We don't yet know how well Ricky Williams and the Vikings' Onterrio "The Whizzinator" Smith will perform in the CFL in 2006, or if they'll ever be the same after returning.

I thought it'd be interesting to take a look at careers of other notable American runners that bolted north. It is interesting that I couldn't find many running backs that were ever the same in the NFL after returning from a Canadian stay.

Is there a CFL curse on running backs? For this discussion, I won't include backs like Lawrence Phillips and Rashaan Salaam, since their CFL stays were pretty much last chances.

However, when taking a look at high-profile running backs that took their game to the CFL, their NFL legacies suggest that Williams and Onterrio may want to sit this season out.

Terry Metcalf

In the 1970s, Cardinals running back Terry Metcalf earned a reputation as one of the more dangerous offensive threats in the NFL.

Somewhat surprisingly in 1978, Metcalf signed with the Toronto Argonauts. He left for the CFL right after reaching three Pro Bowls in four seasons (1974-75, 1977) as a rushing-receiving-return threat for St. Louis.

Unlike Ricky Williams, Metcalf wasn't forced out of the NFL. In fact, Metcalf was probably about to enter his prime. In 1978, the triple-threat was offered a then CFL-record $1.5 million contract, a high number during those days, and of course, he accepted.

Metcalf's first CFL game was apparently amazing; as he gained 297 all-purpose yards in a home win over the Hamilton Tiger-Cats. The remainder of his three seasons weren't, as he put up decent receiving numbers (over 50 catches in 1979 and 1980), but never topped 691 yards rushing in Canada, a number that he eclipsed three different times during his six-year NFL career.

Ricky Williams takes in his first practice with the Argonauts. (CP,Adrian Wyld / Associated Press)

In 1981, Metcalf returned for one more NFL season with the Washington Redskins. He finished as the team's third leading receiver behind Joe Washington and Art Monk, before leaving the game for good after only one season.

It's almost certain that Metcalf's NFL career would've been more prolific had he never ventured north of the border during his prime. Metcalf was projected to be somewhat of a savior in the CFL, a flagship star of the league. Instead, his play proved to be pedestrian up north, which possibly took some toll on his proven rep in the NFL.

Johnny Rodgers

In 1973, the Montreal Alouettes hired a new head coach, straight from the NFL. Marv Levy was the special teams coach of that year's Super Bowl losers, the Washington Redskins. The new coach obviously coveted special teams, and along with Montreal GM J.I. Albrecht, the two men decided to pursue an explosive return man that could blow the CFL game wide open.

The perfect target: Johnny Rodgers, who had won the Heisman Trophy the previous season at Nebraska.

Rodgers, listed at "wingback," was basically a receiver. However, after lining up at I-back during his final college game (with 15 carries in the 1973 Orange Bowl), and carrying the ball well over 100 times in his college career, he also projected as a running back. Also, as time passed, he's sometimes curiously listed in annals as "Nebraska running back." However, having lined up all over the field for the Huskers, Rodgers was as much a running back at Nebraska as Reggie Bush was a wide receiver at USC.

Rodgers, however, was explosive as both a runner and receiver, making him perfect for the Canadian game. Luckily for Levy and Albrecht, the Alouettes had negotiation rights to Rodgers before he was eligible for the NFL draft.

In 1973, Rodgers was the first-round draft pick of the San Diego Chargers. However, the Alouettes made Rodgers a monetary offer that he couldn't refuse, and the Heisman winner took his game to the CFL.

From 1973-76, Rodgers was one of the premier players in the CFL. During four seasons, Montreal won a Grey Cup, while No. 20 garnered CFL Rookie of the Year, All-League, and Most Outstanding Player honors.

With a rich college and CFL resume in hand, Rodgers headed south for the 1977 season, finally joining the San Diego Chargers. Rodgers suffered a number of injuries to his hamstrings in 1977, and badly tore his knee in 1978.

Here's the NFL stat total for Rogers, who is often thought of as one of the more electrifying college offensive stars of the '70s: 4 carries for 49 yards, and 17 receptions for 234 yards. Sadly, if there's such thing as a CFL curse on backs, the curse struck Rodgers.

Billy Vessels

In 1952, Vessels was the first University of Oklahoma player to win the Heisman Trophy, and rush for 1,000 yards in a season. In 1953, Vessels pulled "a John Elway," followed by "a Warren Moon" nearly 30 years before those two references would make any sense.

First, the star running back decided not to play for the NFL's Baltimore Colts (like Elway did later), and accepted an offer to play for the CFL's Edmonton Eskimos (just like Moon). Playing for former Sooner (and future legendary Texas coach) Darrell Royal, Vessels was named the CFL's Most Outstanding Player in his very first season.

Vessels' pro football career was interrupted by military service, and battling injuries after returning to the NFL to play with the Colts. CFL curse or not, the late Vessels is still a legendary figure in Oklahoma football, an honor richly deserved for one of the greatest backs in school history.

Anthony Davis

We remember Anthony Davis as the USC back that scored about 97 touchdowns during legendary 55-24 blowout over Notre Dame in 1974 that still runs every other Friday on ESPN Classic. After rushing for 1,000-yards three times at USC, and finishing second in the Heisman voting in 1974, Davis was originally drafted in the second round by the New York Jets.

In 1975, Davis would first spurn the Jets to dominate the struggling World Football League as the star running back for the Southern California Sun, as he rushed for well over 1,000 yards and scored 18 touchdowns.

Even after the WFL folded, Davis still never wound up with the Jets for the 1976 season. That year, Davis would've possibly started for new Jets head coach Lou Holtz in a backfield that included Ed Marinaro and rookie Clark Gaines. However, the CFL's Toronto Argonauts paid more for his services, and that particular situation seemed like guaranteed stardom for Davis.

At the time, it was a home-run move for both sides, especially considering that the smallish Davis was widely considered one of the more exciting, electrifying players at any level of football at that time.

It didn't work out that way. Davis only played one season of his five-year contract with the Argos. In his lone CFL season, Davis produced when actually on the field. He averaged 4.8 yards per carry, caught 37 passes for 408 yards, managed 26 yards per kick return, and 19.2 yards per punt return.

However, as mentioned in Jeffrey Goodman's 1981 book Huddling Up, Davis' style consistently clashed with Argos coaches. Davis didn't see as much playing time or touches to justify his contract, and an assumed feud with head coach Russ Jackson didn't help. Both sides decided to part ways after only one season, assuming that Davis would buy out part of his contract.

Returning to the NFL in 1977, Davis was never the same. He signed with the Tampa Bay Buccaneers, to play for his college coach John McKay. Davis rushed for 297 yards, second on the Bucs behind former USC teammate Ricky Bell. In 1978, Davis bounced between Houston and the Los Angeles Rams, and did not do anything of note in the NFL before leaving the league to pursue other interests.

In 1983, Sports Illustrated chronicled Davis' comeback attempt with the USFL's Los Angeles Express, but he managed only 32 yards on 12 carries.

More on Ricky

I realize that Ricky Williams is far more proven at the NFL level than any of the backs I mentioned.

However, if a CFL curse does exist for star American backs, don't be surprised if Ricky doesn't break it whenever he comes back. Either way, I'd hate to predict the odds for the Whizzinator.

Eric Moneypenny is a frequent FOXSports.com contributor. You may absolutely not e-mail him at moneypennysucks@yahoo.com.