12.4.12

Mechanics of Muscle Contraction

Muscles

• The term muscle refers to a number of muscle fibres bound together by connective tissue

• A single muscle cell is known as muscle fiber. Each muscle fiber consist of many myofibrils. The myofibrils are divided into functional units i.e. sarcomeres by two Z lines

• Myofibrils consist of thick and thin filaments composed of contractile proteins

• Electron microscopic studies have revealed light and dark bands in the myofibrilss. The lighter areas on either side of the Z—line are I bands and they contain thin filaments. The dark area in the center of sarcomere are called A bands.

• A bands are anisotropic while I bands are isotropic

• Bones provide place for the attachment of skeletal muscle and maintain the optimum length of the resting muscle

• The muscles always begin and end in the bones that touch one another and they never begin and end on the same bone

Muscles

• Resting Length of a muscle:

If a muscle is removed from the body, it assumes a length slightly shorter than the length of the muscle at rest in the body (resting length). The muscle is fixed to its bony attachments in a slightly stretched state

Mechanical Model of a Muscle• Muscle has elastic and contractile components

1. Contractile component: produces force, composed of myofibers

2. Elastic component: absorbs some of muscle's force by stretching. Has two subsets:

Parallel elastic component: membranes running along myofibers (e.g. plasma membrane, sarcoplasmic reticulum)

Series elastic component: tendons and fascia that binds muscle to insertion on bone

Active Tension

• Tension purely due to contraction (of sarcomeres) is known as “active tension”

• This tension is due to the forces generated by many cross-bridge formations. In the cross-bridge theory the force generated is proportional to the number of cross-bridges formed.

• It is the pulling of the actins by myosin heads towards each other that exerts this tension. The magnitude of the tension depends on the frequency of the stimulation and the initial resting length of muscle fibres

• The length of the sarcomeres dictates the overall length of a muscle fibre.

Length-Tension Relation of an Individual Muscle Fiber

• The best way to relate length-tension changes to contractile machinery is to perform this experiment with a single muscle fibre

• An unstimulated muscle fibre is held between a movable clamp and a force transducer. The muscle fibre is then stimulated at various lengths and the force generated is measured

• The force generated is seen to be related to the degree of overlap of actin and myosin filament.

Length-Tension Relation of an Individual Muscle Fiber

The active length-tension curve of a sarcomere is shown as an ascending limb, a plateau and a descending limb. The length-tension curve can be explained by the cross-bridge theory.

• Ascending limb:

In the regions of sarcomere lengths between 1.5 to 2.0 µm, there is a double overlap of thin filaments with thick filaments. This double overlap can be seen as a darker region in the A band at short sarcomere lengths under electron microscope. Tension is low due to the pause in cross-bridge cycling and formation. The actin filaments from opposite ends of the sarcomere begin to interfere with each other and at shortest lengths the Z disc may block or otherwise impede movement of the thick filament

Tension rises as the length is increased from 1.7 to 2.2 µm. This rise has been attributed to progressive decrease in the extent of double overlap of actin filaments

Length-Tension Relation of an Individual Muscle Fiber

• Plateau:

Maximum active tension is produced when sarcomeres are about 2.0 to 2.2 μm long, as seen in 2. This is the optimal resting length for producing the maximal tension. At the plateau of the curve, there is an optimal overlap of actin and myosin filaments and thus the opportunity for maximal cross-bridge formation

• Descending Limb:

By increasing the muscle length beyond the optimum, the actin filaments become pulled away from the myosin filaments and from each other.

Length-Tension Relation of an Individual Muscle Fiber

Descending limb:

The force is reduced due to reduction in overlap of the thin and thick filaments of sarcomere.

At 3, there is little interaction between the filaments. Very few cross-bridges can form. Less tension is produced.

Force tends to approach zero as the overlap is eliminated When the filaments are pulled too far from one another, as seen in 4, they no longer interact and cross-bridges fail to form. No tension results. 

