Forget charisma and skill - the captains of the best teams in sports history shared 7 subtle traits

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The greatest team captains in history relentlessly fight for their teammates. All Blacks captain Buck Shelford, pictured, embodies that ideal.

Superstar athletes, brilliant coaches, a shrewd front office - all three are components of excellent professional sports teams. What you may not have given as much thought to is the role of captain, the player-leader of the team.

Not only is this role also vital, it is the single most important factor for a team's success, argues Wall Street Journal editor Sam Walker in his book "The Captain Class."

Eleven years ago, Walker embarked on a research project to to determine the commonalities of the world's greatest professional sports teams throughout history. An idea for a Journal feature ended up becoming a book, and by the time he was done, Walker had analyzed more than 1,200 teams dating back to the 1880s.

He used a strict and rigorous grading system that resulted in the 16 best teams in history, judged by how consistently they won championships as well as performed at the highest level in their league. Walker was as surprised as most of his readers would be to see that the one common element of these teams was the nature of their captains - or, in the cases where no captain was formally named, their informal player-leaders.

"The most crucial ingredient in a team that achieves and sustains historic greatness is the character of the player who leads it," Walker wrote. These leaders serve as models, teachers, and support to their teammates.

Walker determined that all of these captains shared seven traits: