KIN Muscle Mechanics by Stephen Robinovitch, All Rights Reserved

Transcription

1 KIN Muscle Mechanics 2006 by Stephen Robinovitch, All Rights Reserved

2 Outline anatomical structure eccentric and concentric contractions determinants of muscle force (stimulation frequency, muscle fibre type, muscle length and velocity, crosssectional area) size principle Hill's equation for muscle shortening Hill s active state model of muscle contraction quick-release experiments Huxley's sliding filament model Loeb's "virtual muscle" model

3 Muscle anatomy

4 Eccentric and concentric contractions Concentric (energy generating, positive work) contractions tend to increase joint angular velocity, and increase the total energy of the system Eccentric (energy absorbing, negative work) contractions tend to decrease (or brake) joint angular velocity, and reduce the total energy of the system Concentric (accelerating) contraction of m 2 Eccentric (braking) contraction of m 2

5 Positive and negative joint work direction of joint work depends on polarity of joint torque and direction of joint rotation movements 2, 3, 4, and 5 involve positive work or concentric contractions movements 1, 6, and 7 involve negative work or eccentric contractions B 4 B 7 extension Joint moment (Nm) A 4 A 7 extensor flexor A 1 B 6 B 3 B 2 A 5 B 1 flexion A 6 A 3 A 2 Joint rotation (radians) B 5

6 Muscle force results from interaction between contractile proteins sarcomere: the smallest anatomical unit that contracts like a muscle sliding filament model proposes that muscle force arises from cyclic binding between thick and thin filaments of the sarcomere thin filaments contain actin, troponin C, and tropomyosin thick filaments contain myosin in the absence of calcium, tropomyosin prevents myosin from attaching to actin

7 Calcium is needed for muscle contraction at the onset of an action potential, the sarcoplasmic reticulum (SR; a membrane that surrounds the myofibrils) releases calcium calcium binds to troponin, causing a conformal change in tropomyosin which reveals myosin binding sites on the actin simultaneously, adenosine triphosphate (ATP) is hydrolyzed by ATPase in the myosin head, providing the energy for cross-bridge attachment the SR re-sequesters calcium at the end of the action potential, thereby inducing muscle relaxation

8 Factors affecting muscle force development (a partial list) muscle fibre type number of activated motor neurons, frequency of discharge muscle length velocity of shortening/ lengthening muscle geometry (physiological cross-sectional area (PCSA), angle of pennation)

9 Muscle-nerve interaction a motor nerve enters muscle and splits into numerous axons; each axon contacts muscle fibres each muscle fibre is innervated by only one motor nerve axon, and contracts in response to an action potential in that axon motor unit: a single motor nerve axon and all the muscle fibres it contacts muscle # muscle fibers # motor units av. fibers per motor u. platysma 27,100 1, Brachioradialis 130, Tibialis anterior 250, gastrocnemius 1,120, ,000

10 Stimulation frequency affects muscle force: twitch and tetanus muscle force can be modulated by varying: (1) the number of recruited motor neurons, and (2) the frequency of discharge (i.e., stimulation rate) in motor neurons a single action potential (S 1 ) produces a twitch contraction, a quick rise and slow fall in force a tetanus occurs when a new action potential (S 2 ) arrives before the previous twitch has dissipated, and there is force summation at stimulation frequencies >30/s, there are no twitch transients (fused tetanus)

11 Three types of muscle fibres and motor units, defined by contraction speed, peak force, fatigue resistance

12 Size Principle When a stimulus is applied to the ventral aspect of the spinal cord, the smallest and most excitable motor units are activated first. These tend to be slow (S) motor units which innervate slow oxidative (SO) muscle fibres. Larger FR and FF motor units that innervate FOG and FG fibres are recruited only at high levels of force. Sequence is reversed when force level falls, with largest motor units dropping out first.

