INTRODUCTION: A main difference in cross-country skiing mass-start events compared to individual time-trial events is the possibility to reduce energy expenditure when drafting behind other skiers in packs. In the present study, we model a mass-start event based on the W` balance model and compare the models results with GNSS measurements. This model aims to provide insight into 1) the reasons for race evolvement and pack formation in mass-start events, 2) the corresponding physiological demands, and 3) why different athlete characteristics can lead to success in such events. METHODS: The propulsive power requirements throughout the course of a mass-start event were determined from the power balance equation (Moxnes et al., 2014), with high-end GNSS measurements used as input. Calculations were made both for a skier at the head of the race and for a skier in a pack who benefits from reduced energy expenditure due to reduced drag. These propulsive power requirements were used as input to the W` balance model (Skiba et al., 2021), to calculate combinations of critical power (CP), work capacity above CP (W`) and the time constant t required to finish with the leader. Additionally, time differences throughout the course were calculated for other values of CP, W` and t, and compared to corresponding time differences and pack formations from a mass start event where high-end GNSS data from 42 finishing skiers was available. RESULTS: Model results correspond well to GNSS results in that packs split into smaller packs in the same parts of the course, typically in the longest uphill segments when W` is depleted. Furthermore, the model illustrates how skiers with different physiological abilities, corresponding to different CP, W` and t, can either finish the race with the leading pack, or be forced to drop the leading pack earlier. DISCUSSION/CONCLUSION: Although the W` Balance model has several limitations, it provides novel insight into the relation between race development and skiers` physiological characteristics during a mass-start event. The model illustrates how skiers with lower CP can compensate with a higher W` or lower t. This effect is most likely more prominent in mass-start events compared to individual time-trials, due to the better opportunity to recharge W` after the toughest uphills when exploiting the drafting effect in a pack.
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