The best explanation for the Miami Heat’s improbable 21-5 run isn’t Erik Spoelstra’s dark magic, Hassan Whiteside’s burgeoning maturity, or Dion Waiters’ Super Slayin act. It isn’t Pat Riley’s master plan or visiting teams succumbing to the allure of the city’s nightlife. The explanation is laid out in this terrific Palm Beach Post story about James Johnson’s emergence.

Johnson arrived in Miami weighing 275 pounds. The Heat had him take a shirtless picture so they could use it for a before-after comparison. Johnson thought it was a punishment. Then he picked up the iPad, scrolled through a gallery, and found that the Heat do it for a lot of other players. That was all the motivation he needed.

“I feel like that the more weight I lose, the more I unleash skills that I didn’t think I was capable of having or doing,” Johnson told Anthony Chiang. “Just being able to do more things agility-wise. I can move, cut through smaller spaces, and definitely give more effort out there in the game. I can go from 100 effort to 150 now.”

Johnson is one of many vagabonds thriving on this roster of other teams’ backup plans, fifth choices, and cast-offs. He’s teased us with flashes of brilliance in Memphis and Toronto, only to flicker away as his concentration waned.

How did the Heat see something that those teams didn’t?

By turning Johnson’s body, with his cooperation and dedication, into something new. He is no longer James Johnson, the 275-pound enigma on his sixth team in eight seasons. He is a slimmer person who resembles James Johnson, but is someone else entirely.

Getting in incredible shape — and the shame of looking doughy while the rest of the team is cut — has sustained the Heat’s culture for years. But in a league where rest and recovery are becoming more accepted as training methods, the Heat’s approach is becoming more of a competitive advantage. As this incredible run of form illustrates, there are many ways that being in better shape than your opponent has its advantages.

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It facilitates the magic elixir of ball movement

Every team wants to push the ball and get into its offense quickly. Every player wants to be a quick decision-maker. Every coach wants this sort of ball movement that keeps everybody happy and makes TV commentators swoon.

But too often, these hopes fail because the teams lack the nutrients to power that basketball nirvana. Sustaining those goals for 100 possessions a game while also defending at a high level is exhausting. As players lose energy, they need to walk up the floor to replenish their tanks. And the more tired a person is, the more difficult it is to make clear-headed decisions quickly. There’s a reason we take longer to function when sleep-deprived.

That’s why we can’t separate Miami’s recent offensive success from its emphasis on conditioning. Though the Heat are 20th in offensive efficiency for the entire season, they’re sixth since their winning run began in mid-January. This is despite not having any elite offensive talent — Waiters’ out-of-body experience aside. There’s a reason for that.

It’s not because Spoelstra is a clipboard wizard devising set plays the basketball world has never seen. Though Spoelstra has some unique rotation quirks — combo guards playing small forward, Johnson occasionally as a point center — he’s more orthodox than his reputation suggests. The Heat are not the only team with guards who like to attack off the dribble, power forwards who shoot threes, and big men who roll down the lane.

The real magic is in their execution. The Heat cut hard through their sets. This is the kind of pindown screen that almost every team has in its playbook. The difference is that Wayne Ellington really sprints through it.

There’s a level of precision in Ellington’s change of pace that can only happen if they’re in terrific shape. Otherwise, the details of each movement start to slip, and that makes all the difference.

Because all the Heat players are expected to be in the shape of their lives, they can each react quicker to find the open spaces another player creates. Every team yearns to develop this sort of flow that allows it to transition between one offensive action to the next without any hesitation.

Doing that requires all five players to read the game and act more decisively than their opponents. Every split second spent standing around to recharge the turbo button is a split second that disrupts the timing necessary to exploit a gap.

But because Miami’s players are so well-conditioned, they are not burdened by the need to regather energy. And that’s how you get sequences like this:

Getting in shape makes it easier to play instinctually

Beyond the alignment of set plays, the Heat’s most defining characteristic is how they attack gaps in the defense. The Heat are the NBA’s leader in total drives per game, but they rank dead last in the percentage of time they score on those drives, per NBA.com’s player tracking data. Instead, they rank first by a wide margin in total number of passes thrown off drives and the percentage of time they drive and dish.

