Muscles pull
A muscle contracts to pull through tendons rather than push a bone.
Muscles and Joints
Adjust opposing muscle pulls and watch a hinged forearm model move.
Grades 2–5 · Biology
Your goal
Use flexor pull 2 and extensor pull 0. Observe forearm rotation of 120 degrees from the horizontal guide.
Reduced motion is on. The controls still update the scene.
Flexor pull 0 and extensor pull 0. Forearm rotation is 90 degrees from the rightward horizontal guide. The pulls balance at the middle position in this model.
Muscles can contract to pull on bones through tendons. An opposing pair pulls in different directions around a joint. The model maps the difference between two pull settings to forearm rotation; the rotation guide is not a clinical elbow angle. Real movement also involves loads, leverage, nerves and many muscles. Both muscles can be active together.
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Learn with Muscles and Joints
Muscles and Joints introduces elementary students to an opposing pair of pulling muscles. Adjust flexor and extensor pull settings and watch the forearm rotate around a visible elbow hinge. Stronger flexor pull bends the model arm, while stronger extensor pull opens it. Equal pull settings return the model to its middle position. Goals include movement in both directions and balanced opposing pulls. The diagram measures rotation from a horizontal guide and explains that this simplified relationship is not a clinical elbow measurement or a complete description of muscle mechanics.
What to look for
Try opposite pulls
Increase each muscle separately and compare movement directions.
Balance the pair
Both muscles can be active together in this model.
For teachers and curious learners
Explain how opposing muscles pull bones around a hinge joint. Ask students to predict what will happen, change one setting, and explain what they observe. Learn gives a starting point. Practice and Challenge offer new goals with the same scene.
Illustrative mapping from opposing pull settings to forearm rotation. Loads, leverage, nerves and real muscle mechanics are simplified; rotation is not a clinical joint angle.