Whole-body vibration (WBV) and its effects on the human body have emerged as a dynamic area of study with constantly changing theories concerning application and subsequent physical performance. It has become apparent that short-term exposure may be beneficial in relation to resistance training exercises. One recent area of interest has been the use of WBV in combination with resistance exercise to increase muscle performance. Studies have shown that after acute exposure, several advantageous effects have been observed including increases in strength, power, jump height, flexibility, bone mineral density, and various strength-related hormones (5,7,12,17,27,35). However, a large variation persists among these studies regarding the type of resistance exercise performed during WBV, specifically static and dynamic exercises, as well as training status of the individuals performing WBV exercises. Although studies have shown both of these resistance exercises to produce beneficial physical performance effects, it is unknown whether dynamic resistance exercises performed during WBV produces superior physical performance benefits compared with static resistance exercises or visa-versa. Marin and Rhea (26) in a meta-analysis examined that magnitude of changes in muscular strength was observed by the performance of both acute exercise and chronic vibration training. They commented that across the research reviewed, there can be similar or greater gains in strength observed using vibration training as compared with traditional resistance exercise alone in young and older adults both trained and untrained (26).
Vibration has been shown to significantly activate the musculature of the lower limbs when the quadriceps muscle groups are held at a 100° knee flexion (8). Cardinale and Lim (9) have since identified an optimal vibration frequency of 30 Hz that has been shown to elicit a significant amount of muscular activation in the lower limbs compared with other levels of amplitude and frequency at 100° knee flexion. This combination of frequency and knee flexion, coupled with vibration amplitudes of 2–10 mm during WBV exercise, has since become a common protocol for most studies examining the effects of vibration on changes in lower limb muscular force production (11,14–16,30,31). While daily exposure to such vibration is considered harmful if exceeding 10 minutes (3,4,10,18), this threshold is rarely approached during WBV exercise.
Many studies analyzing the effects of WBV exercise have used the aforementioned parameters of WBV during the performance of static squats, resulting in significant increases in strength (7,16,26,27,30). Similar studies have also been conducted analyzing WBV during dynamic squats and dynamic arm curls (5,15,19,31,34). In addition to the previously mentioned neurological advantages, dynamic resistance exercise may offer several other advantages over a static resistance exercise, including a functional movement pattern and a degree of muscle fiber recruitment. Furthermore, it is observed that after dynamic resistance exercises with WBV, there have been greater increases in muscle force production than after static resistance exercises with WBV (7,16,23,31). However, a comparison of changes in muscle force production after dynamic and static exercise modalities with WBV has yet to be addressed in a single study.
It is conceivable that the exposure of the lower limb musculature to WBV during different modalities of resistance exercise may produce varying effects on quadriceps muscular strength. Although studies have indicated that dynamic resistance exercises may produce greater beneficial performance effects when performed during WBV compared with static resistance exercises, a gap in the literature remains because this has yet to be addressed in a single study. Thus, the purpose of this study was to determine if an acute bout of dynamic squats performed during WBV results in significant increases in quadriceps force production compared with static squats performed under identical conditions. Additionally, we used an orthopedic knee brace to ensure the beginning and ending points in the range of motion (ROM) about the knee. It was hypothesized that there will be an increased force production after performing dynamic squats under WBV as compared with static squats under same conditions. Additionally, regardless of the type of squatting exercise performed, the exposure of WBV will increase force production as compared with normal conditions without WBV.
Experimental Approach to the Problem
Untrained men (N = 4) and women (N = 17), aged 18–25 years completed 4 different experimental protocols. Two of the 4 protocols consisted of performing 5 sets of 10 repetitions of dynamic squats with and without exposure to WBV with 60 seconds of rest between sets (2.5 minutes total). Two of the 4 protocols consisted of performing 5 sets of static squats held for 30 seconds with and without exposure to WBV with 60 seconds of rest between sets. Each protocol was separated by 2 weeks, which has been shown to be an adequate time period of rest between sessions of WBV exercise and resistance exercise (33). The study was conducted as a randomized design to eliminate any order effect of protocol and subjects served as their own controls. A repeated measures design was used to determine if changes in pre- and post-test of quadriceps strength existed. Additionally, a repeated measures with sex as covariate was run to determine any sex differences in strength.
