Final answer:
In elbow flexion against gravity, the biceps must exert a force greater than the weight being lifted due to the mechanics of the lever arm with the elbow as the pivot point. This force depends on the weight's distance from the elbow and the arm's angle. As angles change, so does the biceps' required force.
Step-by-step explanation:
During elbow flexion, the role of the biceps brachii, a two-headed muscle, is vital. This muscle spans across the shoulder and elbow joints, allowing for the flexion of the forearm. Specifically, when lifting an object against gravity, the biceps muscle must contract. In this position, the force required by the biceps is significantly higher than the weight of the object being lifted. This is due to biomechanics, where the elbow functions as a pivot point. As per the principles of lever arms, the biceps, being closer to the pivot point than the weight in the hand, must exert a greater force to produce the necessary torque for movement.
For example, if lifting a weight , the biceps must create enough torque to counter the weight's torque around the elbow joint. At different angles of the forearm to the upper arm, like 60 degrees or 120 degrees, the force exerted by the biceps muscle will change due to the changes in the length of the lever arm and the angle applied.
In summary, when the forearm is at against gravity position during elbow flexion, the biceps muscle must exert a force greater than the weight because of the reduced lever arm. The exact force depends on the distance of the weight from the elbow joint and the arm's angle during the flexion.