13 Active force development in the sarcomere depends on actin-myosin overlap (A): no overlap between actin and myosin, zero developed tension between (A) and (B): tension increases linearly as overlap increases between (B) and (C): maximum overlap & maximum tension left of (C): interference between actin filaments reduces ability of crossbridges to develop tension left of (D): myosin filaments collide with Z-lines and fold, and force declines rapidly

14 Muscle length affects force development in whole muscle the tension developed in a whole muscle is the sum of active force due to muscle contraction and passive force due the passive stiffness of tendon and muscle the passive force is negligible for lengths less that the normal resting length (l 0 ) the active force follows the tension-length behaviour of the sarcomere, and scales with muscle activation

15 Muscle velocity affects force development in whole muscle force (T) is greater during lengthening than shortening contractions the greater the shortening velocity (v), the smaller the force (explains why we cannot lift heavy objects quickly) in the shortening regime, mechanical power output is maximum when T and v are around one-third their maximum values

16 Muscle force-velocity behaviour is described by the Hill Equation An empirical relation that describes the force-velocity behaviour of muscle during shortening is the Hill Equation.

17 Greatest force is developed when lengthening near resting length

18 Hill s active state model of muscle contraction Hill assumed: (1) for a given length, muscle always develops the same peak force T 0 (x 1,t); (2) if the muscle is shortening, some force is dissipated in overcoming inherent viscous resistance B: muscle damping constant, which must be a nonlinear function of shortening velocity and temperature K SE : stiffness of the series elastic component; represents forcedeflection properties of tendon K PE : stiffness of the parallel elastic component; represents forcedeflection properties of sarcolemma, epimysium, perimysium, and endomysium

19 Quick-release experiments for determining the Hill model parameters hold muscle length fixed with the catch stimulate muscle to produce peak (isometric) force T 0 instantly release catch at the instant of release, muscle force is reduced to a value T (where T < T 0 ) that depends on weight in pan

20 Quick-release experiments (cont) there is an instant change (Δ x 2 ) in the length of K SE following release this is followed by a more gradual change (Δx 1 ) in the length of the muscle as T increases, there is a decrease in v (slope of dashed line), reflecting that muscle cannot shorten quickly under high loads combinations of T and v reflect the force-velocity properties of a given muscle

21 Shortcomings of the active state model the model predicts a negative T 0 during the end phase of the twitch (marked by asterisk in diagram at right) the liquid in muscle (water) does not have the required nonlinear damping characteristics the model cannot accurately predict muscle force during lengthening (the slope of the T-v curve is about 6-fold greater for lengthening than shortening, and muscle length increases rapidly when the load exceeds 1.8T 0 )

22 Huxley s sliding filament model Force development is due to stretch of elastic myosin crossbridges, which can form bonds with actin for x<h bonds can be maintained for x>h (tensile) and x<0 (compressive) for (0<x<h), rate of attachment (f) exceeds rate of detachment (g) rate of detachment (g) is slower during lengthening than shortening, thus accounting for greater force under eccentric conditions

23 Gerry Loeb s virtual muscle (J Neuroscience Methods, 2000) each muscle consists of (1) a contractile element, (2) a series elastic element, and (3) a muscle mass instabilities in the model are prevented by the muscle mass, and a small viscosity in the parallel elastic element contractile element consists of several motor units (3-5 for each fiber type), each defined by its order of recruitment, firing frequency, force-lengthvelocity relationships, passive parallel elastic element, and total forceproducing capacity (proportional to total crosssectional area) Matlab/ Simulink-based model for muscle and tendon distributed freely over the Internet at loskeletal/virtual_muscle.html

24 Virtual Muscle (cont) the motor nucleus of the whole muscle receives a single, time-varying neural activation command signal, which activates each motor unit according to the userdefined recruitment order within each motor unit, the frequency of motoneuronal firing is modulated in a realistic manner the activation signal can be based on pre-recorded EMG data, simulated feedback-driven reflex dynamics, or cortical commands

25 Virtual muscle (cont) user defines the properties of each fiber type and the morphometry of each musculotendon element, including the elastic properties of tendon and aponeurosis user then integrates these elements with their defined skeletal dynamics and control system (developed in Working Model)