Older adults show higher increases in lower-limb muscle activity during whole-body vibration exercise

The purpose of this study was to compare lower limb muscle activity during whole-body vibration (WBV) exercise between a young and an older study population. Thirty young (25.9±4.3 yrs) and thirty older (64.2±5.3 yrs) individuals stood on a side-alternating WBV platform while surface electromyography (sEMG) was measured for the tibialis anterior (TA), gastrocnemius medialis (GM), soleus (SOL), vastus lateralis (VL), vastus medialis (VM), and biceps femoris (BF). The WBV protocol included nine vibration settings consisting of three frequencies (6, 11, 16 Hz) x three amplitudes (0.9, 2.5, 4.0 mm), and three control trials without vibration (narrow, medium, wide stance). The vertical platform acceleration (peak values of maximal displacement from equilibrium) was quantified during each vibration exercise using an accelerometer. The outcomes of this study showed that WBV significantly increased muscle activity in both groups for most vibration conditions in the TA (averaged absolute increase: young: +3.9%, older: +18.4%), GM (young: +4.1%, older: +9.5%), VL (young: +6.3%, older: +12.6%) and VM (young: +5.4%, older: +8.0%), and for the high frequency-amplitude combinations in the SOL (young: +7.5%, older: +12.6%) and BF (young: +1.9%, older: +7.5%). The increases in sEMG activity were significantly higher in the older than the young adults for all muscles, i.e., TA (absolute difference: 13.8%, P<0.001), GM (4.6%, P=0.034), VL (7.6%, P=0.001), VM (6.7%, P=0.042), BF (6.4%, P<0.001), except for the SOL (0.3%, P=0.248). Finally, the vertical platform acceleration was a significant predictor of the averaged lower limb muscle activity in the young (r=0.917, P<0.001) and older adults (r=0.931, P<0.001). In conclusion, the older population showed greater increases in lower limb muscle activity during WBV exercise than their young counterparts, meaning that they might benefit more from WBV exercises. Additionally, training intensity can be increased by increasing the vertical acceleration load.     

The addition of synchronous whole-body vibration to battling rope exercise increases skeletal muscle activity

Objectives:

To evaluate the effects of performing battling rope exercise with and without the addition of whole-body vibration (WBV) on muscle activity of the leg, trunk, and upper body.

Methods:

Twenty-eight recreationally active university students completed 20-s of battling rope undulation for 6 separate conditions: 1) alternating arm motion no WBV -Alt_NoWBV; 2) alternating arm motion 30 Hz low amplitude WBV -Alt_30 Hz-L; 3) alternating arm motion 50 Hz high amplitude -Alt_50 Hz-H; 4) double arm motion no WBV -Double_NoWBV; 5) double arm motion 30Hz low amplitude WBV -Double_30Hz-L; 6) double arm motion 50 `Hz high amplitude -Double_50 Hz-H. Electromyography (EMG) was measured for the gastrocnemius medialis (GM), vastus medialis oblique (VMO), vastus lateralis (VL), rectus abdominis (RA), multifidus (MF), biceps brachii (BB), and triceps brachii (TB) muscles.

Results:

The double arm motion during undulation resulted in greater (p<0.05) muscle activity in the VMO, VL, RA, and MF muscles while the GM was more active during the alternating arm motion. WBV at 50Hz increased EMG in all muscles measured vs NoWBV and the 30 Hz condition.

Conclusion:

These results are the first to demonstrate that the exercise stimulus of performing battling rope exercise can be augmented by completing the exercise while being exposed to WBV from a ground-based platform.     

