Arm Weight Lab

This extends the original supported-arm model. Gravity is on by default. Turning it off counterbalances the forearm and load, restoring the original behavior: relaxing both muscles leaves the arm at its current angle, and the triceps must contract to straighten it.

With gravity on, the model compares turning effects (torques) around the elbow, not just muscle force with the load's weight. The biceps pulls near the elbow, while the hand's weight acts farther away. Gravity's turning effect is zero when the forearm hangs vertically and generally increases as it approaches horizontal. A small biceps may start lifting from the bottom but stall later as the turning effects change.

Biceps maximum force is proportional to the size setting (modeled cross-sectional area). Its drawn thickness scales with the square root of that setting. Its belly also thickens as it shortens during a lift. Real strength depends on factors beyond visible muscle size; the settings are illustrative, not a prediction of anyone's ability.

The weight is placed at the hand. Both its mass and its drawn dimensions increase together; dimensions scale roughly with the cube root of mass. Gravity includes an illustrative 1.2 kg forearm as well as the adjustable load. A downward arrow shows weight direction. The comparison bars show upward versus downward turning effects on a shared scale.

The triceps keeps a fixed maximum force and supplies a straightening torque. With gravity on it adds to gravity. It is not needed to make an unsupported raised forearm fall. In real life, controlled lowering can use an active biceps that lengthens under load. This model allows an activated biceps to lengthen if gravity's torque exceeds its torque, but does not include voluntary force control or simultaneous muscle activation.

Motion is damped: the arm moves in the direction of the net torque, slows near balance, and does not swing or bounce. Muscle and tendon attachment geometry determines the turning leverage. This is a teaching simulation; numerical force and size settings are not medical, exercise, or biomechanical measurements.

Tendons keep fixed model lengths and do not actively contract. The two forearm bones are rigid and move together. Muscle heads and attachment locations remain simplified. The anatomy view now includes contoured humerus, radius and ulna shafts and ends, an ulna hook around the elbow, cartilage-colored joint surfaces, tapered fibrous tendons, eight schematic wrist bones, palm bones, and three finger-bone segments per finger (two for the thumb). The hand opens with an empty palm and curls around a dumbbell when a load is present. This is a two-dimensional teaching illustration rather than a dissection-accurate anatomical reconstruction. The movement limits are 0–135°. At a limit, a muscle can remain active without shortening. Reset repositions the arm but keeps the selected experiment settings.

The tendon toggle represents a complete break in the modeled biceps tendon near its radius attachment. The belly can retract and thicken, but its force transmitted to the radius becomes zero. The triceps remains connected. With gravity on, the load may lower the forearm despite visible biceps activity; with gravity off, biceps activity alone cannot move the bone. This model only includes the biceps as an elbow flexor. Real arms also have brachialis and brachioradialis muscles, so a torn biceps tendon does not necessarily prevent elbow bending.

The bones toggle is a cartoon thought experiment about structural support. It removes all drawn arm bones and the elbow hinge, and replaces the rigid arm with a stylized pile of soft tissues on a supporting surface. Muscles can bunch, but neither can lift a rigid forearm or act through a joint that is absent. This is not a simulation of tissue mechanics, circulation, injury, or what an actual arm without bones would look like. The weight is resting on the surface rather than lifted. Both torque bars become zero because there is no modeled bony lever.

Restoring parts with the checkboxes or reset is a simulation reset, not a representation of healing. Reset restores the tendon and bones, relaxes both muscles, and straightens the arm while retaining the weight, muscle-size, gravity, and speed settings.

Background: OpenStax: forces and torques in muscles · Muscle force and cross-sectional area.