Dorsiflexion of the Human Ankle as it Relates to Ski Boot Design in Downhill Skiing

Abstract

This study is part of an on-going series of studies that relate to Alpine or Downhill Ski Boot Design. In current Alpine skiing, the ski boot is an integral part of the ski boot-binding system. One of the roles of the ski boot is to protect the ankle from excessive dorsiflexion during forward falls, as the ski boot is levered out of the heel binding. A boot designer needs to know what the ranges of dorsiflexion are for human ankles so that the allowable forward flex built into the ski boot will not exceed some specified level. That specified level should be such that a large part of the population will not exceed a safe level of dorsiflexion. The stiffening of the ankle by voluntary contraction of the muscles that control the ankle joint cannot be relied upon since the reaction time to contract the muscles will be greater than the time available to the skier under many circumstances. This study looks at the maximum voluntary dorsiflexion of a group of people (n=64) similar to a skiing population. The anatomical and biomechanical posture of the subjects was intended to represent typical skiing situations, therefore the subjects were measured in a weight bearing, flexed knee, upright posture. The age, gender, height, weight and skiing experience of the subjects was recorded as independent variables. The maximum voluntary dorsiflexion of the ankle was the dependent variable. 10 subjects were measured while the knee was kept in a straight or extended posture. The analysis indicates that there is no statistically significant relationship between dorsiflexion and any of the independent variables. The mean dorsiflexion was 42.7 degrees, the 5th% value was 28.5 and the 95th% was 56.7 degrees. The straight knee posture reduces the effective dorsiflexion by 8.5 degrees. Current standards permit as much as 40 to 45 degrees dorsiflexion. The implications are that current standards are excessive, a reasonable limit would be something under 30 degrees. Such a limit, or less, is consistent with the maximum dorsiflexion found in most current ski boots.