The aims of this study were to test the hypotheses that 1) the aerobic energy cost of roller ski skating (CS) is significantly related to level of performance and 2) a significant part of inter-individual differences of CS can be explained by kinematic parameters of skating locomotion. Oxygen uptake, kinematics of the knee and ankle joint, EMG of the vastus lateralis (VL) and gastrocnemius lateralis (GL) muscles, and roller ski velocity were recorded in 13 skiers who roller skied at 19.0 +/- 0.1 km x h(-1). CS was found to be 2.51 +/- 0.35 J x kg(-1) x m(-1) and significantly correlated with the skiers' level of performance (r = 0.61; p < 0.05). Significant relationships were found between CS and 1) knee angular amplitude (r = 0.75; p < 0.01) during the concentric phase, 2) VL average EMG during the concentric phase (r = 0.72; p < 0.01) and 3) VL and GL average EMG during the eccentric phase (p < 0.05). The results of this study showed that a significant part of performance could be explained by the aerobic energy cost of locomotion in skating. It can also be suggested that differences in upper/lower body utilization and/or in mechanical efficiency may explain the differences in CS.