WALTHAM, Mass. -- It's five days before the start of the 2016-17 season for the Boston Celtics, and head strength and conditioning coach Bryan Doo senses that the players, having endured four weeks of training camp battles and seven exhibition games, need a little something out of the ordinary to wake them up before an afternoon practice.

So as Celtics players meander onto the court expecting high knees or lunges, Doo asks a staffer to fetch 17 hand towels, then instructs each player to tuck one into his waistband. The Celtics' roster is split in half, with green-clad starters sent to one end line and white-clad reserves steered to the other. Doo barks out some basic instructions, queuing up a game that is one part capture the flag and another part flag football.

Doo stands at center court like a referee and bellows for the game to begin. Second-year guard Terry Rozier playfully backpedals toward center court, his towel exposed as Jae Crowder streaks toward him from the opposite end. Boston's 15 other players creep cautiously toward center court before someone finally makes a move and then ... pure unadulterated bliss.

Seventeen millionaires revert to school-age children on a playground as they race around the practice floor trying desperately to snatch each other's towel. Unknowingly, they're activating their bodies and minds.

Celtics strength and conditioning coach Bryan Doo, who is entering his 14th season working for the team, stretches out Isaiah Thomas. Jim Davis/The Boston Globe via Getty Images

"Look at it this way: There's probably a minimum of like 100 warm-ups per season, right? So you've got to keep it fresh. I get bored myself," said Doo, who is entering his 14th season with the team and whose drills on the court and in the weight room endeared him deeply to Boston's past stars, including Kevin Garnett.

"What I try to do is get the players so they’re ready to practice, so Coach doesn’t have to deal with them not being focused," Doo said. "I might have a plan in my head, but then I might be like, 'You know what, they're not listening, let's do something different.'"

During Doo's towel-chasing game, Crowder and Amir Johnson eventually trapped Jaylen Brown in a corner, but Kelly Olynyk rushed over to ensure the rookie could race free. The reserves retreated toward the center line as Johnson pursued. When Brown made a sudden charge at All-Star Isaiah Thomas, Johnson -- clearly the crafty veteran of this game -- pursued and plucked Brown's towel, throwing it high in the air to show the starters have won this round.

From the sidelines, Celtics president of basketball operation Danny Ainge and coach Brad Stevens watched with smiles. Players who were set to go through the motions were now fully activated and eager for competition.

"We go into play mode," Crowder said. "We go into play mode, but we’re still getting after it, just playing a little bit. It’s cool to switch it up because it’s a long season. It’s good to smile a little bit."

On any given day behind the closed doors at Celtics practice, a dodgeball game might break out. After one early training camp practice, Ainge was spraying a leftover ball around the gym (and inadvertently drilled an unsuspecting media member with an errant toss, maybe explaining why he chose basketball over baseball for his professional career). Sometimes Doo will replace those soft red dodgeballs with gigantic inflated stability balls for a hilarious variation of the game.

During the summer, Doo will break players into three-man teams and have them compete in what's essentially Ultimate Frisbee. "It's continuous though, so it’s conditioning," Doo explained. "When one team scores, they score, they transition back against another team, while the losing team might have to go ride the bike."

Players often will ask Doo for certain games they've done in the past, but more often he'll simply whip something up based on whatever objects might be available that day. Before one practice this season, Doo broke the team into two groups, one set of attackers rushing from the end line and another group of defenders at midcourt. Players engaged in one-on-one rushes in what essentially was the playground game of tag.

"I think, for me, it’s entertaining. I like to get them to compete because I don’t think these guys compete enough before practice," Doo said. "They have to get into that mindset. It’s hard though, sometimes on certain days, I know I have to talk to some of the [veteran] guys and be like, ‘Are you guys up to this?’"

And depending on what he feels like the team needs that day, Doo might dip into his bag of tricks. Doo, a father of five (four girls and a baby boy) who often works with young athletes, is a master of creating on the spot. The players appreciate how he keeps it fresh.

"It's nice to change things up," Crowder said, "and make guys laugh a little bit."