Healthy men (N = 4) and women (N = 17) aged 18–25 years from the Houston community were recruited to participate in this study. The subjects were considered healthy and not overweight or obese as observed in the following pretesting variables. Mean age (to the nearest year) was 21.0 ± 2.2 years; body height to the nearest 0.5 cm with a stadiometer (Detecto, Webb City, MO, USA) was 163.0 ± 0.1 cm; body mass to the nearest 0.5 kg on a digital scale (SECA Medical Scales and Measuring Devices, Hanover, MD, USA) was 62.8 ± 12.1 kg; body mass index was calculated as 23.6 ± 3.7 kg·m−2; and percent body fat through 3-site skinfold testing was 20.1 ± 7.5%. Subjects had not performed any resistance training within the past year and were considered untrained to control for differences between previous resistance exercise routines, modalities, and training backgrounds. Subjects presented with no orthopedic, musculoskeletal, or chronic conditions would have prevented participation in the study. The subjects completed all testing from February to April in Houston, Texas, during the morning hours between 0700 and 1200 hours. Each participant performed testing at the same time of day each visit. Each subject read and signed an informed consent document reviewed and approved by the Institutional Review Board before participating in the study.
Familiarization and Baseline and Experimental Protocol Strength Testing
Familiarization with the vibration platform, experimental protocols, and isokinetic strength testing on 2–3 different days 1–2 weeks before starting the experimental protocols. The familiarization session occurred between the hours of 0700 and 1200 with each subject returning for subsequent visits at the same time each session. After 10–15 minutes of rest after the familiarization, all subjects performed a standardized warm-up consisting of 5 minutes of cycling on a stationary cycle ergometer without resistance followed by their first baseline quadriceps strength test, which was assessed through knee extension torque (Newton meter) on the Biodex Isokinetic Testing System 3 (Biodex Medical Systems; Shirley, NY, USA). The strength tests consisted of 1 set of 4 isokinetic concentric muscle actions on the dominant leg, composed of 4 repetitions in the concentric/passive concentric mode at a dynamometer speed of 100°·s−1 was performed. The second strength assessment was performed 48 hours later to account for any learning effect. If the first and second strength test values were within 5% difference of each other, the mean value was calculated and used for further comparison with the experimental protocols. A third strength assessment was performed if the first 2 assessments had not been within a 5% difference and the mean of 2 values between third strength assessment and between first or second strength assessment were used for future comparisons. A free-weight squat exercise was not chosen to evaluate muscle strength in this study because of the reliance on the addition of the hip and ankle joints for muscle force production and the fact that subjects were not resistance-trained. Dynamometer knee attachment was adjusted to ensure that its positioning was proximal to the medial malleoli. Range of motion was adjusted from 100° flexion while knee was in flexion to 0° flexion when the knee was in near full extension to simulate the squat exercise ROM. Each extension action by the subject was maximal to assess quadriceps force, and the flexion action was performed passively by the dynamometer.
After the familiarization and initial baseline strength testing, prestrength tests (1 set of 4 repetitions in the concentric/passive concentric mode at 100°·s−1) were performed within 5 minutes before each of the 4 experimental protocol. Poststrength tests were performed within 1 minute after completion of each experimental protocol to evaluate acute mean force production changes that may have been attributable to the respective protocols.
