Final answer:
The biceps brachii is not an efficient elbow flexor in a pronated position because the muscle is also a supinator, which places it at a mechanical disadvantage when pronated. The brachialis overrides the role of biceps in flexing the elbow in this position, and the brachioradialis becomes more effective for elbow flexion between the ranges of pronation and supination.
Step-by-step explanation:
The biceps brachii are not the best flexors at the elbow when the forearm is in a pronated position due to a few anatomical factors. Firstly, when the forearm is pronated, the biceps brachii is in a less favorable position to exert maximal force because its function also involves supination of the forearm. In a pronated position, the radius and ulna are crossed, which places the biceps tendon in a mechanically disadvantaged alignment for flexion.
Secondly, the brachialis muscle, located deep to the biceps brachii, becomes more responsible for elbow flexion in this position since its attachment and action are not affected by the rotation of the forearm. Lastly, the muscle's line of pull is less efficient in a pronated position, reducing the overall torque generated during flexion.
The brachioradialis is more effective in flexing the elbow when the arm is in a mid-position between pronation and supination, taking over the role of producing forceful flexion when the biceps brachii is at a mechanical disadvantage.