Twenty recreationally active volunteers (15 men and 5 women) agreed to participate in the study (mean ± standard deviation [SD] of age, body height, weight, and body mass index: 23.9 ± 2.5 years, 168.4 ± 8.41 cm, 61.8 ± 12.5 kg, and 21.6 ± 3.1 kg/m2, respectively) after being informed about this study protocol. The study protocol was approved by the Ethics Committee for epidemiology of XXX University (approval number: E-2268). All subjects provided informed consent for their participation in the study. “Recreationally active” was defined as participation in at least 150 min of moderate activity per week for at least 6 months prior to the study . Participants were experienced in athletics, basketball, baseball, classical ballet, badminton, football, tennis, swimming, or volleyball. We excluded participants with lower extremity injury and symptoms, previous lower extremity surgery prior to the study, vestibular disease, or neurological impairments.
Study design and procedures
This study used a laboratory-based and repeated-measures test design. To determine the influence of ankle orthotics on jump performance and dorsi-plantarflexion ROM, we used the following experimental protocol with participants in a barefoot condition (no-orthosis) and two orthotics conditions (orthosis 1 and orthosis 2) with different restrictions on dorsi-plantarflexion. Filmista (Nippon Sigmax, Japan) and A1 (Nippon Sigmax, Japan) orthotics were used for orthosis 1 and 2 conditions, respectively. Participants wore correctly sized orthotics on both ankles. A certified orthotist instructed participants on how to wear the orthotics using demonstrations. Figure 1 shows the composition of the orthotics used in this study. Orthosis 1 consists of three thin layers (Figure 1A). It has two surfaces made of soft and hard urethane films with different elasticities. The hard film is found in the middle layer and is designed to limit excessive ankle inversion without restricting the ROM of the ankle joint in the sagittal plane. Orthosis 2 consists of three different straps on the fabric, covering the ankle joint with medial and lateral stays (Figure 1B). Stirrup, biceps, and distal tibiofibular joint straps are applied to prevent excessive ankle inversion, based on the medical taping concept. These ankle orthotics were made by the same manufacturer; however, orthosis 1 had less restriction on ankle inversion than orthosis 2. Participants successively repeated the same jump exercises under the three abovementioned conditions following a randomized order. A jump session was defined as a period during which a participant performed static jumps and RJ under one of the above conditions. All parameters were measured over a 3-day period for each participant. Participants completed a jump session under one condition per day, with a minimum of 24 h of rest between each jump session.
Vertical jump performance tests
Participants performed a 5-min warm-up exercise before undergoing the vertical jump performance tests. They received explanations on how to perform SJ, CMJ, and RJ and practiced these vertical jumps. To perform the SJ, they started by folding their hands and squatting with their hips and knees flexed to approximately 45 and their feet placed a shoulder width apart on a force plate (Technology Service, Nagano, Japan). After 1–2 s, the examiner instructed the participants to jump vertically and forcefully. To perform the CMJ, they began by folding their hands and standing upright with their feet placed approximately a shoulder width apart on the force plate. After 1–2 s, they rapidly descended into a 45 semi-squat position and jumped vertically with maximum effort. Five sets, each of SJ and CMJ, were performed. Participants took as many breaks as needed between each set to avoid fatigue.
The repetitive RJ performance was assessed using the OptojumpTM system (Microgate, Bolzano, Italy), consisting of two infrared photocell bars with one bar acting as a transmitter unit containing 96 light-emitting diodes positioned 3 mm above the ground, and the other bar acting as a receiver unit. Participants were instructed to keep their hands on their hips to avoid upper-body interference, jump, and land on the same spot, with legs extended then flexed, while looking ahead. They were also instructed to maximize the jump height and minimize the ground contact time. In a previous study, this method of RJ assessment achieved interday reliability . When a participant performed the repetitive RJ within a parallel bar configuration, the light from the light-emitting diode was interrupted by the participant’s foot during the jump, triggering the timer in the unit and recording the interruption with a sampling frequency of 1000 Hz. Two sets of seven repetitive RJs were performed with intermittent 5-min resting periods.
