Ronald Acuña Jr. was only 8 years old on Sept. 16, 2006 -- and probably 2,000 miles away from RFK Stadium in Venezuela -- when Alfonso Soriano became the fourth member of the 40-40 club.

There is no fifth. Six weeks from now, a 21-year-old Acuna could be it.

Forget 30-30 -- at 35 home runs and 28 stolen bases, the Braves phenom can get there any day now. With any luck, the stretch run of 2019 will be all about Acuna's chase for 40-40 and the club's youngest membership, currently held by a 22-year-old A-Rod.

MLB's 40-40 Club
Alfonso Soriano (Nationals), 2006: 46 HR, 41 SB
Alex Rodriguez (Mariners), 1998: 42 HR, 46 SB
Barry Bonds (Giants), 1996: 42 HR, 40 SB
Jose Canseco (A's), 1988: 42 HR, 40 SB
Acuna's current pace: 46 HR, 38 SB

To even approach 40-40, it takes rare talent. And Acuna is the rarest bird baseball has, a five-tool player, someone who can do everything. But the 40-40 club requires two of those tools specifically to shine in elite tandem: power and speed. Acuna has them. He more than has them. His power-speed combination is almost unrivaled.

Here's why the Braves phenom's skillset makes him uniquely suited to challenge for 40-40.

First, we'll take the key Statcast metric for both tools, and where Acuna ranks.

For power: Barrels -- those are batted balls with ideal exit velocity and ideal launch angle. A barreled ball is likely to be a home run, or at least an extra-base hit. Hitters with a lot of barrels are going to slug.

For speed: Sprint Speed -- which takes a player's average top speed on competitive baserunning plays. MLB average sprint speed is 27 feet per second. The benchmark for elite speed is 30+ ft/sec. An individual run in the elite 30+ range is classified as a "Bolt."

Acuna has 55 barrels this season. That's second-most of any hitter, behind only Mike Trout, and ahead of MLB's other home run race leaders like Christian Yelich, Pete Alonso and Cody Bellinger. Acuna also has 33 bolts. That's just outside the top 10 in all of MLB -- and the majority of that leaderboard is light-hitting speedsters like Billy Hamilton and Delino DeShields.

There are only nine Major League players this season with even 25 barrels and 10 bolts: Acuna, Trout, Yelich, Bellinger, Fernando Tatis Jr., Trevor Story, Tommy Pham, Starling Marte and Ozzie Albies. That's a list of big-time power-speed threats, including some of baseball's very best players.

But Acuna has way more than 25 barrels and 10 bolts. So let's up the thresholds to the very top tier of MLB players in each category: 40 barrels, which gets us the top 20 sluggers, and 20 bolts, which gets us the top 25 speedsters.

There are only two players who meet that elite power and elite speed threshold: Acuna and Trout.

Acuna isn't just accumulating Statcast's version of counting stats. He's at the top of the game on a rate basis, too. Acuna has barreled 15.3% of his batted balls this season, a better barrel rate than over 95% of the league (ninth of 217 hitters with 200-plus batted balls). His average sprint speed is 29.2 ft/sec, the fastest on the Braves and just outside the fastest 5% of Major Leaguers (23rd of 363 players with 50-plus competitive runs).

One quick way to find the best power-speed players: Start with hitters who have a barrel rate of 10% or higher -- if you're in double digits, you've got power -- and runners with a sprint speed of 28.5 ft/sec or faster, halfway from league average to elite. If you want an MLB regular who meets both criteria, you're already down to just nine.

Players with 10%+ barrel rate AND 28.5+ ft/sec sprint speed in 2019
Ronald Acuna Jr. (ATL): 15.2% barrels | 29.2 ft/sec
Mike Trout (LAA): 19.4% barrels | 29.3 ft/sec
Christian Yelich (MIL): 16.9% barrels | 28.6 ft/sec
Cody Bellinger (LAD): 14.0% barrels | 28.8 ft/sec
Fernando Tatis Jr. (SD): 13.2% barrels | 29.3 ft/sec
Javier Baez (CHC): 13.6% barrels | 28.5 ft/sec
Avisail Garcia (TB): 11.7% barrels | 28.7 ft/sec
Teoscar Hernandez (TOR): 10.8% barrels | 29.0 ft/sec
Dansby Swanson (ATL): 10.9% barrels | 28.8 ft/sec
Min. 200 batted balls and 50 competitive runs

Again, though, Acuna is beyond that tier. He's not just 10+ and 28.5+, he's 15+ and 29+.

At those elite marks, again, there are only two. Again, those two are Acuna and Trout.

It's a perfect duo, really. The best all-around player in the world and the young phenom who looks like he could touch that level of generational greatness.

And the two of them have a link. Trout is the youngest 30-30 club member in history, joining it in his age-20 season as a rookie in 2012. When Acuna joins it at age 21, he'll become the second-youngest next to Trout.

But even Trout has never gone 40-40 (not to say he couldn't, since he's reached both marks individually). Acuna's quest for membership in the club that even Trout hasn't entered, and at so young an age, is a fitting marker of the kind of player he already is, and can be. Few players will ever push for 40-40 once in their careers. Acuna could be doing it for this year and beyond.