In operation the ski is subjected to variable dynamic loads at multiple impact points, and is required to flex elastically to conform to extreme terrain variations. The ski must also represent a stable platform on which the skier can balance at speeds which can exceed 50kph for recreational skiers, and significantly higher for racers and advanced athletes. In the photograph below, the primary forces operating on this platform (the skis) are indicated (for one plane only). As long as the resultant force vector is inside the ski(s) / snow contact points, the skier remains in balance. Any deviation in direction or lateral slippage of the skis requires readjustment by the skier to retain dynamic balance. The strength and rigidity of the ski are therefore key factors determining the performance limits as a function of speed, terrain, weight and skill level of the skier.