Acute effects of local vibration stretching on ankle range of motion,vertical jump performance and dynamic balance after landing

2.1 Participants

Fifteen healthy men (age = 23.5 ± 2.3 years; height = 172.1 ± 6.4 cm; body weight = 67.2 ± 9.9 kg) voluntarily participated in the study. Subjects with a history of a sprain higher than grade II, current ligamentous defects, a history of ligament or joint reconstruction or repair, dysfunction of the vestibular system affecting balance, or trauma (including fracture, myositis ossificans, or burn injuries) were excluded. Three different protocols (SS, V + S, and control) were performed for all subjects in random order on three separate days, with an interval of 24 to 48 hours between tests. This study protocol was approved by Hiroshima University’s Institutional Review Board (study protocol ID number: C-271, date of approval: 2020-6-20). All subjects provided written informed consent before examination.

2.2 Experimental design and procedures

The subjects were randomly allocated to three experimental conditions (SS, V + S, and control). To ensure consistency of data collection among the subjects, stretching of the non-dominant limb was performed for unilateral assessment [12]. The non-dominant limb of each subject was defined by asking about his or her preferred limb when kicking a ball [13]. The warm-up exercise consisted of a 5-min cycling exercise with ergometer air resistance set to 75 W and cadence set to 60 rpm, followed by a complete break of 3-min before the beginning of the test [14].

Stretching was performed with the subjects’ hips and knees in full extension while they stood on a footplate with the ankle at maximum dorsiflexion for both the SS and V + S conditions and with the ankle at an angle of 0 ∘ for the control condition. For the V + S condition, a vibration device (MYOVOLT TM ; New Zealand) was used with small actuators located on each panel. The vibration frequency passed through a range of 0–170 Hz at amplitude of 0–0.12 mm during the 0.5-s, which created a pulsing effect. The device was placed on the muscle belly of the medial head of the gastrocnemius (GM) of the stretching limb and was held in place with a Velcro strap (Fig. 1). Each panel was activated for 0.5 s and deactivated for 0.5 s with a 5-min duration of vibration using the pulsed mode of the device applied to the stretching limb.

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In this study, an ankle joint exercise device for stretching (Rakkun Walk R-1; Maruzen Industrial Co., Ltd. and Hiroshima University, Japan) was applied (Fig. 2). This computerized device enables to control angle and movement velocity of the ankles joints placed on two steps [15]. The two steps are installed horizontally in the base portion, and two actuators control the left and right vertical movements independently. The inclination angle of the step was used to define the dorsiflexion angle. This device was used for both the intervention and functional evaluation.

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We measured the degree of maximum ankle dorsiflexion ROM before and after stretching in the SS, V + S, and control protocols to determine the effect of stretching in each stretching condition. Measurements were taken with the participants in a standing position, with the knee fully extended on the footplate. The ROM was measured three times using a stretching device programmed to automatically move the footplate according to pre-specified angle measurements. The mean value of three trials was used for analysis. The angle at which the subjects began to feel discomfort or pain, or raise the heel was defined as the maximum degree of ankle dorsiflexion.

2.4 Assessment of vertical jump

Each subject performed a maximal voluntary vertical jump on the floor with non-dominant limb in two conditions: (1) from a semi-squatting position (squatting jump, SJ) and (2) from an upright standing position with a downward movement before starting to move upward (counter-movement jump, CMJ).

Vertical jump ability was assessed by measuring jump height during the SJ and CMJ using a jump gauge (Myotest SA, Sion, Switzerland). Subjects were instructed to jump vertically and attempt to land with the same body position and in the same location as takeoff to avoid lateral or horizontal displacement. The participants positioned their arms laterally throughout the entire jump and kept their torso in an upright position to emphasize the use of the leg extensor muscles [16]. The participants could recover for 2 minutes between jumps. The average height of the three jumps was used as the outcome value for each jump test. The standard error of the mean for inter-rater reliability of the composite test was 2.4 cm [17, 18].

2.5 Assessment of dynamic postural stability after landing

Dynamic postural stability after landing was evaluated using a two-leg jump and one-leg landing in the forward direction. In a previous study, this approach achieved an intersession reliability [ICC (3, k)] of 0.86 [19]. The jump height was standardized at 30 cm, and the jump distance was normalized to the subject’s body height [19]. The subjects placed 40% of their height from the end of the force plate, and a hurdle with a height of 30 cm was placed at the midpoint between the force plate and the starting position. The subjects performed the following actions three times: jump in the forward direction using both legs over the hurdle with subsequent landing on the force plate on the non-dominant limb, stabilize as quickly as possible, place both hands on the waist once stabilized, and keep still for 10 s while looking forward. They were permitted five practice trials for each condition to become familiar with the one-leg jump, with 2 minutes of rest after testing. The measurements in each condition were conducted on different days to avoid the subject becoming too familiar with the task and to prevent fatigue. The task was repeated if the subject fell upon landing, contacted the hurdle, or failed to jump. Three successful trials were performed for each condition (control, SS, and V + S) and were used for data analysis.

Ground reaction force data was collected at a sampling frequency of 200 Hz. The global reference system was set so that the anterior-posterior axis referred to the y-axis, the medial-lateral axis to the x-axis, and the vertical axis to the z-axis. A Microsoft Excel macro was used to process the ground reaction force data for calculating the DPSI. These data were passed through a zero-lag fourth-order low-pass Butterworth filter with a frequency cutoff of 20 Hz. The dependent variable was the DPSI, is shown in Fig. 3. The DPSI is the resultant of the anterior-posterior, medial-lateral, and vertical ground reaction forces while it also provides stability indices for the anterior-posterior (APSI), medial-lateral (MLSI), and vertical (VSI) directions. The DPSI was calculated using the ground reaction forces generated during the first 3 seconds immediately following initial contact, which was identified as the instant when the vertical ground reaction force exceeded 5% of the body weight. The force plate data was normalized to body weight [18]. We also used raw data signals to calculate the maximum vertical ground reaction force (vGRF max), which was expressed as the magnitude of the peak force in Newtons divided by the subject’s body weight. The average of three successful trials for each condition was used for further analysis [18].

2.6 Statistical analysis

An a priori power analysis by G*Power software (Heinrich-Heine University, Düsseldorf, Germany) revealed that at least 20 subjects were required for a statistical power of 0.75 at an effect size of 0.80 with an alpha level of 0.05 [19]. Repeated-measures 2 (time) × 3 (stretching condition) analysis of variance (ANOVA) was used to compare changes in ROM in both stretching conditions (SS and V + S) and the control condition. When appropriate, follow-up analyses were performed using paired t -tests between pre- and post-stretching to confirm significant changes within each condition. In addition, repeated-measures 1 (time) × 3 (stretching condition) ANOVA was used to compare the vertical jump and DPSI (DPSI, MLSI, APSI, VSI, and vGRF max) in both stretching conditions and the control condition. When appropriate, follow-up analyses were performed using post hoc tests. An alpha level of 0.05 was the criterion for rejection of the null hypothesis for all statistical tests. Effect sizes were calculated using the Cohen d statistic [19]. Data analysis was conducted using SPSS for Windows, ver. 20.0 (IBM Japan Co., Tokyo, Japan).