Length-Tension Relation of an Individual Muscle Fiber

Passive tension• Parallel elastic elements of the muscle also generate

tension • These elastic elements do not actively generate force

but if it is stretched beyond its resting length it acts just like a rubber band and produces a passive, elastic force

• Passive or resting tension is mainly due to stretching of the series elastic elements i.e. tendons of the muscle

• The force generated is passive as the contractile machinery is not active

• The resting tension increases steeply with an increase in the initial length of the muscle

The curve of total tension (N-shaped) is constructed by stimulating the whole muscle at various lengths and measuring the force generated

Total Tension of a Muscle

Total Tension of a Muscle

• Each of these forces will be the sum of active forces (developed by contractile machinery) and passive forces (due to stretching of elastic elements)

• Forcibly stretching a muscle well beyond its resting length will generate a force higher than that produced by active contraction

Length-Tension Curve of Skeletal Muscle Versus Cardiac Muscle

• The maximal active tension in skeletal muscle occurs when the overlap of actin and myosin filaments is optimal and coincides with the muscles natural resting length

• But in cardiac muscle the resting length is shorter than the optimal length. Due to this the cardiac muscle normally operates on the ascending part of the length-tension curve. Thus small increase in initial length results in large increase in active and total tension

• Unlike skeletal muscle, the decrease in developed tension at high degree of stretch is not due to a decrease in number of cross bridges between actin and myosin, because even severely dilated hearts are not stretched to this degree. The descending limb, if seen is due to the beginning of disruption of the

myocardial fibres

Types of Contraction• Isometric contraction

When a muscle is stimulated such that it develops tension but do not shorten. This is called an isometric contraction (iso = same, metric = measurement or length).

• Isotonic contraction

When a muscle is stimulated such that the muscle shortens with a constant load but its tension remains the same, the contraction is isotonic (iso = same, tonic = tension)

Isometric Contraction: Experimental setup

• A whole muscle is arranged in a muscle bath using a lever arm. The muscle is first stretched to a resting length by a given weight i.e. preload. Then a stop is positioned to maintain the length of the muscle. After this muscle is stimulated to contract, the tension or force developed by the muscle is recorded

• Careful observation reveals that in the isometric contraction, the sarcomeres shorten and stretch the series elastic component even though the muscle as a whole does not shorten.

• Even though the muscle develops tension, but because it does not shorten, it does no external work (work = force x distance moved) but there is internal work being done

• The total tension is the sum of active and passive tension (the curve of total tension is the curve of isometric contraction)

Isotonic Contraction: Experimental setup

• The experimental setup is same. A preload is there to stretch the muscle to a resting length

• There is no restriction on the length of the muscle now and additional load is added to the lever (afterload)

• When the contractile elements shorten they must first stretch the series elastic elements and develop a tension equal to the load before the next increment in tension causes the load to be lifted.

• All of the contraction that occurs before the load is lifted is isometric. Even if the muscle carries no external load, it still must develop a tension equal to its own weight before it can shorten.

• When contractile forces exceed the load, shortening begins; tension remains slightly larger than the load throughout shortening. Shortening stops when active tension drops to the point where it equals the load. At this point, contraction again becomes isometric. The muscle lengthens (is stretched) when the total tension in the muscle falls below the load

Types of isotonic contraction

1. Conentric contraction

A concentric contraction is a type of isotonic contraction in which the muscles shorten while generating force

2. Eccentric contraction

During an eccentric contraction, the muscle elongates while under tension due to an opposing force (load) being greater than the force generated by the muscle

Types of contractions at a glance

Force (tension)-velocity relationship of a muscle

• The force a muscle can generate depends upon both the length and shortening velocity of the muscle

• Force declines in a hyperbolic fashion relative to the isometric force as the shortening velocity increases, eventually reaching zero at some maximum velocity.

• The reverse holds true for when the muscle is stretched – force increases above isometric maximum, until finally reaching an absolute maximum.

• This has strong implications for the rate at which muscles can perform mechanical work (power).

• Since power is equal to force times velocity, the muscle generates no power at either isometric force (due to zero velocity) or maximal velocity (due to zero force). Instead, the optimal shortening velocity for power generation is approximately one-third of maximum shortening velocity

Force (tension)-velocity relationship of a muscle