TITLE: Effects of Whole-Body Vibrations on Neuromuscular Fatigue
AUTHORS: Milos Kalc, Ramona Ritzmann, Vojko Strojnik

[DESCRIPTION]
<div class = "text-blocks"><div class = "text-block"><span style = "font-weight:bold;">Purpose: </span><span>The aim of the study was to investigate the origin and magnitude of neuromuscular fatigue induced by half-squat whole-body vibration.</span></div><div class = "text-block"><span style = "font-weight:bold;">Methods: </span><span>Ten young, recreationally trained adults participated in six fatiguing protocols, each consisting of several sets of 30, 60 or 180 s static squatting superimposed with vibration (WBV</span><span style = "vertical-align:sub;">30</span><span>, WBV</span><span style = "vertical-align:sub;">60</span><span>, WBV</span><span style = "vertical-align:sub;">180</span><span>) or without vibration (SHAM</span><span style = "vertical-align:sub;">30</span><span>, SHAM</span><span style = "vertical-align:sub;">60</span><span>, SHAM</span><span style = "vertical-align:sub;">180</span><span>) for a total exercise exposure of 9-minutes in each trial. Maximum voluntary contraction (MVC), level of voluntary activation (%VA), single twitch peak torque (TW</span><span style = "vertical-align:sub;">PT</span><span>), low- (T</span><span style = "vertical-align:sub;">20</span><span>) and high-frequency (T</span><span style = "vertical-align:sub;">100</span><span>) doublets, and low-to-high-frequency fatigue ratio (T</span><span style = "vertical-align:sub;">20/100</span><span>) were assessed before, immediately after, 15 and 30 minutes after each fatiguing protocol.</span></div></div>

[STEPS]
?. [Warm-up]
warm-up routine consisting of bench stepping ( high) at a frequency of , with a leg exchange each minute
20
0.5 Hz
?. [Warm-up]
rest
?. [Equipment calibration]
We calibrated the  prior to each measuring session.The signal of the dynamometer was connected torunning The same machine has been used in several other studiesWe calibrated the force transducer, by hanging a  weight. We read the Voltage transformation to calculate the exerted torque.
20 kg
?. [Pre experiment procedures]
We invited the subject to seat on the Isometric dynamometer in order to adjust the seating position and lever arm. The subject was positioned in an upright sitting position, the trunk at 100° leaning against the backrest of the isometric dynamometer, fixed by straps over the pelvis and a horizontal pad over the distal third of the thigh. The knee joint axis was aligned with the mechanical axis of the dynamometer. The shin pad was placed just above to the medial malleolus. The right knee joint was fixed at a 60° angle (0° = full extension)
?. [Pre experiment procedures]
Femoral nerve stimulation electrode placementWe invited the participants to flex their hip from in a seated position, while we palpated the iliac fossa and placed the electrode (cathode) into the femoral triangle. A larger self-adhesive electrode placed over the gluteal fold served as an anode.
?. [Pre experiment procedures]
Femoral nerve test stimulationElectrical impulses (single, square wave, 1-ms duration) elicited by a high voltage constant current electrical stimulatorwere used to trigger the muscle response, which was detected as a change in knee extensors torque.
?. [Pre experiment procedures]
We elicited several impulses (3 in average, max 6) at a fixed intensity of   at a frequency of   and slightly moving the cathode in order to find the spot which produced the highest response (highest torque).
20
0.1 Hz
?. [Pre experiment procedure]
The stimulation intensity to elicit the maximum knee extensor isometric twitch was determined in each subject after Warm-up (starting from  ) progressively increasing the stimulation intensity  by  until no further increase in torque was observed despite further increment in current. The current at maximal twitch torque was additionally increased by a factor of 1.5 to obtain a supra-maximal stimulus. This intensity was maintained for the entire visit.
10
10
?. [PRE - assessment (t0)]
?. [PRE - assessment (t0)]
Maximal voluntary contraction with double twitch interpolated techniquesSubjects were asked to perform a 5 s maximal isometric voluntary (MVC) knee extension The signal was smoothed using a 0.5 s window moving average filter and peak torque (MVC) was retained for analysis. The double twitch interpolated technique was performed by superimposing a  doublet on the isometric plateau (TMVC). A second analogous stimulation   (T100) on the relaxed muscle followed after 3 s. The ratio of the amplitude of the TMVC over T100 was then calculated to obtain the level of voluntary activation (%VA):\%VA\;=\;(1\;-\;\frac{T_{MVC}\;-\;MVC}{T_{100}})\;\ast\;100
100 Hz
100 Hz
?. [PRE - assessment (t0)]
The torque change induced by the paired high- (, i.e.  interstimulus interval) and low-frequency (, i.e.  interstimulus interval) supramaximal electrical stimuli were analyzed. The following parameters were obtained: peak torque from doublet (T100), peak torque from doublet (T20) and the low- to thehigh-frequency ratio (T20/100) was calculated using the following formula: T_{20/100}\;=\;\frac{T_{20}}{T_{100}}\ast\;100This ratio was then used as a surrogate of low- to high-frequency tetanic stimulation.
100 Hz
10 ms
20 Hz
50 ms
100 Hz
20 Hz
?. [PRE - assessment (t0)]
The torque change induced by the single supramaximal femoral nerve stimuli was analysed. The following parameters were obtained: 1) the maximum torque value (TWPT);
?. [Intervention]
InterventionThe interventions were performed on  which was switched on (or off for SHAM conditions) at a frequency of . Subjects were instructed to maintain a half-squat position with their knees flexed at an angle of 60°.  Subjects stood with their feet 40 cm apart where the tilting platform reaches peak-to-peak displacement amplitude of 5 mm.
26 Hz
?. [POST 15 assessments - (tf15)]
Repete assessment procedure
?. [POST 30 assessments - (tf30)]
Repete assessment procedure
?. [Data analysis]
A two-way factorial ANOVA was conducted in with the to compare the main effects of time and trial and the interaction effect of time x trial. Generalized eta squared (η2) effect sizes were calculated for the ANOVA main and interaction effects. The emmeans package (Lenth et al. 2018) was used to perform follow-up post hoc analysis. Planned comparisons were performed using Sidak corrected linear contrasts comparing. Statistical significance was set at p
?. [POST assessments - (tf)]
Repete assessment procedure
?. [POST assessments - (tf)]