The Acute Effects of Two Different Foot Positions ...

The Acute Effects of Two Different Foot Positions during Whole Body Vibration on Vertical Jump Performance in Females Kelley K. Menefee, MS, ATC* Department of Exercise and Sport Science, University of Utah, Salt Lake City, UT 84112, USA

Craig Switzler, MS, ATC Department of Exercise and Sport Science, University of Utah, Salt Lake City, UT 84112, USA

Leslie Podlog, PhD Department of Exercise and Sport Science, University of Utah, Salt Lake City, UT 84112, USA

Charlie A. Hicks-Little, PhD, ATC Department of Exercise and Sport Science, University of Utah, Salt Lake City, UT 84112 Edited By: Michael J. Pringle, Orthopaedic Specialists of North Carolina/Lincoln Memorial University, USA www.jampub.com, email: [email protected] *Correspondence Kelley Menefee, MS, ATC, Department of Exercise and Sport Science, University of Utah, 250 S 1850 E, HPER E Room 107D, Salt Lake City, UT 84112, USA, Tel: (801) 581- 7976 Fax: (801) 581-7976 Email:[email protected]

ABSTRACT Whole body vibration (WBV) is a therapeutic training intervention utilized for conditioning and rehabilitation purposes. However, there remains limited understanding on the effect foot position has on the outcomes of this therapy. The purpose of the study was to examine the acute effects of flat-foot vs. on-toes foot positioning during WBV therapy on vertical jump height and relative http://www.timesdispatch.com/making-his-pitch-to-save-arms/article_d47c4a26-a2d6-5d688e01-36156431fd5e.htmlresulted in a statistically significant decrease in vertical jump height (p = .013). Our findings indicate that foot positioning during WBV therapy does not have a significant acute effect on rGRF produced during take-off. However, our findings do indicate that assuming the flat-foot position during WBV therapy does acutely have a significant deleterious effect on vertical jump height. Keywords: Whole body vibration, relative ground reaction force, vertical jump performance. INTRODUCTION Whole body vibration (WBV) is a therapeutic training intervention, becoming popular for conditioning and rehabilitation purposes. Briefly described, WBV is achieved through a vibratory platform that delivers oscillatory movements to the body of varying frequencies around a horizontal axle. Subjects stand on the platform with their feet placed on either side of the axle and Journal of Athletic Medicine • Volume 2 • Number 2 • June 2014 • Menefee et al.

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maintain a steady position while the oscillations of the platform produce a vertical ground reaction force to each foot alternately. The rapidly repeating eccentric-concentric contractions presumably evoked, result in muscular work.1 Other mechanisms responsible for WBV-induced physiological changes include neuromuscular facilitation, tonic vibration reflex, increase gravitational force on muscle, and elevated muscle contractile activities.2-5 WBV is effective in improving muscular strength, vertical jump power, flexibility, and leg blood flow.1, 6-8 However, research is contradictory, showing either no change,9 a decrease,10 or an increase11-12 in these parameters following acute exposure to WBV. The discrepancies in the current literature can be attributed to improper control, differences in vibration duration, frequency, and amplitudes, lack of or use of a warm up, and differing exercise positions.1 The arm countermovement vertical jump (ACMVJ) is an effective measure of lower body functional power,13 and has been utilized as an outcome measure in WBV studies. Torvinen et al,9 found no statistically significant changes in vertical jump height following a four minute WBV intervention after a warm up with treatment parameters that included an initial frequency of 25Hz that increased 5 Hz every minute, ending at 40Hz for the last minute of WBV. Amplitude was kept constant at 2mm for the duration of treatment, and subjects were asked to stand on the platform with a natural standing posture. In contrary Bosco et al,10 reported a statistically significant decrease in vertical jump height after implementing a seven minute WBV intervention after a warm up where position on the WBV plate was standardized to 90° knee flexion and WBV frequency set at 26Hz and amplitude set to 10mm. A follow up study by Bosco et al,11 yielded a statistically significant increase in vertical jump height after implementation of a ten minute WBV intervention with frequency held at 26Hz and amplitude set at 4mm. Warm-up was allowed in this study, and subjects were asked to stand with only their toes touching the platform and knee flexion angle standardized to 100°. These studies highlight the current variability in WBV parameters used in research to date and provide evidence that an assessment has yet to be made regarding ACMVJ performance after acute exposure to WBV therapy. Further, functional muscular strength assessment during vertical jump performance is often examined in the research through ground reaction force production. The vertical jump is a useful measure of determining lower extremity functional strength15 and peak force generated during vertical jumps has been shown to correlate highly with vertical jump height attained.16 However, ground reaction force assessment has not been examined after WBV therapy. Interestingly, foot position has also been generally overlooked in the literature even though it is deemed an important parameter influencing WBV training interventions. A variety of different exercise positions have been researched during WBV1 but investigation has not specifically focused on flat-foot vs. on-toes foot position on the WBV platform. With this paucity on effect of foot position evident in the current literature during WBV therapy, the primary aim of this study was to examine the acute effects of flat-foot vs. on-toes foot positioning during WBV therapy on vertical jump height and relative ground reaction force (rGRF) production in females. We hypothesized that the on-toes foot position would elicit a significant increase in both vertical jump height and rGRF in comparison to the flat-foot position, resulting in a significant difference between the two interventions. Clinically, this study will add to our knowledge regarding WBV training parameters. If the on-toes foot position is identified to be clinically significant, then less attention can be paid to finding the optimal exercise position on the platform and will lead to more streamlined future research. Further, if the on-toes position reveals positive outcomes, it will

