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Notes of week 1 of MNP
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Notes of first week lectures of MNP. Online Lectures with notes and comments.
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Maximal Neuromuscular Performance – week 1College 1
Block 1 – Introduction
Introduction to the concept of power and the importance of having basic
knowledge of fundamental muscle contractile properties to tackle scientific
problems and/or to answer more questions from practice (sport and
rehabilitation) that often involve more complex (multi-joint) movements.
Preparation
- How (which measurements have to be done) are they constructed?
- What is the meaning of these relationships (what do they tell us)?
Force-velocity relation
- As the velocity of muscle shortening increases, so the force sustained by
the muscle rapidly diminishes, eventually reaching a velocity at which
force can no longer be sustained at all; this is the maximum velocity of
shortening (Vmax). The force at zero velocity is the isometric force (referred
as F0).
o The faster the movement, the fewer cross-bridges that will be
attached, the shorter the time during which the myosin head is in
the vicinity of an actin binding site and consequently the smaller the
proportion of cross-bridges that will manage to attach.
o Those cross-bridges that are attached will, on average, be less
stretched and thus generate less force
o A proportion of cross-bridges will be carried into positions where
they oppose movement and thereby reduce the total force.
- Factors affecting the force and power-velocity relationship
o Role of tendons and other series elastic elements during movement
o Effects of angle of pennation and lever rations on muscle speed and
force
o Level of activation
,Length-force relation
- Force decreases relatively rapidly on the left-hand side of the relationship.
This is because at very short lengths the thin filaments start to overlap one
another in the center of the sarcomere and the thick filaments come into
contact with the Z lines.
- The force is proportional to the extent of overlap of the actin and myosin
filaments. It is easy to see that the greater the overlap the larger the
number of myosin heads that can react with actin, and that the force
generated is the sum of all the small forces produced by individual cross-
bridges.
- Factors affecting the length-force relationship:
o Changes in fiber and sarcomere length during isometric contractions
The sarcomeres at the ends of the fiber being shorter than
those in the middle. If the muscle is set to a long length, so
that all sarcomeres on the right-hand arm of the force-length
relationship, the sarcomeres at the ends of the fiber will be
shorter and thus stronger than most.
o Level of activation
As the frequency, and thus the force, increases, so the length
at which maximum force is obtained moves to
shorter muscle lengths.
o Shortening deactivation and lengthening activation
o Muscle morphology
o Angle of pennation
o Joints and levers
Stimulation frequency-force relation
- With increasing frequency of stimulation, the isometric force
increases as the intracellular calcium level increases.
- As the frequency, and thus the force, increases, so the length
at which maximum force is obtained moves to shorter muscle lengths.
Slide 5
An exercise physiologist is approached by a volleyball coach for regular testing of
his athletes for ‘explosive leg extension power’. He knows from his colleague
working in Italy that there the players are regularly tested to determine the force
velocity relationship (and power) curves of their knee extensors. In addition those
players receive training advice based upon the test outcome. His questions are:
1. ’Can you do this too?’ and
, 2. ‘Can you advise me on the explosive strength of my team?’ Thus he also
wants training advice based upon the test results.
How would you tackle this problem? In the following there are several of the
questions you have to answer before you can come up with a reasonable
solution:
- Which factors determine leg extension power? (start with this question)
o Force
o Angle of muscle fibers
- How would you test for explosive power?
o Jumping high
- How can you control these factors during your measurements?
o Starting from the same angle every time
o Don’t use the arms to swing
- How can you determine which of these factors is limiting explosive power
of a specific athlete?
Slide 7 – Padulo approach
Where is the curvi-linearity of the force-velocity relationship seen in
isolated muscles?
Notes: They calculated average force (force pressure plate) and
velocity (linear encoder = displacement measurement) during maximal
pushing using a range of weights.
Comments:
- Squat exercise with different loads or leg press exercise with
different loads
- It shows a linear relationship instead of a curvi-linear relationship
and there is a huge difference between the squat and leg press
exercise while these are the same movements.
- Power-velocity relationship is higher in squatting compared to
leg press.
Slide 8 – well trained strength athlete
Notes: This is an example of the force V
velocity during single leg-press of a well 0
(Po
trained strength athlete obtained by
/ser
Ve
mw
cit
lo
)(
bachelor students during their research
y
project. In this case the displacement and
the first (velocity, v=ds/dt) and second
time derivative (acceleration=dv/dt) of the F
stack of weights (mass, m) of the leg press F 0 F
device (that had to be accelerated against
gravity) were calculated as: F = m x (9.81 o o
m/s2 + dv/dt). Seven different maximal
attempts with different weights were made. r r
Extrapolation (on the left) will give you the c c
theoretical maximal isometric force (F0) and
maximal shortening velocity (v0) for this e e
movement.
Note that here force is plotted on the
horizontal axis because it is the
independent variable and the resulting
( (
N N
) )
, velocity on the y-axis (which is different from the Padulo paper, but otherwise the
resulting relationships are comparable).
Slide 9 – untrained athlete
Notes: This is a similar figure but know for a
untrained woman. With untrained subjects the
measurements always tend to be less consistent
(more spread of the data points around the
fitted lines). Compared to the trained male
subject, it is the force capacity (and not as much
the velocity capacity) which is much lower (F 0 is
less than 50% of the trained male in the
previous slide).
Comments:
- Still a clear linear relationship
- But the force and power are much lower than the male subject from the
previous slide
Slide 10
Notes: A common strategy to compare
and depict data of subjects that have
widely different absolute values, is to
normalize the data: for each subject
maximal force (F0) is set to 1 (or
100%). This is done for the leg press
data of 8 men and 11 women on the
left.
Similary so for the lat pull on the right.
Note that by definition Pmax is
obtained at 50% Fmax
Comments:
- Eventhough it is a different exercise leg press/lat pull, the shape of the
relations are the same
Slide 12
Nowadays we use 3D Inertial Measurements Units (IMUs) to measure linear
accelerations, angular velocities and the earth magnetic field. Here, one is
attached to the shoe of an athlete. IMUs can be used to access linear
accelerations (and thereby velocities) for power determination, but also to
investigate running kinematics (ground contact times, stride frequencies, joint
angles and velocities).
Slide 13 – Question
The Padulo et al. Int J Sports Med. 2017, is a typical example of the common
finding that the force velocity relationship is usually obtained during multi-joint
movements is linear.
How come that the force velocity relation that we obtained in vivo (linear) often is
so very different from what we read in text-books on skeletal muscle function
(hyperbolic relationship)?
Notes: Another obvious question would be: why, when we change the movement
(slightly) e.g. from squat to leg-press, do we end up with a completely different
relationship between force and velocity? (read what Padulo et al. have to say
about this in their discussion).
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