Off-season strength and conditioning programs for collegiate female athletes are instrumental in the preparation for the sport season and are a primary responsibility of the strength and conditioning coach. Common program goals are to decrease injury risk and to improve performance (20). Performance improvements are frequently sought through training to increase strength and power because these make important contributions to athletic development. Strength and conditioning coaches design and manipulate training variables (e.g., sets, repetitions, exercise intensity, exercise selection) depending upon the time of the season in which the training phase takes place and the specific needs of the sport. The sport of softball involves the expression of lower body strength and power in all planes of motion; consequently, power and strength development are of the utmost importance (13). Given that the recruitment of appropriate motor units is the basis of muscular strength and power, when the goal of training is to improve strength and power, training methods should be selected that increase both the frequency of the stimulation of motor units and the number of motor units being activated (4,24,25).
In competitive sport, the ability to generate power quickly is critical to success (13). Efficient use of time is of primary concern to those working with athletes, and conventional training methods often require long periods of training to achieve the desired effects. Strength and conditioning coaches hope to maximize results in minimum time; therefore, they continually seek training methods that increase motor unit recruitment with less training time (24). One therapeutic modality that has become popular recently as a potential training method is the use of vibration. Vibration has been used on the human body in a variety of methods for many years. Some of these methods include massage (27), as a treatment for certain chronic diseases (26), to improve flexibility (9,31), and to attain gains in neuromuscular performance (25,32). Whole-body vibration (WBV) application has gained popularity as of late because of (a) the influx of vibration platforms produced by the sport industry and (b) an increased effort by strength and conditioning coaches to produce greater results in less training time.
The exact mechanism used by WBV to elicit muscular strength and power adaptations is unclear. One frequently cited explanation is that muscular strength and power are enhanced through increased neuromuscular facilitation via increased muscle spindle activation (1,19,24-26). The mechanical action of vibration causes quick, short changes in the length of the muscle tendon complex (28). This vibratory-induced nonvoluntary muscular contraction is referred to as the tonic vibration reflex (16). Although these vibratory-induced mechanisms are not entirely understood, an improvement in neuromuscular performance has been shown (6).
Whole-body vibration has been administered as both an acute bout (1,3,11,23) and as an intervention training method (2,18,19,22,25). The results of acute exposure to WBV are mixed (19,23-25). The findings from studies in which an acute bout of WBV was administered have shown increases (11,23,30) and no change (7,13) in power measures. In part, this may be explained by the selection of different WBV protocols (e.g., total WBV time, frequency, amplitude, exercise selection), and variation in subjects' fitness levels. Specifically, WBV has been shown to improve vertical countermovement jump (VCMJ) (1,3,11,24) and movement velocity (19). The results from studies in which female athletes were exposed to acute WBV have been inconclusive (7-9,13). Elite skeleton athletes (7) and National Collegiate Athletic Association (NCAA) Division I softball players (13) experienced no increase in sprint speed or bat speed, respectively. However, the results from acute WBV research with elite field hockey players showed improvements in flexibility and lower body power (9), and with professional volleyball athletes, an increase in electromyographic activity was observed (8).
Whole-body vibration intervention training has been shown to be successful and of low risk of injury as a method to improve athletic performance in female athletes (2,10,17). Several studies report training effects from regular WBV intervention in competitive dancers (2,33), basketball players (10,17), gymnasts (17), and track athletes (17). These WBV training effects include increases in leg extensor muscular strength and power (2,10,17), VCMJ (2,17,33), squat jump (10), and flexibility (17). It should be noted that conventional strength training (CST) was not included in the aforementioned studies as part of the athletes' training programs. Intervention studies using WBV training have elicited greater improvements in the squat jump (22) and VCMJ (15) when compared with CST methods in college-age adults. However, the above result has not been supported in research studies with athletes (14,18). Elite women sprinters (14) and basketball players (18) experienced no additional improvement in measures of strength and power from regular WBV training compared with CST programs.
