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Summary Maximal Neuromuscular Performance
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Complete summary of the course maximal neuromuscular performance. Should be enough to pass the exam.
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Maximal Neuromuscular PerformanceLecture 1
Whatever question we want to answer, there is muscle-tendon properties
and muscle activation.
Fatigue and training are both of influence on the 3 curves (length-force,
force-velocity and stimulation frequency-force).
Torque = force * moment arm (r)
Higher lengths higher contribution of passive components
Surface electrodes – nerve branches – more stimulation – more mass
stimulated
During isovelocity less myosin heads are attached compared to an
isometric situation so less force is generated
Same force at different muscle lengths is possible around maximal force
Constant force = high frequency stimulated, seperate pulses = low
frequency stimulation. However, both are titanic contractions (sustained
muscle contractions, evoked when the motor nerve that innervates a
skeletal muscle emits action potentials at a very high rate)
Shortening induced force deficit
The decline in force after shortening the muscle is unexpected
This deficit develops during shortening and is consisting of an exponential
phase followed by a linear phase
However, the velocity is constant expect force to decline vertical
1
, Thumb angle
74
(degrees)
36
90
deficit
Force (N)
0
0 1 2 3 4 5
Time (s)
Exponential part; SEC recoil, part of the imposed length change on the
muscle tendon complex is taken up by recoil of the tendons that are
stretched during isometric force development
If we want to measure force by the CE at any specific imposed velocity we
should not measure early in the shortening phase, because the CE velocity
is different (SEC)
Linear part; if we are down the ascending limb of the lenght-force relation
this would also lead to a decline in force in the second phase
The longer the shortening phase linear decline continues
Thumb angle-force relation is relatively flat because the muscle consists of
more small bundles. Distribution of individual bundles determines the total
relation
Even if we are shortening down the ascending limb, the angle-force relation of
the thumb is very flat so something else must be going on.
Shorter length change smaller deficit; low shortening velocity highest
deficit
2
, Linear relationship angular displacement and force deficit
It is force itself rather than velocity which primarily determines force deficit
In the first part SEC dominates, at the fastest velocities SEC determines
the whole shortening phase)
The drop jumps in the article induce low frequency fatigue (LFF)
Linear relationship work(=force * displacement) and deficit
3
, In fatigue; less work is needed for a given deficit (linear relation moved to
the left)
Potential causes shortening induced force deficit
1 Accumulation of metabolites
Unlikely, because after a short period without stimulation,
force recovers immediately not possible for any metabolite
to recover
2 Reduced calcium affinity (tnC) during shortening
Calcium would be back to normal levels after longer sustained
isometric contraction after
shortening. However, there still was
a deficit.
3 Sarcomere length inhomogeneity
When sarcomeres end up at another length (normally A B, sarcomere
inhomogeneity at level C).
- However, with unloaded shortening sarcomere lengths along the fibre
remain more similar (less variation) and there is no shortening deficit.
- Slack length = the length below which the fiber exerts no passive
force: this is a short fiber lengths. The larger the variation in sarcomere
length along a fiber, the greater the force deficit.
- Below slack length inhomogeneity = unloaded shortening
inhomogeneity, and still there is a force deficit sarcomere
inhomogeneity partly contributes to the force deficit (at higher lengths
+ on the descending limb)
4 Stess induced inhibition of cross-bridge attachment
Shortening causes stretched parts of the actin filament to enter the overlap zone;
it is assumed that attachment of cross-bridges to the stretched part of actin is
inhibited in a stress-dependent way amount of force + magnitude of shortening
will influence the magnitude of force depression.
4
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