After the knee, the body part at greatest risk during skiing is not the ankle or the shoulder or the leg. It's the thumb.

Two decades of watching skiers tumble at Waterville Valley, N.H., has uncovered a shift in the frequency of injuries to various parts of the body. Injuries to the thumb have surpassed those to the ankle and lower leg as the number two injury site, according to ski enthusiast Lawrence R. Young of the Massachusetts Institute of Technology.

Among advanced skiers, thumb injuries are the most common type. The injuries usually occur during forward falls on hard snow. Young, an aeronautics professor, suggests that the design of ski poles be examined as a possible contributing factor.

Young's other findings, reported in the MIT publication Technology Review: At Waterville Valley, there were about 3.5 injuiries per 1,000 skiers per day. The more skiers, the more injuries. In 1981 to '82, for example, ther were 2.52 injuries per 1,000 skiers a day on weekdays, 4.43 on holidays. Fatigue seems to contribute to accidents. Injuries peaked between 2 p.m. and 3 p.m., although many skiers had left the slopes by 2 p.m.