In any mammal, there are three different kinds of muscle.
- skeletal muscle
- smooth muscle
- cardiac muscle
An average adult male is made up of 40–50 percent of skeletal muscle and an average female is made up of 30–40%.
Skeletal muscles are typically attached to bones by tendons (stringy tissue) so we can use these muscles to move our bones, to move our bodies. They come in pairs, so that contracting one of the pair of the muscles will move the bone one way – and contracting the other one of the pair will move the bone the other way.
Skeletal muscles are a type of voluntary muscle, which means we can control them and choose when to use them.
Stand tall and sit up straight
As well as being used to move our bones, skeletal muscles are also responsible for maintaining posture (your body’s position when sitting or standing). Postural muscles include
- muscles that hold up your spine
- your leg muscles
- your hip muscles
Controlling your posture is usually subconscious – you don’t need to think about it at all. However, the muscles responsible can also react to conscious control just like other skeletal muscles.
The skeletal muscles of the face are different from the skeletal muscles of the rest of your body. In your face, many of the skeletal muscles attach directly to the skin or to other skeletal muscles – not to tendons or bones. This means that a tiny contraction in one of the facial muscles will pull the skin of your face and therefore change your expression. Because of this unique feature, you can produce countless different expressions, from grinning to smiling, winking, grimacing, raising an eyebrow and so on! Visit the "Spot the fake smile" interactive given in the links below.
Nature of science
Science is empirical - it relies upon observable data, but science is also impacted by the society and culture of time. Human dissection was largely forbidden before the mid 16th century, so it was not possible for scientists to learn more about the muscles of the human body by using human cadavers.
Smooth muscle, or involuntary muscle, is not under your conscious control. Involuntary means that you do not need to think about contracting these muscles (you don’t need to tell your intestinal walls to push food along or tell your eyes to blink.) Smooth muscle is found within the walls of our internal organs and structures such as the stomach, intestines, uterus, bladder and blood vessels. Smooth muscle sustains long or even near-permanent contractions compared with skeletal muscle, which contracts and relaxes in short, intense bursts.
Some smooth muscles can be trained to be under conscious control – for example, young children are trained to control their bladders so teenagers don’t need nappies.
Cardiac muscle is also an involuntary muscle. (Do you need to tell your heart to beat?) It is a specialised kind of muscle found only within the heart. This muscle pumps blood through the body. The average person’s heart beats more than 4,000 times in an hour (figuring on an average of 70 beats per minute), so, by time you turn 70, your heart will beat some two-and-a-half billion times. Cardiac muscle, like smooth muscle, does not tire.