Whole-Body Vibration Exposure
Subjects were exposed to vertical displacement WBV at 30 Hz and 4-mm peak-to-peak amplitude using a Power Plate platform (Power Plate North America LLC, Culver City, CA, USA). This platform has a frequency range of 25–50 Hz with 1 Hz of adjustments allowable. As the platform was set to 30 Hz, the subjects experienced 1 g of force calculated as the acceleration = frequency/amplitude and expressed meter per square second. Standardization of the acceleration of this platform was performed using triaxial accelerometers in a previous study in our laboratory by Abercromby et al. (3) in accordance with ISO standards for WBV evaluation (18). During WBV exposure, subjects wore the same athletic shoes to account for any differences in vibration dampening that personal footwear may have caused (7,16). To ensure the safety of subjects, fine-grade sandpaper was adhered to the platform to ensure adequate grip between the subject's athletic shoes and the platform so as to avoid slippage. Foot placement on the plate was standardized for repeatability during each of the 4 experimental protocols. Our pilot study showed that 5 minutes (10 sets at 30 seconds each) of WBV increased quadriceps force production after dynamic squatting exercise, yet decreased after static squatting exercise. A previous study had also noted that after a single set of WBV exercise (1 set at 30 seconds), only a slight increase (+0.7% from baseline) in jump height performance was documented (11). Thus, an exposure to 2.5 minutes of WBV over the span of 5 sets was chosen to provide adequate stimulation to increase acute quadriceps force production while minimizing muscular fatigue.
Experimental Dynamic and Static Squatting Exercise Whole-Body Vibration Protocols
The following 4 experimental protocols were performed randomly by each subject and were all performed while standing on the platform when exposed to both WBV and no-WBV conditions. Protocol 1 consisted of 5 sets of 10 repetitions of dynamic squatting exercise without WBV totaling 2.5 minutes. Protocol 2 consisted of 5 sets of 10 repetitions of static squatting exercise without WBV totaling 2.5 minutes. Protocol 3 consisted of 5 sets of 10 repetitions of dynamic squatting exercise with 30-Hz and 4-mm peak-to-peak amplitude WBV totaling 2.5 minutes. Protocol 4 consisted of 5 sets of 10 repetitions of static squatting exercise with 30-Hz and 4-mm peak-to-peak amplitude WBV totaling 2.5 minutes. A 60-second rest period was allowed between each set where the subjects remained standing on the platform. Each dynamic and static squatting exercise used body weight resistance with no added external resistance. Each repetition of squat was paced by a 40 b·min−1 metronome. At the beginning of each of the 4 experimental protocols, subjects performed a standardized warm-up as described above. A prestrength test of 1 set of 4 concentric/passive repetitions at 100°·s−1 was assessed followed by a 5-minute resting period during which time subjects were fitted with an orthotic knee brace to the nondominant leg with a ROM limit setting from 100° knee flexion to 0° knee flexion to measure and maintain proper biomechanical properties of the movement.
Subjects began the dynamic squatting exercise without WBV (protocol 1) by standing erect on the platform with knee angles of approximately 175–180° (extension) while holding onto the guidance bar. A metronome set a rhythm of 40 b·min−1. During the eccentric phase of the squat, the subjects lowered their bodies by flexing the knees, hips, and ankles until the knee brace had reached the set limit of knee flexion (∼100°), which lasted for 1 beat of the metronome (or 1.5 seconds). Subjects then performed the concentric phase by extending the knees, hips, and ankles to raise the body from the lowered position to the starting position, which lasted for 1 beat of the metronome (or 1.5 seconds). This pattern was repeated until all 10 squats had been completed, totaling 20 b·min−1 on the metronome. The performance of 1 set of 10 squats lasted for approximately 30 seconds (or 20 beats at 40 b·min−1 on the metronome). Subjects performed a total of 5 sets of dynamic squatting exercises. A 60-second rest period between each set was allowed. Verbal guidance was provided to ensure proper squatting form and balance and to ensure maximum safety. A poststrength test of 1 set of 4 concentric/passive repetitions at 100°·s−1 was assessed within 1 minute of completing the dynamic squatting protocols. The dynamic squatting exercise with WBV (protocol 3) was then performed identically as described above for protocol 1 except it was performed while exposed to WBV for 30 seconds at 30 Hz with 4-mm peak-to-peak amplitude after a 2-week rest period. Over all 5 sets of squatting exercise, the exposure to WBV totaled 2.5 minutes.
