Biomechanics/The Biomechanics Of Skeletal Muscles part 2

Ok, here comes the second part. Here we'll discuss mainly "muscle stretch" or what happens to a muscle when it is stretched passively or actively. We'll also see how this affects crossbridges (links between myosin and actin) and we shall finish this chapter by considering some physical properties of muscles.

Ok, let's go.

Passive stretch
	It is important for cardiac muscles.

	Muscle is attached to 2 supports, one mobile and the other fixed.

	When muscle is stretched, force and length of the muscle are recorded after obtaining equilibrium, to construct a tension-length curve.

	Since the muscle is not actively contracting (no crossbridges) and since it doesn’t require ATP, this is a passive stretch.

Results:

1-	No tension is measured before L becomes greater than L0 (which is quite obvious):

L / L0 > 1

Where L0 = normal length of relaxed muscle (resting length), L = length of muscle at any given time.

Thus, at rest L / L0 = 1, under stretch L / L0 > 1

2-	The curve is not a straight line because it is a biological material (mixture of different tissues). Also, dF / dx is not constant.

3-	Muscles are elastic.

Active stretch
Same experiment, but here, after stretching the muscle to a predetermined length and passive tension, muscle is stimulated to produce a single twitch. Max force developed by the muscle is then measured.



Observation:

1-	The curve obtained describes an active stretch before L0 but an active with some passive stretch after L0. Thus, we must subtract the passive from the active curve (active – passive) to obtain an active twitch tension vs. length.

2-	The single twitch tension is greatest around L0.



Explanation

At L / L0 = 1

Max no. of non-interfering crossbridges can be formed. Myosin only meets actin of the correct polarity and movement is all towards the center of the sarcomere.

H zone is a relatively small portion of the A band.

At L / L0 > 1

Few crossbridges are formed because, the more the muscle is stretched, the less thin and thick filaments overlap.

H zone is close to the size of the A band.

At L / L0 < 1

Few crossbridges are formed because interference occurs. Short length thin filaments can crumple and block access for myosin. Also, myosin may pull on actin from the wrong side of the sarcomere.

No H zone.

Conclusion:

F α total no. of crossbridges. Thus, increase in muscle strength is not due to increase in muscle fiber no., but is due to increase in no. of bundles of parallel myofibrils. Thus, F is directly proportional to cross-sectional area (i.e. total myofibrils).

Tension, cross bridging and muscle elasticity
1) Contraction occurs after a delay. Latent period = time between stimulus and begin of contraction.

Reason: Muscles are not just contractile in nature. They are also elastic, even at L < L0. That’s because the contractile fibers themselves, the z-lines, and anything else in series with contractile components (e.g. tendons) can and will stretch when a contraction puts tension on them.

2) Tension produced by a muscle is not instantaneous: it builds up and then wanes.

Reason: Tension does not appear/disappear instantaneously because Ca2　must diffuse and bind, crossbridges must form, then Ca2　must be removed and crossbridges must be broken. So, diffusion, active transport and conformational changes take time.

Main elements governing muscle elasticity

Contractile Elements (CE)

Contractile fibrils responsible of muscle contraction. During contraction, CE shorten after cross-bridging.

Series Elastic Elements (SEE)

Elements in series with CE. They allow a contracted muscle to reach an L > L0 when it is actively stretched.

During contraction, SEE lengthen slightly.

Parallel Elastic Elements (PEE)

Elements in // with CE. They produce a tension when a muscle is passively stretched past L0.

PEE remain at the same length.

Notes:

	Certain parts of the muscle are both parts of the CE and the SEE. They are functionally separate but physically the same.

	If the CE is not contracting, there are no functionally SEE.

	Max supportable load (F0) = max load that a muscle can support without lengthening = greatest force that CE can produce.

	Viscosity = internal resistance due to rearrangement. It represents a net loss of E. from the system.

Muscle Power Output
	Force = muscular tension = mg (a muscle with a load of 1 kg exerts a force of 9.8 N).

	Work = F d. Since the muscle does not shorten under isometric contraction, the muscle does no external work. It does do internal work though. Likewise, a muscle that shortens against no external load does no external work.

	Power = Work / time = Fv. Again, an isometrically contracting muscle develops no external power, nor does an isotonically contracting muscle with no external load.



	From curve 1 (x vs. t), as the load increases from no load to heavier loads,

o	The distance the muscle can shorten decreases as does the velocity.

o	The latent period increases.

o	At F0, the contraction becomes totally isometric as the muscle doesn’t change in length.

o	The muscle lengthens beyond F0 despite contracting maximally. This is because the elastic elements lengthen and/or the crossbridges are broken by the load.

	From curve 2 (v vs. load), movement of muscle slows down as load increases.

	From curve 3 (P vs. load), there is an optimum load for a given muscle to produce max useful external power. Muscles thus do not contract proportionately faster and faster as the load is reduced. Different muscles give different curves.

Muscles Dimensions (force and speed)
•	F α cross-sectional area of muscle, thus α no. of myofibrils. Thus, F is increased as no. of elements in // increases. That’s because a thick muscle has more crossbridges, so the load is spread over a greater area and thus it is less likely any fiber will break.

•	v is a fn of muscle length. The more the sarcomeres in series, the greater the displacement that occurs when all move at the same time. Because total shortening is the sum of dx of each sarcomere.

•	v also depends on load on the muscle.

Comparison between elastic and compliant materials
Elastic materials

Elastance = Stress / Strain (N / m2)

Tolerates high stress and shows a minor change in shape.

Example: muscles.

Compliant materials

Compliance = Strain / Stress (m2 / N)

A low stress causes a high change in shape. Thus, small forces like surface tension are very significant.

Example: lungs.

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