ἰσχῡρ-ίζομαι · ischyr-izomai — LSJ
make oneself strong, be strong, ἰσχυριζόμενος ὑφʼ ἵππων σίδηρος gaining force from the impetus of the horses, X. Cyr. 6.4.18.
use oneʼs strength, τῷ σώματι Pl. l.c.; esp. in overcoming resistance, πρὸς τὸ πολὺ ἧττον Arist. Pr. 951a13; εἰς τοὺς ἀσθενεῖς Id. EN 1124b23; contend stoutly, ὑπὲρ ἄθλων Ael. NA 15.15; persist or continue obstinately in doing . . , c. part., Th. 7.49: abs., ibid.; esp. by word of mouth, maintain stiffly, obstinately, c. acc. et inf., Id. 3.44, Is. 11.1; ταῦτα Pl. Grg. 495b; ὅτι . . , ὡς . . Th. 4.23, 6.55, Pl. Tht. 172b; περί τινος Id. Sph. 249c.
put firm trust in a thing, rely on it, τῷ ξυνῷ πάντων Heraclit. 114; λόγῳ Lys. 6.35; διαθήκαις Is. 1.3; τῷ νόμῳ Hyp. Eux. 4, D. 33.27; παρασκευῇ Id. 44.3, cf. Isoc. 17.24; feel confidence, Antipho 5.76.