Subjects began the static squatting exercise without WBV (protocol 2) by standing on the platform with a knee flexion angle of 100° while wearing the orthotic knee brace while holding onto the guidance bar. This static squat position was held for 5 sets of 30 seconds each. A 60-second rest period between each static squatting set was allowed. Verbal guidance was provided to ensure proper squatting form and balance and to ensure maximum safety. A poststrength test of 1 set of 4 concentric/passive repetitions at 100°·s−1 was assessed within 1 minute of completing the static squatting protocol. The static squatting exercise with WBV protocol (protocol 4) was then performed identically as described above for protocol 2 except it was performed while exposed to WBV for 30 seconds at 30 Hz with 4-mm peak-to-peak amplitude after a 2-week rest period. Over all 5 sets, the exposure to WBV totaled 2.5 minutes.
Descriptive statistics (means ± SDs) were used to depict basic features of the data. Initial diagnostic methods (histograms and Kolmogorov-Smirnov tests) were applied to identify patterns in the data and assess underlying distributional assumptions. It was determined that normality and homogeneity of the variance existed, and thus the repeated measures analysis of variance and subsequent t-tests were performed to analyze the data. The sample size was determined by power analysis based on a pilot study of the dependent/independent variables to be tested. Power analysis showed that for a power of 0.80, α = 0.05, a correlation of 0.7 between pre- and post-measures, and an anticipated mean difference of 5.0 N·m with a SD of differences of 10.0 N·m (effect size f = 0.25), a sample size of 21 was indicated. Thus, 24 subjects were then recruited by adding an additional 20% for attrition. The final sample size for analysis was 21 subjects. A paired t-test evaluated pretesting to posttesting changes in strength. A 2 × 2 × 2 repeated measures analysis of variance with Tukey post hoc testing for pairwise differences was used to determine if differences in strength across the 4 experimental protocols existed and if they were associated with: (a) effect of resistance exercise on strength change, (b) effect of WBV exercise on strength change, and (c) effect of WBV upon resistance exercise on strength change. All subjects completed at minimum 2 baseline strength testing sessions; however, 10 subjects completed a third baseline testing session to ensure a ≤5% difference between baseline tests. A repeated measures analysis of variance indicated no statistical difference across 4 repetitions for baseline session 1 and baseline session 2, where baseline testing session 1 across 4 repetitions was p = 0.34 and baseline testing session 2 across 4 repetitions was p = 0.42. Thus, the 4 individual repetitions for each session were collapsed into 1 average value used in all subsequent statistical analysis. Additionally, an analysis of variance with gender as a covariate was run to assess the gender differences in strength across the 4 vibration protocols. Data were first expressed as a relative value as newton meter force per kilogram of body mass (N·m·kg−1) to compare strength between genders. Data analysis was performed using SPSS 13 for Windows (SPSS, Inc., Chicago, IL, USA). Significance for the study was set at p ≤ 0.05.
Comparison of 1 Set of 4 Repetitions During Pre- and Post-testing
There was no significant different among the 2 or 3 baseline strength testing performed during familiarization sessions (p = 0.91) so that data averaged into 1 value were used for comparison to any pre- and post-strength tests. Figure 1 showed that there was no statistical difference across the 1 set of 4 repetitions during the pretesting for each of the 4 experimental protocols (prestrength dynamic without WBV, p = 0.55; prestrength static without WBV, p = 0.35; prestrength dynamic with WBV, p = 0.31; prestrength static with WBV, p = 0.34) (Figure 1). Thus, the 4 individual repetitions were averaged into 1 value used in all subsequent statistical analysis and denoted as the pretesting value. After examining the prestrength values statistically, the prestrength values before each of the experimental protocols were thus averaged to obtain 1 overall average strength value for each prestrength test. There was no statistical difference in strength across the 1 set of 4 repetitions during the postsquatting for each of the experimental protocols (poststrength dynamic without WBV, p = 0.16; poststrength static without WBV, p = 0.31; poststrength dynamic with WBV, p = 0.25; poststrength static with WBV, p = 0.39). Thus, the 4 repetition values were averaged into 1 value for subsequent statistical analysis. The values of the 4 repetitions during both the pre- and post-strength testing indicate consistency of the subject to perform each of the 4 repetitions at maximum strength levels. Thus, the actual net gain or loss in strength experienced by each subject was a true result of the individual experimental protocol rather than a learning effect or fatiguing effect of the performance of either the experimental exercise protocols or poststrength tests.
