Bernini's "Davide" was modelled on his own face -- the knitted brow and the clenched mouth

Speranza

Balduccini writes that Maffeo Barberini would hold a mirror while Bernini studied his own face in order to achieve the desired expession.

David's knitted brow and the clenched mouth indicate that moment when the complete physical and psychic resources of teh will are summoned for some great effort.