Acute effects of whole-body vibration on neuromuscular responses in older individuals: implications for prescription of vibratory stimulation

The aim of this study was to analyze surface electromyography activity (sEMG) and rating of perceived exertion (RPE) responses in different muscles while standing on a vibrating platform producing oscillations of different frequencies and amplitudes. Twenty community-dwelling older adults (79.6 ± 3.2 years) took part in the research. Subjects were exposed to 12 different vibration treatments of 15 seconds separated by 1 minute of rest in random order to check the influence of frequency (25, 35, and 45 Hz) and amplitude (1 mm [low] and 3.1 mm [high]) vibration on sEMG signal and RPE. Additionally, the use of a soft pad was also examined for its influence on these measures. Three-factor analysis of variance for RPE and both lower and whole-body sEMGs revealed a significant amplitude main effect (p < 0.01), and soft mat effect (p < 0.01), and a significant frequency main effect (p < 0.01). The major findings were that sEMG and RPE increased with the acceleration of the vibration; moreover, the increments of sEMG were highly correlated with RPE. The results of this study suggest that using the RPE method after each exercise would allow exercise and health professionals to assess the intensity levels that correspond to the level of the vibratory program in older adults.     

Use of whole body vibration in individuals with chronic stroke: Transmissibility and signal purity

This study examined (1) the influence of whole body vibration (WBV) frequency (20 Hz, 30 Hz, 40 Hz), amplitude (low: 0.8 mm and high: 1.5 mm) and body postures (high-squat, deep-squat, tip-toe standing) on WBV transmissibility and signal purity, and (2) the relationship between stroke motor impairment and WBV transmissibility/signal purity. Thirty-four participants with chronic stroke were tested under 18 different conditions with unique combinations of WBV frequency, amplitude, and body posture. Lower limb motor function and muscle spasticity were assessed using the Fugl-Meyer Assessment and Modified Ashworth Scale respectively. Nine tri-axial accelerometers were used to measure acceleration at the WBV platform, and the head, third lumbar vertebra, and bilateral hips, knees, and ankles. The results indicated that WBV amplitude, frequency, body postures and their interactions significantly influenced the vibration transmissibility and signal purity among people with chronic stroke. In all anatomical landmarks except the ankle, the transmissibility decreased with increased frequency, increased amplitude or increased knee flexion angle. The transmissibility was similar between the paretic and non-paretic side, except at the ankle during tip-toe standing. Less severe lower limb motor impairment was associated with greater transmissibility at the paretic ankle, knee and hip in certain WBV conditions. Leg muscle spasticity was not significantly related to WBV transmissibility. In clinical practice, WBV amplitude, frequency, body postures need to be considered regarding the therapeutic purpose. Good contact between the feet and vibration platform and symmetrical body-weight distribution pattern should be ensured.     

Effects of muscular strength, exercise order, and acute whole-body vibration exposure on bat swing speed

The purposes for this study were to investigate effects of acute whole-body vibration (WBV) exposure and exercise order on bat speed and to examine relationship between muscular strength and bat speed. All participants were recreationally trained men (n = 16; 22 ± 2 years; 181.4 ± 7.4 cm; 84.7 ± 9 kg), with previous baseball experience and were tested for 1 repetitive maximum (1RM) strength in squat and bench press. Subjects then participated in 4 randomized sessions on separate days, each consisting of 3 sets of 5 bat swings. Exercises (upper and lower body dynamic and static movements related to bat swing) with or without WBV exposure were performed after sets 1 and 2. Trials were as follows: no-exercise Control (CTRL), upper body followed by lower body exercises without WBV (Arm-Leg NOVIB), upper body followed by lower body exercises with WBV (Arm-Leg VIB), and lower body followed by upper body exercises with WBV (Leg-Arm VIB). Bat speed was recorded during each swing and averaged across sets. Statistical analyses were performed to assess differences across sets and trials. Linear regressions analyzed relationship between strength and bat speed. A significant relationship existed between bat speed and lower body strength (r = 0.406, p = 0.008) but not for upper body strength. The exercise order of Arm-Leg VIB significantly increased bat speed by 2.6% (p = 0.02). Performing identical order of exercises without vibration (Arm-Leg NOVIB) significantly decreased bat speed by 2% (p = 0.039). It was concluded that adding vibration exposure to total-body exercises can provide acute enhancements in bat speed. Additionally, leg strength was shown to influence bat speed suggesting that increasing leg strength may enhance bat speed.     

