Going into training camp, the offensive line is the position group most in flux, and with the most question marks. So, I thought I'd start a review of the various Vikings position groups by looking first at the offensive line.

But when it comes to evaluating this group, and putting all the pieces together, I don't think you can look at this group well without taking a longer-term, multiple-year view. Player development is such an important aspect here, and with some question marks among starters, and a couple of them getting older, that is even more true now.

Last year, the Vikings offensive line was ranked dead last in pass blocking efficiency, which is a measure of the amount of QB pressures (sacks, hits, hurries) compared to the total number of passing plays, weighted toward sacks. Essentially, they were the worst pass blocking line in the league, and gave up the most sacks - 36. That represents a huge decline from 2013, when they were rated 9th in the league, and from 2012 when they were rated 7th. Injuries played a major role in that decline, revealing a lack of depth the Vikings hope to upgrade with this year's draft picks. But what players fit where, and who makes the team and who doesn't, is still up in the air.

But before evaluating the offensive line roster, let's take a quick look at the blocking scheme the Vikings use most, how it works and comparing it with the other widely used blocking scheme, which will help in evaluating linemen later on.

The Power Blocking Scheme

This is the old-school, man blocking scheme, complete with pulling guards and lead blockers. The emphasis is typically on bringing the most blocking to the point of attack, blowing defenders back, and allowing the RB to exploit an over-powered defense. As the name suggests, linemen that are a good fit to this system are bigger and stronger, and must learn more complex rules for blocking the same play based on formations, blitzes, etc. The Vikings have used primarily a power-blocking scheme going back several years- not just since Norv Turner came to town. Matt Bowen does a nice job outlining the power run scheme and many of the plays common to it.

Zone-blocking, by contrast, is focused on linemen blocking the first man into their zone, often double-teaming down linemen initially, then having one lineman move to the second-level, and spreading the defense laterally, by moving as a unit laterally at first, and creating seams for the RB to exploit. Zone-blocking linemen tend to rely more on finesse/quickness and working as a unit, and use consistent, but more complex rules, not as dependent on defensive looks and formations.

Here is a good summary of the differences between man/power blocking and zone blocking. Brian Billick once said that with a power scheme give me good players, and with a zone scheme good coaches. Most likely because a zone scheme can be harder to learn, but a power scheme more difficult to execute.

Even though the Vikings use primarily a power blocking scheme, neither scheme is exclusive. In some situations the Vikings will use a zone blocking scheme, while zone blocking teams will occasionally use a power-blocking scheme, particularly in short-yardage situations.

In general, zone blocking is the current trend, as more teams have adopted this scheme in recent years. It is more common to west coast offenses- Green Bay has used it for years- but also teams like Seattle, and more recently the Dallas Cowboys, who run a Air Coryell offense, which typically includes a power blocking scheme.

One of the reasons Air Coryell offenses typically run a power blocking scheme is to stretch the defense vertically- which not only means protecting against the deep pass, but also having enough defenders in the box to defend against a smash-mouth running attack. That threat can hold linebackers a split second longer, and also draw the strong safety down into the box to prevent the defense from getting out-manned at the point of attack.

With all that in mind, let's look at the current offensive line roster, starting with the returning starters.

An In-Depth Look at Current Starters

Matt Kalil

MATT KALIL - LT Height Weight Age Year Drafted* Salary Cap 6'7" 320lbs 25 4th 2012:1:4 $6.3M PFF Grades Overall Pass Run Screen Penalty 2014 -29.1 -21.1 -4.6 1.5 -4.9 2013 -6 -1 -4.1 -0.5 -0.4 2012 14.3 10 4.6 0.5 -0.8 Sacks Hits Hurries Total PBE** 2014 12 7 36 55 t51st 2013 4 12 33 49 34th 2012 2 2 19 23 6th * Year: Round: Overall Pick ** Pass blocking efficiency, a measure of QB pressures compared to total passing snaps. Ranking among all tackles who played 50%+ of total passing snaps.

