Introduction

Whole body vibration (WBV) has become a popular mode of training for athletes and fitness enthusiasts as either an alternative or a complementary procedure to their routine strength and power training. There are 2 main types of vibration platforms (VP) that produce sinusoidal displacements at set frequencies. The first type, in which the vibrations are synchronously distributed vertically throughout the whole platform, is called a vertical platform. The second type, in which vibrations are centered around a horizontal axis and pivot, with alternating side-to-side vibrations being stronger the further away from the fulcrum amplitudes (A) are measured, is called a rotational platform (14,28). Whole body vibration exercise, in general, can be applied with the subject in either a standing, sitting, or lying position on the VP. The individual may perform static or dynamic exercises loaded or unloaded while the platform delivers vibrations intended to evoke reflexive muscle contractions. It is not known exactly how these mechanisms work or are applied during vibration training, but numerous review articles have described the possible theories with limited evidence for their support (2–3,7–9,14,20,28). One popular theory hypothesizes that g-forces produced by the acceleration of the VP causes stimulation of the primary endings of the muscle spindle (Ia afferents) and excites the α-motor neurons, causing increased recruitment of the homonymous motor units; this allegedly results in a tonic contraction of the muscle referred to as the tonic vibration reflex (TVR) (3,10,12,20).

Most VPs range in frequency (f) from 10 to 60 Hz and displacement from <1 to 10 mm (4,7,8,28). With respect to what WBV frequency is most effective, Hazell et al. (6) investigated the effects of different frequencies on electromyographic (EMG) signals from the lower body during squatting. It was concluded that the WBV stimulus resulted in increased EMG activity as the WBV frequency increased from 20 to 45 Hz. Whether these changes in EMG activity translated into a functional benefit in strength were tested in another study, which showed that a frequency of 50 Hz acutely increased squat 1 repetition maximum (1RM) compared with no vibration or frequencies of 20 and 35 Hz in both trained and untrained participants (23). With respect to determining which amplitude is most effective, review articles have investigated studies which used a range of displacements between 0.6 - 12.5 mm (avg ∼5 mm), resulting in mixed findings without a trend toward higher amplitudes necessarily causing better effects (7,8,28). It is currently uncertain whether there is an optimal amplitude and frequency combination of WBV for strength development.

A systematic review by Rehn et al. (20) found moderate to strong evidence in support for use of WBV on lower extremity strength development and performance. However, most of the studies resulting in the greatest positive responses involved untrained subjects and the elderly, suggesting the changes to be primarily because of neural adaptations that occur early in training (15,22). The studies that did use trained individuals showed little to no difference between vibration and control groups. In another review article, it was concluded that vibration combined with strength training may have provided small additional benefits to power and 1RM, but it seemed that athletes would need to add an external load while undergoing WBV (28). It was also broadly concluded that more research into the long-term use of vibration on trained athletes during loaded exercise is required. To that end, a more recent review by Hortobagyi et al. (8) concluded that WBV training has small and inconsistent acute and chronic effects on athletic performance in competitive and/or elite athletes. In contrast to studying either untrained or high-level athletes, we decided to study recreationally strength-trained male volunteers.

Ronnestad (22) was one of the first who longitudinally studied the effect of WBV back squat training under load on strength and jump performance in recreationally weight-trained men using a Smith machine. An advantage of that study is that it was the first to actually perform the full dynamic exercise under load while in position on the VP rather than holding an unloaded body weight in isometric semi-squat position as was commonly done in other earlier studies. After 5 weeks of training, however, he found no significant difference between the groups in posttest 1RM. He claimed that there was a trend toward greater relative strength in the WBV group. There was also no significant performance difference between groups in relative jump height increase. For our study, we wished to further the investigation of Ronnestad using free-weight back squats instead of Smith machine on 1RM strength and jump performance. Long-term studies on the effects of vibration on jumping performance have shown both increased and not increased jump height according to a review of 9 studies by Hortobagyi et al. (8), who concluded that there was little evidence to suggest a consistent effect of WBV that reliably caused much improvement. In fact, with body-weight-alone exercises, vibration training seemed to provide little or no additional effects on jump performance (21). Body weight exercises alone on a VP produce EMG activity that is less than a maximum voluntary contraction, therefore, to achieve a larger stimulus under vibration, the addition of external loads must be emphasized (5,11,19,21,22,24,27). That being said, many available popular VPs are not constructed to have the user perform exercises with an external load (e.g., barbell with weights placed on the shoulders). Nevertheless, there are commercial VPs that have now been designed on which users can conveniently stand and perform a bar-loaded exercise without interference of an upright console. We used this type of VP for our study.