等长收缩诱发肌肉疲劳及恢复过程中表面肌电信号特征变化规律

运用线性和非线性分析方法分析不同强度等长收缩诱发局部肌肉疲劳及恢复过程中表面肌电信号（surface electromyogram,sEMG）特征的变化规律,探讨影响sEMG信号变化的可能原因和机制.结果显示,在肱二头肌疲劳收缩过程中,sEMG的特征指标平均肌电值（average EMG,AEMG）、平均功率频率（mean power frequency,MPF）、Lempel-Ziv复杂度（Lempel-Ziv complexity,C（n））和确定性线段百分数（Determinism%,% DET）的变化具有良好的规律性.恢复期AEMG没有表现出规律性的变化,MPF、C（n）和?T在恢复期2秒即开始显著恢复,在前10秒恢复很快,随后恢复速度变慢.恢复初期sEMG信号特征的快速变化提示中枢控制因素可能发挥更大作用.     

Effects of whole body vibration on motor unit recruitment and threshold

Whole body vibration (WBV) has been suggested to elicit reflex muscle contractions but this has never been verified. We recorded from 32 single motor units (MU) in the vastus lateralis of 7 healthy subjects (34 ± 15.4 yr) during five 1-min bouts of WBV (30 Hz, 3 mm peak to peak), and the vibration waveform was also recorded. Recruitment thresholds were recorded from 38 MUs before and after WBV. The phase angle distribution of all MUs during WBV was nonuniform (P < 0.001) and displayed a prominent peak phase angle of firing. There was a strong linear relationship (r = -0.68, P < 0.001) between the change in recruitment threshold after WBV and average recruitment threshold; the lowest threshold MUs increased recruitment threshold (P = 0.008) while reductions were observed in the higher threshold units (P = 0.031). We investigated one possible cause of changed thresholds. Presynaptic inhibition in the soleus was measured in 8 healthy subjects (29 ± 4.6 yr). A total of 30 H-reflexes (stimulation intensity 30% Mmax) were recorded before and after WBV: 15 conditioned by prior stimulation (60 ms) of the antagonist and 15 unconditioned. There were no significant changes in the relationship between the conditioned and unconditioned responses. The consistent phase angle at which each MU fired during WBV indicates the presence of reflex muscle activity similar to the tonic vibration reflex. The varying response in high- and low-threshold MUs may be due to the different contributions of the mono- and polysynaptic pathways but not presynaptic inhibition.     

Effects of whole-body vibration on bone properties in aged rats

Objective:This study aimed to explore optimal conditions of whole-body vibration (WBV) for improving bone properties in aged rats.Methods:Eighty-week-old rats were divided into baseline control (BC), age-matched control (CON) and experimental groups, which underwent WBV (0.5 g) at various frequencies (15, 30, 45, 60 or 90 Hz) or WBV (45 Hz) with various magnitudes (0.3, 0.5, 0.7 or 1.0 g) for 7 weeks. After interventions, femur bone size, bone mechanical strength and circulating bone formation/resorption markers were measured, and trabecular bone microstructure (TBMS) and cortical bone geometry (CBG) of femurs were analyzed by micro-CT.Results:Several TBMS parameters and trabecular bone mineral content were significantly lower in the 15 Hz WBV (0.5 g) group than in the CON group, suggesting damage to trabecular bone. On the other hand, although frequency/magnitude of WBV did not influence any CBG parameters, the 0.7 g and 1.0 g WBV (45 Hz) group showed an increase in tissue mineral density of cortical bone compared with the BC and CON groups, suggesting the possibility of improving cortical bone properties.Conclusion:Based on these findings, it should be noted that WBV conditions are carefully considered when applied to elderly people.     

Acute effect of whole-body vibration on power, one-repetition maximum, and muscle activation in power lifters

