  The National Football League's (NFL) 2011 collective bargaining agreement
(CBA) with its players placed a number of contact and quantity limitations on
practices and workouts. Some coaches and others have expressed a concern that
this has led to poor conditioning and a subsequent increase in injuries. We
sought to assess whether the 2011 CBA's practice restrictions affected the
number of overall, conditioning-dependent, and/or non-conditioning-dependent
injuries in the NFL or the number of games missed due to those injuries. The
study population was player-seasons from 2007-2016. We included regular season,
non-illness, non-head, game-loss injuries. Injuries were identified using a
database from Football Outsiders. The primary outcomes were overall,
conditioning-dependent and non-conditioning-dependent injury counts by season.
We examined time trends in injury counts before (2007-2010) and after
(2011-2016) the CBA using a Poisson interrupted time series model. The number
of game-loss regular season, non-head, non-illness injuries grew from 701 in
2007 to 804 in 2016 (15% increase). The number of regular season weeks missed
exhibited a similar increase. Conditioning-dependent injuries increased from
197 in 2007 to 271 in 2011 (38% rise), but were lower and remained relatively
unchanged at 220-240 injuries per season thereafter. Non-conditioning injuries
decreased by 37% in the first three years of the new CBA before returning to
historic levels in 2014-2016. Poisson models for all, conditioning-dependent,
and non-conditioning-dependent game-loss injury counts did not show
statistically significant or meaningful detrimental changes associated with the
CBA. We did not observe an increase in injuries following the 2011 CBA. Other
concurrent injury-related rule and regulation changes limit specific causal
inferences about the practice restrictions, however.