At this point, despite substantial research efforts, there is no consensus regarding the effectiveness of WBV as a training tool to improve strength and performance compared with an equivalent heavy weight–training program in the absence of WBV. The inconsistency of results may be partly due to the variability in subject parameters such as training status and sex, exercise selection, and vibration training protocols with respect to frequency, amplitude, duration, and external load. It also needs to be considered that WBV may simply not work effectively under all conditions, e.g., with trained exercisers lifting heavy loads (1,5,7,8,19,21,22,24,27). Based on the mixed results of previous research, it is yet unknown whether there are additive ergogenic effects of WBV training on strength or performance that can reliably be achieved when variables of interest are measured and controlled. To narrow the scope of variables examined in this study, we recruited only recreationally resistance-trained male subjects who had been squat training under load for at least 2 months to minimize neural adaptations upon start-up. We used a manufactured preset low-displacement (<1.0 mm) VP that to our knowledge is popular and currently in use by athletes in college and professional facilities, as well as by fitness enthusiasts in fitness centers (VibePlate, Lincoln, NE, www.vibeplate.com Website Accessed October 3, 2017).

The present study was designed with the aim of testing 2 groups of recreationally strength-trained men, after an aggressive squat strength training protocol to determine whether WBV would elicit greater maximal strength gains in squat 1RM and improved standing broad jump (SBJ) performance from training on a VP compared with conventional squats on the floor.

Methods

Experimental Approach to the Problem

Participants were randomly assigned to either 1 of 2 training groups, conventional squat training (SQT; n = 10) or squat training plus WBV (SQTV; n = 10; 9 completed the study). They were required to complete 12 squat workouts at least 48 hours apart (training occured 2 d/wk for 6 weeks) with 4 sets of progressing loads between 85 and 95% 1RM. Pre- and posttesting for subject's 1RM squat and SBJ distance tests were performed and regarded as dependent variables for maximal strength and performance comparison analyses.

Subjects

Twenty recreationally strength-trained men (all data are reported as mean ± SD: age 22.3 ± 1.66 years (range 20-27 years), height 178 ± 8.49 cm, body mass 80.6 ± 13.62 kg: Table 1) were recruited via posted flyers and verbal announcements during exercise science courses at Central Michigan University. These subjects reported routine exercise 3–6 d/wk in a variety of physical and sport activities. A post hoc power analysis was used to verify whether a sample size of 16 or more subjects (i.e., 8 per group) was sufficient to detect the effect of the intervention on the 1RM result based on the observed pre- and posttest mean values and SDs and suggested that the data could be interpreted with a moderate effect size (ES) level of 0.78 and power level of 0.9 when significance was set at an alpha level of 0.05. Our sample size is consistent with other similar studies that have investigated loaded WBV training on squat strength (5,11,19,22,24,27).

Table 1.: Baseline participant characteristics (mean values ± SDs).*

Inclusion criteria included the participant's ability to squat 140% of their body mass (with verified proper lift mechanics at a depth of the top of the thigh within 10° parallel with floor), no previous experience with WBV training, and that they had currently been resistance training for at least 2 months immediately before the study, to minimize initial neurological strength gains as a variable. They volunteered and were selected based on their interest, familiarity, and technique competency with the parallel barbell back squat. Exclusion criteria included any current musculoskeletal injury or injury within the past 6 months or other serious health problems as determined by answering a medical/health history questionnaire. This study received ethical approval from the Central Michigan University Institutional Review Board. Written informed consent and a health screen questionnaire were received from all participants.