The purpose of this study was to investigate the acute effect of whole-body vibration with a frequency of 50 Hz (WBV(50Hz)) on peak power in squat jump (SJ), 1 repetition maximum (1RM) in parallel squat, and electromyography (EMG) activity and compare them with no-vibration conditions in power lifters. Twelve national level male power lifters (age 24 ± 5 years, body mass 110 ± 24 kg, height 179 ± 7 cm) tested peak power in SJ and 1RM in parallel squat while they were randomly exposed to WBV(50Hz) or to no vibration. These tests were performed in a Smith Machine. Peak power output was higher while performed with a WBV(50Hz) compared with the no-WBV condition (p < 0.05). This increase in power output was accompanied by higher EMG starting values and EMG peak values of the investigated thigh muscles during WBV(50Hz) (p < 0.05). There was no difference between adding WBV(50Hz) and no-vibration conditions in 1RM parallel squat. In conclusion, the results of this study suggest that the application of WBV(50Hz) acutely increases peak power output during SJ in well strength trained individuals such as power lifters. This increase in power was accompanied by an increased EMG activity in the quadriceps muscles. However, in 1RM parallel squat, there was no difference between WBV50Hz and no-vibration conditions. Therefore, adding WBV(50Hz) has no acute additive effect on 1RM parallel squat in power lifters and, based on the present findings, may thus not be recommended in the training to improve 1RM in power lifters. However, WBV(50Hz) seems to have an acute additive effect on peak power output and may be used in well strength trained individuals for whom a high power output is important for performance.     

Differences in muscle activation between submaximal and maximal 6-minute rowing tests

This study aimed to establish the differences in muscle activation between a 6-minute simulated race (all-out test) and a submaximal (blood lactate [LA] concentration 4 mmol·L(-1)) 6-minute effort (submax test) on a rowing ergometer. Eleven healthy, well-trained subjects performed the submax test followed after 1-hour rest by the all-out test. Surface electromyographic (sEMG) signal of muscles gastrocnemius medialis (GC), rectus femoris (RF), vastus lateralis (VL), biceps femoris, gluteus maximus (GM), erector spinae (ES), lower latissimus dorsi (LD_lo), upper latissimus dorsi (LD_up), brachioradialis (BR) and biceps brachii (BB), and other biomechanical, biochemical, and respiratory parameters were monitored during rowing. During the all-out test, the subjects covered a longer distance with larger average power output, higher stroke frequency, LA concentration, and oxygen consumption compared to the submax test (p < 0.05). During the submax test, the average rectified values (ARVs) of sEMG signal increased significantly only in the RF and LD_lo muscles. During the all-out test, the ARVs of the RF, VL, and GM muscles increased (p < 0.05), whereas the MDFs of the RF, ES, and LD_lo muscles decreased (p < 0.05). Compared to the submax test, the ARVs of the GC, RF, VL, LD_lo, LD_up, and BB muscles were significantly higher during the all-out test. However, only for the RF muscle, the all-out test resulted in a significantly lower MDF value compared to the submax test. The most involved muscles that would need special attention in training seem to be the leg and shoulder girdle extensors and arm flexors but not the trunk and hip extensors.     

A comparison of the effect of two types of vibration exercise on the endocrine and musculoskeletal system

Background: Whole body vibration (WBV) is a novel training intervention but a comparison of different methods of WBV has rarely been performed. Aim: To compare the short and medium term effects of two regimens of WBV on endocrine status, muscle function and markers of bone turnover. Patients and Methods: Over a period of 16 weeks, 10 men with a median age of 33 yrs (range, 29,49), were randomised to stand on the Galileo platform (GP) or Juvent1000 platform (JP) 3 times/wk. The total study duration was 16 weeks with measurements performed in a 4 week period of run-in, 8 weeks of WBV and a 4 week period of washout. These measurements included an assessment of anthropometry, body composition, muscle function and biochemical markers of endocrine status and bone turnover. To assess immediate effects of WBV, measurements were also performed at 60 mins before and 5, 30 and 60 mins after WBV. To assess immediate effects of WBV, measurements were also performed at 60 mins before and 5, 30 and 60 mins after WBV. Results: GP at 22 Hz was associated with an immediate increase in serum GH, rising from 0.07 μg/l (0.04,0.69) to 0.52 μg/l (0.06,2.4) (p=0.06), 0.63 μg/l (0.1,1.18) (p=0.03), 0.21 μg/l (0.07,0.65) (p=0.2) at 5 mins, 20 mins and 60 mins after WBV, respectively. An immediate effect was also observed in median serum cortisol which reduced from 316 nmol/l (247,442) before WBV to 173 nmol/l (123,245) (p=0.01),165 nmol/l (139,276) (p=0.02) and 198 nmol/l (106,294) (p=0.04) at 5 mins, 20 mins and 60 mins after WBV, respectively. Median serum CTX reduced significantly after 8 weeks of WBV training in the GP group from 0.42 ng/ml (0.29,0.90) pre-WBV to 0.29 ng/ml (0.18,0.44) at the end of WBV training (p=0.03). Over the 8 weeks, there was a reduction in median serum cortisol in the GP group from 333 nmol/l (242,445) (pre-WBV) to 270 nmol/l (115,323) (WBV) (p=0.04). None of the changes observed in the JP group reached statistical significance. Neither group showed any significant effect on muscle function, IGF-1, testosterone, leptin, CRP, creatine kinase, insulin or other markers of bone turnover. Conclusion: WBV can stimulate GH secretion, reduce circulating cortisol and reduce bone resorption. These effects are independent of clear changes in muscle function and depend on the type of WBV that is administered.     

