Kinesiology 201 Solutions
Muscle Mechanics
Tony Leyland
School of Kinesiology
Simon Fraser University
1. The stabilizing component of a muscle force is a component of muscle force that
acts parallel (non-rotary force) to the long axis of the bone [1] and which tends to
push the articulating surfaces together. [1]
e
tiv
c
A
Resting Length
Tension
100%
Pa
ssi
ve
Te
nsi
on
Diagram = 4 marks
Total Tension
ion
ns
Te
2. The active tension is generated by the
cross-bridges (you should look up the
sliding filament theory of muscular
contraction if do not understand this
answer). [2] The passive tension is
provided by the cell walls and
connective tissue arranged parallel to
the
muscle
fibres
(sarcolemma,
epimysium,
perimysium
and
endomysium).
This connective tissue
helps reduce the likelihood of muscle
damage when the muscle is being pulled
by an external force. [2]
170%
Muscle Length as Percentage of Resting Length
3a
Force
Diagrams = 2 marks each
3b
Lengthening
Lengthening
Velocity
Power
Velocity
Shortening
Shortening
c) The muscle is working eccentrically [only 1 mark if this is all that was mentioned]
Power is a scalar but the term “negative power’ just means that the muscle is
working against an external force (eccentrically) to reduce the total energy of the
system. [3]
1
4.
The right upper chest muscles (pectoralis major and minor, serratus anterior), right
anterior deltoid muscle, biceps brachii, and left quadriceps. To a lesser extent the
rights side abdominal muscles also. [2]
For the chest muscles discussed, the rest of the body driving ahead of the arm initiates
the cycle. If you look at frame (l) you can see that the arm is basically "left-behind" as
the right leg and hips have driven forward. Basically, the inertia of the upper arm/javelin
system (Newton's 1st law) is used to stretch these muscles out as the rest of the body
advances.
For the quadriceps group, the stretch is caused by the eccentric work of the muscles in
slowing the downward velocity of the body when the left foot plants {frame (k)}. This is
similar to the stretch shortening cycle initiated in plyometric training. [3]
b) The benefits of the "stretch-shortening cycle" are an increase in positive work done
by the muscle due to: [4 x 1 mark]
Ø return of stored energy from passive elastic structures within the muscle (crossbridges and connective tissue (70-75% of increase?)
Ø prior activation (time to develop force reduced)
Ø initial increased force potentiation (eccentric contraction)
Ø reflex augmentation (stretch reflex)
The type of diagrams you could have used if you required would be ones like the force
velocity curve of human skeletal muscle (above) that shows the benefit of eccentric
contractions in terms of increased force output. The time-activation & electromechanical
delay graphs would also highlight the finite time it takes to develop tension when
activating a muscle from rest. This is shown by the force-time profile graph. [4]
E l e c t r o m e c h a n ical D e l a y
Myoelectrical Activity
Force
Stimulus Response of CC
Force-Time Profile
Force
Tension
Threshold
Stimulus Strength
Time
2
5.
The area is 15 cm2 and the strength of
this muscle is 60 N/cm2 so the total force
produced is 60 x 15 = 900 N. [2] As can be
seen from the diagram opposite the tension in
the tendon in this case is:
900 x cos15 = 900 x 0.966 = 869 N [2]
Ffibres
Ftendon
15o
Ftendon = Ffibres x cos 15o
6. From lecture slides.
Ø Electro-mechanical delay
Ø Length-Tension Relationship
Ø Velocity-Tension Relationship (Muscular Power)
Ø Prior Contraction History
Ø Recruitment, Frequency, Synchronization (activation level)
Ø Elastic Elastic Energy (storage and recoil)
Ø Muscle Temperature
Ø Muscle Fibre Type
Ø Angle of pennation 6 x 1 mark
7. In this question all of the points in question 6 will affect force and in addition the:
Ø insertion point, and
Ø line of action of muscle and joint angle …..will affect muscle torque.
In other words force x force arm).
6 x 1 mark for muscle factors and 2 marks for moment are factors
8. Not necessarily (and almost certainly not).
a. Tendon insertion points could (would?) be different hence you need different
forces to get same torque.
b. Length of the forearm and hands could be different, if longer the 20kg produces
more torque that must be opposed by the muscle.
c. Mass of the forearms and hands may be different which would have a slight
affect on torque as opposing torque required is due to segment mass as well as
load.
d. Distribution of mass in limb could be different (hence different C of g location).
e. % distribution of forces between the three main flexor muscles may be different
(i.e. optimization of muscle action could be better in one individual).
f. Technique is important: jerks; higher accelerations; etc., require more force than
a “smooth” movement.
g. Velocity of muscle shortening (similar to f but even with a smooth movement
performed quicker you need more power – same amount of work done over less
time). If you perform the movement in less time, you need a larger acceleration
of the limb and load segment (more power) and hence more muscle torque
(which for a given moment arm means more muscle force).
h. Co-contraction of forearm extensors would mean more force from the flexors is
required. Similar to point e but not exactly the same point.
i. Muscle architecture may be different. Obviously, angle of pennation within
muscles will change the force required. However, I believe all three of these
muscles are fusiform muscles (that have a parallel fibre arrangement). However,
I will accept this point as 201 is not an anatomy course and there still could be
architectural differences between these muscles for our two subjects.
6 x 1 mark for fact and 6 x 1 mark for explanation
3
9. Isotonic strictly speaking means constant force but is used to describe weight
training with a constant load. [2] The muscle force is not constant for the following
reasons
• the load is usually stationary and needs a given force above mg to move
(accelerate) it, hence once the load is moving it needs less force to keep it
moving (Newton’s laws). [2]
• the lever arm of the muscle-joint system changes throughout the movement so
that less force is usually needed mid-range than at the beginning of the
movement (at the end of the movement the load is generally moving reasonable
speed so the force requirement doe not go up much). [2]
Obviously safety is an issue but it is
not
strictly
speaking
a
biomechanical issue (the force
applied if you drop the weight on
you toes is, but you know what I
mean). The main disadvantage
from a muscle biomechanics point
of view is as stated above that the
muscle is not stressed throughout
the whole range of movement (see
diagram opposite). The movement
tends to become easier at later
ranges of movement. [2]
Max torque
Barbell
Nautilus
Redrawn from Smith, 1982.
The advantages are that for
stability and balance more muscles must be recruited (especially around the trunk).
These stabilizer muscles help protect the joint from injury as well as helping coordinate
the movement pattern. [2]
The coordination of the movement is more realistic with free weights so although this is a
skill component it does relate to efficient activation of the muscle groups to effect a
controlled movement. [2]
[Max 10 marks]
10. This is a slightly different question than 4 b). When doing plyometrics you initiate a
stretch shortening cycle so there is an enhancement of the positive work done during
the concentric phase of the movement. Four marks would have been available for
listing the benefits of the stretch shortening cycle as listed in answer 4b.
But more would be needed. The main benefit of the training is the increased force
during the eccentric phase. This increases the overload on the muscles and other
structures (tendons, ligaments, etc). Overload is one of the main principles of athletic
training. [3]
The second benefit for plyometric loading is that it involves very common movement
patterns seen in sports activities so training this way promotes the neural activation and
coordination of the muscles. [3]
4