Pneumatically-driven stretching machine for ankle dorsiflexion: safety concepts and effectiveness test involving healthy young subjects

Abstract

Stretching is a major treatment for restoring the ankle range of motion while decreasing the ankle stiffness. An automatic stretching machine can provide long-term continual stretching at home. However, no commercial stretching machines are currently available, and therefore, no safety guidelines exist for them. In this study, we prototyped a stretching machine using a chassis from a commercial foot exerciser for preventing disuse syndrome, and then proposed three safety concepts as general requirements for stretching machines. The concepts were determined based on risk assessments, which included (1) preventing excessive force from being applied to the foot, (2) responding to the ankle stiffness and the range of motion of each different individual, and (3) preventing unintended operations by the user. The above concepts allowed us to develop a stretching machine taking into account safety measures. Further, the stretching effect of the prototype was tested on sixteen healthy young subjects ( 22.3 ± 3.6 years ). We found that the passive resistance of the subjects at specific dorsiflexion angles decreased significantly after stretching the ankle using the prototyped machine.

Effective position of the rotation axis of an ankle stretching machine and the effect of misalignment

“…Yamada et al developed a machine for 3D foot motion for dorsiflexion, abduction, and eversion using McKibben-type pneumatic actuators and wire-driven mechanisms Kimura et al, 2017). The authors of the present study have also developed an ankle stretching machine by modifying a commercial foot exerciser with a pneumatic actuator (Shiraishi et al, 2020) and demonstrated its effectiveness in decreasing the passive dorsiflexion resistance in young healthy subjects. In the present study, we use this developed machine.…”

“…The stretching machine ( Fig. 1) was based on the chassis of a commercial foot exercise machine for ankle dorsi/ plantarflexion (Shiraishi et al, 2020). When the bellows (i.e., pneumatic actuator) were pressurized, it expanded in the longitudinal direction, and the foot was dorsiflexed with rotation of the moving part of the machine (Fig.…”

“…Thus, stretching was continued only if the user intended so. The safety measures are detailed in (Shiraishi et al, 2020).…”

“…For example, the contact force components at the maximum dorsiflexion angle for Case 1 were f x = −4.9 ± 0.5, f y = 16.0 ± 1.5, and f z = 68.2 ± 3.6 (N) (mean ± standard error). According to Mao et al (2017b), an inherently safe threshold of tangential traction for skin abrasion caused by repeated rubbing motion is 40 kPa for the continuous stimulation of 300 s, which was the duration adopted in our previous study (Shiraishi et al, 2020). Considering the surface area of the forefoot plate (1.75 × 10 −3 m 2 ), this threshold value corresponds to f y = 70 (N), which is substantially greater than the value observed in the experiment.…”

Principal motion analysis of manual stretching techniques for the ankle joints

“…In a future work, the foot-stretching technique used by each physical therapist and its effectiveness for the patient should be analyzed simultaneously to establish an effective stretching method. Moreover, several previous studies developed automatic machines for ankle stretching 15 , 16 , 17 , 18 ) . Although currently, no such machine has been commercialized, the analysis of foot-stretching techniques in this study will help in their development.…”