The influence of whole body vibration on the plantarflexors during heel raise exercise

Whole body vibration (WBV) during exercise offers potential to augment the effects of basic exercises. However, to date there is limited information on the basic physiological and biomechanical effects of WBV on skeletal muscles. The aim of this study was to determine the effects of WBV (40 Hz, 1.9 mm synchronous vertical displacement) on the myoelectrical activity of selected plantarflexors during heel raise exercise. 3D motion capture of the ankle, synchronised with sEMG of the lateral gastrocnemius and soleus, was obtained during repetitive heel raises carried out at 0.5 Hz on 10 healthy male subjects (age 27 ± 5 years, height 1.78 ± 0.04 m, weight 75.75 ± 11.9 kg). During both vibration and non vibration the soleus activation peaked earlier than that of the lateral gastrocnemius. The results indicate that WBV has no effect on the timing of exercise completion or the amplitude of the lateral gastrocnemius activity, however significant increases in amplitudes of the soleus muscle activity (77.5–90.4% MVC P < 0.05). WBV had no significant effect on median frequencies of either muscle. The results indicate that the greatest effect of WBV during heel raise activity is in the soleus muscles during the early phases of heel raise.     

The acute effect of different frequencies of whole-body vibration on countermovement jump performance

Whole-body vibration (WBV) has been shown to elicit acute and chronic improvements in neuromuscular function; however, there is little conclusive evidence regarding an optimum protocol for acute WBV. The aim of this study was to compare the effects of acute exposure to different frequencies of WBV on countermovement jump (CMJ) height. Twelve recreationally trained men (age, 31 ± 8 years; height, 177 ± 12 cm; weight, 83.0 ± 6.9 kg) completed maximal CMJs pre- and post-WBV in a half-squat position for 30 seconds. In a blinded design with randomized testing order, participants were exposed on different days to frequencies of 0, 30, 35, and 40 Hz. Significant main effects were found for time (pre-to-post WBV, p < 0.01) and frequency * time interaction (p < 0.01), with post hoc analysis highlighting that there was a significant mean improvement of 6% in CMJ as a result of WBV at 40 Hz but no significant change at other frequencies. This study demonstrates that for recreationally trained men, an acute 30-second bout of vertical WBV at 40 Hz and 8-mm peak-to-peak displacement significantly enhances explosive jumping performance in comparison to other frequencies. Acute vertical WBV for 30 seconds at 40 Hz may be incorporated into strength and conditioning training to enhance explosive power; however, the exact mechanisms for improvements remain to be elucidated and further well-controlled investigations on chronic WBV training and using well-trained athletes are recommended.     

Acute effects of whole-body vibration on jump force and jump rate of force development: a comparative study of different devices

The goal of this study was to compare the acute effects of whole-body vibration (WBV) delivered by 3 devices with different mechanical behavior on jump force (JF) and jump rate of force development (JRFD). Twelve healthy persons (4 women and 8 men; age 30.5 ± 8.8 years; height 178.6 ± 7.3 cm; body mass 74.8 ± 9.7 kg) were exposed to WBV for 15 and 40 seconds using 2 professional devices (power plate [PP; vertical vibration] and Galileo 2000 [GA; oscillatory motion around the horizontal axis in addition to vertical vibration]) and a home-use device [Power Maxx, PM; horizontal vibration]). The JF and JRFD were evaluated before, immediately after, and 5 minutes after WBV. The JF measured immediately after 40 seconds of vibration by the GA device was reduced (3%, p = 0.05), and JRFD measured after 5 minutes of rest after 40 seconds of vibration by the PM device was reduced (12%, p < 0.05) compared with the baseline value. The acute effects of WBV (15 or 40 seconds) on JF and JRFD were not significantly different among the 3 devices. In conclusion, our hypothesis that WBV devices with different mechanical behaviors would result in different acute effects on muscle performance was not confirmed.     

