U1L20 – MECHANICS AND ENERGETICS OF CONTRACTION
TWITCHES AND TETANI
A single action potential produces a twitch contraction. However we don’t usually move our muscles
in little twitches and jerks, we do it in low, smooth contractions – tetani.
FORCE – FREQUENCY RELATION
EXPERIMENT – Abductor pollicis (supplied by ulna nerve)
* Put electrodes on arm
* Stimulate ulna nerve
*Get abductor to contract
* Measure force with force transducer
* Hand in clamp – clench hand as hard as can for a short amount of time = maximum voluntary
contraction (with the brain driving the muscle)
Can use a stimulator to drive the muscle:
 Brief pulses trigger just one action potential – tetani
o Stimulating infrequently (1/sec)  separate little twitches are seen; brief contractions,
turning on in 50ms, turning off in 100ms
 Turning up to 10Hz – unfused tetanus
o APs are travelling down at a rate of 10/s  see little blips each time an AP hits the
muscle, gives a lot more force and doesn’t relax completely. Can see a ripple at the
same frequency you’re stimulating but it’s not relaxing completely
 Turn up to 40Hz – fused tetanus
o Much larger contraction, no ripples on top at all. This is the way we normally contract
our muscles. When we lift a heavy weight, we don’t see any oscillations in the
muscle, you just get a smooth force like this.
 100 Hz, makes essentially no difference to the force, saturated something in the muscle.
Presumably now all the cross bridges are doing their thing and you can’t get any more force
out of the muscle.
This is an important characteristic of any muscle that tells you something about how it responds to
action potentials coming from the brain.
Conclusions about tetani:
 Maximum voluntary contraction (MVC) is very similar/almost identical to the maximum
stimulated contraction
o There are only a finite number of muscle fibres and cross-bridges in a muscle, and
it’s not really surprising that you could stimulate them all. If the brain couldn’t activate
them all, then what would be the point in having them?
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Maximum force occurs at about 40 Hz using an external stimulator. If one measures the rate
of APs produced by the brain during an MVC it is similar
o The brain doesn’t waste AP’s, it only sends the optimal number down to get maximal
force
The brain has two main mechanisms for regulating force;
o Fraction of total number of motor units activated
 Can go from smallest motor unit up to all, including the biggest
o Firing rate of motor neurons
At maximal activation all muscles produce roughly similar forces per cross-sectional area (2 –
4 kg/cm2)
o Men and women and babies and adults are just as strong as each other. The only
difference is the cross-sectional area of the muscle. Bigger area = greater force.
Evolution has packed roughly the same number of cross-bridges into the same unit
area, and therefore per unit area all muscles give roughly the same force (there are
minor variations, but they are not important at this stage)
LENGTH – TENSION RELATION
For intact muscle –
Experiment is often done on a muscle taken out of a frog and isolated.
Tendon at one held is fixed, and the other is moveable (i.e. can be
stretched). Can get tetani from this isolated muscle.
The muscle can be stretched by moving one end. When you do that,
in general the resting tension goes up a little, and the developed
tension when stimulated goes up a lot more  muscles care about
what length they’re at.
At the fourth tension, the developed force is actually smaller than previously. When we stimulate
again, we can see a time dependant component to the resting force – it is very stiff immediately, but
then slowly relaxes and the developed force gets smaller still.
This information can be plotted 3 ways and they all tell us something different about muscle properties
 Force of resting tension – goes smoothly up and steeper and steeper as you stretch more
and more. This is to protect muscles. You don’t want them getting ripped apart, so they have
lots of connective tissue in them which protects them from big stretches. Resting force comes
from the external connective tissue surrounding the muscle, and titin (elastic protein inside
the muscle)
 Total force has a complex shape
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Subtracting resting from peak – get just the developed – very characteristic and interesting
shape (goes up to max then falls all the way down to zero). This is observed because
contraction force depends on thick and thin filaments and cross-bridge
In skeletal muscle –
The dimensions of elements contributing to sarcomere length:
We already know that force is only produced where thin and thick filaments overlap, and the crossbridges can attach. It stands to reason that if you stretch a muscle out to the point where there is no
overlap between thin and thick filaments - which would happen at 3.6microns - you would expect to
see no force whatsoever.
As you make the overlap more and more and more, the force increases until there is perfect overlap
between the thin and thick filaments, ignoring the bare zone. It goes up uniformly until 2.3microns. It
stops rising at exactly 2.3 and plateaus, because even though we’re getting more and more overlap,
the ends of the thin filaments are in the bare zone, so it makes no difference. Force is proportional to
the area of interaction between cross bridges and thin filaments (i.e. overlap)
At 1.6microns the thick filaments bump into the Z line – there is a linear decline from there down.
The intermediate region does not correspond perfectly to any degree of overlap.
The range in muscle length is 4 fold, but your muscles only change two fold at the very most, possibly
even only 1.5. Evolution has arranged so that muscles constantly work at or near the peak of their
length-tension curve – there’s no point having all these cross-bridges if they’re not going to be used.
