Meaning of "clench" in "clenching the muscles in his forearms."

This is a passage from a novel:

Neal had both hands on the counter, clenching the muscles in his forearms. Like he was retroactively bracing himself for bad news. His head was hanging down, and his hair fell away from his forehead.

From OALD, clench is defined as:

when you clench your hands, teeth, etc., or when they clench, you press or squeeze them together tightly, usually showing that you are angry, determined or upset

From the definition, I understand what it means when you clench your teeth, but I don't understand what clenching mean in this passage when she said clenching the muscles in his forearms.