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Joints and Muscles
Summary
Joints and muscles
Joints and muscles đĻ´đĒ
In this chapter, you will learn:
To find and name hinge joints and ball-and-socket joints.
About X-ray photographs of joints (Science in context).
How antagonistic muscles move the bones at a hinge joint.
Do you remember? đ¤
State three important jobs of the skeleton (all your bones).
The skeleton's jobs are to protect organs, support the body, and allow movement.
Point to these bones in your body:
Skull (head bone)
Jaw (mouth bone)
Rib cage (chest bones)
Hip (pelvis)
Spine (backbone)
A leg bone (like the femur)
An arm bone (like the humerus)
How can you tell a muscle from a bone?
A muscle is soft and can move, but a bone is hard and is still.
How do pairs of muscles make bones move?
Muscles work in pairs (one pulls, the other relaxes) to move a bone.
LET'S TALK đŖī¸
What are the most common bones that people break?
The collarbone (clavicle) and bones in the arms and legs are often broken.
Do a survey (ask around) in class to see what bones have been broken.
Extend this to thinking about family and friends to try to give a fuller answer.
If someone has broken a bone, ask them how they broke it, how it was fixed, and how long it took to heal.
Joints đ§
The place where bones meet is called a joint.
In some joints, like those in the skull, the bones are fused (stuck) together and cannot move.
Most joints, however, allow some movement.
Some joints, like the elbow or knee, are called hinge joints because the movement is like the hinge on a door.
The bones can only move forwards and backwards.
A few joints, like the hip and shoulder joints, are called ball-and-socket joints.
This is because the end of one bone forms a round shape, like a ball, that fits into a cup-shaped socket.
This allows movement backwards and forwards, from side to side, and even circular movement, like when you move your arm in a circle.
Two examples of the hinge joint are the elbow joint and the knee joint.
Two examples of the ball-and-socket joint are the hip joint and shoulder joint.
Figure 1.3 The human skeleton.
DO YOU KNOW? đ¤
The strongest bone in the body is the femur (thigh bone).
It is between your hip and knee joint.
Questions â
1. Use Figure 1.3 to find each marked joint on your own body. Can you tell which type of joint each one is by seeing how they move? Explain your answer.
A is the shoulder joint, a ball-and-socket joint (allows circular movement).
B is the elbow joint, a hinge joint (only allows movement forward and backward).
C is the hip joint, a ball-and-socket joint (allows circular movement).
D is the knee joint, a hinge joint (only allows movement forward and backward).
2. Label each letter shown on Figure 1.3 as either a hinge joint or a ball-and-socket joint.
A: Ball-and-socket joint (shoulder)
B: Hinge joint (elbow)
C: Ball-and-socket joint (hip)
D: Hinge joint (knee)
Science in context: X-ray photographs of joints đ¸
When a joint is hurt by an illness like arthritis or an injury, doctors investigate by taking X-ray photographs of the joint to help plan a course of treatment.
X-rays are a type of energy that can pass through flesh (like muscles and skin), but they are stopped by the material in bones.
This is what makes bones visible on X-ray photographs.
Figure 1.4a Photograph showing an X-ray of an elbow joint.
Figure 1.4b Photograph showing an X-ray of a hip joint.
Questions â
3. Why would a doctor use X-rays to investigate first, rather than cut open the flesh around the joint to have a look?
A doctor uses X-rays first because they are a non-invasive way to see the bones inside the body. Cutting open the flesh is a risky surgery and should only be done if needed.
4. Here are X-ray photographs of four joints. Which joints are:
a. Hinge joints
Elbow joint (Figure 1.4a) and Knee joint (Figure 1.4c)
b. Ball-and-socket joints?
Hip joint (Figure 1.4b) and Shoulder joint (Figure 1.4d)
Figure 1.4c Photograph showing an X-ray of a knee joint.
Figure 1.4d Photograph showing an X-ray of a shoulder joint.
LET'S TALK đŖī¸
What problems do people have with unhealthy joints?
Problems include pain, stiffness, and less movement.
Think about older people you may know who have arthritis (sore joints).
Or think of a time when you damaged a joint of your own in an accident or in a sporting injury.
Muscles in a hinge joint đĒ
Exercise the muscles in your upper arm to move your lower arm as shown in Figure 1.5.