Procedures

Testing and training was performed in the Human Strength and Conditioning Laboratory of the Health Professions Building at Central Michigan University. Subjects reported for a pretesting day for group assignment and familiarization to the testing and training protocols. The volunteers were assigned to 1 of the 2 groups of 10 using a random card draw method, with one person eventually dropping out from the WBV group because of time constraints. It was confirmed that all participants were already regular users of the exercise equipment and stations in the laboratory used for warm-up, back squat, SBJ, and other testing procedures. During the pretesting familiarization day, individuals were also instructed on proper lifting technique for this study, and those assigned to SQTV were allowed to stand on the VP to experience the sensations of acceleration forces of vibration frequency and amplitude. The VibePlate (VibePlate 3048) VP was used for this study because it is commercially available and is a popular VP used by athletes of many college and professional teams that we are familiar with. It also has the convenience of allowing external load exercise without design interference of movement as occurs with console stand VPs. Subsequent testing on a different day included subject's height, body mass, SBJ distance, and squat 1RM measurements, in that order. Before testing, subjects warmed up on a True upright cycle (750U; True Fitness Technology, St. Louis, MO, USA) for 5 minutes. The workload was adjusted and initially set to 60% of the individual's body weight (lb) in watts. The aim was to achieve a heart rate (HR) between 140 and 160 b·min–1 by the third minute. If after 2 minutes HR was below 130 b·min–1, the workload was increased to 70% of the body weight, and if after 3 minutes, it had not reached 130 b·min–1, it was increased to 80% of body weight. Thereafter, the determined appropriate workload was used immediately for warm-up purposes before all workouts.

The SBJ was used to measure leg power in the horizontal plane and was performed according to an established, reliable method (intraclass correlation coefficient [ICC] = 0.95; coefficient of variation [CV] = 2.4%) (13). The subject placed the toes of both feet on the back of the starting line, and with a simultaneous arm swing and crouch, then jumped forward as far forward as possible, ensuring a 2-footed landing. Subjects had to “stick” the landing for the trial to be counted. If not, the trial was disregarded and another completed. No restrictions were placed on body angles attained during the preparatory phase of the jump or the degree of arm swing used. Distance was measured using a standard tape measure, which was the perpendicular line from the front of the start line to the posterior surface of the back heel at the landing. Subjects had 3 warm-up attempts at the SBJ before their fourth and final jump was marked and measured.

In preparation for the 1RM squat, subjects used a wooden dowel to perform 10–15 form “air squats” in the squat rack. During this time, individuals were measured for their squat depth based on safe maintenance of a neutral lumbopelvic angle (confirmed that all participants were within 10° of thigh parallel with floor), and the stop bars were set at one hole below their determined ideal depth for a safety catch. Subjects were then instructed to perform a 1RM following a standard protocol outlined and recommended by the National Strength and Conditioning Association (17) and previously shown to be highly reliable (ICC = 0.99; CV% = 2.1) (1). Because it was not known if squat training loads would differ in the SQTV group, when WBV was superimposed, they completed one additional day of pre-1RM testing on the VP after at least 2 days rest. However, 1RM determined without vibration was compared statistically within and between both groups before and after. To eliminate the potential variability of intertester reliability, pre- and posttesting was administered on the same equipment, with identical subject positioning, and was consistently conducted by one of the authors. The subjects were given identical verbal encouragement throughout the testing sessions and subsequent training also by the same researcher. To standardize the damping of the vibration by footwear, individuals wore the same shoes during the test sessions and training.

