"He reckons he's in the best condition of his life," Rennie said. "He's had a bit of a tidy-up of his knee and hasn't played much this season but he's played 100 tests and over 100 Super Rugby games and we think that experience will be invaluable for us."
Rennie said he had spoken often to Muliaina, who had made it clear that he was keen to return to his former franchise.
Sonny Bill Williams' unavailability had opened the door for both signings, Rennie said. Fruean, 25, who played alongside Williams at the Crusaders in 2011, has a similar physical presence.
"We were pretty keen to get a big midfielder ... we always knew if Sonny didn't come over we had an option to lure Robbie up and it certainly had a bearing on that."
Rennie said the Chiefs had investigated Fruean's heart problem. "We've got cardiologists reports and specialists reports. They've been really positive."
He added: "We're willing to be reasonably patient there. We've got a lot of cover in midfield."
Andrew Horrell, Charlie Ngatai, Tim Nanai-Williams and Bundee Aki are all midfield options, but none have the size or power of Fruean at his best.
Rennie said Tom Marshall, another who has moved from the Crusaders, was also capable of playing in the midfield. However, he prefers to play fullback, Muliaina's specialist position.
Fruean said: "I am thankful for the chance to return to rugby at this level and am looking forward to the Chiefs environment and the beginning of a new and important chapter for me."APNZ