[Image showing the arm moving from straight out to bent up, demonstrating a hinge joint movement] Figure 1.5 The muscles in your arm allow the hinge joint of your elbow to move like this.
The muscle action is what causes movement across a hinge joint.
To understand this, we need to look more closely at what is going on under the skin.
Muscles are made up of tissues that have the power to move.
It can contract (get shorter) or relax (return to its normal length).
A muscle is attached to two bones across a joint.
When a muscle gets shorter (contracts), it creates a pulling force.
This moves one of the bones, but the other one stays still.
For example, the biceps muscle in the upper arm is attached to the shoulder blade and to the radius bone in the forearm.
When the biceps shortens (or contracts), it pulls on the radius and raises the forearm.
Figure 1.6 Biceps on arm bones.
Figure 1.7 Triceps on arm bones.
DO YOU KNOW? đ¤
Muscles are attached to bones by tendons (strong cords).
They do not stretch or shorten when the muscles do.
Many scientists believe that the strongest muscle in the human body is the masseter muscle (or jaw muscle) which you use when you bite your food.
Muscles which move your bones are called skeletal muscles.
There are two more types of muscle:
Smooth muscle which moves food along your digestive system.
Cardiac muscle which forms the heart and pumps your blood around your body.
Antagonistic muscles đ¯
A muscle cannot lengthen or stretch itself.
It needs a pulling force to stretch it again.
This force is provided by another muscle.
The two muscles are arranged so that when one contracts (pulls), the other one relaxes (lets go).
In the upper arm, the triceps muscle is attached to the shoulder blade, humerus, and ulna.
When the biceps contracts, it raises the forearm and the triceps relaxes.
When the triceps contracts, it lowers the forearm and the biceps relaxes.
The action of one muscle causes the opposite effect on the other muscle and causes movement in the opposite direction.
The two muscles are therefore called an antagonistic muscle pair (muscles that work against each other).
The action can be summarised as follows:
Biceps contracts, triceps relaxes, lower arm raised.
Biceps relaxes, triceps contracts, lower arm lowered.
Questions â
5. Draw a diagram showing both the biceps and the triceps fully shortened (contracted).
This diagram would show the biceps as thick and short, pulling the forearm up to the upper arm. The triceps would also be shown as thick and short, pulling the forearm back.
6. Using dotted lines, draw on the position of the forearm when the biceps is fully shortened.
The forearm would be shown bent upwards, close to the upper arm.
CHALLENGE YOURSELF đ¤¯
How does a muscle feel when it contracts and relaxes?
Stand up and let your left arm hang down by your side.
Spread out the fingers of your right hand and push them into your biceps muscle (the one at the front of your upper arm).
Move your fingers around a little to feel the muscle.
Raise your left forearm (bend your arm), keeping the upper arm still, and feel the muscle with your fingertips.
Lower your forearm again and feel the muscle.
Describe any changes that you felt in the muscle.
Can you feel changes in the triceps muscle? â
This activity is part of thinking and working scientifically by setting up and carrying out a scientific enquiry.
Scientists often use an observation from an activity to set up an investigation.
The changes felt in the biceps during the 'Challenge Yourself' activity can be used to set up a hypothesis to investigate the triceps muscle.
Hypothesis
When you feel your triceps muscle and raise and lower your arm, you should feel a change in the muscles.
Prediction
Use the results of the challenge (feeling your biceps) to predict what might happen to the triceps muscle when you raise and lower your arm.
Prediction Example: When the forearm is lowered, the triceps will feel short and hard. When the forearm is raised, the triceps will feel long and soft.
Planning your enquiry
What will you do to test the hypothesis and prediction?
Plan Example: I will feel my triceps muscle while raising and lowering my forearm to see if it changes hardness and length, just as I did with my biceps.
Investigation and recording data
Write down how the triceps felt when you:
a. raised the forearm (bent the elbow).
The triceps muscle felt soft and long (relaxed).
b. lowered the forearm (straightened the elbow).
The triceps muscle felt hard and short (contracted).
Examining the results
Compare your observations when the forearm is raised and lowered.
Conclusion
Compare the examination of the results with the hypothesis and your prediction, and draw a conclusion.