Each training day began similar to testing with a warm-up on the cycle followed by 10–15 “air squats” using a wooden dowel while standing on the floor for the SQT group or VP for the SQTV group. The VP was set at 50 Hz for all training squats. Subjects in the SQTV group performed all squats while receiving WBV and were instructed to step off the VP between sets so that vibration occurred only while exercising under load. There is no amplitude adjustment on the VibePlate, and the default peak-to-peak displacement was determined to be <1 mm when loaded. Participants performed back squat exercises in a Cybex Power Rack (Cybex International, Medway, MA, USA) that was outfitted with safety bars and easy adjustments to accommodate subject height differences. Individuals in each treatment group trained under strict form and were supervised at every workout by the same trained investigator twice per week for a total of 12 sessions, with at least 48 hours between each session. Each individual was instructed to refrain from performing additional resistance training on the legs during the study period. They were, however, not restricted from engaging in any other physical activities required in their academic programs, intramural sports, or strength training of their upper bodies. Both groups followed a nonundulating, linear progressive resistance exercise regimen that was based on well-established guidelines for strength development outlined by the National Strength and Conditioning Association and is listed in Table 2 (17).

Specifically, in set 4 of each training session, participants were to perform 2 repetitions (reps) or more at 95% maximum but were encouraged to lift as many reps as possible. If only the 2 required reps were achieved, the weight load was not increased in the next training session. For every rep past 2 reps, the training load was increased to 2.25 kg (5 pounds) per each additional rep in the next training session (e.g., 2 additional reps in set 4 would result in a 4.5 kg (10 pound) increase in the next training session for each set). Rest periods were 2–3 minutes between each set (17). No SBJ training took place in either group throughout the study period. After completion of the 12 training sessions, subjects reported for a postmeasurement day of body mass, SBJ distance, and 1RM determination in that order following the same warm-up and measurement procedures as the baseline tests.

Statistical Analyses

Unpaired sample t-tests were run for each variable to determine whether there were any initial between-group differences for the premeasurements. On completion of the training period, paired sample t-tests were then run for each variable to determine within-group significant differences from pre- to postmeasurements of 1RM, SBJ, and body mass. A mixed analysis of variance (ANOVA) (SPSS for Windows, V. 15.0; IBM Corp., Armock, NY, USA) was used to determine whether main effects or an interaction were found and if so, to compare the significant differences between the 2 groups. A p value of 0.05 was used to establish the criteria for statistical significance. Effect sizes for the 1RM were calculated using the formula Cohen's d = (M2–M1)/SD pooled (26). Unpaired t-tests were implemented to determine whether there was a training load difference between groups during the last week of training.

Results

There were no significant differences between groups for any variable of interest (age, body mass, 1RM, or SBJ) at the start of the study. Height differences between groups were significant (Table 1). All 19 subjects who completed the study (19/20) performed 100% of the training sessions. Paired sample t-tests revealed within-group significant differences (p < 0.001) in 1RM for both groups (Table 3), with very large ES (SQT = 1.54, SQTV = 1.38). Standing broad jump performance increased by an average of 5–6 cm in both groups pre-to-post but this change was not significant in either group (SQT; p = 0.199, SQTV; p = 0.087). Body mass increased in the SQT group (1.34 kg; p = 0.001), but not in SQTV (−0.18 kg; p = 0.915) with no difference between groups (p = 0.094). There was no difference between groups in the final average training loads for the fourth set (SQT = 174.32 vs. SQTV = 176.26 kg; p = 0.428). A mixed ANOVA was used to compare 1RM change pre-to-post between the SQT and SQTV groups. Although there was a time main effect (p < 0.001), there was no significant interaction within groups (p = 0.875).

