Final answer:
The moment arm is 4.0 cm when the forearm is at a 90° angle to the upper arm due to the biceps tendon attachment. The actual length of the moment arm may vary depending on the angle of the forearm relative to the force applied by the biceps muscle.
Step-by-step explanation:
The moment arm in biomechanics is the perpendicular distance from the axis of rotation (in this case, the elbow joint) to the line of action of the force (tendon attachment point). If the tendon for the biceps muscle is connected to the forearm at a point 4.0 cm from the elbow joint, we must consider the orientation or angle of the forearm to the applied force to determine whether the actual moment arm is longer, shorter, or equal to 4.0 cm.
Typically, in biomechanics, the moment arm may change with the angle of application. If the biceps is contracting with the forearm at a 90° angle to the upper arm, the moment arm will indeed be 4.0 cm. However, if the forearm is at a different angle, the moment arm may be shorter due to the projection of the force vector on the axis perpendicularly.