The phrase "strength side" refers to the part of someone or something that is strong or powerful. It suggests focusing on those strong aspects or qualities to succeed or overcome challenges. Full definition
This is why the major clubs, including Arsenal, do not play full strength sides in most of the rounds. (justarsenal.com)
We'd been reported by Brentford for fielding an under strength side against them in the new League Cup competition. (uptheclarets.com)
As far as the team news is concern Barcelona don't have any major injury worries and I full expect Enrique to name his normal full strength side with Neymar, Messi and Suarez starting upfront while Rakitic, Iniesta and Busquest to playing in central roles while the Enrique might have to decide weather to start with Dani Alves or stick with Sergio Roberto. (totalsportek.com)