Table 3.: Maximal strength and standing broad jump summary (mean values ± SDs).*

Discussion

The results of the current study showed that 6 weeks of squat training with superimposed WBV did not offer any appreciable advantage in any outcome measure when compared with identical strength training alone. This contrasts with earlier work that found improvements from unloaded vibration training primarily in the untrained, the elderly, and where no proper control group was used (Rehn et al., 2006). The weight lifting regimen used in our study was a linear progressive resistance overload model and resulted in highly significant (p < 0.001) 1RM percent increases for both groups (SQT = 23% vs. SQTV = 24.6%), with very large ES. Despite some studies showing effectiveness of WBV, studies limited to those comparing loaded barbell squats with and without WBV have primarily not shown an additive effect of vibration on strength or performance (5,11,19,22,24,27). Ronnestad (22) conducted a 5-week study similar to the present study also with recreationally strength-trained subjects and found similar results in that loaded squats concurrent with WBV (f = 40 Hz; A = 3 mm) were not significantly better than conventional squats. However, it was stated that there was a trend of WBV squats working better than conventional squats. The conclusion was then made that WBV showed promise of being “superior” regarding development of maximal strength and explosive power with more subjects (7 in each group) over a longer period of training. There are a few key differences between our 2 studies. Ronnestad (22) had subjects performing back squats using a Smith machine. It has been shown that free weight squats averaged 43% more muscle activation in the involved musculature than squats performed at the same weight in a Smith machine (25). That said, the overall percent changes in 1RM were similar to our study, in that there was a 52.7 kg (31.5%) increase with WBV and 36.4 kg (24%) increase without.

A study by Weier and Kidgell (27) had untrained participants assigned to loaded squat training with or without WBV (f = 35 Hz; A = 2.5 mm) for 4 weeks and showed significant increases in squat 1RM over time but detected no differences between groups. Kvorning et al. (11) also showed no additive effect of WBV (f = 20–25; A = 4 mm) on strength or jump performance after 9 weeks of heavy squat training with relatively untrained weightlifters. Rosenberger et al. (24) trained recreationally active subjects with 6 weeks squatting with heavy loads, and although all dependent variables of interest increased from the protocol followed, they showed no difference in thigh muscle hypertrophy or strength and no difference in jump performance between those who trained with progressively intense WBV (f = 20–40; A = 3–4 mm) or without. Preatoni et al. (19) studied high-level female athletes undergoing 8 weeks of periodized WBV (f = 25–40; A = 4 mm) while squatting with a low-intensity load vs. squatting alone with a moderate load and showed no differences between groups in isometric or dynamic maximal force output or power generation as measured by a vertical jump performance test. Finally, Goodwill and Kidgell (5) compared 2 training groups for 3 weeks performing single-leg heavy squats with or without WBV (f = 35 Hz; A = 2.5 mm) and showed no differences in 1RM strength.

Although 5 of the above-cited studies, as well as ours, used heavy-load squats as the method of strength training with subjects of varying fitness status using somewhat different WBV equipment and protocols, all resulted in similar findings. Superimposed WBV did not augment the improvement in strength or performance beyond that induced by conventional squatting alone. We must consider the possibility that WBV training simply may not work under these heavy-load squat conditions.

It can be argued that the proportion of stimulus that comes from WBV as part of the total conditioning stimulus provided by high-resistance weight training is so small that it cannot substantially augment strength or athletic performance or act as an additional ergogenic aid (7). The nature of the stimulus provided by WBV may also not be specific enough to the motor skills involved in weightlifting or jumping tasks (7). It also must be noted that acceleration energy transmitted to the muscles through WBV rapidly dissipates as it progresses from the foot through the ankle and knee to the muscles of the hip (16,18). Although this was not directly quantified in the current study, Pel et al. (18) reported a dramatic reduction in transmission of WBV-induced vibration from the ankle to the knee and hip at a range of frequencies above 20 Hz. Along these lines, Rosenberger et al. (24) concluded that effects of 6 weeks of loaded WBV resistance exercise are distance dependent (the closer to the VP the stronger the expected effect), and their study demonstrated increased strength and cross-sectional area of the plantar flexor muscles, but not the quadriceps musculature.

