Class 9 BiologyChapter 3NCERT 3.4 and 3.4.1
The musculoskeletal system: muscles pull, bones move
A muscle can only pull. So how do you ever straighten your arm again?
Think it over
- Put one hand on the front of your upper arm and bend your elbow. What do you feel happen, and what happens at the back of the arm?
- Roughly how much of your body weight do you think is bone: a twentieth, a seventh, or a half?
- You can swing your arm in a big circle but not your forearm. What is different about the shoulder and the elbow?
In one minute
- The musculoskeletal system is the team of bones, muscles and joints, held together by cartilage, tendons and ligaments.
- It holds you upright, moves you, keeps your posture and protects delicate organs, all under the control of the nervous system.
- Muscles pull on bones through tendons; the force turns the bone at a joint.
- An adult skeleton is about 12 to 15 per cent of body weight on average.
- A joint is where two or more bones meet. Joints allow movement but cannot move bones by themselves; muscles do that.
Bones, muscles and the bands that join them
Six kinds of parts work as one system whenever you move: bones, muscles, joints, cartilage, tendons and ligaments. Together they are the musculoskeletal system. It keeps you standing, moves you, holds your posture and shields delicate organs, and it takes its orders from the nervous system.
The rule that makes it work: muscles pull, they never push. A muscle is joined to a bone by a strong, flexible band, the tendon. Shorten the muscle and its tendon tugs the bone, which then pivots at a joint. Drag the slider to bend the arm.
A muscle can only pull. The front muscle bends the elbow; the back muscle straightens it. When one contracts, the other relaxes.
Our drawing. Both muscles get their orders from the nervous system (yellow lines). You may also meet their names: biceps at the front, triceps at the back.
Since a muscle can only pull, one muscle can bend the elbow but cannot straighten it again. Muscles therefore work in pairs that pull in opposite directions: when one contracts, its partner relaxes and is stretched.
- Tendon
- Strong, flexible band joining a muscle to a bone; passes on the muscle's pull.
- Contract
- A muscle getting shorter (and thicker) as it pulls.
Quick check
What happens when the muscle at the front of your upper arm contracts?
Show the answer
B. It pulls the forearm up through its tendon and the elbow bends A contracting muscle shortens and pulls; the tendon carries the pull to the forearm bone, which swings up at the elbow.
How much of you is bone and muscle?
Weigh an adult and, on average, about 12 to 15 per cent of the reading is skeleton; the exact share shifts with age, gender and build. Muscle is a bigger share: on average about 40 to 50 per cent in adult men and about 30 to 40 per cent in adult women. Put in a body weight to turn those averages into kilograms.
- Bone (12 to 15 per cent)
- 6 to 7.5 kg
- Muscle (40 to 50 per cent)
- 20 to 25 kg
- Everything else
- 17.5 to 24.0 kg
Working: 50 kg × 0.12 = 6 kg and 50 kg × 0.15 = 7.5 kg of bone.
These are adult averages from the book. For a growing student they are only a rough guide. Everything stays on this page; nothing is saved or sent anywhere.
Collect the class results and compare. They differ because people differ in age, height, build and how active they are, and the percentages are only averages: for teenagers who are still growing they can be quite different. Treat your answer as a rough estimate, not a measurement.
Quick check
An adult weighs 60 kg. Using 15 per cent, about how much of that weight is bone?
Show the answer
B. 9 kg 15 per cent of 60 kg is 0.15 × 60 = 9 kg.
The musculoskeletal system in action
Move your elbow, shoulder, knee, neck, fingers, toes and wrist one at a time. Which can turn all the way round, which can turn part of the way, which only bend? Fill in the table the way Table 3.5 asks, then compare.
| Body part | Complete rotation | Partial rotation | Bending | Other: turning, side-raising, up and down |
|---|---|---|---|---|
| Elbow | No | No | Yes | No |
| Shoulder | Yes | Yes | Yes | Yes |
| Knee | No | No | Yes | No |
| Neck | No | Yes | Yes | Yes |
| Fingers | No | No | Yes | Yes |
| Toes | No | No | Yes | Yes |
| Wrist | No | No | Yes | Yes |
- Elbow. Hinge joint: bends and straightens only.
- Shoulder. Ball and socket joint: forwards, backwards, sideways and full circles.
- Knee. Hinge joint, like the elbow.
- Neck. Pivot joint turns the head part way; the neck also nods and tilts.
- Fingers. Finger joints bend; fingers can also spread apart.
- Toes. Toes bend up and down and spread a little.
- Wrist. The wrist bends and moves side to side. Turning the palm over comes mostly from the forearm.
Some parts move freely in many directions and others only one way. The reason is the kind of joint at each place. Wherever two or more bones meet, there is a joint. It allows movement, but it cannot move the bones on its own: the pull of muscles does that. The next page looks at the types of joint.
Quick check
Your elbow only bends and straightens. In Table 3.5, which column gets a "Yes" for the elbow?
Show the answer
C. Bending The elbow cannot rotate fully or partly; it only bends, like a door on its hinge.