Modern football has changed the way full-backs are supposed to play. Tactical innovations and managerial demands are such that full-backs are now amongst the fittest players on the field.They are asked to contribute to attack and defense, and have to be comfortable playing as a center-back or a wing-back if necessary. For clubs like Liverpool, their full-backs are among the main sources of attack, bombing forward and providing width and creativity.The likes of Kyle Walker, Luke Shaw and Kieran Tierney have also been utilized as center-backs in a three-man defense this season, further highlighting the demands placed on the player.There have some fine performances from left-backs this season. Without further ado, we look at the top 5 left-backs in the world this season.#5 Kieran Tierney (Arsenal)Kieran Tierney has done well at ArsenalIt is a little unfortunate for Scotland that two of their best players, Kieran Tierney and Andrew Robertson, are both left-backs.A highly-coveted talent during his time at Celtic, Tierney moved to Arsenal in 2019 for a fee of £24 million. After a slow start to his Arsenal career due to injury issues, the left-back has established himself as an important cog in Mikel Arteta's set-up. A good crosser and a combative presence at the back, the Scotland international's leadership ability has also been appreciated.3 – Kieran Tierney is the first player to assist three goals in a single international match at Hampden Park since Kevin De Bruyne did so against Scotland for Belgium in September 2019. Wand. pic.twitter.com/F3RuJ3M296— OptaJoe (@OptaJoe) March 31, 2021The 23-year-old has also been utilized as a left-sided center-back in a three-man defense, predominantly for his country. If things go right, Tierney could soon find himself wearing the captain's armband at Arsenal.#4 Jordi Alba (Barcelona)Jordi Alba in action for BarcelonaMany believed Jordi Alba's time at Barcelona was coming to an end, but he has enjoyed a good season for the Catalan club so far.Regarded as one of the best left-backs in the world during his prime, there were suggestions that Barcelona needed to upgrade on the left-back position after some subdued performances from Alba. The 32-year-old, however, has given a strong reply through his performances this season He has made 26 appearances in La Liga, scoring three goals and providing four assists.Which left-back would you choose for your #ElClasico combined XI? ⚡️🔁 @JordiAlba💜 @ferland_mendy#LaLigaSantander pic.twitter.com/Ff49nIfLS0— LaLiga English (@LaLigaEN) April 6, 2021The Spain international has managed to keep Junior Firpo out of the first XI. While there is no denying that Barcelona need to invest in a younger left-back, Alba has shown that he can still contribute at the highest level.Also Read: 5 most valuable players in the Premier League this season