Intervention

A total of twenty sessions of WBV training were performed by each training group. WBV training was performed on a commercial vibrating platform (Power Plate Pro5, Power Plate North America Inc, USA) that produces simultaneous vibration in 3 planes, left to right, front to back, and up and down. Each training session lasted 40 min and consisted of 10 min warm-up (5 min cycling and 5 min stretching exercises), 20 min of intermittent WBV exercise, and 10 min cool-down (5 min cycling and 5 min stretching exercises). Since prolonged exposure to a continuous vibration is known to suppress the tonic vibration reflex and decrease muscle activation and force, an intermittent vibration protocol was used in this study. Specifically, the intermittent WBV training protocol consisted of 10 sets x 1 min WBV exercise, with 60 s rest between sets and an additional 2 min rest after the first 5 sets. The frequency of WBV exercise was progressively increased during the training programme from 25 Hz (1^st to 6^th training session), to 30 Hz (7^th to 12^th training session) and 35 Hz (13^th to 20^th training session), while the peak-to-peak displacement of vibration was kept unchanged throughout the study (4–6 mm). The selection of the frequencies and amplitudes was based on the fact that most studies that have used synchronous vibration applied frequencies of 25–35 Hz and peak-to-peak displacement 4–6 mm [16, 22, 23].

The two WBV training programmes had the same training volume (total training sessions: 20), but differed only in the frequency of training per week. The Lf-WBV group trained three times per week, with at least 48 hours between sessions, so that the total of 20 training sessions were completed in 7 weeks. The Hf-WBV group trained five times per week on consecutive days, and a total of 20 training sessions were completed in 4 weeks. The CG did not perform any training, but followed the general programme of the academy, as did the subjects allocated to the two training groups.

During the WBV sessions, the participants wore non-slippery socks in order to avoid any dampening effect attributable to footwear [9]. The participants were instructed to place their feet equidistant from the rotation axis and maintained an upright position with their feet in full contact with the platform and their knees flexed at 70°. The knee angle was controlled during the WBV training sessions with a goniometer (Lafayette Gollehon; Lafayette Instrument Co., Lafayette, IN, USA). Participants were instructed to direct their head and eyes forward, breathe normally, and place their hands on the machine’s handles without any grasping.

At the completion of WBV, most participants mentioned a hot sensation at the bottom of their feet (the part closest to the vibration plate), and a few participants (n = 5) in the first 3–4 training sessions reported minor pruritus, which subsided within a few minutes of vibration. None of the participants complained of any other side-effects in response to WBV training.