The Oklahoma tackling drill puts three men — a blocker, a tackler, and a ball carrier — in a space about as wide as a phone booth with a single objective: Win the battle at the point of attack. It's a simple yet primal exercise in football grit that NFL teams run as a way of measuring the ruggedness and strength of one player against another.
 
The drill, in many ways, is what NFL training camp is all about. A punishing endurance test in which teams whittle down rosters from 90 to 53 players in the span of four sweltering summer weeks, and the preseason is the ultimate reality show in which only the biggest, fastest, and strongest will survive final cuts. Contracts are rarely guaranteed, and one day’s backup, through a twist of fate — or the tear of a knee ligament — can be tomorrow’s starter.
 
Tom Brady got his shot to play when Patriot's starting quarterback Drew Bledsoe was knocked out of a game early in 2001. Had it not been for the Jets’ Mo Lewis, Brady may have flamed-out like a long list of sixth rounders before him. When the Arizona Cardinals lost 10-year veteran Darnell Dockett for the season earlier this week with a knee injury, it opened the door for the lesser-known Frostee Rucker to see a lot more playing time.
 
This kind of cruel reality regularly plays out on NFL practice fields. With training camps becoming more intense amid the high stakes of the hard-cap NFL, and with fewer actual practice sessions allowed by league rules, today’s preseason has boiled over with hyper-competitive drills like the Oklahoma, and the fight for jobs has produced a rash of serious, season-ending injuries.
 
According to a study completed last year by the Edgeworth Economics consulting firm, serious preseason NFL injuries have been on the rise for the last decade. Since 2004, there have been an average of 62.4 season-ending injuries suffered each summer before the regular season even starts. This summer there have already been 19 serious injuries with another two weeks to go before the real games start. Here’s a closer look at the carnage:

Darnell Dockett, DL, Arizona Cardinals (knee) — Out for the season after tearing his ACL in practice Aug. 18.

Marquis Spruill, LB, Atlanta Falcons (knee) — Out for the season after suffering a torn ACL in practice on Aug. 6.

Kapron Lewis-Moore, DE, Baltimore Ravens (Achilles) — Out for the season after suffering a serious Achilles injury during practice on Aug. 10.

Brent Urban, DE, Baltimore Ravens (knee) — Out for the season after suffering a torn ACL in practice July 30.

Tyler Gaffney, RB, Carolina Panthers (knee) — Expected to miss the 2014 season after tearing the lateral meniscus in his left knee during practice on July 25.

Marquess Wilson, WR, Chicago Bears (shoulder) — Out until at least November following surgery to repair a fractured clavicle suffered in practice on Aug. 4.

Marvin Jones, WR, Cincinnati Bengals (foot) — Out until October after undergoing surgery to put a pin in the broken foot he suffered in practice on Aug. 10.

DeMarcus Lawrence, DE, Dallas Cowboys (foot) — Out until October after undergoing surgery to repair a broken foot suffered in practice on July 29.

Jared Abbrederis, WR, Green Bay Packers (knee) — Expected to miss the 2014 season after tearing his ACL in practice on July 31.

Fendi Onobun, TE, Jacksonville Jaguars (quad) — Will miss the 2014 season after undergoing surgery to repair a torn quad suffered in training camp on Aug. 17.

Chris Jones, DT, New England Patriots (ankle) — Expected to miss the remainder of training camp after a teammate fell on his ankle in a preseason game on Aug. 7.

Dee Milliner, CB, New York Jets (ankle) — Out until the regular season with a high-ankle sprain suffered in practice on Aug. 10.

Shaq Evans, WR, New York Jets (shoulder) — Will miss the entire 2014 season after having shoulder surgery following an injury suffered in practice July 31.

Jermaine Cunningham, LB, New York Jets (Achilles) — Out for the season after suffering a torn Achilles tendon in a scrimmage on Aug. 2.

Alvin Scioneaux, San Diego Chargers (knee) — Will miss the 2014 season after tearing his ACL in a preseason game Aug. 7.

LaMichael James, RB, San Francisco 49ers (elbow) — Expected to miss the entire preseason after dislocating his elbow in practice on July 27.

Isaiah Pead, RB, St. Louis Rams (knee) — Out for the season following an ACL tear suffered on a non-contact play in a preseason game on Aug. 16.

Mason Brodine, TE, St. Louis Rams (ankle) — Out for the season after suffering a broken ankle in a preseason game on Aug. 7.

Charles Sims, RB, Tampa Bay Buccaneers (ankle) — Expected to miss more than half the season following surgery to repair an ankle injury suffered in practice on Aug. 12.