulse/innervations
V. Motor unit (muscle) unable to relax/increase the force
W. Tetanus/tetanic for powerful contraction
X. Muscle spindles detect changes in muscle length/speed of contraction
Y. Send information to brain/CNS
Z. Compares information to long term memory to ensure correct force applied/past
experiences
• AA.Spatial summation
• BB.rotating the frequency of the impulse to motor units to delay fatigue/some work while
others rest or equiv.
Name
Speed of contraction
Force of contraction
Fatigue
Structure
Type I
Name
Type IIa
Type IIx
Fast Oxidative
Glycolytic (FOG)
Fast Glycolytic
(FG)
Speed of contraction
Slow
Fast
Fastest
Force of contraction
Less
High
Highest
Fatigue
Slow to fatigue
Quick to fatigue
Quickest to fatigue
Structure
Smaller in size,
more blood
capillaries for
carrying oxygen.
Bigger in size – more myosin filaments
which are also thicker.
More developed sarcoplasmic reticulum –
quicker release of calcium.
Different myosin ATPase – releases
energy quicker for the powerstroke.