Gender differences in knee stability in response to whole-body vibration

The purpose of this study was to determine whether there are kinematic and electromyographic (EMG) differences between men and women in how the knee is controlled during a single-legged drop landing in response to whole-body vibration (WBV). Forty-five healthy volunteers, 30 men (age 22 ± 3 years; weight 76.8 ± 8.8 kg; height 179.0 ± 6.8 cm) and 15 women (age 22 ± 3 years; weight 61.0 ± 7.7 kg; height 161.9 ± 7.2 cm) were recruited for this study. Knee angles, vertical ground reaction forces, and the time to stabilize the knee were assessed after single-legged drop landings from a 30-cm platform. Surface EMG data in rectus femoris (RF) and hamstrings (H) and knee and ankle accelerometry signals were also acquired. The participants performed 3 pretest landings, followed by a 3-minute recovery and then completed 1 minute of WBV (30 Hz to 4 mm). Before vibration, the female subjects had a significantly higher peak vertical force value, knee flexion angles, and greater H preactivity (EMG(RMS) 50 milliseconds before activation) than did the male subjects. In addition, although not significant, the medial-lateral (ML) acceleration in both knee and ankle was also higher in women. After WBV, no significant differences were found for any of the other variables. However, there was a decrease in the RF to H activation ratio during the precontact phase and an increase in the ratio during the postcontact phase just in women, which leads to a decrement in ML acceleration. The gender differences reported in knee stability in response to WBV underline the necessity to perform specific neuromuscular training programs based on WBV together with instruction of the proper technique, which can assist the clinician in the knee injury prevention.     

Alternative to traditional stretching methods for flexibility enhancement in well-trained combat athletes: local vibration versus whole-body vibration

This study aimed to compare the effect of local vibration (LV) and whole body vibration (WBV) on lower body flexibility and to assess whether vibration treatments were more effective than traditionally used static and dynamic stretching methods. Twenty-four well-trained male combat athletes (age: 22.7 ± 3.3 years) performed four exercise protocols – LV (30 Hz, 4 mm), WBV (30 Hz, 4 mm), static stretching (SS), and dynamic stretching (DS) – in four sessions of equal duration 48 hours apart in a randomized, balanced order. During a 15-minute recovery after each protocol, subjects performed the stand and reach test (S&R) at the 15th second and the 2^nd, 4^th, 6^th, 8^th, 10^th and 15^th minute. There was a similar change pattern in S&R scores across the 15-minute recovery after each protocol (p = 0.572), remaining significantly elevated throughout the recovery. A significant main protocol effect was found for absolute change in S&R scores relative to baseline (p = 0.015). These changes were statistically greater in LV than WBV and DS. Changes in SS were not significantly different from LV, but were consistently lower than LV with almost moderate effect sizes. After LV, a greater percentage of subjects increased flexibility above the minimum detectable change compared to other protocols. Subjects with high flexibility (n = 12) benefited more from LV compared with other methods (effect size ≥ 0.862). In conclusion, LV was an effective alternative exercise modality to acutely increase lower extremity flexibility for well-trained athletes compared with WBV and traditional stretching exercises.     

Vibration transmission to lower extremity soft tissues during whole-body vibration

In order to evaluate potential risks of whole-body vibration (WBV) training, it is important to understand the transfer of vibrations from the WBV platform to the muscles. Therefore, the purpose of this study was to quantify the transmissibility of vibrations from the WBV platform to the triceps surae and quadriceps soft tissue compartments.     

