Alec Titan
The human body is a fascinating machine, with muscles that contract and relax to move our bones and enable us to perform a variety of physical activities. However, despite their strength and versatility, muscles can only pull; they cannot push. This may seem counterintuitive, especially when we consider actions like opening a door by pushing. So, how does this work? When a muscle contracts, it shortens and thickens, but this only results in movement because it is attached to bones via tendons. When the muscles in your upper arm contract, they pull your lower arm bones towards your shoulder, bending your arm. To straighten your arm, a different set of muscles pulls your arm in the opposite direction. This interplay of flexors and extensors is what allows us to move our limbs with ease.
| Characteristics | Values |
|---|---|
| How muscles move body parts | By contracting and then relaxing |
| Whether muscles can push bones | No |
| Whether muscles can pull bones | Yes |
| What happens when a muscle is relaxed | It is longer and thinner |
| What happens when a muscle is tensed | It contracts to get shorter and thicker |
| What happens when the biceps muscle is contracted | The elbow bends |
| What happens when the triceps muscle is contracted | The elbow straightens |
| What happens when the upper arm muscles are tensed | The lower arm bones are pulled towards the shoulder, bending the arm |
| What happens when the muscles of the upper arm tighten | The arm bends |
| What happens when the tricep muscle tightens | The arm straightens |
| What happens when the facial muscles are contracted | The eyebrows are raised or the nose is wriggled |
| What happens when the tongue is stuck out | The facial muscles attached to it contract |
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What You'll Learn
Muscles can only pull, not push
Our muscles move our bodies by pulling or relaxing. When a muscle is relaxed, it is longer and thinner. When you tense a muscle, it contracts, becoming shorter and thicker. If the muscle was not attached to anything, this would not have much of an impact. However, muscles are attached to our bones by tendons. When you tighten the muscles in your upper arm, they get shorter, pulling your lower arm bones towards your shoulder and bending your arm.
Muscles can only pull bones, not push them back to their original position. They work in pairs of flexors and extensors. The flexor contracts to bend a limb at a joint. Then, when the movement is completed, the flexor relaxes, and the extensor contracts to extend or straighten the limb at the same joint. For example, the biceps muscle, at the front of the upper arm, is a flexor, and the triceps, at the back of the upper arm, is an extensor. When you bend your elbow, your biceps contract. Then, the biceps relax, and the triceps contract to straighten the elbow.
Facial muscles are different from those in the rest of the body. Instead of attaching directly to bone, they attach under the skin, allowing for small, precise movements. The tongue is an example of this, being made of a group of muscles that work together to allow speech and chewing.
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Muscles contract and relax to move body parts
Our muscles move by contracting and relaxing. They are attached to our bones by tendons, and they can only pull on these bones to create movement. When a muscle contracts, it becomes shorter and thicker. This pulls the bone it is attached to, causing movement. However, muscles cannot push; they can only pull.
For example, when you bend your arm, the muscles in your upper arm contract, pulling your lower arm bones toward your shoulder. To straighten your arm, a different set of muscles must pull your arm in the opposite direction. These muscles are called extensors. In the case of your arm, the tricep is the extensor. When the flexor contracts to bend the arm, the extensor relaxes. Then, when the movement is complete, the flexor relaxes, and the extensor contracts to straighten the arm again.
The bicep is the flexor in this scenario, and it is located in the front of the upper arm. The tricep, or the extensor, is at the back of the upper arm. This pair of muscles works together to allow the bending and straightening of the arm.
The mechanism of muscle contraction involves a multistep molecular process within the muscle fiber. It begins when acetylcholine binds to receptors on the muscle fiber membrane, opening membrane channels. This allows an influx of sodium ions into the muscle fiber cytoplasm, triggering the release of calcium ions. The calcium ions diffuse into the muscle fiber, and the relationship between the chains of proteins within the muscle cells changes, leading to contraction.
The contraction of skeletal muscle is a voluntary process, controlled by the brain and nervous system. Smooth and cardiac muscles, on the other hand, are involuntary and regulated by reflexes and the autonomic nervous system. Smooth muscles are found in the walls of blood vessels, the digestive system, and various organs, and they help maintain blood pressure and move food through the body. Cardiac muscles make up the heart's walls and contract rhythmically to pump blood.
