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Los Angeles Raiders running back Bo Jackson carries the ball against the Seattle Seahawks during an NFL game on Dec. 1, 1987.

(AP Photo)

The NFL Scouting Combine starts this week, and the fastest of the pro football prospects will be sprinting to beat the 40-yard time of East Carolina running back Chris Johnson, who flashed across the distance in 4.24 seconds in 2008. This is considered the combine's "modern" record for the 40.

But was Johnson's 40 the fastest ever run at the combine? Maybe, even though it isn't the fastest 40 time recorded at the combine. That contradictory statement could be true because of changes in the way the 40 is timed at the combine and changes in the combine itself.

During the 1986 combine at the Superdome in New Orleans, Auburn running back Bo Jackson recorded a 4.12-second 40, a feat so extraordinary it's become part of Jackson's legend, a subject for conjecture, speculation and comparison, so much so that current combine stories qualify references to the 4.12 with words such as "reportedly," attribute the time to "rumor," or describe it as an "urban legend."

Football fans can watch the prospects working out at the combine on the NFL Network starting on Friday. Their results in the various drills will be available almost instantaneously on NFL.com. That kind of coverage hasn't always been the case. The NFL does not keep official records of combine performances. The NFL.com database on the combine starts with the 2006 workouts. Top-10 lists of the best times and distances in the various combine drills go back to 1999. But the combine started in 1982.

Upon closer examination, the question isn't if Jackson was clocked at 4.12 seconds at the 1986 combine. It's easily verified. For example, USA Today had a front-page story about the feat. The question is: Was that time accurate?

The 40-yard sprints at the combine have had semi-electronic timing since 1999. It's not true electronic timing because while the clock is stopped electronically at the finish line, it's started by hand on the first movement by the runner. That's because the combine participants aren't reacting to a starter's gun. Instead, they begin running when they are ready.

The assumption has been that Jackson's 4.12 was a hand-timed 40. The International Association of Athletics Federations says to add 0.24 seconds to hand-timed races to convert to the probable electronic timing. So do we assign Jackson a 4.36-second time and declare Johnson the combine 40 champ? It's not that simple, because Johnson's 4.24 isn't a true electronic time either - the clock was started by hand.

Also, Jackson told ESPN's Dan Lebatard in 2012 that he ran a 4.13-second 40 at the 1986 combine that was electronically timed.

In recent years, Jackson's 40 has been a subject of comparison to the times of world-class sprinters to show, oddly enough, that he couldn't have run that time and that he most certainly could have.

The extrapolations are apples-to-oranges comparisons. The track sprinters wear spikes and run on a surface made for speed. The combine's football players run in cleats on a synthetic playing field (in Jackson's case in 1986, Astroturf). Also, a sprinter such as Usain Bolt, the comparison runner of choice these days, still will be accelerating 20 yards past the 40-yard mark in a 100-meter dash.

Jackson is famous for playing baseball in the Major Leagues and football in the NFL, but he also was a track standout - good enough to qualify for the NCAA indoor championships in the 60-yard dash twice and reaching the semifinals at the national meet while at Auburn.

"My favorite sport was track and field," Jackson told an interviewer, "because I was always competing against someone on an individual basis, but I found out down the road that the pay scale was not that of football and baseball."

Bo Jackson winning 100-meter race at track meet in Florida. Circa 1984. (Wil McIntyre, SI) pic.twitter.com/ZD8e3jq7DJ — Suite Auburn (@SuiteAuburn) March 4, 2013

Taking Jackson's fastest time of 6.18 seconds in the 55-meter dash in Auburn's track and field records and projecting it to 40 yards (an inaccurate exercise since the math counts every yard as having been run at the same rate), he shows enough speed to run a 4.11 40. Of course, the 55 meters was almost certainly electronically timed and run in track, and not combine, conditions.

For the curious, Bolt's world-record 9.58-second 100 meters would be a 3.50 40 when converted in the same manner. In 1986, when Jackson ran his combine 40, Calvin Smith was the world's fastest human, and his world-record 100-meter time of 9.93 seconds would be 3.63 seconds when reduced to 40 yards. All track times, though, include reaction time - that is, the tenth of a second or so it takes the runner to register the sound of the starter's gun and break from the blocks. At the combine, there is no reaction time in the final clocking because the stopwatches don't start until the runner does.

While showing Jackson, Bolt and Smith can pick them up and lay them down, those mathematical gyrations don't contribute to finding an answer to who ran the fastest 40 at the NFL combine - Jackson or Johnson or, to muddy the waters more, Eastern Kentucky wide receiver Rondel Melendez, who was credited with a 4.24 40 in 1999.

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Some who don't think the 4.12-second time can be accurate point to Jackson's scouting report by the Chicago White Sox from 1985. Jackson received a 7 grade for speed, which is the rating for "very good," and not an 8 for "outstanding," and he was timed at 4.1 seconds over the 30 yards from home to first, which, of course, involved getting out of the batter's box after hitting the ball, not exploding from a four-point stance as runners do at the combine.

There's no doubt Jackson was fast. The 1985 Heisman Trophy winner had the NFL's longest run of the year in three of his four seasons in the league, and he stole 27 bases in 33 attempts in his best big-league season. But is he the fastest runner in the history of the NFL Combine? Maybe only Bo knows.