The American Birkebeiner (AB) is the largest annual Nordic ski event in the United States showcasing the long-course competitions of the 50K freestyle and 55K classic technique. Purpose: The purpose of this investigation is to describe the effect of aging on the finish times of skiers who have completed at least one AB race from 1999 to 2019 inclusive.
Methods: We followed 22,004 (3.3 men:1 women) individual skiers >18yo using publically available data. Finish times were standardized to 50K for freestyle and 55K for classic. A subset of the top 3 finish times at each year of age was selected and mean (N=3) finish times were fit to multivariate adaptive regression splines (MARS) in R for sex and technique. A single knot was iteratively fit to the model with the lowest mean square error (MSE) to show the age at which finish times increase significantly (Table 1). Results: Freestyle finish times for women are significantly higher compared to men at each year of age becoming progressively more pronounced at age 57 compared to age 65 in men (Fig. 1). Similarly, women skiing classic style (Fig. 2) reach an earlier breakpoint at age 61 compared to men at age 70. Conclusion: Overall, women’s AB finish times are higher than men and begin a sharp increase in finish time 8-9 years before men.

Table 1: MARS analysis

Technique	Sex	Age_Span	Slope	R2
Freestyle	Men	18-65	0.013	0.95
		65-81	0.143
	Women	18-57	0.007	0.93
		57-71	0.157
Classic	Men	18-70	0.017	0.96
		70-83	0.248
	Women	18-61	0.013	0.90
		61-72	0.196