Types of muscle contraction
Force of weight = force developed by muscle Muscle length does not change
Force of weight < force developed by muscle Muscle allowed to shorten
Force of weight > force developed by muscle Muscle resists to stretching
Mechanical properties
Contractile properties - ability to contract (develop force)
Elastic properties - ability to recoil to normal length
Classical model of muscle mechanics – Hill (1938)
PEC
CC SEC
PEC
CC SEC
CC behavior
The force-length properties
The force that a muscle generates varies with its length Isometric condition
a bell-shaped force-length curve
Force
CC behavior
The force-velocity properties
Due to the cross-bridges breaking and reforming in a shortened condition The force that a muscle generates varies with the velocity
F = 0
Maximal shortening velocity
No experimental data at high velocities = wrong evaluation of maximal shortening velocity
Maximal unloaded shortening velocity Slack test procedure
0 500 1000 0 10 20 30 40 50 60 70 Time F o r c e A B D A=Muscle activated B=Slack initiated C=Muscle Unloaded D=Muscle inactivated C
* * * * * * *
V
0= slope / FL
Duration vs. Displacement
50 70 90 110 130 150 170 0 10 20 30 40 50 60 70 80 90 100Duration of Unloaded Shortening (mS)
F ib er D is p lac em en t (u m )
CC behavior
The power-velocity properties
Power = force x velocity
velocity F o rce concentric velocity F o rce concentric Pmax = 1/3 Vmax
PEC
CC SEC
PEC – passive elastic properties of muscle, derived from muscle membranes (epimysium, perimysium, endomysium, sarcolemma)
PEC behavior
The passive force-length properties
Hysteresis
The passive force that a muscle
generates depends on the amount of connective tissue
CC + PEC behavior
The global force-length properties
PEC
PEC
CC SEC
SEC – elastic properties and force transmission, composed of 2 fractions
SEC active fraction – localized in actin-myosin cross-bridges
SEC behavior
The tension-extension properties
CC SEC
0 500 1000 0 10 20 30 40 50 60 70 Time F o r c e A B C A=Muscle activated B=Slack initiated C=Muscle inactivated
Variation in length vs. variation in force
Different slacks of different amplitudes
Variation in length V a ria tio n in fo rce
Stretch-shortening cycle (SSC)
Eccentric contraction followed immediately by concentric contraction
Present in motor daily activities such as walking, running
Eccentric phase: storage of energy Concentric phase: restitution of energy
Compliant SEC Stiff SEC
Variation in length V a ria tio n in fo rc e
good storage of energy poor storage of energy
long coupling time between eccentric and concentric phases: poor force transmission
energy loss
short coupling time between eccentric and concentric phases: good force transmission
Brief reminder
Type I
Human muscle fiber types
Type IIa
Type IIx
PEC
PEC
CC SEC
Consequence on Stretch-shortening cycle
Variation in length V a ria tio n in fo rce
Slow muscle: stiffer SEC Decrease in coupling time between stretching and shortening Better force transmission
Evaluating the mechanics of human
skeletal muscle
Utilization of specific ergometers
Example of elbow flexion
Schematic view
Elbow flexors
Example of measurements
Torque (T) Angle (A) Angular velocity (AV)
Construction of torque-angle relationship (Force-length relationship)
Construction of torque-angle relationship
Elbow flexion =Resistance of the device
Angle T o rq u e
Construction of force-length relationship from torque-angle relationship
Torque (T) = Force (F) x lever arm (d) F = T/d
d
Sinus α = d/muscle length (L) L = d/sin α
d
α
d can be measured = distance between the elbow and the bone
Construction of torque-angular velocity relationship
Elbow flexion >Resistance of the device
Angular velocity T o rq u e
Elbow flexion <Resistance of the device
Construction of force-velocity relationship from torque-velocity relationship
Torque (T) = Force (F) x lever arm (d) F = T/d
d
Sinus α = d/muscle length (L) L = d/sin α
d
α
d can be measured = distance between the elbow and the bone
Quizz
1- During knee extension
the quadriceps femoris is in concentric condition and the biceps femoris is in eccentric condition
the quadriceps femoris is in eccentric condition and the biceps femoris is in concentric condition
the quadriceps femoris and the biceps femoris are both in concentric condition
2- The Hill’s model is composed of
three components: two in series (SEC and PEC) and one in parallel (CC)
three components: two in series (SEC and CC) and one in parallel (PEC)
four components: CC, SEC, CEP and SSC
Quizz
3- CC
is the force generator, and is composed of the actin-myosin cross-bridges
is the force transmitter, and is composed of the actin-myosin cross-bridges
is the force transmitter, and is composed of the muscle connective tissue
4- PEC
reflects the active elastic properties of muscle and is derived from muscle membranes
has an influence on the force-length relationship
reflects the passive elastic properties of muscle and is composed of the muscle connective tissue
Quizz
5- SEC
is the force generator, and is composed of 2 fractions
has an influence on the force-length relationship
is the force transmitter, and is composed of the muscle connective tissue
6- The force-velocity relationship
is relative to the CC
in concentric condition, force increases with velocity; whereas in eccentric condition, force decreases with velocity
depends on the constitution of the muscle
Quizz
7- The tension-extension relationship
is relative to the PEC
allow the calculation of the stiffness
do not depend on the constitution of the muscle
8- The stretch-shortening cycle
is present when running
is an eccentric contraction followed immediately by concentric contraction
is a concentric contraction followed immediately by eccentric contraction Several good answers are possible per question
Quizz
9- The force-length relationship in Humans
can be obtained from the torque-angular velocity relationship
can be obtained from the torque-angle relationship
is a bell-shaped curve
Quizz
10- Who will win the next football world cup
France
France
France