In other words, the Heat spend a lot of time attacking a gap, drawing the defense, then passing on the move for someone else to do the exact same thing. Eventually, they make defenses dizzy and find an open shot for someone.

Mastering this style requires players to make rapid decisions on the move, which is much easier when said players are in great physical condition. There have been numerous studies suggesting that exercise improves decision-making skills. Human beings who work out more find they are generally more decisive. (This is when I took a gym break to fix writer’s block. It worked).

Every professional athlete exercises far more than the average human being, but even a marginal improvement within this elite selection sample can make a huge difference. There’s a cognitive reason Johnson can “go from 100 effort to 150 now.”

Proper decision-making in an NBA setting means relying on instinct. The moment a player has to stop and think about his next move, some 6’9 wing player with a 7’2 wingspan has cut it off. That tends to happen when players are tired.

But Heat players never tire, so they never lose their ability to play off instinct. Even players standing on the opposite side of the court know to cut into different positions to open up spot-up threes like this:

In that sense, physical condition, and not player alignment, sustains Miami’s offensive success without elite offensive talent.

It also helps them put the clamps on the other end

Hard-nosed defense has sustained Miami’s culture ever since Alonzo Mourning patrolled the paint. Even when the offense was floundering early in the season, the Heat were defending the hell out of teams. They rank fifth in defensive efficiency and third since their winning run began.

It’s tempting to attribute Miami’s defensive success to Whiteside. Yes, Whiteside is a towering presence inside, swatting more than two shots a game and altering countless others. But for the second straight season, the Heat allow fewer points per 100 possessions with Whiteside out of the game than with him on the floor — and the difference is more pronounced this year.

This isn’t to pick on Whiteside, who is still a force. This is to illustrate that the backbone of Miami’s defense is on the front lines. And it’s here that Miami’s conditioning really shines:

That’s Rodney McGruder sliding in front of a dribble hand-off, then hounding Kemba Walker like a gnat. Try as they might, the Hornets can’t get rid of him. The end result is a bricked contested jumper, and Whiteside barely had to stray from his preferred area.

Heat perimeter players just don’t give up on plays. Only 13.3 percent of shots taken outside of 10 feet against them are classified as having no defender within six feet of the shooter, according to NBA.com’s player tracking data. That’s nearly a percentage point better than the next-best team. Only the Jazz allow more catch-and-shoot opportunities, and no team allows fewer three-point attempts per game.

The shots the Heat frequently allow look a lot like this:

That miss happened because Josh Richardson moved his feet, didn’t get slammed off his path at the point of contact, and fought back into position to put a hand up. Because he did, the other four Heat defenders could stay at home, removing the gaps that other teams exploit for drives and open threes.

Less-conditioned defenders fail in all three areas. They aren’t quick enough to jump over those ball screens, they lose balance fighting through picks, and they lack the will to rally back when beat the first time. How many times have you seen your team’s guard recoiling toward midcourt as his man speeds off a high screen? That never happens to the Heat.

The Heat have several players known for their defense, but even players like Goran Dragic and Waiters are hounding ball-handlers with more vigor than in previous stops.

There’s nothing complicated about this approach schematically. The difference is that Heat players have enough energy to actually pull it off time and time again.

And that has a cumulative effect. Like a great distance runner, the Heat keep pace over the first three quarters, then turn in a devastating kick as the other team tires. Miami’s fourth-quarter defensive rating of 98.7 points allowed per 100 possessions is nearly three points higher than second-place San Antonio. Because of their commitment to conditioning, they have gas in the tank as their opponent runs on fumes.

It remains to be seen if the Heat’s recent surge began soon enough to steal a playoff berth. Sunday’s loss to Indiana puts the Heat a game back of the eighth-place Bucks, who have also been playing well recently. They’ve needed several of Waiters’ daggers to survive late in games, when even their free-flowing offense sometimes needs a bailout option. If they do make the playoffs, their style won’t be as effective without opponents on long road trips and back-to-backs.

Back when they were 11-30, we all assumed the Heat would pack it in and begin tanking. Turns out they’re not conditioned to do that.