In addition, it has been shown that the true frequency or amplitude of WBV imposed on the body can differ from the assumed values of the vibration device, particularly when considering additional loads imposed on the VP, which include the subject's body mass and barbell weights, which may dampen the effect (18,19). Related to this, Wilcock et al. (28) identified that researchers often do not state whether they are referring to peak displacement (amplitude) or peak-to-peak displacement in reporting their methods. Thus, the reader must be aware that all VPs are not equal and the amplitude data published in some of these articles may not always be accurately reflected and may not have been corrected based on subject body mass and when external loads are applied. That being the case, the manufacturer of the VibePlate states in their literature that the amplitude is 2.0 mm displacement (VibePlate 3048, Website Accessed October 3, 2017), but does not specify whether it is peak-to-peak amplitude that is being referred to. Because in this case the amplitude is preset by the manufacturer, it is also important to know what frequency they used or whether an external weight load (simulating a subject together with barbell) was placed on the VP when amplitude was measured. A multiple scanning laser Doppler vibrometer was used to take measurements on a similar VibePlate to that used in this study, and data collected revealed that with an external mass of 63.5 kg placed on the center of the VP at low frequency (10 Hz), the peak-to-peak amplitude was 1.8 mm (near what the manufacturer claims), but at a higher frequency (32 Hz), the peak-to-peak amplitude was reduced to a maximum of 0.6 mm but varied with a large amount of nonuniformity from one side of the plate to the other (between 0.3 and 0.6 mm), with the smallest vibrations being at the location of the loaded weight. In addition, an unloaded measurement taken at 53 Hz showed that the VP vibrated with a peak-to-peak amplitude <0.3 mm with low variability. On the upside, frequency (Hz) measurements across a range were consistent and accurate (B. Feland, 9 personal communications, April 15, 2016–October 19, 2017). Preatoni et al. (19) also found that the reported VP displacement and subsequent g-forces were less than what was reported by the manufacturer of the VP and varied according to external load. Therefore, it is reasonable to conclude that the VP used in this study may not have provided a displacement great enough to increase the g-forces to stimulate an increased squat demand of individuals in the SQTV group and subsequently stimulate a TVR to potentiate muscle contraction intensity beyond those achieved by subjects in the SQT group. That said, other researchers who chose to study directly measured actual VP displacements (via accelerometer) that were higher did not document better outcomes (5,19,27).

Furthermore, although muscle stiffness was not measured in the current study, the heavy training load used (i.e., 85–95% of maximum voluntary strength) would have caused intense increases in muscle contraction that is speculated to result in increased soft tissue stiffness, which may act to lessen muscle resonance and consequently dampen the effects of WBV (18,27). For individuals who cannot lift such heavy loads (such as the elderly, individuals subjected to prolonged bed rest or other microgravity environments and/or those with a musculoskeletal disorder), WBV may or may not enhance strength (20,24). Given the identical prescription of training load for both groups, it seems that the mechanism for increased strength in the current study was facilitated by the heavy training load and not the simultaneous exposure to WBV.

The present findings show that WBV using a low-amplitude VP seem to yield little, if any, ergogenic benefits compared with heavy weight training alone on increasing strength development or on improving jump performance. It should also be noted that WBV did not, at least, result in an ergolytic effect. It is clear that future studies are needed to understand what the best strength protocols to follow are to possibly benefit from WBV stimulation. With new technological developments always underway, standards are also needed to guarantee the quality of devices being sold to athletic teams and the general public. To that end, it is also suggested that more appropriate manufacturing quality control should be applied to make sure that the vibration dose (especially A and f) reported on the VP's display, and as device specification details on the manufacturer's instructional materials, accurately reflect actual measured values (19). Future studies may need to take into account the different masses of their subjects and external weight loads to discern individual effects of training with WBV. More work is needed before we can declare with finality what the true effect of WBV training is.

Practical Applications

A 6-week squat training program designed to increase strength can produce significant increases in 1RM for recreationally resistance-trained men. Individuals looking to improve squat performance are encouraged to use conventional loaded squats. Fitness trainers and strength coaches who are operating under the assumption that WBV provides a significant benefit may be considering the purchase and adoption of a VP as an alternative or a complementary procedure to routine strength and/or power training for their clients or athletes. Such additional procedures seem to provide limited ergogenic benefits, and we cannot recommend their implementation in programs where there is a concern over resource management. Our study and the others reviewed showed that there were little compelling scientific bases for this conditioning practice. Squat training with or without WBV also does not seem to improve broad jump distance. Squat training alone should not be relied on to improve SBJ, despite dramatic increases in strength, as it is apparently not specific enough to produce results.