Final answer:
Performing a bicep curl with your arm supinated and wrist flexed may feel harder due to the inefficient position for the bicep muscles to exert force. The biceps must exert greater force because of their shorter lever arm to the elbow pivot point, but this force reduces as the weight approaches the body due to decreased torque.
Step-by-step explanation:
Performing a bicep curl with your radioulnar joint supinated and your wrist flexed may feel harder because of the torque and force dynamics involved in the motion. When you supinate your forearm, you are positioning your palms to face upwards, which is the most effective position for bicep muscle force application.
However, adding wrist flexion can create unnecessary strain, making it less efficient. The force placed on your bicep muscle is greater when performing curls given the bicep's proximity to the elbow pivot compared to the weight in your hand.
This decrease in lever arm length requires the bicep to exert a greater force to move the weight, especially as the weight gets closer to the body, the force necessary from the bicep decreases due to the decrease in torque demand.