Belgium international Lukaku endured a six-match run without a goal before a last-minute leveller in Tuesday's 1-1 draw with West Ham in the FA Cup, and was dropped to the bench in recent defeats to Newcastle United and Hull City.
However, after finding the net against West Ham, Lukaku also impressed in Saturday's 1-1 Premier League draw with Manchester City and only a fine save from Joe Hart prevented him from getting on the scoresheet.
Manager Martinez believes the striker initially struggled to play to the best of his ability this term due to his commitments at the World Cup last year.
However, the Spaniard feels Lukaku is now starting to reach the heights that convinced Everton to fork out £28 million for his services in the close-season after he had impressed on loan from Chelsea last term.
"I think Romelu is a player who needs to be at his best physically, the power and pace in his play means he can't contribute by being half-fit," said Martinez.
"When he came back from the World Cup he wanted to contribute to the team but he wasn't 100 per cent in his physical abilities.
"Now we're seeing consistently a strong footballer who plays for 90 minutes. When he was in the races with [Martin] Demichelis and [Eliaquim] Mangala he looked always in a strong position, the footwork was good.
"He's someone who wants to take responsibility for the team and he looked back to his very best physically."
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