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Matt Kalil is coming off by far his worst year since being drafted, with a -29.1 overall PFF grade, and having given up 12 sacks- nearly twice that of his first two years combined. In 2013 he had an overall -6.0 PFF grade, and in 2012 a +14.3 overall grade. CCNorseman did a quantitative analysis of Kalil over the years a few months ago, which isn't encouraging, suggesting that Kalil's performance varied inversely with his level of competition, and which predicts an even worse year for Kalil this season against even better competition, if the trend continues.

I prefer to take a qualitative approach, as I think that can give a better understanding of why Kalil is failing to perform to expectations, and which may lead to a different prognosis for Kalil in the future.

Leading up to the 2012 draft, Kalil was pretty much seen as the best offensive lineman in the draft and likely a top 5 pick, which he was. Kalil had all the measurables- height, weight, length, frame, build, agility, and footwork and technique were solid. His tape in college was impressive. There were some concerns about his ability to anchor and his limited knee bend, but these were minor critiques, not major issues. He did have more than one knee surgery in college, although this was not seen as a major issue (perhaps it should have been?), and he did not have any off-field issues. He was generally viewed as a safe, reliable, plug-and-play left tackle and likely future pro-bowler able to start day one and for the next 10 years.

Well, he did start day one, and he did make the pro-bowl his rookie year, as an alternate and played. Kalil also earned a solid +14.1 overall grade from PFF.

Comparing Kalil's rookie year to other tackles drafted recently in the top 10 picks, there were two LTs taken in the 2014 draft- Greg Robinson (#2) and Jake Matthews (#6). Matthews was the worst rated LT by PFF, coming in at -36.8. Robinson was near the bottom as well, at -24.1 in the 9 games he played at LT.

In 2013, there were two tackles taken #1 and #2 in Eric Fisher and Luke Joeckel. Fisher finished the year with a -21.5 rating at RT, while Joeckel earned a -7.7, mostly at RT, before going down to an ankle injury after 5 games. Both moved to LT in 2014, and they continued to perform poorly, earning -17.5 and -15.8 PFF grades respectively.

In fact, since Kalil was drafted in 2012, no top 10 tackle has had even remotely as good a year as Kalil did his rookie year. Not even close. They've all sucked. Every damn one of them. The last one to work out was Tyron Smith, also from USC, drafted #9 by the Cowboys in 2011, although he had a sophomore slump in 2012.

In 2010, Russell Okung was taken #6 by the Seahawks, and Trent Williams #4 by the Redskins. Okung finished -0.6 overall, while Williams came in at -18.2. Since then Okung has had one good year and a few average, while Williams quickly improved and has been very good ever since.

In 2009, two tackles were taken in the top 10 picks, both struggled for a couple years before doing okay at RT. In 2008, Jake Long was taken #1, and, except for 2012 and 2014, has been a stud. Joe Thomas was taken #3 in 2007 and has been a stud ever since. But, as all of the above makes abundantly clear, that is the exception, not the rule for left tackles taken in the top 10 the past 8 years.

I go through all this recent history to make an obvious point: most left tackles struggle in their early years- even the top rated ones. Some go on to do very well, others don't. In general, however, the ones that don't were never good. They never had a good year. They just never made the jump to NFL caliber, for whatever reason.

But Kalil has had a good year- his rookie year- which is a bit unusual for top left-tackles in recent years. Kalil's predecessor at USC was the only other one who had a good rookie year going back to 2008- but that was at RT. He struggled his second year when he moved to LT, and didn't do nearly as well as Kalil that year. His third year, however, he flourished, while Kalil had a horrible year. Two left tackles. Both out of USC. Both top 10 draft picks. Both good rookie years, both mediocre sophomore years. Now one looks like a perennial pro-bowler, while the other one's future is very much in doubt. Hmmm. Is this really a story of the cream rising to the top, or is there some other explanation? I think the latter. Here's why:

Toward the end of his rookie year, Kalil started to have some knee (and back) issues, which limited him in practices on a couple occasions, and then developed pneumonia in December, which caused him to drop from 310 all the way down to 280 in the off-season. While he was able to gain the weight back, it did appear to effect Kalil's strength throughout pre-season and into his sophomore year. As Arif Hasan pointed on in October 2013, Kalil was faring much worse against opponents in close quarters rather than split wide, and generally against the bull rush than the outside move, owing to his reduced strength and slightly worse knee bend effecting his anchor and leverage- but still possessing quick feet to counter the outside move. The knee bend issue may have been due to the worsening knee issue bothering him, and causing him to alter (worsen) his technique, exacerbating a weaker part of his game even in his rookie year- which was also noted by scouts leading up to the 2012 draft.

