An outcome measure to quantify passive stiffness of the ankle

The primary aim of this research project was to develop a method to quantify a specific variable of neuromuscular function, passive stiffness of the ankle. It was then used to determine if values for the outcome measure varied with age and sex in a sample of elderly men and women between 55 and 85 years old. Subjects were screened for medical conditions contraindicative to testing, e.g. severe arthritis, other history of bone or muscle disorders, ankle fracture, neural disorder causing spasticity or contracture. A rotatable footplate which recorded position and resistive torque with a potentiometer and strain gauge, respectively, was linked to an electric, computer-controlled torque motor. Passive elastic stiffness, defined as the slope of the passive resistance curve at 10 degrees dorsiflexion (in N.m/degree), was greater in males, (p < .001), and increased with age (p = .002). There was a significant effect of age group on passive range of motion (ROM) values for ankle dorsiflexion (p < .001), but also a significant sex by age interaction (p = .001). Females showed a large drop from a mean of 19.3 degrees +/- 3.2 in the youngest group (55-60 yr) to 12.1 degrees +/- 5.5 in the 81 to 85 yr olds. Corresponding mean values for the middle-aged males were 15.4 degrees +/- 4.3 versus 13.1 degrees +/- 3.5 of dorsiflexion in the group of 81 to 85 yr old men. It was concluded that passive ROM of the ankle into dorsiflexion showed decreases in the elderly, which seemed to reflect less complaint tissue structures.