In summary, the results from the majority of acute and intervention studies, in which only WBV was administered, have shown increases in power measures for female athletes in the postvibratory period. Because no single WBV protocol has been clearly defined as the best for improving lower body strength and power in female athletes, the protocol selected in this study was similar to the most commonly used WBV settings in previous studies. The purpose of this study was to investigate the effect of WBV training in conjunction with CST on the strength and power measures in NCAA Division III softball players. Specifically, would it be more beneficial for strength and power development to include regular WBV training as a consistent part of off-season resistance training programming? It was hypothesized that the inclusion of WBV with CST would increase muscular force and power, which would be reflected in significantly improved performance test scores.
Experimental Approach to the Problem
This study was designed to determine the impact of inclusion of WBV training on lower body strength and power during an off-season strength and conditioning program for collegiate softball. After random assignment to group 1 or group 2, the athletes were placed under the direct supervision of a Certified Strength and Conditioning Specialist (CSCS) while they participated in two 3-week training phases that included 1 WBV and CST combined phase and 1 CST-only phase. Group 1 (WBV 1) underwent the 3-week WBV protocol first followed by the 3-week CST, whereas group 2 (WBV 2) completed the 3-week CST first followed by the 3-week WBV protocol. All the athletes performed both training protocols. The total program lasted approximately 9 weeks as scheduled: pretesting (week 1), phase I: WBV or CST (weeks 2-4), posttest 1 (week 5), phase II: WBV or CST (weeks 6-8), and posttest 2 (week 9). The 3 testing sessions (pre, post 1, post 2) were conducted at approximately the same time of the day, and tests were administered on the same day. All the training sessions were scheduled at the same time of the day. The athletes were experienced lifters who were familiar with the resistance training exercises included in the program and the performance tests that were administered during the testing sessions. Lower body strength was measured by 3 repetition maximum (3RM) squat testing, and 1RM back squat values were estimated (4). Lower body power was measured by standing long jump (SLJ) distance and VCMJ height. Adequate recovery time between tests and trials was provided. Consistent testing order was SLJ, VCMJ, and 3RM squat. Data were included in an analysis if the subject completed all pretests and posttests for that particular variable.
Nine NCAA Division III (age 20.44 ± 0.88 years, height 163.97 ± 5.70 cm, and mass 67.13 ± 9.82 kg) female softball players who were familiar with the current strength and conditioning program and had a minimum of 1 year of formal collegiate strength and conditioning training experience volunteered to participate in this study. None of the women had participated in WBV training before this study. All were medically cleared for Intercollegiate Athletic participation, had the risks and benefits explained to them beforehand, signed an institutionally approved consent form to participate, and completed a medical history form. Those who had severe musculoskeletal injuries of the lower body or spinal injuries within 6 months before the study were excluded as subjects. Twelve athletes were initially recruited for the study. Three subjects were unable to complete the study for reasons unrelated to the protocol and were excluded from statistical analysis. The Springfield College Institutional Review Board for Human Subjects approved this study.
Estimated Maximum Strength
The athletes were tested using the 3RM test for the back squat. Pretesting occurred 1 week before the start of the phase 1 (WBV or CST) training program. The athletes completed a supervised (CSCS) warm-up before testing, to ensure that the exercises were performed correctly. Standard weight lifting power racks (Power Lift, Jefferson, IA) were used for the 3RM tests. Safety spot bars were individually set in the power racks for each athlete to ensure that each subject squatted to the desired depth of 90° knee flexion. The athletes took a timed rest of 3 minutes between each maximal effort set. Weight was increased based upon the performance of the previous attempt, and the athlete continued to perform sets of 3 repetitions until failure or until it was determined that she could no longer squat safely with proper form. After 2 failures, testing was stopped, and the best lift was recorded. The 1RM was estimated (4) from the best 3RM load. If <3 repetitions were completed with proper form, that number was used to estimate the 1RM.
