Age ain't nothin' but a number

This wasn't the first time LT found the end zone, and it certainly won't be the last. Drew Hallowell/Getty Images

One of the toughest assignments in grading free agents is determining which aging veterans have hit the career-ending performance wall and which ones still have some distance to go before they will crash into that barrier.

That is a difficult call to make at any position, but it's especially problematic when reviewing running backs. This quandary has led to the establishment of several rules of thumb on when a ball carrier's days are numbered: 2,000 career rushing attempts, 30 years of age and a noticeable decline in yards per attempt are all helpful guidelines that are used to assist in this decision-making process.

Each of them likely was a factor in the offseason releases of LaDainian Tomlinson and Thomas Jones by the San Diego Chargers and New York Jets, respectively -- and in the Pittsburgh Steelers' not pursuing Willie Parker for a new contract after the expiration of his last deal.

As useful as those metric dynamics can be, they really don't answer the question of whether the running back in question still can tote the rock effectively.

It was this quandary that me to conduct a tape-review study of running back effectiveness last offseason. The central theme of the analysis revolved around determining the level of impact that good or poor blocking has on a ball carrier's performance.

I did this by looking at the yards per attempt totals for every running back when he received good blocking (i.e., when every blocker won his point-of-attack block) and when he received poor blocking (i.e., when at least one blocker lost his POA battle with a defender).

The results of this analysis were clear.