We may complain when we strain them or boast about how we work out to build them, but how often do we stop to consider just how amazing our muscles are? They’re what allow us to move, lift and manipulate objects; they pump our blood and help us digest our food; they let us express ourselves by talking, writing or smiling; they help us to see and they enable women to give birth.
As the engines of the body, muscles turn energy into motion. But, of course, there are different muscles for different tasks, just as there are different motors for different vehicles. Each type of muscle has a specific role in the body.
There are three types of muscle in the human body:
- Perhaps the most specialised is cardiac muscle which is only found in our heart. It is dedicated to making short involuntary contractions at intervals (so we don’t consciously need to think about what it’s doing), which makes it ideal for pumping blood around our bodies, even when we’re asleep. It is also capable of huge feats of endurance and consistency: a human heart beats approximately three billion times during a lifetime. Heart muscle can stretch in a limited way, like smooth muscle, and contracts with the force of a skeletal muscle.
- Smooth muscle is responsible for contractions in our blood vessels, digestive system, airways and, in females, the uterus. Its speciality is its ability to stretch and sustain tension. It also contracts on an involuntary basis, meaning that our nervous system controls it automatically. Normally, there is little we can do to stop such contractions, as any women entering labour will testify!
muscle is what most of us recognise as muscle: it’s what we go to the gym
to build and what enables us to run, jump, eat and grasp objects. As their name
suggests, skeletal muscles are attached to the skeleton and their function is
movement. Because skeletal muscles only pull in one direction, they are grouped
together in pairs around the framework of our skeleton: when one of the pair contracts, the opposing muscle relaxes; thus, one muscle moves the bone in one direction and the other moves it back the other way. We’ll all be aware that skeletal muscles usually contract voluntarily: when you think about contracting them, your nervous system tells them what to do. They can make a short, single contraction (twitch) or a long, sustained contraction (tetanus). Occasionally they contract involuntarily, for instance when we experience cramps or spasms.
Because our muscles are such important mechanisms for our bodies, they have evolved into incredibly sophisticated organs. Not only are they moderately efficient at turning stored chemical energy into kinetic energy (there is also huge heat loss, as anybody undertaking strenuous exercise will know) they are incredibly resilient and capable of modification. For instance, they grow in size when we exercise them and they can heal themselves when they are damaged.
Humans have a high capacity to expend energy for many hours doing sustained exercise. Research tells us that skeletal muscle burns 90mg of glucose a minute in continuous activity, generating approximately 24W of mechanical energy along with some 76W of heat energy.
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