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Notes of week 2 of MNP
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notes of the second week of maximal neuromuscular performance. Slides with notes and comments from Jo.
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College 4Block 7 – Superimposed stimulation I
Given the importance of neuromuscular activation for force and power production
block 7 will be about the interpolated (or superimposed) twitch technique that is
used to check the level of voluntary activation (also called neural activation, or
muscle activation) during maximal contractions.
Slide 2
We have seen that to construct the concentric part of the force velocity (power)
relation, the method of using short activated contractions seemed to work in
fresh and fatigued muscle.
What would potentially be the biggest problem with short activated contractions?
Notes: Answer: Maximal muscle activation (intracellular Ca2+ concentrations),
particularly for high velocities (low forces), consequently only a short phase of
isometric torque development is needed before shortening has to start.
Therefore, during electrical stimulation very high frequencies are used (to make
intracellular Ca2+ concentrations as high as possible). However, during very vast
voluntary contractions, submaximal activation levels are very likely to occur, this
may also occur with electrical stimulation with a high frequency (up to 300Hz),
because still only a few pulses can be applied before the start of shortening.
Therefore, maximal saturation Ca2+ is hard to reach in a very short time.
Comments: We can use short activated contractions to construct valid force-
velocity relationship both in fresh and fatigue muscle.
Slide 3
Two modes: long- and short activated contractions
Notes: the right side of this figure shows enlargements of
the shortening pars of long and short activated
contractions depicted at the left side. At lower shortening
velocities (e.g. 229 deg/s at the top) during long activated
contractions (shortening starts from the isometric force
plateau), we can back-extrapolate forces from the linear
phase to the beginning of shortening, as a way of
correcting for shortening induced force deficit which
develops during shortening. This gives us a valid indication
of the maximal force the CE can generate at that
particular velocity. Note that the backwards extrapolated
forces are indeed similar to the force generated during
short activated contractions (cot and arrow in the top right side figure coincide).
At (very high) velocities of shortening (bottom 458 deg/s), backwards
extrapolation is not possible: because of the short duration, SEC shortening
dominates the entire shortening phase of long activated contractions. There is no
clear linear phase of force decline during long activated shortening at the higher
speeds, thus backwards extrapolation is not possible at high speeds. Thus, to
construct the fast-speed-part of the force velocity relationship we need
to use short activated contractions
Comments:
- At the time we start shortening (lower left figure) we have maximal
calcium saturation in the satellite cell of the muscle fiber, so there is
maximal stimulation of the muscle cell at long activation. However, with
short activation the start of activation is almost directly followed by the
start of shortening.
, - To get high amounts of calcium in the cell with short activation, there is a
higher frequency used. Although they tried to get as high calcium
saturation as possible, it is questionable whether calcium saturation and
muscle activation is maximal. Especially during voluntary activation it is
hard to reach maximal activation.
Slide 4
With electrical stimulation we can use very high stimulation frequencies (300Hz)
to make activation (intracellular [Ca 2+]) as high as possible in the short time
available during short activated dynamic contractions.
With volitional effort we can never be sure about the level of muscle activation.
Thus:
- Short activated contractions are the best option for constructing force
velocity relationship of a muscle(group), since we minimize the effects of
both SEC recoil and shortening induced force deficit.
- However, we have to be aware that, particularly during volitional effort,
muscle activation may determine force output to a great extent, especially
at high shortening speeds (with little time for activation).
- Consequently, any force-velocity relationship constructed with short
activated voluntary contractions may be a function of activation as well as
of muscle fiber contractile properties.
Comments: We are not certain that we have maximal activation during all of the
shortening velocities. This problem might even become larger with voluntary
activation.
Slide 5 + 6
With volitional effort we can never be sure about the level of muscle activation.
Muscle activation may very well be submaximal during short lasting contractions,
but also during isometric contractions during which subjects have several
seconds to increase [Ca2+], the maximal possible forces may not be attained due
to insufficient muscle activation.
Moreover, we already have seen in the previous block that eccentric force
production is greatly affected by neural activation.
We do want to have information about muscle activation e.g.
- Following training, did athletes become stronger/more powerful because of
increased muscle activation or because their muscles (CE) became
stronger and/or faster?
- Does the volleyball coach want increased muscle mass for his players or
improve activation? (this may be even more relevant for road cyclists that
have to compete in the mountains)
N.B. similar questions can be asked in rehabilitation of for instance stroke
patients.
Slide 7 – voluntary activation
We have seen that it can be questioned if (all) subjects are able to maximally
activate their muscles during voluntary contractions when there is only a short
time available, as is the case during what we called ‘short-activated’ contractions
that we for example used to construct the concentric part of the force-velocity
relationship.