Comparison of Baseline and Prestrength Values
There was no statistical difference between the average baseline level of strength and any of the 4 experimental prestrength values (dynamic without WBV, p = 0.79; static without WBV, p = 0.935; dynamic with WBV, p = 0.997; static with WBV, p = 0.913) (Figure 2). The prestrength value of each subsequent experimental protocol had matched to the baseline strength value before any of the experimental testing protocols were performed (Figure 2). This showed that there was no order effect of the experimental protocols and no interference of the previously performed protocols on subsequent prestrength values.
Comparison of Pre- and Post-strength Values With and Without WBV
Quadriceps muscle strength was significantly affected by the type of resistance exercise performed by each subject and use of WBV. Compared with the corresponding prestrength values, there was a significant decrease in strength after completing static squatting exercise with (p = 0.0002) and without (p = 0.0004). WBV and dynamic squatting exercise without WBV (p = 0.0003) and a significant increase in strength after dynamic squatting exercise with WBV (p = 0.006) (Figure 3). A significantly greater decrease (p = 0.002) in strength after the static squatting exercise as compared with the dynamic squatting exercise was observed (Table 1).
When expressed as a pre- to post-strength percent value, there was a significant increase (p = 0.003) in strength after the dynamic squatting exercise with WBV (+3.9%) as compared with the dynamic squatting exercise without WBV (−7.4%) (Figure 4). However, there was no significant (p = 0.53) difference between static squatting without WBV (−15.83%) and with WBV (−20.71%) although strength was decreased as a result of static squatting exercise (Figure 4). There was a significant (p < 0.00) increase in strength after the combination of dynamic squatting exercise with vibration (+3.9%) compared with static squatting exercise with vibration (−20.71%). There was also a significantly greater (p = 0.00) decrease in strength after the combination of static squatting exercise without vibration (−15.83%) as compared with dynamic squatting exercise without vibration (−7.39%) (Figure 4).
Sex Differences in Strength With and Without WBV
When expressed as relative strength (Newton meter force per kilogram body mass), there was a significant sex effect for only the static with WBV (p = 0.02) and static without WBV (p = 0.03). For the static without WBV, men had a more significant relative percent change from pre- (men = 2.03 ± 0.47 N·m·kg−1; women = 1.92 ± 0.36 N·m·kg−1) to post-strength (men = 1.63 ± 0.63 N·m·kg−1; women = 1.73 ± 0.36 N·m·kg−1) as −34.99% and for women as −11.32% (Figure 5). For the static with WBV protocol, men had a more significant percent change from pre- (men = 2.05 ± 0.67 N·m·kg−1; women = 1.61 ± 0.86 N·m·kg−1) to post-strength (men = 1.91 ± 0.37 N·m·kg−1; women = 1.70 ± 0.41 N·m·kg−1) as −46.19% and for women as −14.01% (Figure 5). When expressed as relative strength, there was no sex difference in the dynamic WBV protocols (dynamic without WBV, p = 0.33; dynamic with WBV, p = 0.34) (Figure 5).
