What this tool does
Interactive single-segment static equilibrium with a movable muscle attachment. Pick a preset — elbow flexion, ankle plantarflexion, or knee extension — then drag the sliders to see how segment angle, muscle insertion distance, line-of-action angle, and external load change the required muscle force, mechanical advantage, and joint reaction force.
Step by step
- Pick a preset — elbow, ankle, or knee — to load a realistic starting geometry and muscle.
- Adjust the geometry with the segment angle, length, and mass sliders.
- Move the muscle — change the insertion distance from the joint and the line-of-action angle, and watch the required muscle force respond.
- Add an external load at the distal end to see the effect on muscle force and joint reaction force.
- Read the outputs: required muscle force (and × bodyweight), mechanical advantage, and joint reaction force, with a live diagram and equilibrium check.
- Export or save — download the diagram as PNG, the values as CSV, or save the session as JSON to reload later.
Tips
- Drag the insertion distance from 4 cm down to 1 cm — the muscle force climbs steeply. Short moment arms trade force for speed and range of motion.
- The equilibrium check (ΣFx, ΣFy, ΣM) should all sit at ≈ 0 (green); the joint reaction force is calculated to balance the diagram.
- Sign convention: anti-clockwise moments positive, +x rightward, +y upward.
Full reference →