Conclusion: The results support the hypothesis because the triceps muscle felt different when the arm was raised versus lowered, showing it changes state (contracts or relaxes) to move the arm.
Is your conclusion limited in some way? Explain your answer.
Yes, the conclusion is limited because I only used one person's feeling to check the results, and I did not use special measuring tools.
What improvements could be made? Explain the changes that you suggest.
Improvements could be to have more people check the feeling of the muscle, or to use a machine to measure the actual change in the muscle's size.
Questions â
7. Think about what you found out about the biceps in the challenge and the triceps in the investigation. Did you find any patterns or trends (rules)? Explain your answer.
Yes, the pattern is that muscles work in an antagonistic pair. When one muscle (biceps) contracts to pull the arm up, the other (triceps) relaxes. When the second muscle (triceps) contracts to pull the arm down, the first (biceps) relaxes. The trend is that muscles are only active when they pull, not when they push.
Modelling muscles đ ī¸
This activity helps you apply your thinking and working scientifically skills by creating and using a model to show how the joints and muscles work together to make your arm move.
Figure 1.8 shows an idea for making a model of an arm using elastic bands for muscles.
Figure 1.8 Model of an arm, using elastic bands for muscles.
Use the idea shown to make a model and show how the muscles work.
Here are some suggestions to help you make your model. You should discuss them with your teacher and perhaps search the internet for further ideas to make the model.
The bones could be made from straws or wooden craft sticks.
The joint could be joined at the hinge with a carefully inserted drawing pin (thumb tack) which has its protruding point covered by a piece of sticky tack.
Thin elastic bands could be selected and held in place on the vertical stick by a notch cut into the stick by your teacher.
You may simply wish to try and hold the lower parts of the elastic band to the horizontal stick.
Describe the analogies (things that are similar) used in your model.
Analogies: The straws/craft sticks are like the bones. The drawing pin is like the hinge joint. The elastic bands are like the muscles (they can only pull, like a contracting muscle).
What are the strengths and limitations of your model?
Strength: It clearly shows how a pulling force from a pair of things (elastic bands) causes movement at a hinge joint.
Limitation: It is a simple model and does not show the true structure of tendons, cartilage, or skin.
CHALLENGE YOURSELF đ
Use the Internet to find out about the common sport injuries in tennis, athletics, cricket, football, and any other sport of your choice.
Are some injuries found in more than one sport? Explain your answer.
This is a good opportunity for you to examine the sources of evidence you choose. Can you find any bias (one-sided opinion) in any of them?
Science extra: Muscles and sports injuries đ¤
If a muscle is suddenly stretched hard, an injury called a pulled muscle can occur.
One set of muscles which can be damaged in this way are the hamstring muscles on the back of the upper leg.
They may be damaged by running hard in a sporting race.
Another set of muscles which can also be damaged in sports are the muscles on the inside of the top of the leg from the hip to the knee in an area called the groin.
Groin injuries, called groin strains, can be caused by kicking, jumping, running, and twisting at the same time.
LET'S TALK đŖī¸
Look at the two 'Science extra' sports injuries in this chapter, then find out how frequent (often) sports injuries are in your class.
Make a list of injury types and how they happened, and then discuss what was done to help people recover from them.
Summary â¨
Hinge joints, such as elbows and knees, move like the hinge on a door.
Ball-and-socket joints, such as shoulders and hips, allow more circular motion.
X-rays can be used to take photographs of joints (Science in context).
Antagonistic muscles move the bones at a hinge joint.
End of chapter questions â
1. Name two hinge joints.
Elbow joint and Knee joint.
2. Name two ball-and-socket joints.
Shoulder joint and Hip joint.
3. How is a hinge joint different from a ball-and-socket joint?
A hinge joint only allows movement forwards and backwards (like a door). A ball-and-socket joint allows movement in many directions, including a circle.
4. When the biceps contracts, what happens to:
a. the triceps?
The triceps relaxes (gets longer).
b. the lower arm?
The lower arm is raised (pulled up).
5. What do you understand by the term 'a pair of antagonistic muscles'?
A pair of antagonistic muscles are two muscles that work against each other to move a joint. When one muscle contracts (pulls/shortens), the other muscle relaxes (relaxes/lengthens) to allow the movement.
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