KS3 · Biology

Biomechanics: forces in the skeleton

Hold a bag of sugar in your hand, then hang it on your forearm near your elbow. Same bag, same weight. So why is one harder to hold up?

Biomechanics · the arm as a machine

Your arm is a machine: find the pivot, the pull and the distance

Showing 3 layers: Bones and joint, Muscles and tendons, Load and distance

Layers

Explore

tap a part ↓

A 50 N bag of sugar held 0.35 m from the elbow.

Tap each part of the arm to see its job. Switch off the muscles to see the bones and joint underneath.

From contraction to movement

Reason it through

Why does your forearm lift when your biceps contracts?

Link 1 of 4

First link · your turn

Your biceps gets the job of lifting your forearm. What is the first thing it does?

Locked — reveal the link above first

Antagonistic pair: biceps and triceps

Bending the arm (forearm moves up)vsStraightening the arm (forearm moves down)

The most-mixed-up row is first.

Focus

Which muscle contracts?

Bending the arm (forearm moves up)

Biceps

Straightening the arm (forearm moves down)

Triceps

The insight

The muscle doing the pulling is the one that contracts, and only one of the pair does at a time.

Which muscle relaxes?

Bending the arm (forearm moves up)

Triceps

Straightening the arm (forearm moves down)

Biceps

What carries the pull to the forearm bone?

Bending the arm (forearm moves up)

The biceps' tendon

Straightening the arm (forearm moves down)

The triceps' tendon

How does the forearm move?

Bending the arm (forearm moves up)

Turns up, about the elbow

Straightening the arm (forearm moves down)

Turns down, about the elbow

Your turn: fill the gaps

The moment of a dumbbell

You hold a 2 kg dumbbell in your hand, 40 cm from your elbow. What is its moment about the elbow? (Use 1 kg = 10 N.)

The forearm turns about the elbow, so the elbow is the pivot. Distances are measured from there.
missing step

Which line is step 2?

Force: 2 kg × 10 = 20 NForce: 2 N, because the mass is 2 kgForce: 2 kg ÷ 10 = 0.2 N

What do you really think?

Lifting a bag of sugar

You're holding a bag of sugar in your hand and bending your arm to lift it.

Which of these is right? Pick the one closest to what you think right now.

My biceps is fixed straight onto my forearm bone, so it tugs the bone directly.My biceps and triceps both contract together to lift the bag.To work out the bag's turning effect, I measure the distance from my shoulder to the bag.My biceps contracts and pulls a tendon, which pulls my forearm bone round my elbow.

How sure are you?

I'm sureI think soI'm guessing

My biceps is fixed straight onto my forearm bone, so it tugs the bone directly.

This is a common misconception

There's a link in the middle. Muscles aren't attached directly to bones: tendons join them. The biceps pulls its tendon, and the tendon pulls the forearm bone.

My biceps and triceps both contract together to lift the bag.

This is a common misconception

They're a pair, but they take turns. To bend your arm, the biceps contracts while the triceps relaxes. To straighten it, the triceps contracts and the biceps relaxes.

To work out the bag's turning effect, I measure the distance from my shoulder to the bag.

This is a common misconception

The forearm turns about the elbow, not the shoulder. The elbow is the pivot, so measure from the elbow to where the force acts.

My biceps contracts and pulls a tendon, which pulls my forearm bone round my elbow.

This matches the evidence

Spot on. The pull goes muscle → tendon → bone, and the bone turns about the elbow, the pivot.

A tendon sits between muscle and bone, the pair take turns, and distances start at the pivot.

Put it in your own words

Biomechanics in sport

A sports scientist is working with a sprinter. Explain how biomechanics can be used to help the sprinter improve their performance. [4 marks]

Your answer

0 words · your answer stays on this page and is not sent anywhere.

