DEVELOPMENT OF A CLASSIFICATION SYSTEM ON ANKLE-JOINT STIFFNESS

Abstract

AIM OF THE PROJECT Aim of the project is to develop a classification system for the ankle stiffness in sideward rotations of the fool. MOTIVATION In a previous study (1) a classification system for (running) shoes has been developed on the aspects stability and shock absorption based on a mechanical test protocol. If corresponding features can be quantified for the (lower) legs of the runner, running shoes can be more carefully selected. For example, a runner with less stiff ankles may need shoes which have high stabilizing properties, whereas runners with stable ankles may ask for (more) shock absorbency of their shoes. Consequently, classification systems may contribute to the reduction and /or prevention of running-related injuries. In the present project we have focused ourselves on the stiffness of the ankle. Method: An apparatus has been constructed to quantify the resistance of the foot against sideward rotations. The foot of a person to be tested in a sitting position is placed on a platform and fixed by a special technique. The lower leg is kept in vertical position by straps. The leg is allowed to rotate and translate to some extent around and along the vertical axis of the lower leg. By rotating the platform, the foot can be moved through plantar/dorsal flexion, abduction/adduction and inward/outward rotation. In the first two planes of motion discrete positions of the foot are prescribed while the inward/outward motion is imposed in a motor-driven way. The axes of rotation can be adjusted to individual differences. The axial load on the lower leg can be varied in discrete steps. In this project, the moment of force (M) as a function of rotation (Φ) is measured during inversion/aversion cycle in two positions of the foot, viz. neutral position (0 degrees flexion, 0 degrees adduction) and flexed position (20 degrees plantar flexion, 10 degrees adduction) without axial loading of the tibia. At present, the left ankles of a group of 14 volunteers without ankle complaints and 6 with ankle complaints are measured. Results and Discussion:Based on the specific and reproducible non-linear M- Φ relationship several parameters are deduced such as primary stiffness, mobility, and secondary stiffness for aversion and inversion. One of the results is that in both foot positions the primary stiffness shows to be linear related to the mobility ( a higher stiffness results in a lower mobility.) In the flexed position the primary stiffness is less than in neutral foot position. Using the primary stiffness results of the “sound” group in the flexed foot position a starting classification is made in terms of “stiff”, “neutral”, and “lax”. Depiction of the results of the “injured” ankles in this classification shows that most of these ankles belong to the “lax” category. Biomechanical and epidemiological studies are necessary to evaluate the ultimate effects of combinated foot/shoe classifications.