Lower Body Power
The SLJ and VCMJ tests were used to measure lower body power. Before the SLJ test, the subjects completed a specific dynamic flexibility warm-up. The warm-up served both the SLJ test and VCMJ test. For the SLJ, an SLJ mat (SBP Products, Canada) was used. The athletes were encouraged to use a countermovement of the body and arms. The athletes completed 2 jumps with the best trial recorded. A 3-minute rest was given between the SLJ test and the VCMJ test.
To measure the VCMJ height, the athletes performed the exercise using a pressure-sensitive jump mat (Just Jump or Run System, Probotics, Inc., Huntsville, AL, USA). The best trial of 2 jumps was recorded. A 3-minute rest was given between the end of the VCMJ test and the start of the warm-up protocol for the 3RM back squat.
The WBV was administered via the pro5™ vibration platform (Power Plate®, Northbrook, IL, USA). The athletes performed static holds at a visually monitored knee angle of approximately 130° because this is a common position from which to execute movement in a variety of sports (athletic stance). Exposure time was limited to 30-second sets at a frequency of 35 Hz and low amplitude (2-4 mm of peak-to-peak displacement). The WBV was administered 2 d·wk−1 for 3 weeks during lifting sessions that were separated by 48 hours. After each set of back squat within the training session, the athletes completed a 30-second set of WBV. Although no optimal WBV protocol has been identified, the one used in this study was developed based upon several studies that showed positive results with athletes as subjects (2,8,9,17).
Total WBV volume throughout the 3-week training phase was as follows: First week, first day: 5 × 30 seconds (total = 150 seconds), second day: 3 × 30 seconds (total = 90 seconds); second week, first day: 6 × 30 seconds (total = 180 seconds), second day: 3 × 30 seconds (total = 90 seconds); third week, first day: 7 × 30 seconds (total = 210 seconds), second day: 4 × 30 seconds (total = 120 seconds).
Off-Season Strength and Conditioning Training Program
The athletes followed a 9-week off-season strength and conditioning program that was designed and implemented by the CSCS assigned to their team. All the training was done together as a team in the varsity weight room during the softball off-season (September-November). Specifically, the athletes performed 2 whole-body lifting sessions per week separated by 48 hours. Training sessions occurred at the same time and on the same days each week. To remain in the off-season program, the athletes were required to make up any missed workouts under the direction of their CSCS within 1 week of being absent. Weight selection for weekly workouts was based upon percentages from pretesting. Squatting occurred during both weekly lifting sessions and during each 3-week phase the training intensities for squat ranged from 55 to 90% 1RM on day 1 and 45-80% 1RM on day 2 (Table 1).
Descriptive statistics were computed for each of the variables. To evaluate changes from pretest to posttest in each dependent variable, a total of three 2 × 2 mixed factorial analysis of variance were calculated to compare the dependent variables across groups and testing periods. The 2 independent variables included the timing of the WBV within the program (WBV 1 and WBV 2) and testing period (posttest 1 and posttest 2). The alpha level was set at p ≤ 0.05. All the statistical procedures were conducted using the Statistical Package for the Social Sciences (SPSS 17.0 for Windows, SPSS, Inc., Chicago, IL, USA). Intraclass correlation coefficient was 0.84 between SLJ 1 and SLJ 2 and 0.93 between VCMJ 1 and VCMJ 2. The strength of effect sizes was 0.34 for SLJ, 0.67 for VCMJ, and 0.51 for 1RM squat.
The primary purpose of this research was to determine if differences existed between performance testing results after the inclusion of WBV training in the CST of a current strength and conditioning off-season program for a group of NCAA Division III softball players. It was hypothesized that the WBV training phase would produce greater increases in lower body strength and power than the CST-only phase would.
The descriptive statistics for performance testing-dependent variables for all the athletes at pretest, posttest 1, and posttest 2 for WBV 1 and WBV 2 are reported in Table 2.
