The mechanical rotation axes of joint exercisers are believed to operate better when they match the biomechanical axes of human joints. However, there are few studies regarding ankle stretching machines. Further, the maleffects of rotation axis misalignments are not well known. Hence, we investigate the effective positions of rotation axes for ankle stretching machines and the effects of misalignments using a pneumatic-driven stretching machine developed in our previous study (Shiraishi et al., 2020). Eight healthy young males (ages 23.3 ± 1.4 years) participated in stretching exercises while the relative positions of the rotation axes between the machine and ankle were changed via plates installed under the heel. The stretching machine dorsiflexed the feet of the participants, and the dorsiflexion angles and three-axial forces applied to the forefeet were recorded. The measured values at the maximum dorsiflexion angle were evaluated by two-way analysis of variance and/or regression analysis. We determined that the rotation axis of the machine must be placed 7 mm above the lateral ankle because the normal force applied to the forefoot and maximum dorsiflexion angle were large, whereas the friction force was moderate. Further, the relationships among the dorsiflexion angle and contact forces were investigated via covariance selection. The three-axial forces significantly decreased as the axis of the machine was lowered below the ankle. Additionally, the force normal to the sole had large positive effects on the dorsiflexion angle and friction force of the sole, which could damage the skin. The misalignment of the rotation axis increased the contact force at the sole when the axis of the machine was above the ankle or decreased the efficiency of force transmission from the stretching machine to the user's foot when the machine's axis was below the ankle.

Keywords

Ankle contracture, Stretching, Ankle stretching machine, Rotation axis, At-home rehabilitation

Paper information

Yuma SHIRAISHI, Shogo OKAMOTO, Naomi YAMADA, Koki INOUE, Yasuhiro AKIYAMA, Yoji YAMADA, “Effective position of the rotation axis of an ankle stretching machine and the effect of misalignment”, Journal of Biomechanical Science and Engineering, Vol.15, No.4 (2020), p.20-00202 doi:10.1299/jbse.20-00202. Final Version Released on November 09, 2020, Advance Publication Released on August 20, 2020.

This study focuses on observing and analyzing the occurrence of a fall for humans during normal gait, which perhaps results in severe injury. Previously, the insufficiency of a recovery step, which diminishes the forward rotation of the body after tripping, was suggested as a factor for the failure of fall avoidance motion. However, although it was identified that a slow and/or short recovery step resulted in the failure of fall prevention, the physical process of tripping and falling during the gait was not analyzed sufficiently. In this study, the subject's reaction motion and fall process against tripping became clearer because the fall motion, which included the phase closer to the ground contact, could be recorded for a longer time than that in most previous studies. Although the subject attempted to mitigate the forward angular momentum and the descent of body induced by tripping by holding on his recovery foot, it was not effective, especially when the recovery step length was short. Among such trials, the larger forward inclination of the body, the excessive forward movement of the center of mass from the support point, and the smaller ground reaction force of the recovery foot in the normal direction was observed. Because the short recovery step resulted in the limitation of the moment arm, which affected the reaction torque of the recovery foot, it became difficult to decelerate the body rotation and prevent the fall. Furthermore, it was also suggested that the forward lean of the body and the decrease of the ground reaction force increased the impact speed and effective mass, which affected the impact force. The observation of such an advanced fall phase also contributes to a more realistic simulation of the human reaction motion for a more precise estimation of the fall injuries.

Keywords

Trip and fall, Reaction motion, Severity estimation, Fall experiment, Unavoidable fall

Paper information

Yasuhiro AKIYAMA, Kento MITSUOKA, Shogo OKAMOTO, Yoji YAMADA, “Experimental analysis of the fall mitigation motion caused by tripping based on the motion observation until shortly before ground contact”, Journal of Biomechanical Science and Engineering, Vol.14, No.1 (2019), p.18-00510. doi.org/10.1299/jbse.18-00510. Final Version Released on May 10, 2019, Advance Publication Released on April 18, 2019.