Although studies have indicated that dynamic resistance exercises may produce greater beneficial performance effects when performed during WBV compared with static resistance exercises, to our knowledge, no studies have been performed addressing this issue in a single study. The key findings of the study are threefold. First, it was demonstrated that quadriceps muscle strength was significantly decreased immediately after the performance of 5 sets of static resistance squatting exercise and performance of 5 sets of 10 repetitions of dynamic squatting resistance exercise of equal time duration. This indicates that regardless of the type of squatting resistance exercise performed, either static or dynamic or equal duration, there was a significant decrease in muscle strength. Second, it was demonstrated that the addition of WBV to the dynamic squatting resistance exercise, not the static squatting exercise, resulted in an increase muscular strength. This was opposite to the effect of performance of the dynamic squatting exercise alone, where a decrease in muscle strength was observed. Third, it was demonstrated that addition of WBV to the static squatting resistance exercise resulted in a decrease in muscle strength, similar to the performance of the static squatting exercise alone. This is contradictory to the effect of vibration on muscle strength after the performance of the dynamic squatting exercise in which no decreases in strength were observed. The results suggested that there was a loss in muscular strength after the performance of both the dynamic and static squatting exercises. Furthermore, the results indicate that during a dynamic squatting exercise, the addition of WBV to the resistance exercise protocol appeared to enhance the gains in strength observed after squatting. This is in conflict to the results of strength change in the dynamic squatting exercise without WBV, which resulted in a significant loss in muscular strength. The fatiguing nature of the full squatting activity in these nonresistance-trained men and women, both dynamic and static in position, seemed to have contributed to the loss in muscular strength. These findings of an immediate loss in strength was potentially due to the fatiguing nature of the types of squatting exercises performed and probably was to be expected on some level. Data outcomes did not support one hypothesis, as it was observed that the addition of WBV to the static squatting resistance exercise in untrained individuals did not result in an improvement in acute muscle strength compared with performance of the static squatting exercise alone.
The parameters of the dynamic strength testing used in this study were similar to previous studies that have demonstrated significant increases in knee extensor strength after both dynamic and static resistance exercises with 35–40 Hz and WBV after resistance training (15,16). Many studies examining WBV have demonstrated significant changes in strength using a type of static strength tests (i.e., isometric muscle actions) (23,27,34,35). Verschueren et al. (36) used both types of testing (dynamic and static) to measure significant changes in strength after exercise with 35–40 Hz WBV after resistance training. However, a static strength test was not chosen for this study because of its inability to measure strength changes only over a full ROM. In other words, there is only a carryover of strength for every ±15° about the angle of contraction, and thus multiple isometric or static strength tests would need to be performed for a translation to strength over a full ROM to be realized. A dynamic strength test, however, is capable of measuring strength changes over a joint's full ROM. A dynamic strength test was then chosen due to the simulation of the functional movement pattern of the knee extension during the squatting exercises, which were subsequently performed during each resistance exercise protocol in the study.
After the performance of the acute dynamic (−7.39%) and static (−15.83%) resistance exercise protocols without WBV, there was a significant decrease in quadriceps strength observed. It has also been postulated in previous studies that any gains in strength due to neurological changes would occur after approximately 1 to 4 weeks of chronic resistance training (1,29), and furthermore, any gains in strength due to morphological changes would occur after 6 to 12 weeks of chronic resistance training (2,21,22). In this study, subjects observed a significantly greater decrease in quadriceps strength after the performance of 5 sets static squats compared with the 5 sets of dynamic squats (Figure 2). This result was also expected because it has been demonstrated that the performance of static squats is generally more exhaustive in nature than performance of dynamic squats. A potential mechanism of this result could be due to constant contractile state that the quadriceps musculature must sustain to maintain the designated position of 100° knee flexion, as opposed to the constantly changing knee angle experienced during dynamic squats, especially as that experienced in nonresistance-trained individuals.
The combination of WBV and dynamic resistance exercise resulted in a significant increase in quadriceps strength compared with the dynamic squats performed without WBV. This is in agreement with previously published studies (23,25–27,31). After a 9-week training period involving 28 moderately trained adults, Kvorning et al. (23) reported similar increases in maximal voluntary contraction in both the group performing resistance training without WBV and the group performing resistance training with WBV. Mahieu et al. (25) showed that the combination of WBV and dynamic resistance training led to increases in musculature about the knee and ankle joints in young elite athletes. Ronnestad (31) also reported significant increases in jump height and 1 repetition maximum strength after a 5-week training period combining WBV and weighted dynamic squats. Although these studies demonstrated an augmentation in physical performance after several weeks of training, our study has conclusively shown that such improvements may occur after just 1 acute session of dynamic squatting exercise with the addition of WBV.