The reliability of determining the onset of medial gastrocnemius muscle activity during a stretch-shorten-cycle action

The instant at which a muscle increases its level of activity from baseline represents the onset of muscle activity. Accurate identification of muscle onset allows determination of temporal and amplitude characteristics of the surface electromyography (sEMG) signal. This investigation determined the intra- and inter-tester reliability for determining the onset of medial gastrocnemius (MG) activity using visual and automated methods. One hundred hop cycles, performed at 2.2 Hz, were selected from sEMG recordings (bandpass filtered 50–500 Hz and full wave rectified) of ten participants who performed three trials of single-leg hopping. The onset of MG muscle activity was identified by 3 separate investigators on two separate occasions and an automated method (10% of the peak activation amplitude). The duration of the anticipatory period, from muscle onset to initial ground contact, was then determined. Intra-tester (ICC from 0.72 to 0.95) and inter-tester reliability (ICC from 0.70 to 0.88) were high as was comparison to the automated method (ICC = 0.90). These findings indicate that visual onset detection was highly reproducible between testing sessions, independent investigators and comparable to an automated method. These methods may be used reliably to determine the onset of MG muscle activity during a stretch-shorten-cycle muscle action.     

Effect of different rest intervals after whole-body vibration on vertical jump performance

Whole-body vibration (WBV) may potentiate vertical jump (VJ) performance via augmented muscular strength and motor function. The purpose of this study was to evaluate the effect of different rest intervals after WBV on VJ performance. Thirty recreationally trained subjects (15 men and 15 women) volunteered to participate in 4 testing visits separated by 24 hours. Visit 1 acted as a familiarization visit where subjects were introduced to the VJ and WBV protocols. Visits 2-4 contained 2 randomized conditions per visit with a 10-minute rest period between conditions. The WBV was administered on a pivotal platform with a frequency of 30 Hz and an amplitude of 6.5 mm in 4 bouts of 30 seconds for a total of 2 minutes with 30 seconds of rest between bouts. During WBV, subjects performed a quarter squat every 5 seconds, simulating a countermovement jump (CMJ). Whole-body vibration was followed by 3 CMJs with 5 different rest intervals: immediate, 30 seconds, 1 minute, 2 minutes, or 4 minutes. For a control condition, subjects performed squats with no WBV. There were no significant (p > 0.05) differences in peak velocity or relative ground reaction force after WBV rest intervals. However, results of VJ height revealed that maximum values, regardless of rest interval (56.93 ± 13.98 cm), were significantly (p < 0.05) greater than the control condition (54.44 ± 13.74 cm). Therefore, subjects' VJ height potentiated at different times after WBV suggesting strong individual differences in optimal rest interval. Coaches may use WBV to enhance acute VJ performance but should first identify each individual's optimal rest time to maximize the potentiating effects.     

Myoelectric reactions to ultra-low frequency and low-frequency whole body vibration

5 healthy males were exposed to vertical sinusoidal whole body vibration (WBV) at 5 frequencies (F1 = 0.315 Hz, F2 = 0.63 Hz, F3 = 1.25 Hz, F4 = 2.5 Hz, F5 = 5.0 Hz) and 2 intensities (I1 = 1.2 ms-2 rms, F1-F5; I2 = 2.0 ms-2 rms, F2-F5). Erector spinae EMGs were derived at the levels of the first thoracic (T1) and third lumbar (L3) spinous processes, rectified and synchronously averaged, as were the accelerations of the seat and the head. WBV induced vibration-synchronous EMG activity (T1 and L3) which exceeded the activity without WBV during enhanced gravitation and decreased during lowered gravitation from F1 to F3. At F4 and F5, these phase relations changed drastically, thus suggesting a different trigger mechanism. The extreme average EMG-amplitudes remained nearly constant at F1 to F3 and increased at higher frequencies. Maximum EMG activity was higher at I2 than at I1. WBV from F1 to F3 is supposed to cause tonic muscular activity triggered by the otoliths; at higher frequencies, stretch reflexes probably gain additional importance. The results hint at an increasing sensory conflict with decreasing frequency of WBV and are interpreted within the theoretical framework of different modes of motor control. Relations between transmissibility and muscle activity suggest the usefulness of including time-variant spring-characteristics into biomechanical models.