Martinez has brought in four players so far this summer, with Antolin Alcaraz and James McArthur this week joining Mauro Boselli and on-loan Ali Al Habsi.
The Spaniard says he is not finished there as he aims to improve on last season's 16th place.
"There is still a long way to go before the [transfer] window closes," Martinez said. "I expect a few people to come in and hopefully we will balance the squad and be a stronger group of players than we were last season.
"We want to create a very competitive environment at Wigan Athletic and to do that we need to improve the squad in every window.
"We will do that and we are always working to do that."
Wigan yesterday presented Alcaraz and McArthur to the media at a press conference at the DW Stadium.
Paraguay defender Alcaraz, 27, completed his free transfer from Club Bruges when he was granted a work permit on Thursday.
Midfielder McArthur, 22, followed when he signed a four-year contract yesterday to complete a move from Hamilton believed to be worth around £500,000.
Martinez said: "James McArthur is a young, exciting prospect who we have been following for a long time and someone I think is going to go on to full international honours with Scotland.
"He has a winning mentality, a very strong character and is at a young age.
"Antolin was one of the best centre-halves at the World Cup.
"We spotted him really early and managed to persuade him to sign for Wigan Athletic before the World Cup.
"We signed him on a free and he is a really exciting prospect for us."
Martinez feels the pair will both fit well into a group which has returned for pre-season training in good spirits.
He added: "The boys have come back in very good shape and we are delighted with that.
"I am happy with the players that have come in and happy with the players that have stayed.
"They have really taken ideas on board and taken responsibility.
"I am very pleased with that aspect but we will carry on working and carrying on bringing players to remain as competitive as we can."