Our findings of a significant decrease after WBV and static resistance exercise are similar to results reported in previous studies (12,13). In such studies, one of long-term resistance training and one of acute resistance exercise, subjects were exposed to similar parameters of vibration (30 Hz, 8 mm) and biomechanical properties of static resistance exercise (110° knee angle) as in this study. Results in one longitudinal study demonstrated no significant changes in knee extensor strength, whereas results of one acute study demonstrated a significant decrease (−7%) in force production. Contrary to this, other studies using nearly identical parameters of vibration and static squat positioning have reported increases in lower limb power production (7,16). Although studies examining the acute effects of WBV during static squatting exercises have reported mixed results (7,12), a key deficiency in these studies has been the lack of a static resistance exercise control group. By including this control group in this study, it has been effectively demonstrated that the addition of WBV to an acute static resistance exercise results in a significant decrease in quadriceps strength compared with static resistance exercise without WBV in healthy adults with no or limited resistance training experience. This potentially suggests that WBV may induce greater levels of muscular fatigue during acute bouts of static resistance exercises than performing the resistance exercise without the addition of WBV.
Although previous studies have analyzed the effect of WBV concomitantly with the performance of either static or dynamic resistance exercises alone (11,19) or in combination (33,36), to our knowledge, this is the first study to have directly compared the use of WBV during the 2 different types of resistance exercises individually. After a 2-week study of exposure to various resistance exercises with the addition of WBV, including dynamic squats and light intensity static squats, Torvinen et al. (33) reported significant gains in isometric leg extensor strength and functional jump height. A study by Verschueren et al. (36) using a variety of static and dynamic movements during WBV reported significant increases in isometric and dynamic leg extensor strength (torque) after 5 weeks of static and dynamic squats with WBV. As most of the exercises performed in these studies were dynamic in nature, it could be theorized that the positive strength gains accounted for in these studies may have been due to the overriding influence of the performance of dynamic squats with WBV compared with static squats with WBV. Marin and Rhea (26) in their meta-analysis review commented that the type of muscular action (i.e., isometric or dynamic) performed while exposed to WBV may play an important role in improvements in muscular strength. They conclude that performing dynamic muscular actions, such as the dynamic squatting exercise used in this study, while under exposure to WBV, may be needed to elicit the greatest benefits in muscular strength (26). They suggest that these dynamic exercises influence the specificity of the motor unit recruitment during those dynamic exercises, and thus influence strength outcomes (26).
Based on covariate analysis, there seemed to be a sex effect only in the static experimental protocols and not the dynamic protocols such that from the sample size, that men experienced a greater percent change in pre- to post-WBV relative lower body strength (Newton meter force per kilogram body mass). However, given the small and unequal sample size, this cannot be state-definitive. Fagnani et al. (16) showed a significant change in the isokinetic leg strength of competitive athletic women using similar WBV parameters (35 Hz at 4-mm displacement) while training at 90° knee flexion. However, the study by Fagnani et al. (16) used an 8-week WBV training protocol, whereas this study used an acute WBV training protocol under 4 different conditions in non–resistance-trained women. Ye and Yuen (37) showed significant improvements in trunk extensor strength after 4 minutes of 25-Hz WBV exercise at only 10° knee flexion and that men responded with greater strength outcomes to the vibration exercise. Bosco et al. (6) examined competitive women volleyball players performing dynamic leg press after WBV of 26 Hz at 10 mm displacement for a total of 10 nonconsecutive minutes (10 times for 60 seconds each) on 1 leg at 100° knee flexion. They showed significant improvements in lower body force production; however, this was in a trained population and this study examined untrained women. The data from this study are consistent with these limited previous studies (16,37) in that while maintaining a squatted position (static experimental protocol) simultaneously exposed to WBV showed that improvements in a dynamic lower body leg exercise can be observed in young women. Although, a sex difference existed in both the static with and without WBV and not the dynamic conditions, generalizations about the sex differences need to be interpreted with caution due to the small subject size of men versus women. Research is limited examining a combination of untrained and trained men and women (26) in the same study; thus making comparisons difficult at best to generalize until further research has been examined.
