1Bones are levers
A lever turns about a pivot. In the forearm, the elbow is the pivot, the biceps gives the effort, and the ball in the hand is the load.
leverpivoteffortload
Lesson 13.3 notes
Levers and muscle forces
Bones as levers, the moment of a force, and measuring the forces muscles exert.
2The moment of a force
The turning effect of a force is its moment. Moment = force × distance from the pivot. A 20 N ball 35 cm from the elbow has a moment of 700 N cm.
moment
3Balanced moments
To hold the ball still, the biceps' moment must equal the ball's (we ignore the forearm's weight). The biceps pulls 5 cm from the elbow, so it needs 700 ÷ 5 = 140 N.
momenteffort
4Small pull, big movement
Because the biceps pulls close to the elbow, a small, quick contraction moves the hand a long way, fast. The forearm lever trades force for speed and range of movement.
lever
5Standing on tiptoe
On tiptoe, the foot is a lever. The pivot is the ball of the foot. The calf muscle pulls up on the heel (the effort) to lift your weight (the load).
pivoteffortload
6Measuring muscle forces
Forces are measured in newtons (N) with a force meter. A hand-grip meter measures how hard the hand squeezes. A newton meter's spring stretches further for a bigger pull.
newtonforce meter
7A fair test
To compare muscles fairly, test the same person, the same way every time. Do each test three times and work out the mean. The leg muscles gave the biggest force.
fair testmean
Explore: Forearm lever lab
Change the load and where it is held, and see how hard the biceps must pull to keep the forearm still.
Common mistake: The biceps pulls harder than the load it holds
Holding a 20 N ball 35 cm from the elbow, the biceps pulls with about 140 N, because it is attached just 5 cm from the elbow. The two moments are equal; the two forces are not.
Remember
- Bones are levers and joints are pivots. Muscles give the effort that moves the load.
- Moment (N cm) = force (N) × distance from the pivot (cm). When something is balanced, the clockwise and anticlockwise moments are equal.
- Muscle forces are measured in newtons (N) with a force meter, which measures the force at the hand or foot, not the pull inside the muscle. Repeat each test and work out the mean.
Going further at GCSE