Passive metatarsophalangeal joint stiffness: Reliability of a novel portable device

The objective of this study was to introduce a novel, portable test device for measurements of passive metatarsophalangeal joint stiffness.

A total of 19 healthy students participated in the study (6 f, 13 m, 24 ± 6 yrs, 75.4 ± 9.6 kg, 1.79 ± 0.08 m, sport/week: 8.1 ± 4.8 hrs, forefoot length: 8.0 ± 0.6 cm). Passive MPJ stiffness was measured twice in one day for within-day reliability and on two separate days within a week for between-day reliability, using a custom-made, pneumatic-driven device (Figure 1, left). Toes were positioned on a forefoot plate, with the first MPJ aligned with the plate axis and the shank oriented vertically. A pneumatic cylinder (Festo, Esslingen, Germany) generated pressure (1 bar ≙ 41 N) and moves the forefoot plate upwards into toe dorsiflexion. Pressure was regulated by a pressure control unit and valves (Festo, Esslingen, Germany). The angle of the forefoot plate was measured using a potentiometer (MCP05, linearity 1%, Megatron, Putzbrunn, Germany). The MPJ angle was defined as the angle between the rearfoot and the forefoot plate. Stiffness was defined as the joint moment needed to deform the toes by 60° dorsiflexion, normalized to forefoot length. We used slow angular velocities of around 50°/s to avoid triggering the muscle’s stretch-reflex and tendon’s viscoelastic response [4]. In a pretest, the force needed to deform the forefoot plate to 60° was determined. To take a possible creep-effect into account and to additionally minimize velocity effects, the mean of the last 5 repetitions was used for statistics. Statistics: Intraclass correlation coefficient (ICC 3,1).

ICC indicated both excellent test-retest reliability for comparing MPJ stiffness between-day (ICC = 0.993, 95% CI: 0.98-0.99, p < 0.001) and within-day measurements (ICC = 0.999, 95% CI: 0.99-0.99, p < 0.001, Figure 1, right). In addition, there were no significant differences (p > 0.05) between the three measurements of mean MPJ angle, angular velocity or stiffness (Table 1).