As you’ll see in the chart below, regardless of how we sliced the data, there was no statistically significant difference in injury rates between mobile and conventional quarterbacks. Quarterbacks of both types tend to lose 11 to 14 percent of their starts to injury. Even without counting the thus-far injury-free Kaepernick, three of the four tests produced a lower injury rate for mobile quarterbacks. The gap, though, is small enough that a statistician would call it zero.
This was an intriguing finding. We were worried, though, that we had made too coarse a distinction between quarterback types, so we turned to regression analysis to understand if any combination of variables might explain why we see high injury rates in some quarterback seasons and low rates in others. Again, we found no relationship between rush share and starts missed due to injury. This held even when we controlled for age and weight—neither of which were found to have an effect on health—and also for injuries in the season prior, which did turn out to be a weak but statistically significant predictor of injury in any given year.
It turns out that the only gameplay variable that explains injuries with any statistical significance is sacks. On average, a 1 percent increase in sack share—the percentage of plays called for the QB that end in a sack—is associated with a 2.6 percent rise in starts missed due to injury (0.7 percent standard error). This link holds when we use the career-wise dataset and when we use sacks per start instead of sack share.
Skeptics might protest the main finding for two reasons. First, running quarterbacks could have shorter careers because of their bruising style of play. If this were true, though, we’d expect them to sustain more injuries while in the league, and they don’t. Second, maybe types of injury are different between mobile and conventional quarterbacks. We tallied up all the start-quashing injuries suffered by each quarterback in our sample, not double counting any repeated damage across years to the same body part, and classified each into four categories: head/neck, hands/arms/shoulders, torso/back, and feet/legs/groin.
A comparison using this admittedly simple procedure reveals almost no difference between injury-type distributions for the two groups. In each group, the lower body took the most punishment, and just more than 10 percent of injuries affected the head or neck. If mobility is a major determinant of quarterback health, we would expect this comparison to reveal a difference, but the injury patterns are remarkably similar.
In sum, it seems that standing in the pocket is just as dangerous as scrambling around. Yes, RGIII left the Redskins’ playoff game with his knee twisted so badly that you hoped Fox was experimenting with in-game CGI. But when we take the long view, serious injury doesn’t discriminate based on one’s ability to race.
At least one puzzle remains, though. Since sacks are the only significant predictor of injury other than prior medical history, it is tempting to tell a story in which mobile quarterbacks evade sacks more successfully, thus compensating for the injury risk inherent in rushing upfield. Yet the dataset reveals that mobile quarterbacks are sacked slightly more often than are conventional QBs. We can show that mobility doesn’t have a negative effect on health, but we can’t explain exactly how running quarterbacks are able to avoid additional risk.
We aren’t necessarily jumping on the read-option bandwagon, either. There’s still plenty to debate regarding the effectiveness of mobile quarterbacks. They may be less useful in two-minute drills, and their main advantage—speed—may wane quickly with age. But we hope to have shown that, based on the evidence available, conventional fears about quarterback rushing and injury risk may be overblown. If a general manager wants to protect a top draft pick, he should shore up the offensive line and cross his fingers.