The mark scheme

Scientists study how the sprinter's skeleton and muscles interact (work together) as they move. (1 mark)
They measure the force exerted by different muscles. (1 mark)
They measure the energy needed to move. (1 mark)
They use these measurements to advise the sprinter. (1 mark)

Model answer

Biomechanics looks at how the sprinter's skeleton and muscles interact as they run. The scientist measures the force exerted by different muscles and the energy the sprinter needs to move. They then use these results to advise the sprinter on how to improve.

Study the interaction, measure the forces and the energy, then advise.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

What you need to know

Biomechanics is the study of how the body moves: muscles, bones, tendons and ligaments all work together.
When a muscle contracts it shortens and pulls. Tendons attach muscle to bone, so the pull reaches the bone through a tendon.
Muscles work in antagonistic pairs, like the biceps and triceps: when one contracts, the other relaxes.
A moment is the turning effect of a force. A pivot is the point something turns around; in the arm it is the elbow.
moment (N m) = force (N) × distance from the pivot to the force (m).

The big picture

Muscles move bones by contracting and pulling on tendons, which pull the bone so it turns about a joint acting as a pivot. Biceps and triceps are an antagonistic pair: one contracts while the other relaxes. The turning effect of a force about a joint is its moment: moment = force × distance from the pivot.

Key points

1A muscle isn't attached directly to a bone: its pull reaches the bone through its tendon.

2Bend the arm: biceps contracts, triceps relaxes. Straighten it: triceps contracts, biceps relaxes.

3The bone turns about the joint, so the joint is the pivot — the elbow, for your forearm.

4Moment = force × distance, with the distance measured from the pivot to where the force acts.

5Turn a mass into a force before you use it: in this lesson, 1 kg = 10 N.

6In sport, scientists measure the force exerted by different muscles and the energy needed to move, then use it to advise athletes.

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Worked example

Problem

Priya carries a 3 kg watermelon resting on her palm, 30 cm from her elbow, with her forearm level. Calculate the watermelon's moment about her elbow. (Use 1 kg = 10 N.)

⚠ Watch out

Putting the mass straight into the equation. A 3 kg bag is not a 3 N force: turn kilograms into newtons first (1 kg = 10 N in this lesson), and measure the distance from the pivot, not from anywhere else.

🧠

Memory hook

Muscles pull, tendons pass it on, bones turn. For the turning effect: find the pivot, find the force, measure between them — then multiply.

✓

Check yourself

From memory: what joins a muscle to a bone? What does the triceps do while the biceps contracts? What is the moment of a 1.5 kg bag held 0.4 m from the elbow?

Flashcards

(13)

What is biomechanics?

The study of how the body moves: muscles, bones, tendons and ligaments working together.

What happens to a muscle when it contracts?

It shortens, and as it shortens it pulls.

What do tendons do?

They attach muscles to bones. The muscle pulls the tendon; the tendon pulls the bone.

What is an antagonistic pair?

Two muscles that work together: when one contracts, the other relaxes.

Which antagonistic pair moves your forearm?

The biceps and the triceps.

Bending vs straightening your arm: which muscle contracts each time?

Bending: biceps contracts, triceps relaxes. Straightening: triceps contracts, biceps relaxes.

What is a moment?

The turning effect of a force.

What is a pivot, and where is it in your arm?

The point an object rotates around. In your arm, it's the elbow joint.

Write the equation for a moment.

moment = force × distance, where the distance is from the pivot to the force.

What are the units of moment, force and distance?

Moment in newton-metres (N m), force in newtons (N), distance in metres (m).

How do you turn a mass into a force in this lesson?

Multiply by 10: 1 kg = 10 N, so 6 kg is 60 N.

A force is moved twice as far from the pivot. What happens to its moment?

It doubles.

How is biomechanics used in sport?

Scientists look at how skeleton and muscles interact, measure muscle forces and the energy needed to move, and use this to advise athletes.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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The moment of a dumbbell

Lifting a bag of sugar

Biomechanics in sport

More KS3 Biology topics