Fig 1.
Panel (a) shows the equipment and position used to assess the stiffness of the ankle joint: (A) to lock the foot in position, (B) to lock the knee in position, (C and D) metal blocks used to support the foot, (E) to lock lever 1, (F) to set the initial position of lever 2, (G) weight to calibrate lever 2 that can be moved to left and right, (H) standard weights (loads), (I) to adjust lever 1 height. Panel (b) shows the mass-spring model of the system: (F) reaction force, (m) mass of the system, (K) “apparent” stiffness, (C) “apparent” damped coefficient and (X) displacement of the mass. Panel (c) the diagram of the moment arms: The reaction force is represented by (F), the displacement around the ankle by (Δx and ΔX), and the distance between ankle joint-forefoot and ankle joint-rearfoot by (r1) and (r2) respectively.
Fig 2.
Panel (a) shows a schematic representation of the stiffness (k) with its constituents (km and kt). Considering the ankle’s moment arm (Fig 1c), the ‘apparent’ parameters (i.e., K and C) are transformed into ‘true’ parameters (i.e., k and c). Panel (b) illustrates the k-ƒ curve obtained from the experimental data (i.e., red squares). The (kd) parameter was obtained with the slope of the k-ƒ curve at its origin and used to estimate muscle stiffness (km = kdƒ). Tendon stiffness (kt) was obtained through the horizontal asymptote at high values of the k-ƒ curve.
Fig 3.
Model implemented in Simulink software to estimate velocity and position-time curves.
Table 1.
Comparison of stiffness, natural frequency and damping coefficient assessed using three impulses of different magnitude.