The study aimed to investigate sex differences in double poling (DP) ergometer performance among youth elite cross-country skiers and biathletes and determine if these sex differences may be explained by upper-body strength and endurance capacities. Thirteen female and nine male youth elite cross-country skiers and biathletes (age: 16.7 ± 1.7 years; VO_2max: 60.7 ± 6.3 mL·kg⁻¹ min⁻¹), matched for relative performance, completed a test battery evaluating upper-body strength and power along with various endurance parameters on a DP ergometer. Testing included one-repetition maximum (1RM) in upper-body exercises, maximal oxygen uptake (VO_2max) running test, and DP ergometer incremental test to exhaustion and sprint tests to determine peak oxygen uptake (VO_2peak-DP), maximal lactate accumulation rate (vLa_max), and power. Body mass and body composition were measured using bioelectrical impedance analysis. The findings demonstrated that the absolute differences in maximal strength, peak, and mean DP power outputs from both sprint and incremental tests to exhaustion (29%–38% difference), as well as maximal and peak oxygen uptake (29%–31%) between male and female athletes, were considerably reduced (2%–12%) following normalization to fat-free mass (FFM). Correlations of absolute and FFM normalized VO_2max, VO_2peak-DP with peak and mean power output from both sprint and incremental test to exhaustion during DP ergometry were significant (r_xy = 0.69–0.87) and remained consistent after correction for sex (r_xy-z = 0.61–0.84). These findings suggest that sex performance differences are primarily attributable to absolute differences in maximal aerobic power, maximal strength, and FFM.