How Muscles Work
Agonist vs. Antagonist
Muscle pairs: Muscles are grouped together in pairs on your skeleton
Relaxed or contracted: When one muscle of a pair contracts, the other relaxes
Pulling muscles
Skeletal muscles only pull in one direction. For this reason they always come in pairs. When one muscle in a pair contracts, to bend a joint for example, its counterpart then contracts and pulls in the opposite direction to straighten the joint out again. Without this arrangement you wouldn't be able to straighten your legs when you walk or bend your fingers to grip something.
Ex. Biceps vs. Triceps…
When your biceps muscle in your upper arm contracts, it pulls your lower arm in towards your shoulder. However, when it relaxes, your biceps cannot push your arm back out. To do this, your triceps muscle, on the underside of your upper arm, contracts and straightens your arm out. If your triceps muscle wasn't there, your arm would stay drawn in permanently.
Skeletal muscle: Produces movement, maintains posture, stabilises joints and generates heat
Smooth muscle: Found in the walls of hollow organs
Cardiac muscle: Exists only in your heart
Three types of muscle
You've got around 650 muscles in your body, and they make up roughly half of your bodyweight. These muscles can be divided into three different groups: skeletal, smooth and cardiac. All of these muscles can stretch and contract, but they perform very different functions.
Contraction
In a relaxed muscle, thick and thin myofilaments overlap each other a tiny bit. When a muscle cell is stimulated by a nerve impulse, these myofilaments slide past each other until they completely overlap. This makes the muscle cell shorter and fatter. The more shortened muscle cells there are in a muscle, the greater the contraction of the muscle as a whole.
Skeletal muscle
The tissue most commonly thought of as muscle is skeletal muscle. Skeletal muscles cover your skeleton, giving your body its shape. They are attached to your skeleton by strong, springy tendons or are directly connected to rough patches of bone. Skeletal muscles are under voluntary control, which means you consciously control what they do.
Just about all body movement, from walking to nodding your head, is caused by skeletal muscle contraction. Your skeletal muscles function almost continuously to maintain your posture, making one tiny adjustment after another to keep your body upright. Skeletal muscle is also important for holding your bones in the correct position and prevents your joints from dislocating. Some skeletal muscles in your face are directly attached to your skin. The slightest contraction of one of these muscles changes your facial expression.
Skeletal muscle generates heat as a by-product of muscle activity. This heat is vital for maintaining your normal body temperature.
Smooth Muscle
Smooth muscle is found in the walls of hollow organs like your intestines and stomach. They work automatically without you being aware of them. Smooth muscles are involved in many 'housekeeping' functions of the body. The muscular walls of your intestines contract to push food through your body. Muscles in your bladder wall contract to expel urine from your body. Smooth muscles in a woman's uterus (or womb) help to push babies out of the body during childbirth. The pupillary sphincter muscle in your eye is a smooth muscle that shrinks the size of your pupil.
Cardiac Muscle
Your heart is made of cardiac muscle. This type of muscle only exists in your heart. Unlike other types of muscle, cardiac muscle never gets tired. It works automatically and constantly without ever pausing to rest. Cardiac muscle contracts to squeeze blood out of your heart, and relaxes to fill your heart with blood.
Exercise
All types of exercise involve using your muscles to generate movement. Regular exercise can increase muscle size, strength and endurance. But not all exercise produces the same effect.
Endurance exercise
Endurance exercises like running or cycling make your muscles stronger and less likely to get tired. This sort of exercise doesn't only benefit your skeletal muscles. It also:
- Enlarges your heart so it can pump blood around your body more quickly
- Increases the capacity of your lungs so you can breathe in more oxygen with each breath
- Improves your digestion
- Makes your metabolism more efficient
- Strengthens your skeleton
- Improves your coordination
Endurance exercise isn't, however, the best type of exercise to do if you want to make your muscles bigger.
Bodybuilding
You can increase muscle size by doing resistance exercises. Resistance exercise involves using your muscles to move an extremely heavy load. This type of exercise forces your muscles to contract as hard as they can, which leads to an increase in the number of contractile filaments within your muscle cells. This makes your muscle cells enlarge and, as a result, it makes your muscles bigger.
Endurance and resistance combined
Some forms of exercise offer the benefits of both endurance and resistance exercise. Swimming improves endurance, and as you have to push against water to move forward, it is a resistance exercise, too.
Energy is directed to your muscles during exercise
If you're doing intense exercise, other systems in your body focus their efforts on helping your muscles. Your heart beats more quickly so that it can pump more blood to your muscles, to provide them with more oxygen and nutrients. Your digestive system slows down so that it does not use up energy that your muscles need.
Two fibres: There are two different fibres in muscle
Slow twitch muscle fibres: Contract slowly, but keep going for a long time
Fast twitch muscle fibres: Contract quickly, but rapidly get tired
Endurance or speed
Slow twitch muscle fibres are good for endurance activities like long distance running or cycling. They can work for a long time without getting tired. Fast twitch muscles are good for rapid movements like jumping to catch a ball or sprinting for the bus. They contract quickly, but get tired fast, as they consume lots of energy.
Most of your muscles are made up of a mixture of both slow and fast twitch muscle fibres. But, your soleus muscle in your lower leg and muscles in your back involved in maintaining posture contain mainly slow twitch muscle fibres. And muscles that move your eyes are made up of fast twitch muscle fibres.
Dark and white meat
Chickens have fast and slow twitch muscle, too. Dark meat, like in chicken legs, is mainly made up of slow twitch fibres. White meat, like in chicken wings and breasts, is largely made up of fast twitch muscle fibres. Chickens use their legs for walking and standing, which they do most of the time. This doesn't use much energy. They use their wings for brief bursts of flight. This requires lots of energy and the muscles involved tire very quickly.
Slow twitch and fast twitch muscle fibres make energy in different ways
Muscles that contain a lot of slow twitch fibres are red, because they contain lots of blood vessels. Slow twitch muscle fibres rely on a rich supply of oxygenated blood as they use oxygen to produce energy for muscle contraction.
Fast twitch muscle fibres don't use oxygen to make energy, so they don't need such a rich blood supply. This is why fast twitch muscles are lighter in colour than muscles that contain a lot of slow twitch muscle fibres.
Fast twitch muscle fibres can produce small amounts of energy very quickly whereas slow twitch muscles can produce large amounts of energy slowly.