The objective of rollerskiing is to simulate cross country skiing on snow as close as possible both in terms of technique and working the appropriate muscle groups. Rollerskis are a compromise. Classic rollerskis don't simulate the kick action well which can lead to bad technique and skiers don't have the classic guide rails making balance more difficult. Skate skis have rounded wheels so skiers have to pay attention to landing the ski flat. In both cases a fall on the ground is much less forgiving compared to snow.