Muscle Fundamentals: Components, Characteristics and Contraction

Structure of skeletal muscle cell

Kinesiology is the study of the anatomy and physiology of body systems that create movement. It includes the study of things like skeletal muscle contraction, tendon placement, joint structure, and the connective tissues that hold everything together. Massage students will need to have a good understanding of muscle components and characteristics, not only for the MBLEx, but to be an effective massage therapist. It’s also important to understand how our massage and bodywork treatments may affect our clients’ movement.

The kinesiology content area on the MBLEx represents 12% of the massage exam. This section tests your understanding of muscle tissue characteristics, components and contractions. You will need to know the specific muscle locations and attachment points, and also joint structure and function.

I will also point out some key root words, prefixes and suffixes that will help you to decode and remember important terminology for the kinesiology content area.

This post focuses on the components and characteristics of skeletal muscle tissue, and how contraction occurs.

Structure and components of muscle

Structure and components of skeletal muscles

There are different types of muscles in the body, including skeletal muscle, smooth muscle and cardiac muscle. Since this post is about kinesiology, I will focus on skeletal muscle tissue and leave cardiac and smooth muscle for another post.

Let’s start by looking at a single muscle. The biceps for example.

Every skeletal muscle is surrounded by an epimycium. This is a dense, fibrous connective tissue that envelops the entire muscle to protect it from injury or friction from surrounding structures.

The epimycium merges with the muscle tendons to form a continuous structure. Tendons attach the muscle to bone to create movement at the joints. The epimycium also connects to the surrounding fascia, as well as the deeper connective tissue within the muscle (the perimysium and endomysium).

Skeletal muscles are made up of bundles of muscle fibers, called fascicles. Each fascicle has it’s own protective sheath called perimysium.

Fascicles are composed of individual muscle fibers (also called muscle cells). These muscle fibers, or cells, have a protective sheath called endomysium. Like other cells of the body, skeletal muscle cells have a nucleus, need a blood supply, and need to communicate with the nervous system in order to function properly.

It will help to memorize these prefixes:

  • “Epi-“ means over or upon.
  • “Peri-” means around or near.
  • “Endo-” means within, inward, or inner.

Structure of an individual muscle cell

An individual muscle cell is also sometime called a muscle fiber, or myocyte. There are three types of muscle cells: skeletal, cardiac and smooth. For the MBLEx, it is only important to be familiar with the basic differences between these types of muscle cells.

Skeletal muscle cells are striated and have many nuclei, so they are “multinucleate”. An individual muscle cell is usually about 1-2 inches in length, but can be up to 10-12 inches.

Cardiac muscle cells are also striated. However they only contain a single nucleus.

Smooth muscle is not striated. It is found in the walls of hollow organs and tubes throughout the body. For example: intestines, stomach, esophagus, urinary bladder, and blood vessels.

Unless specified, the content on this page refers to skeletal muscles, since that is the focus of massage therapy techniques.

Every muscle cell is composed of hundreds of myofibrils. These myofibrils that are the filaments that contract and shorten the muscle, which creates movement of the skeleton.

There are two main types of myofibrils: thick and thin. The thick myofibrils are made up of myosin, and the thin filaments are made of actin (more about these in the next section).

Sarcomeres are repeating units in skeletal muscle, which are divided by two Z lines. These units are composed of thick and thin myofibrils.

Sarcomere actin and myosin filaments

*The sarcomere is the basic functional unit of striated muscle tissue.

Sarcomeres are what give skeletal muscles the striated (striped) appearance when viewed under a microscope.

Characteristics of muscles

Muscle cells have 4 primary properties:

Contractility. Muscle tissue has the ability to contract and shorten. Muscles can actively shorten, but they cannot actively lengthen themselves. They must rely on their antagonist muscle(s) or another external forces such as gravity to help them lengthen.

Excitability. This is the characteristic of muscle tissue that describes the muscle’s ability to respond to a stimulus. When a motor neuron sends a signal to the muscle, it contracts.

Extensibility. This means that muscle tissue can be stretched. When there is tightness of a muscle group, for example at the hamstring or pectoral muscles, it is usually not the muscle tissue itself that is the problem. The tightness usually comes from restrictions in the fibrous connective tissue (fascia) that surrounds the muscle tissues. This is why massage techniques like myofascial release are so effective and lengthening muscles and restoring range of motion. This is also why low-load, long duration (LLLD) stretching is an effective stretching technique and reduces fascia restrictions.

Elasticity. This is the ability of muscle tissue to recoil or come back to its original length after being stretched.

Basics of muscle contraction

Muscle contraction is the activation of muscle fibers and increase of muscle tension. This may result in the shortening of the muscle, as in the case of concentric contraction.

However, a muscle may stay the same length when contracted (isometric contraction), or may even lengthen as it stays contracted (eccentric contraction). Eccentric contraction occurs when the force applied to the joint is greater than the force produced by the muscle(s) acting on that joint.

In order to contract, motor neurons (efferent nerves) send a signal from the brain or spinal cord to the muscle, telling it to contract.

The sliding filament theory explains the process of muscle contraction. The motor neuron signals the thick and thin filaments (myofibrils) to slide past each other. This increases tension in the muscle, and will cause a contraction.

Muscle shape and fiber direction

The size, shape and fiber direction of muscles depends on the action that the muscle needs to perform, and the bones that the muscle has to attach to.

There are 4 basic skeletal muscle shapes:

Parallel (fusiform). The length of the fascicles run in a direction that is parallel to the direction of the muscle.

Pennate muscles have short fascicles that run in an oblique direction to the central tendon, to which they attach. The word pennate means “feather-like”. There are three types of pennate muscles:

  • Unipennate: the fascicles insert onto one side of the tendon. Example: extensor digitorum longus.
  • Bipennate: the fascicles insert obliquely on both sides of the central tendon. These muscles look sort of like a feather. Example: rectus femoris.
  • Multipennate: multiple fascicles that are obliquely positioned. Example: deltoid.

A convergent muscle has a triangular shape. It has a broad origin and converges toward a single tendon. The pectoralis major is an example of a convergent muscle.

Circular muscles are also called sphincters. These are found at openings to the gastrointestinal tract (orbicularis oris, and sphincter ani externus). Circular, or sphincter, muscles are also found throughout the body, throughout the GI tract, at the openings of organs, and at blood vessels. These internal sphincters however are made of smooth muscle, and controlled by the autonomic nervous system.

Conclusion

This blog post contained some of the fundamental information about muscle components, characteristics and contraction that massage therapists should know for the MBLEx.

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