Matthew 26:40

οὐκ ἰσχύσατε; Had you not the ἰσχύς—the physical strength to watch? This was an instance of failing to serve God with their strength (ἐξ ὅλης τῆς ἰσχύος, Mark 12:30). ἰσχύω, not a mere synonym of δύναμαι, seems always to retain some sense of physical power, cp. οἱ ἰσχύοντες, ch. Matthew 9:12; ὥστε μὴ ἰσχύειν τινὰ παρελθεῖν διὰ τῆς ὁδοῦ ἐκείνης, ch. Matthew 8:28; σκάπτειν οὐκ ἰσχύω, Luke 16:3, ‘am not strong enough to dig.’

Note that the verb is in the plural. As Peter took the lead in the promise of devotion, Jesus by naming him singles him out for rebuke. St Mark has ‘Simon (the name of the old life), sleepest thou? Couldest not thou watch one hour?’