After heavy bicep workouts, most guys show reduced muscle stiffness in a specific part of the bicep—especially when the arm is stretched out—meaning that area might get worked harder during certain...

When resistance-trained men perform bicep curls with their shoulders pulled back, the lower part of the biceps muscle becomes stiffer in most individuals, which may reflect localized mechanical stress or early tissue changes from intense eccentric contractions at stretched muscle lengths.

After performing heavy bicep curls with the shoulder extended, the lower part of the long head of the biceps muscle shows a larger decrease in stiffness compared to other parts of the muscle, indicating it experiences more mechanical strain during the exercise.

In men who regularly lift weights, the biceps muscle becomes stiffer when the shoulder is moved from a bent position to a straightened position, due to the muscle being stretched longer.

Performing heavy resistance exercises with the muscle fully stretched, like bending the elbow while the shoulder is extended, creates uneven stress patterns in the biceps muscle, especially near its lower end, and this pattern could help guide how to design strength training or recovery programs.

If you're into weight training, where you position your shoulder affects how stretched your biceps are and how stiff they feel before exercising — especially when your arm is pulled back behind you.