Lesson 6 ·Class 9 · Skeleton — Movement and Locomotion · Flow Lab
Muscles and Levers
Muscles can only pull — so they work in pairs.
Bend your arm and straighten it again. Two muscles take turns.
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💪 Step 1 of 5 · Arm relaxed
Biceps and triceps are attached across the elbow joint by tendons. Neither is contracted.
Free play — step through the process at your own pace, and move the slider to see what changes. Switch to when you're ready to test yourself.
Because a muscle can only pull, a second muscle is needed to pull the bone back — an antagonistic pair.
What this lesson covers
Challenges to clear
- Advance to the moment that shows why muscles work in pairs.
- Step through bending and straightening.
The task
Bend your arm and straighten it again. Two muscles take turns.
What this lesson covers
- Arm relaxed — Biceps and triceps are attached across the elbow joint by tendons. Neither is contracted.
- Biceps contracts — The biceps shortens and pulls the forearm up. A muscle can only pull, never push.
- Triceps relaxes — At the same moment the triceps relaxes and lengthens, allowing the movement. The pair is antagonistic.
- The bones act as a lever — The elbow is the fulcrum, the muscle supplies the effort and the load is in the hand — a lever system.
- Straightening the arm — To straighten, the triceps contracts and the biceps relaxes. The roles simply swap.
Hint
Because a muscle can only pull, a second muscle is needed to pull the bone back — an antagonistic pair.