The 2016-17 season has been the year of the triple-double. The league has set an all-time record with 110 triple-doubles, recorded by 22 different players. And there are still seven days left in the season.
Russell Westbrook has led the way and, with four games remaining, is one triple-double from setting a new record for most in a season. He's also just six assists away from becoming the second player in history to average a triple-double over a full season.
With Westbrook, James Harden and John Wall all doing it this season, there are 86 instances in NBA history of a player averaging double-digit assists. All 86 have been by (31 different) guards.
But Westbrook's 10.7 rebounds per game are just the fifth instance of a guard averaging double-digit boards. Oscar Robertson did it three times and Tom Gola averaged 10.4 in the 1959-60 season.
Westbrook has done it while playing at a much slower pace than the league played at in the early 60s. The league averaged 215 shots per game when Robertson averaged 30.8 points, 12.5 rebounds and 11.4 assists in the '61-62 season. This season, the league has averaged 171 shots per game.
But there are a couple of recent trends that have helped Westbrook average double-digit rebounds and helped guards account for 75 of the league's 110 triple-doubles this season.
The offensive rebounding trend
Two fairly simple points here ...
1. Guards are more likely to grab defensive rebounds than offensive rebounds. Over the last five seasons, guards have grabbed 31 percent of available defensive boards, but only 20 percent of available offensive boards.
2. Over the last five seasons, league-wide offensive rebounding percentage has continually dropped. Increasingly, coaches have prioritized transition defense over second-chance opportunities. This season, offensive rebounding is at an all-time low, with offensive players grabbing just 23.3 percent of available rebounds.
The league started counting offensive and defensive rebounds separately in the 1973-74 season.
So, as defensive rebounding rises, so does guard rebounding.
The 3-point shooting trend
Longer shots produce longer rebounds. Since guards generally play further from the basket, they're more likely to grab rebounds off long shots than short shots.
According to SportVU, over the last three seasons, guards have accounted for 35 percent of defensive rebounds off 3-point shots and 31 percent of defensive rebounds off 2-point shots. So as the league shoots more threes, guards grab more rebounds.
And of course, the league is taking more threes than it ever has. This season, 31.6 percent of all field goal attempts have been from 3-point range. It's the sixth straight season that that rate has hit an all-time high, and this year's increase (from 28.5 percent last season) is the biggest increase we've seen since the league went to a shorter 3-point line in 1994.
When you combine the defensive rebounding trend with the 3-point shooting trend, you have more rebounds going to guards. This season, guards have grabbed 29.9 percent of all available rebounds, up from 29.1 percent last season, 28.0 percent 10 years ago, and 24.5 percent 20 years ago.
Free throws, free rebounds
The free throw factor is specific to the two guys leading the league in triple-doubles.
If you've watched the Thunder, you may have noticed that, when an opponent is shooting free throws, Westbrook's teammates often focus solely on clearing their man out of the way, so Westbrook can grab an uncontested rebound. Two examples from the Thunder's March 31 loss to San Antonio are here and here.
According to SportVU, Westbrook had grabbed 91 defensive rebounds off missed free throws through Tuesday, 26 more than any other player in the league. That's an average of 1.18 per game, which is 0.78 more than he averaged last season (0.40).
If you subtract those extra 0.78 defensive rebounds per game, Westbrook's rebounding average (through Tuesday) would drop from 10.7 to 9.9 per contest.
James Harden isn't exactly innocent in regard to rebounds on missed free throws. Through Tuesday, he was tied for seventh in the league (second behind Westbrook among non-bigs) with 58 defensive rebounds off missed free throws.
By comparison, Kyrie Irving had just five defensive rebounds off missed free throws through Tuesday. Isaiah Thomas had four, Stephen Curry had three, and John Wall had two. Maybe they need to talk to their bigs about helping out their rebounding numbers a little bit.
In total, Westbrook leads the league with 8.6 uncontested rebounds per game, according to SportVU. Only 20.2 percent of his rebounds have been contested, the third lowest rate among 107 players who have averaged at least 5.0 boards per game. Harden's 21.6 percent is the sixth lowest rate.
What Westbrook has done has been historical. There's no doubting his value to the Thunder (who have been 12.6 points per 100 possessions better with him on the floor than they've been with him off the floor) and he's about to be the first player in 55 years to average a triple-double over a full season. But in registering 10.7 rebounds per game, he has had some help from a couple of league-wide trends and from teammates willing to sacrifice their own numbers.
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