We also have strong indications that voluntary activation may be a limiting factor
during maximal eccentric contraptions with larger muscles when very high forces
can be reached.
,Comments
- When calcium levels don’t reach maximal saturation levels, the output of
the muscle during voluntary effort will be lower than the theoretical
maximal output that the muscle is capable of.
- During eccentric contractions we get inhibiting effects of the Golgi tendon
organs, and than it might be difficult to maximally drive our alfa
motoneurons, and submaximal calcium levels might be the result and the
force that we measure is lower than the theoretical maximal force our
muscles can deliver.
Slide 8 + 9 (in addition to slide 5+6)
Voluntary activation not only is an important factor in relation to the construction
of the force-velocity relationship. Neural activation always is important, as it
determines force (power) output of our skeletal muscles.
Information on muscle activation during voluntary effort is usually
obtained with EMG and/or with ‘the interpolated twitch’ technique
Slide 12
Neuromuscular activation, particularly during volitional effort, may compromise
force output, especially at high movement execution speeds with shorter
duration and therefore little time to increase intracellular Ca 2+ to maximal
saturating levels.
What evidence is there that muscle activation may limit force production?
What methods do we have to measure voluntary muscle activation?
Let us start with isometric contractions of several seconds duration (there is
plenty of time for the subject to build up intracellular Ca 2+.
Slide 13
Interpolated ‘twitch’ technique (ITT) or
superimposed ‘twitch’ stimulation, actually is a
doublet (and it the papers often triplets are
applied) why?
VA (%) = (1 – superimposed ‘twitch’
torque / resting ‘twitch’ torque) * 100%
VA = (1-6/60) &100% = 90%, which suggests
that 90% of MTC is used during maximal voluntary effort
Notes: with doublets (triplets) forces are higher which increases signal to noise
ratio.
MT(G)C = maximal torque (generating) capacity
Comments
- Maximally extend the leg (dynamometer is at a fixed angle). The isometric
force is wobbly (neural activation is changing). With stimulation you get a
smooth line because action potentials are arriving at a regular intervals so
calcium levels are high continuously. During voluntary activation this
changes.
- It checks whether the calcium levels in the muscle are maximal.
- The green lines at 3000ms are extra pulses with short interval, it released
extra calcium in the muscle, it leads to a small increase of force on top of
the voluntary force.
o The force just before the impulses was not maximal, because the
force could increase after the impulses.
- This is compared to the same stimulated in the resting muscle resting
‘twitch’
, Slide 15 – Superimposed triplet stimulation
Comments
- The equation is based on the assumption that there
is a linear relationship between the super imposed
torque and the volitional torque
o Superimposed torque the extra force that
you get after the doublet/triplet stimulation.
- Voluntary torque at low level with a
superimposed stimulation it leads to a high
additional force. With an increase of the level of
voluntary contraction, the level of the
superimposed stimulation decreases.
- When the calcium levels are maximal and thus the
volitional torque cannot be increased after a superimposed stimulation, the
MVC = MTC.
Slide 16 +17
The ITT assumes there is a linear relation between voluntary
effort and superimposed torque (pink) which probably is not
the case (blue) (see also Kooistra et al., 2007)
Notes: MTC = maximal torque capacity, also called MTGC
(maximal torque generating capacity (or MFGC, maximal
force generating capacity or MFC, maximal force capacity)
Different Journals and or authors use different terms to
indicate the same. Quite a few subjects (even the untrained)
are able to activate their muscles close to maximal (blue
diamonds). Whenever this is the case we observe that the
relation between voluntary and superimposed torque
response becomes ‘non-linear’ at the very high forces. If your blue subject (like
the pink one) would not have been able to generate voluntary moments above
133Nm, we would have predicted that his MTC would be about 160N. however,
when the blue subjects exerts 160Nm, there still is a superimposed response (of
9%). Even at 200Nm there still is a small superimposed response indicating that
voluntary activation was just under 100%. Hence, MTC is considerably higher
than 160Nm (in this case about 210Nm!).
Comments
- The pink subject was asked to voluntary maximally activate the muscle,
but couldn’t reach a higher torque than 133Nm. With the linear
relationship this would be 83% of maximal voluntary activation, so 17% of
the muscle force is not used/cannot be recruited by this subject.
o In 90-95% of the subjects there is a linear relationship
- However, there are subjects with a exponential relationship, because these
subjects are able to reach a higher volitional activation. Even with the
highest voluntary activation, there was still a hump (increase) in force after
the superimposed stimulation.
- There is a limitation with the superimposed technique, but this only
emerges when we have subject with very high capacity of voluntary
activation of the muscles.
- Overestimate voluntary activation or underestimate the torque capacity
capacity when using the linear relationship, while the subject would
actually have the exponential relationship.
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