A potential mechanism underlying the immediate changes in strength after a single session of WBV with resistance exercise is not entirely understood. It has been speculated that because of the acute nature of the effects, the origin of such changes in muscle strength after WBV exercise must be related to a neural adaptation because any morphological changes could not occur in such a short time period (10,27,28). Previous studies have hypothesized that WBV provokes length changes in the skeletal muscle, thus stimulating the muscle spindles and subsequently causing a tonic vibration reflex (15,30,31). It could then be suggested that the sudden stretch of the skeletal muscle during a dynamic squatting exercise, combined with the additional stimulation of WBV, might provoke a supramaximal level of muscle strength, resulting in the immediate increase in strength observed in this study. This is further supported by the observed decrease in muscle strength after WBV and static squats. As the length of the quadriceps muscle is not altered, but maintained, during the static squatting position, any additional reflex response by the muscle spindles may not be expected. It could also be hypothesized that WBV may invoke too great of a stimulation to the affected musculature during static squats, thus providing a potential explanation as to the excessive fatigue and decreased muscle strength observed in static squats with WBV as opposed to static squats alone. Given the methodology of this study, potential mechanisms can only be theorized as additional invasive measures need to be examined in future studies.
Regarding study limitations, first, it is recognized that the changes in strength are a result of an acute session of resistance exercise and an acute exposure to WBV. It is not known if these results would be consistent with exposure to chronic resistance training protocols or chronic exposure to WBV. Another limitation to the study design may be that all of the subjects were not resistance-trained and thus the experimental protocols were novel to the subjects. Although previous studies have demonstrated significant changes in muscle function after exposure of WBV in trained individuals performing both dynamic and static squatting exercises with WBV, future research should examine the changes in muscle strength after the currently used experimental protocols in resistance-trained subjects. In this study, while quadriceps strength was examined, squatting is a resistance exercise that affects the musculature of the entire leg not just a single muscle or group of muscles (i.e., quadriceps). It was not determined if the addition of WBV to the dynamic squatting exercise may have altered the function of the antagonist muscle groups (i.e., hamstring muscle group) or various assisting muscle groups, which may have contributed to the recorded findings. Finally, in recruiting for this study, the resultant sample population consisted of a majority of women. As it has been hypothesized that women are less susceptible to gains in muscle mass and have lower blood androgen receptors (20,32), this might contributed to the practical application of such results. Additionally, to provide a greater generalization based on sex, there needs to be a greater sample size for males and females. However, because the study was performed on an acute basis, it is difficult to conclude if any hormonal differences might have contributed to the generalization of the results.
Overall, the results of this study provided evidence that WBV has varied effects in altering muscle strength in untrained individuals according to the type of resistance training with which it is combined. It has been shown that an acute bout of dynamic squats with WBV can potentially lead to an increase in quadriceps strength after exposure to a single session. However, it would appear that the addition of WBV to an acute bout of static squats does not potentiate an increase in strength and may, in fact, contribute to a further decrease in strength due to the high fatiguing nature of the combination of techniques. Based on these results, it can be postulated that untrained young adults performing an acute bout dynamic squats with WBV can be immediately beneficial in producing higher levels of quadriceps strength compared with static squats with WBV.
As a dynamic squatting exercise under exposure of WBV seems to immediately potentiate neuromuscular functioning, the combination of dynamic exercises and WBV could be used as a potential warm-up procedure before acute or possibly longer term resistance training sessions. Our current research protocols used in conjunction with WBV seemed to be initially fatiguing. However, it is not known how or if the effects of the WBV using our resistance exercise protocol will have positive benefits with subsequent resistance training sessions. An acute set of static squats with WBV could also be used before dynamic resistance training sessions, potentially to increase force production in lower body exercises. The inclusion of WBV to dynamic types of resistance exercise can potentially be an additional mechanism by which to increase strength during certain types of resistance training as it has been demonstrated in this study that WBV has varied effects in altering muscle strength in untrained individuals according to the type of resistance training with which it is combined. It should be taken into account, although, that the results of this study have reported an acute effect, and any kind of residual or lasting effect on strength would warrant multiple posttests.
The authors would like to thank all of the subjects in this research study and Dr. Daniel O'Connor for assistance with statistical analysis. The results of this study do not constitute endorsement of the product by the authors or the NSCA.
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