No significant interaction (p > 0.05) was found for training phase; therefore, data were collapsed into 1 group. Regardless of whether WBV training occurred in phase 1 or phase 2, all the athletes experienced significant (p < 0.05) mean gains in SLJ (Figure 1) from pretest to posttest 1 (162.28 ± 16.74 vs. 169.34 ± 15.44 cm) and pretest to posttest 2 (162.28 ± 16.74 vs. 170.18 ± 15.90 cm). The VCMJ (Figure 2) values increased significantly (p < 0.05) from pretest to posttest 1 (41.05 ± 4.29 vs. 43.69 ± 3.51 cm). Additionally, the estimated 1RM squat (Figure 3) increased significantly (p < 0.05) from pretest to posttest 1 (75.50 ± 17.56 vs. 79.29 ± 16.09 kg) and from posttest 1 to posttest 2 (79.29 ± 16.09 vs. 83.08 ± 17.94 kg).
No significant interaction (p > 0.05) was found in the average percent change between testing periods and the WBV 1 group and the WBV 2 group for SLJ, VCMJ, or 1RM squat values; therefore, the groups were collapsed. All the athletes experienced significantly (p < 0.05) greater percent changes from pretest to posttest 1 (4.50 ± 2.94%) than from posttest 1 to posttest 2 (0.54 ± 3.50%) in SLJ. A similar significant (p < 0.05) result was found with the VCMJ from pretest to posttest 1 (6.83 ± 6.31%) and posttest 1 to posttest 2 (−1.33 ± 5.30%). The estimated 1RM back squat had no significant (p > 0.05) mean difference in the average percent change from pretest to posttest 1 (5.75 ± 6.52%) or posttest 1 to posttest 2 (4.63 ± 4.97%).
This is the first training study to examine whether or not the inclusion of WBV in a CST off-season program designed for NCAA Division III softball athletes would result in greater increases in strength and power. The hypothesis that the WBV training phase would produce greater increases in lower body strength and power than the CST-only phase would produce was not supported.
The inclusion of WBV into a current strength and conditioning program of trained female athletes has not been widely researched nor have consistent program design parameters been well established. Finding and developing a protocol that elicits the most beneficial training response to the athlete is vital. The optimal frequency of vibration can lead to the synchronization of motor unit firing, whereas a higher than optimal frequency of vibration can decrease motor unit synchronization (19). The WBV settings in this study (35 Hz, 2-4 mm) were comparable with those commonly reported in WBV training studies with female athletes (10,14,17,18) as were the WBV sets (30 seconds) and rest periods between sets (30-60 seconds). However, it is possible that the WBV protocol used in this study might have been insufficient in some fashion. The sample size (n = 9) in this study was small. Although this is not uncommon for studies with trained athletes (7), the small number may have affected the results. Certainly, having a larger sample size would have increased the statistical power, because the strength of effect size in this study ranged from 0.34 to 0.67.
Relatively few studies have been located that investigated the effect of regular WBV training on strength and power in trained female athletes (2,10,17,33), and unlike this study, CST was not included as a part of the training program. It was hypothesized that having the subjects undergo WBV as part of the training session in between sets of squats would be more beneficial in recruiting and synchronizing motor units and thereby increasing muscular force for subsequent sets, which is consistent with the findings of previous research (21), which reported significant differences as a result of WBV training in recreationally trained men. Although the proof of the effectiveness of the placement of the WBV between sets was not substantiated in this study, theoretically, WBV would appear to be effective based on the concept of increased motor unit recruitment. Our results indicate that no added benefit is obtained by including WBV in a CST program, which is similar to the findings of previous research with female sprinters (14) and basketball athletes (18). It should be noted that these 2 training studies did not fully integrate WBV training into the CST program of the athletes, but instead used it as either an activity before CST (14) or as an alternative training method to CST (18).
