NEUROMUSCULOSKELETAL SYSTEMS
LECTURE 8
SKELETAL MUSCLE
MECHANICS
Medical Science
Dr R G Luckwill
MOTOR UNIT RECRUITMENT
100%
Recruitment
% of Motoneuron
Pool used
50%
0%
0%
70%
100%
% Max. Voluntary Contraction (MVC)
Time-course of Contraction
Tetanus
Contractile
activity
Single
twitch
If a muscle fiber is
restimulated
after it has completely
relaxed, the second
twitch is the same
magnitude as the first
twitch.
Action
potentials
Twitch
summation
If a muscle fiber is
restimulated before
it has completely
relaxed, the second
twitch is added on to
the first twitch,
resulting in
summation.
Stimulation
ceases or
fatigue
begins
If a muscle fiber is stimulated
so rapidly that it does not have
an opportunity to relax at all
between stimuli, a maximal
sustained contraction known
as tetanus occurs.
Figure 8.16
Page 267
Tetanic Contractions
80 Hz
25 Hz
10 Hz
1 Hz
Tetanic Contractions
Increase in frequency of action potentials
leads to:
1.
Greater force
2.
Smoother contraction
3.
Faster development of force
MOTOR UNIT FIRING
100
Firing Rate
(Hz)
50
0
0%
70%
100%
% Max Voluntary Contraction (MVC)
Regulation of Voluntary Force
RATE
Frequency of Action Potentials
RECRUITMENT
Number of Motor Units used
Figure 8.17
Page 268
Force
Transducer
STIM
STIM
Isometric
Tension
Heavy
Medium
Light
1
Length
Light
Medium
Heavy
Time
Maximum velocity of shortening
(zero load)
Maximum load
(zero velocity of shortening
i.e., isometric contraction)
Figure 8.21
Page 275
Velocity
Power
1/
3
max
1/
3 max
Concentric & Eccentric Contractions
Concentric
Eccentric
Biceps
Insertion
of biceps
Fulcrum
for lever
Figure 8.21 (2)
Page 271
Muscle Fibre Types
Type I
Slow Twitch - Oxidative Red
Used for endurance
Type IIa
Fast Twitch - Oxidative
Type IIb Fast Twitch - Glycolytic White
Used for explosive action
Fibre Types
Type of Fibre
Type I
Type IIa
Type IIb
Slow
Fast
Fast
Colour
Red
Red
White
Oxidative phosphorylation
capacity
High
High
Low
Mitochondria
Many
Many
Few
Capillaries
Many
Many
Few
Enzymes for anaerobic
glycolysis
Low
Intermediate
High
Glycogen
Low
Intermediate
High
Diameter
Small
Intermediate
Large
High
Intermediate
Low
Speed of contraction
Fatigue resistance
Muscle fiber
Blood
Figure 8.22
Page 273
Contractile
component
(sarcomeres)
Series-elastic
component
(connective
tissue/tendon)
Load
Load
Figure 8.18
Page 269
Extension
Triceps
contracts
Origin
of triceps
Triceps
Insertion
of triceps
Origin of
biceps
Flexion
Biceps
Insertion
of biceps
Biceps
contracts
Figure 8.19
Page 269
Fulcrum
Figure 8.21 (1)
Page 271
Fibre Length