Fig 1.
The custom-built shoe with an active insole.
The system consisted of a foam sole, an active insole, a portable battery, and the main body. The portable battery could be attached to the bottom of the active insole. Mechanical vibration was applied to the metatarsal head and heel using the actuators in the active insole, which could be placed inside the shoes and connected to the charging port of the shoes. The red circle shows a magnified view of the charging port that can be opened and connected to a custom-made power cable.
Fig 2.
Demonstration of the vertical jump task.
(A) The initial position of the participant on the force platform before the jump. Placing their feet shoulder-width apart, participants bent their knees and lowered their bodies so that their fingers touched the toe section of the shoes. (B) The highest position after the experimenter gave the participants a verbal signal to jump. The participants jumped as high as possible, extending their arms upward. Eight non-invasive EMG sensors were used to measure the muscle activation levels of the tibialis anterior, gastrocnemius medialis, rectus femoris, and biceps femoris of both the legs. The placement sites of the EMG sensors, the vibrating shoe, and the force platform are highlighted in green, blue, and light gray colors, respectively.
Fig 3.
An illustration of the overall process of selecting the reference points for the calculation of vertical jump height (VJH) of a representative participant.
(A) The time profile of the vertical ground reaction force (VGRF) during a single trial of the jumping task. The moment of vertical jump takeoff was defined as the moment when the VGRF value reached its minimum (t1) (denoted by a red star inside a red circle). (B) The section of the VGRF time profile that started at 3 s before t1 and ended at t1. This section was used to calculate the body weight (BW) of the participants. (C) The standard deviation (SD) of the VGRF. The VGRF section in (B) was divided into time bins of 50 ms (denoted by blue squares). The SD of the VGRF during each time bin was calculated. The initial 2 s of the section are enlarged on the right side to illustrate the time bins more clearly. The time bin with the least SD of the VGRF is highlighted by a red square. The BW was estimated as the mean VGRF during the red time bin. (D) The moment of jump initiation (t0). The moment after which the VGRF began to be equal to or lower than 99% of the BW for the second last time within the VGRF time profile in (B) was selected as t0 (denoted by a red star in a red circle). The green line denotes the BW of the participant. The integration of the difference between the VGRF and BW from t0 to t1 was defined as the net impulse during the jump motion, which was used to calculate the VJH.
Fig 4.
The effect of active insole vibration.
(A) The effect of vibration on the vertical jump heights (VJHs) of 20 participants under two experimental conditions (ON: Vibration at 130% of the sensory threshold, OFF: No vibration). The paired t-test concluded that vibration increased the VJH. (B) The effect of vibration on the ground contact duration before takeoff. (C) The effect of vibration on the average vertical ground reaction force (VGRF) during the ground contact before takeoff. (D) The effect of vibration on the integrated electromyography (EMG) of the prime movers (tibialis anterior [TA], gastrocnemius medialis [GM], rectus femoris [RF], and biceps femoris [BF]). The error bars and * denote standard errors and statistically significant difference (p < 0.05), respectively.
Fig 5.
The body weight (BW) and standard deviation (SD) of the vertical ground reaction force (VGRF) during the BW calculation phase.
(A) The BWs of 20 participants under the two experimental conditions (ON: Vibration at 130% of the sensory threshold, OFF: No vibration). (B) The SD of the VGRF during the BW calculation phase of 20 participants under the two experimental conditions. The error bars and NS denote standard errors and no statistically significant difference (p > 0.05), respectively.