How Does A Muscle Contract?
The bones and joints form an important part of the skeletal system. However, they would be incomplete without the use of muscles.
Muscles are long bundles of contractile tissue that form the basis of all movements and give contour to the body.
The three important types of muscles found in the human body are:
- skeletal muscles;
- cardiac muscles;
- smooth muscles.
Types of muscles
Let us take a look at the types of muscles found in the human body.
The first type of muscles the skeletal muscles or striated muscles. They enable us to carry out many movements. There are more than 650 skeletal muscles in our body.
A skeletal muscle usually has two ends: a fixed end where the muscle originates and a movable end that pulls another part.
Skeletal muscles joint bones together and are held in place by tendons. Tendons are chords of tough tissue that connect bones to muscles.
Skeletal muscles often work in pairs while one contracts the other relaxes. For example, the bicep muscle in the upper arm contracts when the lower arm is raised up. At this time the tricep muscle relaxes to straighten the arm out again. The triceps muscle contracts while the bicep muscle relaxes. Such muscles that cause opposing movements are known as antagonistic muscles. They are found in the thigh as well.
As muscles contract, they become shorter and thicker. The process of contraction is a result of a complex series of events that take place within the muscle cells. Muscles are composed of long thin cells called muscle fibers. They are packed in bundles and run through the entire length of the muscle in the body.
Each muscle contains individual contractile units called myofibrils. Myofibrils are cylindrical multinucleated cells. Myofibrils are composed of smaller units known as sarcomeres. Each sarcomere contains two long fibrous proteins.
The thicker filament of protein is known as myosin, while the thinner filament is known as actin. When a muscle receives nerve impulses these filaments slide over each other and the muscle contracts. The filaments in the sarcomeres are responsible for the striation of skeletal muscles.
Skeletal muscles can contract and relax rapidly but are easily fatigued. As skeletal muscles are controlled by the somatic nervous system. They can be consciously controlled. Such muscles are also referred to as voluntary muscles. When you push your shoulder blades together, you are using your trapezius muscle. This is an example of a voluntary muscle.
The second important type of muscle is the cardiac muscle, also known as the myocardium. It is found exclusively in the heart. Cardiac muscles make up the walls of the heart and a capable of quick and rapid movement. Unlike skeletal muscles, cardiac muscles do not tire easily. With every contraction of the cardiac muscles, blood is pumped out of the atria and ventricles into numerous blood vessels. Cardiac muscles are thinner and shorter than skeletal muscles and have a branch-like structure. However, they too contain a shaky myosin which gives the cardiac muscles the striated appearance. The cardiac muscle cannot be controlled consciously. So, we refer to it as an involuntary muscle
The third type of muscle in the human brain is the smooth muscle or the non- striated muscles. They can be found in structures such as the esophagus and organs such as the stomach and the intestines. Smooth muscles perform slow long-term contractions. For example, peristalsis is a result of contraction of smooth muscles.
Unlike skeletal muscles, smooth muscles are not under our control. They are involuntary muscles and non-striated. However, there are some muscles that are both voluntary and involuntary. The eyelids are an example of this type of muscle.
Movements in our body are made easier due to the lever action in the skeleton. The level of the first order is where the power and the weight act with a fulcrum in between. This is represented by the extension of the arm at the elbow by the action of the triceps muscle. The lever of the second order is where the fulcrum and power are at each end with a weight in between. This is represented by the calf or gastrocnemius muscle that raises the weight of the body on the toes.
The level of the third-order is where the fulcrum and the weight are at each end with par in between. This is represented by the bicep muscle when flexing the arm.
Thus, the various muscles of the human body make movement and locomotion possible.
Muscle Contraction Types
As muscles contract, they pull on tendons, which in turn pull on bones. This creates force and movement.
There are three types of contractions:
This seems like a pain to memorize until you give some thought to what the words mean. ISO means equal. So, when we categorize our muscle contractions we name them according to what stays the same throughout the contraction.
We measure three things as we categorize contractions:-
- tension across the muscle;
- the length of the muscle;
-the amount of energy required by the muscle.
In isotonic contractions, the muscle tension stays the same. The length of the muscle changes as the muscle contracts and pulls on the bone. The amount of energy required by the muscle may change, but the tension across the muscle stays the same.
If the force the muscle is producing is greater than the opposing force, the muscle shortens. This is called a concentric contraction. If the force the muscle is producing is less than the opposing force, the muscle lengthens. This is called an eccentric contraction.
So, if this biceps muscle is being used to do curls, energy goes into the muscle to cause it to contract and pull on the tendon, which pulls on the bone. On the way up the muscle, force overcomes the weight of the dumbbell. This is a concentric contraction. On the way down the muscle is still contracting and creating a force to oppose the dumbbell. If it didn't the arm would just fall with the weight of the dumbbell. But the weight of the dumbbell is overcoming the force created by the muscle and the muscle lengthens. This is an eccentric contraction.
So, isotonic contractions are the regular weight lifting contractions that we normally think of.
Concentric isotonic contractions occur as the muscle overcomes the opposing force and shortens.
Eccentric isotonic contractions occur when the force created by the muscle is less than the opposing force and the muscle lengthens.
In isometric contractions, the muscle length stays the same. The tension between the muscle may change and the energy required by the muscle may change in order to maintain the length, but the length stays the same.
Think of the metric system being used to measure length. Isometric maintains an equal length. If this biceps muscle wants to maintain a constant length it may require more tension and more energy depending on how much weight is added. But the length stays the same and the bones the muscle is playing on don't move.
Doing pull-ups requires isotonic contractions. The invisible chair requires isometric contractions. The pull-ups are a dynamic exercise, where there is movement. The invisible chair is a static exercise where there is no movement. In isokinetic contractions, the energy required by the muscle stays the same throughout the contraction. The tension across the muscle may change and the length of the muscle changes.