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.
Enable JavaScript to change the conditions and run the interactive experiment.
Muscles pull through tendons. A pull close to a joint can balance a much smaller weight farther away. Holding a pose still requires tension, even when the joint is not moving.
If the hand is held still, is the muscle necessarily doing nothing?
During an isometric hold, tension can balance external torques while the joint angle stays fixed.
The elbow lets the forearm rotate. Biceps brachii and brachialis help flex it; triceps brachii helps extend it. Tendons transmit muscle tension to bone. The virtual bench uses one equivalent flexor instead of claiming to separate every real muscle.
With the forearm horizontal, a load farther from the elbow creates more torque. Slide the load inward while keeping its mass the same. The required equivalent muscle pull falls because the external moment arm is shorter.
A muscle can generate tension while staying about the same length, shortening or actively lengthening. Lifting and lowering in the motion view are prescribed illustrations. The numerical force comparison applies only to the horizontal hold.
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.
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.
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.
Without holding a weight, bend and straighten one arm gently within an easy range. A paper arm is an alternative.
Pause in a comfortable pose. Notice that you can maintain the position even while nothing visibly moves. Do not force a stretch or keep holding to fatigue.
Sketch two bones and their joint. Add a string on each side to represent opposing pulls. Connect the idea to the virtual lever.
Can you observe muscle activity without a lifting challenge?
Optional observation, not a health test or exercise prescription. Skip body movement if uncomfortable; use the paper analogy instead.
Worked example 9.4 supplies the horizontal-arm parameters and reference values.
OpenStax · Muscles and torquesAnatomical roles of biceps, brachialis and triceps; the render is a functional analogy, not a reproduction of these figures.
OpenStax · Upper-limb musclesATP and calcium have distinct roles in cross-bridge cycling. Molecular dynamics are not modeled.
OpenStax · Muscle contractionIndependent subject review is pending.
Read the sources and model assumptions
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