Pervious training studies varied from 4 to 24 weeks in length using a progressive overload or a periodized plan with varying loadings (2,10,21-23,26,28,30,32,35). Aagaard (1) reported significant increases in force/time integrals from the onset of contraction to 200 milliseconds following 14 weeks of variable load resistance exercise. The greatest percent increase was seen in the early (0-80 milliseconds) time phase. Significant increases were also reported in contractile impulse and signal amplitude and rate of EMG rise. The present study was of a shorter duration (6 weeks in length with 12 total workouts) and was periodized to emphasize maximal strength development (force production) during the first 3 weeks and then maximal power during the final 3 weeks. During the first 3 weeks, when heavier loads were used relative to 1RM measures (70-88% of 1RM), subjects were instructed to push as forcefully as possible against the bar in an attempt to maximize acceleration and dynamic RFDs against the heavy load (19,24,36-38,42).
During the present study, measures of RFD taken during the early stages following the onset of contraction revealed interesting between-group differences. When performing the statistical analyses on measures of ISORFD 0-30 and 0-50 milliseconds, repeated measures ANOVAs were used as no significant group differences were found at baseline (p > 0.05). The remaining analysis performed on ISORFD 0-80, 0-100, 0-150, and 0-250 milliseconds; PISORFD; TPISORFD; and RFDinitial required repeated measures ANCOVA as significant differences were seen between groups at week 1 (p < 0.05). With this in mind, data for ISORFD from 50 milliseconds and upward were covariate weighted by the respective week 1 ISORFD values in an attempt to “normalize” baseline values between groups. Such normalization shifted the magnitude of the changes from week 1 to week 7 favoring the control group (going from reductions to improvements), while attenuating changes for the SQT group (further reducing changes).
When comparing between-group measures for ISORFD 0-50 milliseconds, no significant differences were seen; however, a strong trend was seen favoring the SQTV group, which produced a small 2.4% improvement (p = 0.070). Both the CG and SQT groups exhibit similar percent decrements (−24.4 and −24.9%, respectively). The trend suggests that WBLFV facilitated early phase force/time characteristics, which could be viewed as an increase in starting strength. The trend continued between 0 and 80 milliseconds favoring the SQTV group, which was the only group that saw an actual improvement at week 7. Such a result would seem to favor the addition of WBLFV to the resistance training if a further preferential neuromuscular adaptation regarding early-phase RFD was desired. Looking at ISORFD 0-100 milliseconds, both the CG and SQTV groups saw improvement (+16.2 and +9.6%, respectively), whereas the SQT group saw a decrement between weeks 1 and 7 (−24.6%); however, there were no significant differences between groups, suggesting that there was a high degree of inter- and intrasubject variability. The use of ANCOVA instead of ANOVA due to week 1 differences in ISORFD 0-100 milliseconds favored the CG. Measures of ISORFD 0-150 milliseconds revealed improvements of 20.6 and 16.7%, respectively, for the CG and SQTV groups and a 15.1% reduction for the SQT condition.
Practically, if maximal strength is the main desired outcome, asynchronous firing of motor units appears to be more economical than stimulus-driven motor unit synchronization. The opposite would appear to be true if high RFDs over short time periods are required such as during punching or sprinting (1,2,9,15,18,20-24,28-30,33,36,38,44).
Measures of RFD from the onset of contraction to initial peak in force (ISORFDinitial) revealed significant between-group differences favoring the addition of WBLFV. Following ANCOVA adjustment (covariate week 1 RFD at initial peak values), SQTV values were significantly greater than SQT at week 7 (p = 0.041, mean difference 1,994.2 N·s−1). The time at which initial peak was achieved varied nonsignificantly at week 1 from 204.7 ± 61.6 milliseconds to 352.4 ± 41.8 milliseconds, which equated to an average value of 299.8 milliseconds across groups. This force/time variable was recorded and analyzed because it may represent an isometric measure of “explosive strength,” that is, the ability to recruit and then maximize firing frequency of high-threshold motor units during the first initial explosive drive.
Analysis of PISORFD measures between groups produced no significant differences, although a strong trend was seen in favor of adding WBLFV (p = 0.067), with the SQTV group being the only condition to see a practical improvement (+13.0%) at week 7. The mean difference at week 7 between SQTV and SQT (p = 0.072) was 1,642.4 N·s−1, which equated to a 29.9% total difference (SQTV 13% increase and SQT 16.9% reduction). Also, of interest was the −10.5% reduction in the time of onset of PISORFD for the SQTV group, suggesting a trend favoring WBLFV application. A practical increase in the PISORFD coupled with an earlier onset of such a contractile phenomena would appear to be a preferential adaptation in “explosive strength” expression. At week 1, the time of onset of PISORFD for the SQTV group started at 144.8 milliseconds while at week 7 was 128.4 milliseconds, which equated to a 16.5-millisecond (−11.4%) reduction. It is possible that adding WBLFV to SQTV increased alpha motor neuron excitability and synchronization of high-threshold motor units prior to and then in-between sets of resistance exercise, leading to an increased neuromuscular training stimulus above resistance training alone. Practically, the ability to produce a combination of increased PISORFD at an earlier onset would allow an athlete to produce a similar impulse but at an earlier time point and over a shorter duration.