Analysis of the vertical jump performance
SJ and CMJ performances were analyzed using the VGRF, recorded by a force plate with a sampling frequency of 1000 Hz. Based on previous studies , jump height [cm], rate of force development (RFD) [N/s], vertical impulse [Ns], peak power [W], and maximum VGRF (VGRFmax) [N] were calculated using MATLAB (R2020b, Math Works GK, Tokyo, Japan). The jump height was estimated as follows: jump height = (1/2 × Tair × g)2 × (2g)-1. Tair represents the flight time [s] from the force record on the force plate, and g the acceleration due to gravity (9.81 m/s2). The RFD was calculated as: RFD = (VGRFmax – minimum VGRF [VGRFmin]) / Δt1, where Δt1 indicates the change in time [s] between 20% and 80% of the total time from the VGRFmin to the VGRFmax. VGRFmin was defined as the lowest value of the VGRF during the contact phase before increasing to VGRFmax. VGRFmax was defined as the peak of the VGRF occurring for the first time if two peaks were applicable. The net vertical impulse was calculated as VGRF × Δt2 – body weight ×Δt2, where Δ2 indicates the change in time [s] from the point at which the VGRF equated with the body weight to the point at which VGRF fell below the body weight. Peak mechanical power was calculated from the vertical jump height and body weight as (60.7 × jump height [cm]) + (45.3 × body height [cm]). The RFD and VGRFmax were normalized using the body weight [kg] to calculate the relative RFD and relative VGRFmax.
OptojumpTM proprietary software (OptojumpTM Next software, version 22.214.171.124, Bolzano, Italy) was used to automatically calculate RJ performance variables (jump height [cm], contact time [s], and RJ index (RJ index) [m/s]). The RJ index was estimated as: RJ index = 1/8 × g × Tair2 / contact time. Of the seven repetitive jumps in the second set, the second, third, fourth, and fifth jumps were included in the analysis. The individual performance variables per RJ (second, third, fourth, fifth, and sixth jumps) and their means were used for further statistical analysis.
Analysis of the sagittal ankle joint motion
Two-dimensional motion analysis for sagittal ankle joint motion was performed simultaneously with the vertical performance test parameters (SJ, CMJ, and RJ). Three reflective markers were placed on the dominant leg on the lateral aspect of the tibial plateau, lateral malleolus, and lateral aspect of the base of the fifth metatarsal . The markers were applied by the same examiner. A video camera was positioned at a distance of 1.5 m perpendicular to the edge of the force plate or OptoJumpTM device to capture the trajectory of the marker from the sagittal plane with a sampling frequency of 240 Hz. ImageJ software (National Institutes of Health, Maryland, USA) was used for the analysis. The maximum dorsiflexion angle, plantarflexion angle at toe-off, and dorsi-plantarflexion ROM were calculated by connecting three points , as shown in Figure 2. The ROM was defined as the difference between the plantarflexion angle at toe-off and the maximum dorsiflexion angle. For each joint angle, the mean angle from the five jumps was used as the representative value. Additionally, the individual angle of the second set of RJ was used for further analysis.
A one-factor (type of orthosis: no-orthosis, orthosis 1, orthosis 2) repeated-measures multivariate analysis of variance (MANOVA) was used to determine the influence of ankle orthotics on the variables of jump performance and ankle ROM. MANOVA was conducted for each type of jump (SJ, CMJ, RJ). A univariate analysis of variance (ANOVA) and Tukey method for pairwise comparisons were conducted on any significant findings.
To determine the difference in the effect of ankle orthosis use on each RJ-related variable (jump height, contact time, and RJ index) and the angle for sagittal ankle motion by the number of RJs, a two-factor (type of orthosis: no-orthosis, orthosis 1, orthosis 2 × number of RJ: second, third, fourth, fifth, and sixth) repeated-measures MANOVA was initially conducted. As a follow-up analysis, a two-factor ANOVA was conducted on any significant findings. If the main or interaction effect was observed, the Tukey method of pairwise comparisons was also performed. Additionally, to explore the relationship between the RJ-related variables and the angle of sagittal ankle motion during RJs, a Pearson’s product moment correlation coefficient was computed for the variables that were significant by pairwise comparisons. A statistical significance was defined as p < 0.05 in all statistical tests used in this study.