Between the two issues- loss of strength and the knee issue, Kalil also lost some confidence in his ability to perform, and the mean streak he had shown at times during his rookie year and in college. Knowing you're not at your best can do that. That off-season, Kalil had arthroscopic surgery on his knee which caused him to miss all of the OTAs and mini-camp in 2014- which in turn got him behind in learning Norv Turner's new offensive scheme. Added to that, Kalil's knee never really healed correctly, and the swelling and inflammation bothered him the rest of the year, exacerbating technique issues he had the previous year, and further reducing his confidence. Perhaps another downside to Kalil's extended injury issues, and his diminished technique that resulted from it, was that there was now extended tape on it that opponents could use against him - and without a reliable counter. So, while Kalil finished the year a lot better than he started- perhaps becoming more comfortable in the new system, perhaps because his knee wasn't bothering him as much, perhaps both- he still ended the year with far and away his worst performance of his early career, with a -29.1 overall PFF rating, and having given up nearly twice as many sacks as his first two years combined.

So, with his knees still effecting his performance, Kalil had both of them scoped this off-season, and also underwent an Regenexx procedure, reportedly to help with joint pain- that may have been effecting his knee bend and technique issues the past couple years. Unlike last year, Kalil has not missed any time this off-season in terms of OTAs and mini-camp, and has not been restricted or limited either.

Dave Campbell recently reported about Kalil's travails and this off-season, with a few quotes this month regarding Kalil:

"Proof of the pudding will come out when we get the pads on," coach Mike Zimmer said. "But his sets have been better. He looks much more confident. He looks much better, doing the things we're trying to get him to do."

Read more here: http://www.bellinghamherald.com/sports/article24658135.html#storylink=cpy

"I think he knew it wasn't his best year," left guard Brandon Fusco said. [regarding Kalil's 2014 season] "You never know if it was his injuries or whatever he was doing, but he looks great to me. I told a bunch of other people it's probably the best I've ever seen him. He's going to have a great year. I can just tell."

Read more here: http://www.bellinghamherald.com/sports/article24658135.html#storylink=cpy

"From a mental standpoint, physical standpoint, it's the best I've felt in a long time," he said. "That's all I can say." - Matt Kalil

Read more here: http://www.bellinghamherald.com/sports/article24658135.html#storylink=cpy

While I think it is appropriate to discount the happy talk that permeates practices this time of year, particularly from coaches and players about themselves, the quote from Fusco still stands out as an honest assessment to me. We'll likely find out in a little over a month, as in past years when Kalil struggled, he struggled in pre-season and training camp as well.

My feeling is that if Kalil is really healthy, and the knee procedures have been successful, he will have a comeback year. Whether that comeback is to his 2013 level, 2012 level, or something else remains to be seen.

The on-going concern for Kalil, however- even if he does have a nice comeback year this season- will remain his knees. He has had multiple surgeries on them, although none of the major ACL/MCL variety, and that could continue to effect his performance, durability, and longevity. The fact that the Vikings picked up Kalil's 5th year option- no bargain at $11m given his recent performance and guaranteed in the event of injury- is perhaps a sign that the Vikings feel this isn't a major issue. We'll see. The fact that Kobe Bryant has had a similar procedure done with his knees over the years suggest it is manageable, however.

Bottomline, there are a several compelling signs that Kalil's decline the past couple years has been injury-related, and not simply Kalil not being up for the job. Knee issues resulting in him using less knee-bend in anchoring and, along with some strength loss, losing leverage battles in pass protection, and getting less explosion on run plays. Missing practice/install time last off-season also may have contributed to his particularly poor start last year, as he was still thinking on the field, particularly when the QB changed protections at the line. But, finally having had a chance to get up to speed with the new offense, and should Kalil indeed be healthy, expect his performance to improve significantly this year.

In part II, I'll look at the rest of the starters, and how they may fit together going forward.