As good as the top teams were in the American League last season, they may be even better this year.

Four of the five playoff teams made significant additions to their rosters: The Boston Red Sox added J.D. Martinez, the New York Yankees traded for Giancarlo Stanton, the Houston Astros made a move for Gerrit Cole and the Minnesota Twins added several quality players as well.

These teams are not settling with what was good enough last season. They're going for it all this year. Everyone is chasing the Astros, who recorded their first World Series victory in franchise history, but anything can happen in the American League this year.

Here are five storylines to watch in what promises to be a highly competitive season.

Five American League storylines to watch in 2018

1. Will the addition of Martinez truly affect Boston's fortunes? Adding Martinez to any line-up makes it better. The 30-year-old outfielder hit a career-high 45 home runs in 2017 and promises to give the Red Sox a huge boost on the power side. But as much as a boost as he may give, the Red Sox experienced a multitude of problems last season. Mookie Betts had a down year, Xander Bogaerts never figured it out, Jackie Bradley Jr. regressed and Dustin Pedroia was never healthy. Does Martinez fix all of their problems?

2. How many wins will/won't Giancarlo Stanton add to the Yankees 2017 total? It's easy to pencil in the Yankees for 100-plus wins with the addition of the National League MVP. However, the Yankees did win 91 games last season, and they play in a good division that includes an improved Red Sox team. Will Stanton help New York win more games, or could they even lose a couple more?

3. Will the Cleveland Indians take a step back? The Indians have their most important core intact. Corey Kluber, Carlos Carrasco and Trevor Bauer give Cleveland a potent 1-2-3 in their rotation. However, Cleveland lost Carlos Santana and Jay Bruce, and Danny Salazar already is going through injury problems. The Indians were a team driven by pitching in 2017, but you can't win unless you score, and the Indians could have a really hard time scoring with the loss of those two big bats.

4. Will the Astros experience a World Series hangover? It is really hard to win back-to-back World Series titles. No team has done it this century, and many that win don't make it back the next year. Houston don't seem like a team that will go through the hangover, but there is a reason teams struggle to get back to the Series. It's really hard to get there.

5. What will come of Shohei Ohtani? The favourite to be American League Rookie of the Year may not even be with Los Angeles Angels in May if he plays in April as he has in spring training. The games before the season mean nothing, but Ohtani has also shown nothing as of yet. Will he figure things out anywhere early in the season without even mediocre results? Or does he need more seasoning in the minors to truly contribute to the Angels?