Why is my brachialis muscle sore, but not my bicep?

The brachialis muscle is sore because it is being worked more than the bicep during certain exercises or activities. The brachialis muscle is located underneath the bicep and is often engaged when performing movements that involve bending the elbow, such as hammer curls or reverse curls. If the brachialis muscle is being used more than the bicep, it can become sore while the bicep may not be as affected.