Back to the experimentTHE EVIDENCE BEHIND THE EXPERIENCE

A small distance. A big internal pull.: sources & model

Move a load closer to an elbow on a transparent teaching arm. See the force change, then explore how muscles can work without visible movement.

Scientific review · independent subject review pending

The source and model records are available for inspection. No external scientific reviewer has signed off yet.

muscles-joints-1 · content 1 · setup format 1

What supports the explanation?

A short muscle moment arm can require a large tension and joint reaction.

Worked example 9.4 supplies the horizontal-arm parameters and reference values.

OpenStax · Muscles and torques

Flexors and extensors act through different attachments around joints.

Anatomical roles of biceps, brachialis and triceps; the render is a functional analogy, not a reproduction of these figures.

OpenStax · Upper-limb muscles

Sliding filaments explain shortening without shrinking the filaments.

ATP and calcium have distinct roles in cross-bridge cycling. Molecular dynamics are not modeled.

OpenStax · Muscle contraction

What this model assumes

Original articulated mechanical arm analogy, not a medical scan or a detailed anatomical attachment map. One equivalent flexor stands for several muscles.
The fixed horizontal statics model is kept separate from prescribed lifting/lowering. Bone lengths remain fixed.
No individual strength, exercise, clinical or injury predictions.
The textbook balance: Fₘ×0.040 = 2.50×9.80×0.160 + m_load×9.80×r_load, in SI units. The fixed arm/hand mass, its center of mass and the equivalent moment arm are a worked-example geometry, not measurements of you.
The elbow also supplies a force: At a 4 kg load and 0.38 m load arm, the equivalent flexor force is 470.4 N. Vertical balance needs a 406.7 N downward elbow reaction. Internal forces can greatly exceed the hand load.
One equation cannot identify every muscle: Several muscles can pull at once and their moment arms vary with pose. This one-flexor static example cannot estimate an individual biceps force or predict fatigue, injury or performance. At molecular scale, actin and myosin slide relative to one another; the filaments do not shrink.

What has been checked

Analytical reference cases, conservation or transition invariants, finite drawing commands, bounded setup parsing, discovery and route integrity are checked automatically. These checks do not establish anatomical fidelity, learner outcomes or browser/device compatibility. Independent subject review, learner trials, comprehensive accessibility review and browser video encoding checks remain pending.

Each source supports the associated claim. Sources do not certify this implementation or its visuals.

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