Final answer:
The statement is false; changing the angle of the elbow does change the biceps muscle's moment arm. The moment arm is dependent on the angle of the elbow, and it affects the mechanical advantage and the force the biceps can exert.
Step-by-step explanation:
Changing the angle of the elbow does indeed change the moment arm of the biceps muscle, making that statement false. The moment arm is the perpendicular distance from the axis of rotation (elbow joint) to the line of action of the force (biceps muscle). When the elbow is flexed at 90°, the biceps muscle has an optimal length for force generation. As the angle of the elbow changes, the length of the biceps muscle changes, thus affecting its force. The moment arm for the biceps will be largest when the forearm is perpendicular to the force of the muscle; when the elbow is fully flexed or extended, the moment arm becomes smaller.
Moreover, when you lift a weight with your arm, the torque created by the biceps muscle must counter the torque from the weight. As the forearm moves closer to vertical, the distance from the weight to the elbow decreases, reducing the torque from the weight and, in turn, lessening the force required from the biceps to hold or lift the weight. Hence, the angle at which the forearm is held affects the mechanical advantage of the biceps muscle and alters the moment arm.