Evaluating the Chase Utley Rule: The Association Between a Rule Change and Collision-Related Injuries in Major League Baseball

Methods

Results

The number of collision-related injuries in each season is shown in Table 1. In the seasons before the rule change (2010-2015), the median number of collision-related injuries was 126 per season. In the seasons after the rule change (2016-2019), the median number of collision-related injuries for defensive players decreased to 112 injuries per season, which was an 11% decrease (difference in location, 23.5 [95% CI, 7.0-52.0]; P = .019) (Table 1).

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Table 1

Number of Infield Collision Injuries in Major and Minor League Baseball

	Infield Collision Injuries, n
Season	All Infield Bases	First Base	Second Base	Third Base
Before rule change, median	126	49.5	58.5	15
2010	113	43	56	14
2011	131	50	59	22
2012	130	49	60	21
2013	132	54	63	15
2014	121	58	51	12
2015	122	49	58	15
After rule change, median	112	42.5	37.5	15
2016	86	40	31	15
2017	114	53	44	17
2018	112	45	53	14
2019	78	34	29	15

This reduction was concentrated among injuries at second base and occurred in 2016. In the seasons before the rule change, the median number of collision-related injuries for defensive players at second base was 58.5. In the seasons after the rule change, the median number of collision-related injuries for defensive players at second base decreased to 37.5 injuries, which was a 36% decrease (difference in location, 19.5 [95% CI, 5.0-31.0]; P = .019) (Table 1 and Figure 1).

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Figure 1.

Median number of collision-related injuries per season at second base before and after the rule change.

In contrast, the median number of collision-related injuries at first base decreased by only 14.1%, from 49.5 before the rule change to 42.5 after (difference in location, 9.0 [95% CI, –4.0 to 18.0]; P = .16), and the median number of collision-related injuries per season at third base was unchanged at 15 per season (Table 1 and Figure 2). The median number of days lost because of collision-related injuries was largely unchanged after the rule change (Table 2).

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Figure 2.

Total number of collision-related injuries by base per season, 2010-2019.

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Table 2

Median Number of Days Missed

	Days Missed, Median (25th-75th Percentile)
Season	First Base	Second Base	Third Base
Before rule change	2 (1-9)	2 (1-9)	3 (1-9)
2010	3 (2.25-5.75)	3 (1-11)	3 (1-7)
2011	4.5 (2-10)	4 (1.25-11.75)	4 (1-7)
2012	2 (1-7)	2 (1-7)	3 (1-12)
2013	1 (1-7.5)	2 (1-10)	3 (2-9.75)
2014	2 (1-3.5)	2 (1-8.75)	2 (1-10.75)
2015	1 (1-2)	3 (1-7.75)	2 (1-8.25)
After rule change	2 (1-8)	2 (1-5)	2 (1-8)
2016	2 (1-5.5)	2 (1-3)	2 (1-5)
2017	2 (1-3)	2 (1-3)	2 (1-4)
2018	1 (1-2)	3 (1-13)	2 (1-10)
2019	2 (1-2)	2 (1-12)	4 (1-11)

Discussion

Rule 6.01(j) was implemented with the goal of improving player safety, focusing on defensive players at second base. The present study found that rule 6.01(j) was associated with a significant reduction in the number of collision-related injuries to defensive players in professional baseball and likely enhanced player safety. However, the median number of days missed because of these injuries was not statistically significantly different after the implementation of rule 6.01(j).

There are 2 prior studies that have highlighted the role that professional league rule changes may play in improving player safety. First, Green et al ⁶ studied a similar rule change that targeted slides into home plate. In their study of MLB and Minor League Baseball players, the authors found that decreasing the number of collisions at home plate was not associated with a meaningful change in the median number of days missed because of those collisions. Second, Ruestow et al ¹⁰ analyzed how a rule change in the National Football League influenced injury rates on kickoff attempts. The authors used data from publicly available injury reports to quantify the number of injuries before and after a change in the starting yard line for kickoffs. The authors found that there was a statistically significant decrease in the incidence of these injuries, and they supported their conclusions by demonstrating that the incidence of injuries on punt attempts (another kick-related play that was not affected by the rule change) was unchanged during the study period.

We performed 3 additional analyses to confirm our principal finding that the number of collision-related injuries in the infield decreased as a consequence of rule 6.01(j). First, it is possible that any observed reduction in the number of collision-related injuries was caused by some unobserved overall change in player safety that was unrelated to rule 6.01(j). If that were true, then we would expect the number of collision-related injuries to decrease at a similar rate across all 3 infield bases (home plate was excluded). Unlike at first or third base, however, collisions at second base can be used to impede double plays, which may confer a strategic advantage that does not exist for collisions at first or third base. The implementation of rule 6.01(j) eliminated that strategic advantage. Therefore, if any observed change in player safety were attributable to rule 6.01(j), then we would expect a disproportionate reduction in the number of collision-related injuries at second base relative to first or third base. To address this hypothesis, we found that the reduction in collision-related injuries was heavily concentrated among defensive plays at second base, where we observed a 36% reduction. In contrast, much smaller (14% reduction at first base and no change at third base) and non—statistically significant reductions in collision-related injuries were observed at first and third bases. Taken together, these findings suggest that the reduction in collision-related injuries is more likely the result of rule 6.01(j) in eliminating the strategic advantage of colliding with the defensive player at second base than some other, unobserved, broader change in player safety that would have manifested similarly across all 3 infield bases.

Second, if a reduction in the number of injuries was effected by the rule change, then we would expect players to respond by changing their play pattern almost immediately after the rule was implemented. To address this question, we qualitatively compared the evolution of the number of collision-related injuries in the years before the rule change. If the trend was that the number of collision-related injuries was decreasing before the rule change, then it is possible that any observed reduction in collision-related injuries in the seasons after the rule change was attributable to the preexisting trend rather than to the rule change itself. However, if rule 6.01(j) were a meaningful driver of change in the number of collision-related injuries, then we would expect a sharp discontinuity in the number of such injuries in the 2016 season. We found the largest season-to-season change in the number of collision-related injuries between the 2015 and 2016 seasons, a finding that supports the hypothesis that rule 6.01(j) was responsible for the reduction in collision-related injuries over the course of the 2016 to 2019 seasons.

Finally, if rule 6.01(j) improved player safety, then we might expect that change to be reflected in a change of the severity of collision-related injuries for defensive players. To address this question, we compared the median number of days missed because of infield collisions in the seasons before and after the rule change. We did not observe a statistically or clinically relevant difference in median days missed because of infield collisions. This finding is in keeping with that of Green et al. ⁶ In the present study, the likely explanation for this finding is that the median number of days missed before the rule change was only 2 days and that, therefore, there was little room for improvement in injury severity (when measured via days missed) as a result of rule 6.01(j).

Conclusion

MLB and the MLB Players Association agreed to implement rule 6.01(j) at the start of the 2016 season with an eye toward improving player safety during slides. The rule was designed primarily to protect defensive players in the infield. The results of this study suggest that the rule was successful in achieving its aim, especially for defensive players at second base. This outcome is a cause for optimism that continued collaboration between MLB and the MLB Players Association can lead to meaningful improvements in player safety.