FORCE – VELOCITY RELATION
Muscles can contract –
 Isometrically (at constant length) - Producing as much as force as possible in those
conditions, but muscle staying at same length
o Trying to lift something up that’s unmoveable – bicep is contracting but cannot
shorten, because bone is attached to muscle
 Isotonically (at constant force and shortening at a constant velocity)
o Try and pick up a light force, can lift it up very quickly. Heavier, lift slower.
 Eccentrically (when a contracting muscle is stretched by a force greater than it can resist) –
o If the muscle was contracting isometrically, then suddenly had a 100kg weight
applied; the muscle would be stretched, even though trying to resist it. This is what
happens when walking down a mountain – you use your muscles to decelerate
yourself, so stretching contracting muscles
When v = 0, it is an isometric contraction, usually called P0.
As you start shortening the muscle, force goes down in this characteristic curved way
When F=0 (lightest load) get very fastest velocity – typically about 6 times the length of the muscle
per second
You can apply a weight to the muscle so great that it is stretched, so as well as having velocity in the
shortening direction it also has velocity in the stretching direction. Very high forces are developed
when you stretch muscles.
Power = work/time = force x velocity
* Note that this is zero at both Vmax (because P=0) and P0 (because V = 0)
Power output peaks at intermediate force. When muscles are going too fast or too slow there is
virtually no power. To actually do any work, muscles need to not just produce force, but use it.
It’s the way the cross-bridges work that underlies the force-velocity relationship
Attached – no force
Power stroke – powered by ATP hydrolysis, where head twists round, stretches spring in arm and
produces force
Detached – been through power stroke and waiting for another suitable actin site to appear within its
range of motion
* Only power stroke actually produces force
When a muscle is isometric most of the cross-bridges are attached.
 4/7 in power stroke – 4/7ths of maximum (though it’s not actually feasible for all of the crossbridges to be attached at any one time)
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Pretty realistic of a tetanus – about 50% of cross bridges are in the right section, therefore in
power stroke & contributing to force.
At Vmax, because thick and thin filaments are moving so fast, most have had to detach, and are
having to wait for another site to come by to which they can attach
 Because things are rushing past, quite a lot are in that intermediate state where they’re
detached.
 Shortening velocity will be fast because each can immediately do a power stroke and move
shortening on a bit
 Going to be very little force, because only 1 is in power stroke, in contrast to 4
 In very fast movement, only a few cross bridges attached and only a little force is produced.
THE WAY MUSCLES CONSUME ATP
ATP is the ultimate source of energy in all cells.
It is consumed by a number of reactions. Most important are –
 Cross bridge cycle - Actin/myosin ATPase
o 1 ATP/CROSSBRIDGE cycle
 SR Ca2+ pump, once the Ca2+ has been released into the myoplasm to start contraction,
pumps it back into SR to start relaxation
o 1 ATP/2 Ca2+ pumped
+
 Na pump (Na/K ATPase); keeps all the ions right after action potentials
o 1 ATP/3 Na+ out and 2 K+ in
Resting ATP consumption is very low and mainly due to the Na+ pump.
During maximal contraction ATP consumption increases roughly 50-fold –
 Cross bridge cycling consumes roughly 2/3 of the additional consumption
 SR Ca2+ pump consumes roughly 1/3 of the additional consumption
This is why we get hot and have a huge increases in blood flow when we exercise, because
everything has to be geared up to meet this enormous increase in the metabolic rate of muscles.
HOW MUSCLES RE-SYNTHESISE ATP
The ATP concentration in muscle cells isn’t very high – only 6mM.
The maximum rate of ATP consumption is 2mM/s (when cross bridges and SR Ca pump are working
at maximal rate).
Therefore, if not replenished, all ATP would be consumed in 3s (and the muscle would go into rigor) –
need ATP for cross-bridges to detach, and if they don’t detach muscles will rip themselves apart.
This does not happen because there are various sources of ATP –
Phosphocreatine rephosphorylates ADP. Each produces an ATP.
* Raises the activity time to 18 seconds
Glycogen is a store of carbohydrates inside muscles. The very important thing is that it can broken
down in two quite different ways.
When muscles contract maximally, they get very hard, and so stiff they stop blood supply – at the very
moment when it needs the most, it can get no blood or oxygen supply. Muscles have been designed
with pathways for energy that don’t require oxygen.
Glycogen can be broken down without oxygen, producing lactic acid, and 3 ATP. There is enough
glycogen in a typical muscle to last 2 -3 minutes – total of 2.18 to 3.18 without oxygen (all the
anaerobic pathways).
If you put a cuff on your arm so there is no blood supply, your arm will last a few minutes before it
runs out of all energy, and it will get very sore, and you will stop the experiment long before rigor
happens.
This is the most important.
Glycogen can be broken down WITH oxygen present (like with running – muscles contract, no blood
supply; relax – big rush of blood supply; get oxygen when muscles relax).
It is much, much more efficient to use glycogen aerobically (39ATP instead of 3). This produces
enough energy even when the muscles are working maximally for a couple of hours.
Understanding fuel usage is central to understanding why muscles fatigue, and particularly why
muscles get weaker when you use them intensively.