Matā taʿtarik fīhi al-farāʾiṣu tarʿad uses matā to introduce a conditional situation. Taʿtarik states the condition and tarʿad its consequence: when the sensitive muscles become agitated, trembling follows.
This is not an empirical claim that everyone reacts alike in battle. It makes the place appear extremely frightening, thereby intensifying the poet's assertion of personal composure.