Broadly speaking, the muscular system of the human body is composed of three distinct types of muscles - skeletal muscles, smooth muscles, and the cardiac muscle. The skeletal muscles are what we know as voluntary muscles, and these muscles are those, that we can train and develop by using muscle-building methods. The smooth muscles are the involuntary muscles. The cardiac muscle is somewhere between the involuntary and voluntary muscles, as although, it functions like the former, while its structure is very similar to the latter. Before we proceed on a detailed discussion of these muscles, let's take a brief look at each of these three types of muscles.
The Three Types of Muscles
Skeletal Muscles: These muscles, attached to the bones by tendons, are known as voluntary muscles, as these are actively, if not consciously (since many of our physical activities are performed instinctively also), used to induce motion to the skeletal structure for the purpose of performing various movements and to maintain a stance or posture. Examples of such muscles, include, the biceps (located in the front portion of the upper arms), triceps (located in the back parts of the upper arms) and quadriceps (located in the front portion of the thighs).
Smooth Muscles: These are known as involuntary muscles, as they perform their functions without relying upon human will, to perform an action. The smooth muscles are usually located on the inner sides of various organs, such as, on the inner walls of the esophagus, stomach, blood vessels, bladder, etc. These muscles, as opposed to the voluntary ones, perform their functions with mechanical precision, without involving conscious or instinctive intervention of the human will. Examples of involuntary muscles are the visceral and iris muscles.
Cardiac Muscle: The cardiac muscle is located on the inner walls of the heart and in the myocardium. The cardiac muscle structure is closer in resemblance to skeletal muscles, however, the cardiac muscle function resembles that of the involuntary muscles. Due to this dual aspect, cardiac muscles cannot be grouped either under involuntary or voluntary muscles.
How Are These Muscles Controlled?
As the name suggests, the function of voluntary muscles is regulated by the human will, whether it is conscious or instinctive. These muscles are characterized by the transverse structure of their stripes. These muscles are controlled by opposing motions of expansion and contraction. This is achieved depending upon the way, one set of muscles is arranged opposite to another set. For instance, consider the arrangement and movement of biceps and triceps.
Suppose, you are to lift your arm upwards to point a finger towards the sky or the ceiling (depending upon whether you are indoors or outdoors). Now, while you read this, if you try performing this motion, you'll notice that while your triceps (located on the back of the upper arms) get stretched or expand, your biceps (located on the inner side of the upper arm) get contracted to help your arm achieve the desired pointing-towards-the-heavens posture.
Now, this is a motion that, most often, we consciously perform. An example of an instinctive or unconsciously habitual motion may include raising our hands to cover our eyes from the sudden blinding headlights of an oncoming heavy vehicle. On such occasions, we don't consciously process the thought that we must cover our eyes. As soon as the light hits the eyes, the hand comes to their rescue! In both cases, the fibers of the voluntary muscles get signal from nerve endings, which carry signal from the brain to perform a particular motion. Most of the time, such a signal is sent by the brain as a response to the chemistry of the thought process (that causes hormonal shifts and causes neurons to emit electrical signals).
That, precisely, sums up what voluntary muscles are and how they function. Since these muscles are literally slaves of our whims, it is up to us to use them properly to avoid stress, strain, and injury to them. A proper diet rich in proteins coupled with a healthy workout regime is the best way to nourish these muscles and keep them fit.