This properties refers to a muscle fiber’s ability to stretch without damage.

(to a certain extent..)

When a muscle is stimulated above its threshold, it will start to contract, which generates tension between its origin and insertion points.

If that tension created is great enough to overcome the resistance of the object opposing it, the muscle will shorten and movement occurs.

The following animation gives us a rough sketch of the property of contractility:
http://youtu.be/CepeYFvqmk4?t=56s

Both muscle tissue and neurons exhibit this property, which is defined by the ability to respond to certain stimuli by producing electrical signals called action potentials (or nerve impulses). These signals travel along the plasma membrane, due to the presence of specific ions (Na+ and K+) and the local charge they create.

The electrical response of a muscle fiber stimulated beyond the threshold level.

As a little biomechanics overview: the different muscle sizes, shapes, and fiber arrangements, are all designed to give us different kinds of mechanical advantage.

Fiber Arrangements

• Parallel: These fibers run parallel along the single longitudinal axis of the bone, usually appearing longer with just two attachment points. The mechanical advantage here, is that they can contract over a greater distance at higher velocities. Of course, it sacrifices mobility and force for speed and efficiency. This fiber arrangement comes in 5 different shapes.

• Pennate: This arrangement can be found in muscles that have more than two distinct attachment points, or else, insert obliquely into a tendon. There are also 3 different types pennate arrangements, all of which tend to allow for multiple lines of pull and a greater cross-sectional area. This translates into an advantage of greater force production over shorter distances and longer intervals. In this case, we sacrifice efficiency and speed for mobility and force.

Lever Class

Another determinant in mechanical advantage is lever class. Specifically, the bones act as the lever arms while the muscle is the effort. Any force of resistance acting against the body, or specific muscle group, is the “load”.

• Class One: An example of this class is the seesaw, where the fulcrum is in between the load and the effort. There are very few class one levers in the body.
• Class Two: Like a wheelbarrel, the load sits between the fulcrum and the effort.
• Class Three: This is by far the most common lever in the body, and occurs when the axis of effort is situated in between the load and fulcrum.

The advantages and disadvantages of each lever are situational, and depend upon the length of the lever arms. Refer here to learn more.

Direction of Pull

Finally, putting these two structural aspects of the muscle together, we see that line of pull is a big factor in determining which particular motor units are recruited as well as the relative forces that can be produced.

Epimysium

Finally, this is the connective tissue surrounding the entire muscle.

Perimysium

This is the connective tissue surrounding fascicles.

Fascicles

When I call it the “functional unit” I mean that an individual muscle fiber exhibits all 4 properties of muscle tissue. In other words, it’s the fundamental building block of skeletal muscle.

Anything smaller than the singular fiber and we are talking about its component parts (e.g. structural proteins), which will exhibit different properties under different conditions and typically aren’t as well understood.

Endomysium

This is the connective tissue surrounding muscle fibers.

These are simply the contractile components of muscle and include in their structure, myofilaments.

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