Previous researchers have reported an immediate increase in the VCMJ height post-WBV (9) and improvements in strength and VCMJ for female athletes participating in an 8-week WBV training (17). Although all the subjects in this study showed improvements in the SLJ, VCMJ, and estimated 1RM back squat, no differences were found between groups across testing periods. This finding is comparable with previous results from WBV and CST training with basketball players (18) and sprinters (14). It is unclear why the VCMJ and SLJ experienced the greatest gains during phase I of the training, yet the back squat values continued to improve at the same rate throughout the 9-week training program. No normative data were located for NCAA Division III softball players; however, the measured improvements in lower body strength (squat +10.4%) and power (SLJ +5%, VCMJ +5.5%) were consistent with published results from a 12-week training program for NCAA III female athletes that reported a 10% increase in 1RM squat, a 5% increase in VCMJ, and a 2% increase in SLJ after training (20).
The overall volume of loading because of the WBV may have affected the softball athletes in this study. The number of sets of WBV was increased in direct proportion to the squat sets, which increased the overall volume of the training and may have contributed to a fatigued state. A total WBV time per workout of 360-720 seconds has been recommended for chronic power improvement (24). The total WBV time per workout for the softball players was well under that recommendation because it ranged from a low of 90 seconds (3 sets × 30 seconds: first week, second day) to a high of 210 seconds (7 sets × 30 seconds: third week, day 1). This total amount of WBV also appears to be in range with what has been used previously with female athletes (2,10,14,17,18,33), but the squatting volume is higher. Repetition-volume (sets × repetitions) of squats decreased by 8% from week 1 to week 3. However, squatting load-volume (sets × repetitions × load) increased by 21% from week 1 to week 3. Although this progression resulted in significant gains from pretest to posttest 1, it is possible that repeating the same program for a second 3-week phase with no adjustment may have been too much. Not only was the WBV included with the CST but also the softball players were performing additional lifting exercises representative of a typical off-season workout. It has been suggested that the addition of WBV may lead to fatigue and reduce power output (22). In a counterbalanced crossover design, untrained men performing half-squats for 5 sets of 10 repetitions exercised at a higher oxygen consumption and perceived the exercise to be more difficult than the non-WBV condition (12). When investigating the long-term effects of vibration training, Rønnestad (28) recommended that subjects refrain from performing additional leg exercises. Although restricting exercise might not be practical for the competitive athlete participating in an off-season strength and conditioning program, reducing CST training volume on WBV days may be an approach worthy of consideration.
The effects of acute WBV are transient and diminish by 10 minutes after the exercise (1,5,23). To date, the length of time WBV training effects remain after cessation of WBV has not been addressed. The 1-week allotted between the phase I and phase II training cycles was prescribed arbitrarily to allow time between protocols and to permit the completion of the off-season training program within the allotted time frame of NCAA guidelines. There is a need for research to examine the length of time WBV training effects remain and what an appropriate “wash-out” period between protocols might be. The impact that these “wash-out” periods may have on overall performance is unknown.
The exact mechanism used by WBV to elicit muscular strength and power adaptations is unclear, and the scientific evidence on the benefits of WBV training is mixed (29).
Protocols vary widely, the number of long-term studies remains limited, and the majority of research has been conducted with untrained or recreationally trained subjects. In research with athletes, the timing of the WBV implementation has varied considerably in relation to the athlete's training program (e.g., preseason, off-season, season). Future study that addresses the aforementioned areas is necessary.
The results of this study suggest the following in trained female softball athletes: (a) Gains in maximal lower body power and strength may be attained over a 9-week structured strength and conditioning program. (b) The inclusion of WBV training into lower body strength training sessions was no more beneficial than using solely CST methods for strength and power development. The athletes in this study did not express concerns or experience injury while training with WBV; therefore, WBV appears to be safe for use during off-season strength and conditioning programming. Future research is warranted to examine further the effect of WBV intervention in conjunction with CST programming.
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Keywords:Copyright © 2011 by the National Strength & Conditioning Association.
motor unit recruitment; power; softball; resistance training; women