Skeletal Muscle Mechanics

Transcription

1 Skeletal Muscle Mechanics

2 Brief reminder

3 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

4 Mechanical properties Contractile properties - ability to contract (develop force) Elastic properties - ability to recoil to normal length Classical model of muscle mechanics Hill (1938) CC SEC PEC Phenomenological model does not take into account the architecture of the muscle

5 CC SEC PEC CC force generator, composed of the actin-myosin cross-bridges

6 CC behavior The force-length properties Isometric condition The force that a muscle generates varies with its length

7 a bell-shaped force-length curve Due to the cross-bridge overlapping

8 Force Cross-sectional area

9 CC behavior The force-velocity properties Isotonic condition

10 The force that a muscle generates varies with the velocity Due to the cross-bridges breaking and reforming in a shortened condition

11 Maximal shortening velocity F = 0 No experimental data at high velocities = wrong evaluation of maximal shortening velocity Slack test procedure Maximal unloaded shortening velocity

12 1000 B D F o r c e 500 A A=Muscle activated B=Slack initiated C=Muscle Unloaded D=Muscle inactivated C Time

13 Different slacks of different amplitudes Duration vs. Displacement Fiber Displacement (um) * * * * * * * V 0 = slope / FL Duration of Unloaded Shortening (ms)

14 CC behavior The power-velocity properties Pmax = 1/3 Vmax Force concentric Force concentric velocity velocity Power = force x velocity

15 CC SEC PEC PEC passive elastic properties of muscle, derived from muscle membranes (epimysium, perimysium, endomysium, sarcolemma)

16 PEC behavior The passive force-length properties The passive force that a muscle generates depends on the amount of connective tissue Hysteresis

17 CC + PEC behavior The global force-length properties CC SEC PEC

18 CC SEC PEC SEC elastic properties and force transmission, composed of 2 fractions SEC active fraction localized in actin-myosin cross-bridges SEC passive fraction localized in tendons

19 SEC behavior The tension-extension properties SEC CC

20 1000 B C F o r c e 500 A A=Muscle activated B=Slack initiated C=Muscle inactivated Time

21 Mostly due to the SEC active fraction: cross-bridge elasticity

22 Definition of SEC stiffness or SEC compliance Different slacks of different amplitudes Variation in length vs. variation in force Variation in force Variation in length

23 Stretch-shortening cycle (SSC) Eccentric contraction followed immediately by concentric contraction Present in motor daily activities such as walking, running SEC used in such phenomenon

24 Eccentric phase: storage of energy Compliant SEC Concentric phase: restitution of energy Stiff SEC good storage of energy long coupling time between eccentric and concentric phases: poor force transmission Variation in length Variation in force poor storage of energy short coupling time between eccentric and concentric phases: good force transmission energy loss

25 Brief reminder Human muscle fiber types Type I Type IIx Type IIa Slow muscles have different mechanical properties compared to fast muscles

26 CC SEC PEC

27 CC SEC Variation in force PEC Variation in length Consequence on Stretch-shortening cycle Slow muscle: stiffer SEC Decrease in coupling time between stretching and shortening Better force transmission

28 Evaluating the mechanics of human skeletal muscle Utilization of specific ergometers To test different muscle groups

29 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-angular velocity relationship (Force-velocity relationship)

30 Construction of torque-angle relationship Elbow flexion = Resistance of the device Torque Angle

31 Construction of force-length relationship from torque-angle relationship Torque (T) = Force (F) x lever arm (d) F = T/d d d can be measured = distance between the elbow and the bone Sinus α = d/muscle length (L) L = d/sin α d α can be found in the scientific literature α

32 Construction of torque-angular velocity relationship Concentric Elbow flexion > Resistance of the device Eccentric Elbow flexion < Resistance of the device Torque Angular velocity

33 Construction of force-velocity relationship from torque-velocity relationship Torque (T) = Force (F) x lever arm (d) F = T/d d d can be measured = distance between the elbow and the bone Sinus α = d/muscle length (L) L = d/sin α d α can be found in the scientific literature α

34 Several good answers are possible per question 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

35 Several good answers are possible per question 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

36 Several good answers are possible per question 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

37 Several good answers are possible per question 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

38 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

39 Several good answers are possible per question Quizz 10- Who will win the next football world cup France France France

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