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Muscles are attached to bones by tendons
Tendons play a crucial role in connecting muscles to bones, allowing for the movement of body parts. Muscles themselves can only pull or relax; they cannot push. When a muscle contracts, it becomes shorter and thicker. If the muscle were not attached to anything, contracting it would not result in any significant movement. However, because muscles are attached to bones by tendons, they can pull on those bones and create movement. For example, when you bend your elbow, the biceps muscle in the front of your upper arm contracts, pulling the lower arm towards the shoulder and bending the arm.
To straighten the arm back out, a different set of muscles is required to pull in the opposite direction. In this case, the tricep muscle, located at the back of the upper arm, contracts and pulls the arm back to a straight position. This process of muscles working in pairs, with one muscle contracting to bend a joint and another muscle contracting to straighten it, is common throughout the body. The contracting muscle is called the flexor, while the muscle that straightens the limb is the extensor.
The musculoskeletal system, comprising bones, muscles, and joints, enables us to perform everyday physical activities. Muscles help to hold the skeleton together, give the body its shape, and facilitate voluntary movements. Skeletal muscles, in particular, can contract quickly and powerfully, but they can also tire easily. Smooth muscles, on the other hand, are found in the walls of the stomach and intestines, where they help with digestion, and in blood vessels, where they maintain blood pressure.
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Muscles work in pairs of flexors and extensors
Muscles can only pull bones and cannot push them back to their original position. Therefore, muscles work in pairs of flexors and extensors. The former contracts to bend a limb at a joint, and the latter extends or straightens the limb at the same joint. For example, the biceps muscle, a flexor, contracts to bend the elbow, and then relaxes. The triceps muscle, an extensor, then contracts to straighten the elbow.
Flexor muscles are a type of skeletal muscle that bends or decreases the angle between two bones or body parts at a joint, resulting in flexion. They contract, pulling on tendons attached to bones, and bringing the connected bones closer together. Extensor muscles, on the other hand, are also a type of skeletal muscle that straightens or increases the angle between two bones or body parts at a joint, causing extension movements. They also contract, pulling on tendons attached to bones, but they move the connected bones away from each other.
Flexors and extensors work together to facilitate quick changes in direction and provide balance and stability. They also contribute to maintaining posture and smooth movement. For example, when performing rapid activities like jumping out of the way, flexors help with rapid deceleration, while extensors aid in accelerating and changing direction. Extensors play a vital role in providing the force required for efficient running and jumping. The push-off phase during these activities relies on extensor muscles such as the calf muscles and quadriceps.
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Muscles can't push bones back to their original position
Muscles can only pull bones and cannot push them back to their original position. This is because muscles move body parts by contracting and then relaxing. When a muscle contracts, it gets shorter and thicker. When it relaxes, it lengthens and thins out. If a muscle is not attached to anything, contracting it will not result in any movement. However, muscles are attached to bones by cord-like tendons. When a muscle contracts, it pulls on the bone, causing movement.
Take the example of bending and straightening your arm. When you bend your arm, the biceps muscle in the front of your upper arm contracts, pulling the bones of your lower arm towards your shoulder. When you straighten your arm, the biceps muscle relaxes, and the triceps muscle at the back of your upper arm contracts, pulling your lower arm bones away from your shoulder.
The triceps and biceps muscles work in tandem as a pair of flexors and extensors. The flexor (biceps) contracts to bend a limb at a joint, and when the movement is completed, it relaxes, allowing the extensor (triceps) to contract and extend or straighten the limb at the same joint.
To summarise, muscles can only pull and cannot push. When a muscle contracts, it pulls on the bone it is attached to, causing movement. When the muscle relaxes, the bone returns to its original position, but the muscle itself does not push the bone back.
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Frequently asked questions
Muscles can only pull or relax. They cannot push. When a muscle is relaxed, it is longer and thinner. When you tense the muscle, it contracts, getting shorter and thicker.
Muscles move body parts by contracting and then relaxing. Muscles can pull bones, but they can't push them back to their original position. So, they work in pairs of flexors and extensors.
When you bend your elbow, the bicep muscle (flexor) contracts. Then, the bicep relaxes, and the tricep muscle (extensor) contracts to straighten the elbow.
Muscles are attached to bones by cord-like tendons. Tendons are long, string-like tissues that attach muscles to bones.
The bicep and tricep are examples of muscles that pull. When the bicep contracts, it pulls the lower arm bones toward the shoulder, bending the arm.