The analysis of force measures from weeks 1 to 7 expressed as a relative percentage of MVC (MVC = 100%) revealed a significant trial effect for Finitial (%), with week 1 measures greater than week 7 measures. Practical trends were seen for the SQT condition with regard to increased capacity to express force at integrals from the onset of contraction up to 250 milliseconds. The practical trend to “shift” the training-induced adaptation to earlier time integrals may be a result of the vibration stimulus facilitating reflex-induced motor unit discharge characteristics such as increased synchronization and doublet discharge prior to the resistance exercise (1,2,13,26).
In conclusion, it appears that the training adaptations seen for both experimental groups were of a similar magnitude when all components of force up to MVC were considered. The difference appears to be in how the neuromuscular adaptation was distributed with the addition of WBLFV seeming to favor the early force/time components over peak force expression (MVC). This would be a case of the macrostate (bigger picture, overall training adaptation) being directly related to the micromanipulation (addition of WBLFV with the intent to elicit an acute PAP state). Sale (33) suggested that a PAP stimulus can slightly reduce the resultant MVC (high frequency force expression) while preferentially improving the RFD and force generated at lower activation frequencies, resulting in a leftward shift in the force/time curve (7,15-20,23,24,29,30,33,36,38,41,44). High inter- and intrasubject variability may account in part for the moderate nonsignificant group differences for measures of PISORFD; however, significant improvements were seen in ISORFDinitial, indicating a preferential adaptation in “explosive strength” expression. Practical trends also favored the addition of WBLFV with regard to improved PISORFD and other earlier force/time characteristics.
Baseline ISORFD ability appeared to significantly affect the resultant training adaptation. Potentially, varying the vibration frequency, amplitude, exposure time, and time points of application prior to and then in-between sets of resistance exercise could lead to greater group delineation resulting in preferential adaptations. Also, different results may be seen between groups with longer (>8 weeks) training periods as well as greater subject numbers (>15) and equality of numbers between groups. The appropriate selection of amplitude, frequency, and duration, coupled with the athlete's background resistance training status and fatigue state, would all appear to be important factors to consider when designing a combined resistance and WBLFV protocol. More chronic combined resistance and WBLFV training studies are needed using male and female subjects of varying resistance training backgrounds in order to work out the appropriate “dose response” for such a combined training approach.
The type of WBLFV plate used would also appear to have an impact on its relative effectiveness as an ancillary aid to resistance exercise. Plates using pivot/wobble mechanisms or variable stochastic resonance could provide a different stimulus, so further studies are warranted comparing WBLFV mode of application (40,44).
Applying WBLFV between sets of exercise rather than during resistance exercise is a novel concept that can readily be used by strength and conditioning practitioners who have access to WBLFV platforms. The results from the present study suggest that there is some practical merit to applying WBLFV prior to and between sets of resistance exercise. The application of WBLFV between sets, rather than during sets of resistance exercise, may be more appropriate, potentially leading to a greater resultant acute neuromuscular stimulus and subsequent chronic adaptation at similar volume loads. For coaches wanting to use such a method with highly trained athletes, potentially using a variable “dose” with adjustments in frequency, amplitude, and period of application throughout a periodized resistance model may be based on knowledge of the athlete's background and training status. Also, holding off on vibration application between sets until force or power output drops below a predetermined level may be a practical approach to help preserve dynamic RFD while allowing for PAP typically seen between the first and second sets of resistance exercise. The use of a linear position transducer or modified accelerometer attached to a barbell may be a practical way of assessing fluctuations in force, power, and velocity to give the coach or researcher an objective means of monitoring PAP and fatigue during successive repetitions and sets of exercise.
The use of Olympic lifts (Snatch, Clean, and Jerk) and their derivatives (full snatch and clean pulls, as well as pulls from the low waist and mid thigh positions, respectively) in conjunction with WBLFV would appear to be the next logical area to study because such semiballistic lifts produce some of the highest dynamic RFDs and peak and mean power outputs of any total body movements.
This project was funded by internal funds allocated by the College of Arts and Sciences at the University of Oklahoma. The results of the present study do not constitute endorsement of the Power Plate Next Generation whole body vibration platform by the authors or the National Strength and Condition Association.
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