Major League Soccer could become the latest North American professional sports league to see a work stoppage. But unlike the NBA, NFL and NHL, whose expiring collective-bargaining agreements have led to lockouts in recent years, the MLS could be hit by a players strike.

The Major League Soccer Players Union wants the league’s new contract to address two main issues: a hike in the minimum salary, and an ability for players to pursue free agency. On both topics, labor and management are far apart.

The strike looms as the MLS continues to rapidly expand. Salaries are on the rise, but the gains haven’t been well-distributed. High-profile players make well into six, and even seven, figures. In 2014, the Orlando City Soccer Clubs Brazilian star Kaka banked $7.17 million, while the Toronto FC’s Michael Bradley earned $6.5 million. Still, the league minimum stood at $36,500 last year, with 54 of 570 players making less than $37,000.

MLS is the only major sports league in North America to prohibit free agency, which allows players to negotiate and sign with any team under certain conditions. Since the league officially owns each of its 20 franchises, players have contracts with the league, not individual teams. That means their ability to move between franchises is limited.

MLS President Mark Abbott told the Los Angeles Times that this structure -- unique among top-tier soccer leagues around the world -- is directly responsible for the league’s success.

“The structure that we have has served as the foundation for the tremendous growth that we’ve seen in the last 20 years,” he said. “And it is that very structure ... [that] leads to a greater investment in players every year.”

Last year, the league signed a landmark eight-year television deal with ABC, ESPN, FOX and Univision worth $720 million. In 2014, average attendance hit an all-time high of 19,151.

Players say they just want their share of the wealth. “The league is growing, and in no way do we feel like we’re being greedy,” Tally Hall, an Orlando City goalkeeper, told the L.A. Times.

With union density on the decline, lockouts are more frequent than ever. By contrast, strikes, reflecting confidence on the part of labor, are increasingly rare.

Meanwhile, the MLS Players Union also threatened to strike after the last collective-bargaining agreement expired in 2010, before reaching a last-minute deal. As contract negotiations roll on, the regular season is scheduled to kick off March 6.