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provide evidence to support utilizing this foot position when designing effective WBV training interventions. METHODS PARTICIPANTS Twenty-four females (19.87 ± 1.26 years; 171.75 ± 8.56 cm; 66.11 ± 7.00 kg) currently participating in collegiate athletics volunteered for the study. All participants were in an off-season training period. Participants were screened for exclusion criteria using a medical history questionnaire addressing recent musculoskeletal issues as well as systemic conditions listed as contraindications by the whole body vibration manufacturer. These contraindications include pregnancy, pacemakers, acute hernia, uncontrolled diabetes, hip/knee implants, non-healed fracture, discopathy/ spondylitis, detached retina, unstable heart rhythms, epilepsy, thrombosis/clots, and advanced arthritis. All experimental procedures were explained to the participants verbally, and all participants gave written informed consent. The study was approved by the University Institutional Review Board. DESIGN The independent variable of foot position (flat-foot vs. on-toes) during WBV therapy was investigated to examine effect on the following dependent variables: ACMVJ height (cm) and rGRF during take-off (N/Kg). Sample size for this randomized crossover study with repeatedmeasures design was determined through a power analysis for ANOVA with α of 0.05, power of 0.8 and an effect size 0.25. PROCEDURES Participants were familiarized to the equipment, procedure, and testing requirements prior to their first testing session. During the course of the study, participants were not permitted to undertake any vigorous power or strength training, and to prevent interference from variations in daily biorhythms, participants performed the tests at the same time each day. At least 24 hours between testing sessions was required for recovery. Participants wore the same shoes for each testing session, but shoes were not standardized between all subjects. Warm up was prohibited prior to testing sessions to reduce the possibility of influencing the outcome of the study. Immediately after signing the informed consent, three baseline ACMVJ were performed with a ten second rest period provided to the participants between jumps. Each vertical jump height was manually recorded to the nearest half inch using a Vertec® Jump Trainer (Power Systems, Knoxville, TN, USA). This method of calculating vertical jump height has been shown to be a valid and reliable measure.14 An AMTI (AMTI BP400600 Model, Watertown, MA, USA) force plate was used to calculate pGRF during take-off of each vertical jump. GRF data was collected at a sampling rate of 1000 Hz. pGRF has been shown to be a reliable measure of functional strength in the lower extremity.15 The average of the three pGRF were then normalized to body weight to yield average rGRF. After baseline ACMVJ height and pGRF measurements were completed, the participants then performed the first randomly selected WBV intervention. A schematic representation of the experimental design is provided in Figure 1.


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Day 1

Day 2

Pre-Test Measurements (ACMVJ Height and pGRF)

Pre-Test Mesaurements (ACMVJ Height and pGRF)

Randomized WBV Intervention (Flat-Foot or On-Toes Position)

Randomized WBV Intervention (Flat-Foot or On-Toes Position)

Post-Test Measurements (ACMVJ Height and pGRF)

Post-Test Measurements (ACMVJ Height and pGRF)

Figure 1: A schematic illustrating the experimental procedure design. The intervention order was allocated in a randomized, balanced design. Participants performed WBV therapy on a Dynatronics VF5 Model vibration platform (Dynatronics Corporation, Salt Lake City, UT, USA). The participants stood on the platform in two different positions, separate foot positions for each intervention (Figure 2). For the ‘flat-foot’ intervention, participants stood on the platform with their feet shoulder width apart in natural standing posture with their feet flat on the platform (heels touching platform). For the ‘on-toes’ intervention, participants stood on the platform with their feet shoulder width apart, in an isometric squat with their knees at approximately 120°, and on the balls of their feet (heels not touching platform). Each position was held for 30 seconds, repeated 10 times, with 30 seconds of rest between repetitions, for a total of five minutes of vibration. Frequency was set at 26Hz, and amplitude at 6mm due to positive outcomes seen in other acute WBV studies using these parameters.1, 17-18


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Figure 2: Foot position for WBV intervention therapy; Flat-Foot position (left), On-Toes position (right). Immediately after completing the intervention, participants completed three further ACMVJ, measuring vertical jump height and pGRF during take-off. A 24 hour time period was required between intervention sessions. Participants returned the following day to complete the second intervention during which the exact same testing procedures and measures were conducted as the first testing session. All participants completed both intervention sessions. STASTICAL ANALYSIS Kinetic data was first processed using a 2nd order low pass Butterworth digital filter with the cutoff set at 10Hz. Data was then processed using customized software to obtain pGRF values. pGRF data was then normalized to participant body weight to yield average rGRF. The average of the three ACMVJ and the average of the three rGRF for all participants’ pre and post each intervention was then used for statistical analysis. A repeated measures analysis of variance (ANOVA) was utilized to determine whether there were statistically significant differences between foot positions on ACMVJ height and rGRF. Significance was considered to be at or greater than the 95% level of confidence (p ≤ 0.05). All statistical analyses were performed using a specialized statistical software package (SPSS for Windows version 20.0, SPSS, Chicago, IL, USA). RESULTS Means and standard deviations for each dependent variable (ACMVJ and rGRF), pre and post WBV intervention can be viewed in Table 1. The results revealed no statistical significance between foot position and rGRF (p = .231), but there was a statistically significant difference Journal of Athletic Medicine • Volume 2 • Number 2 • June 2014 • Menefee et al.

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between foot position and ACMVJ height (p = .036). Specifically, the flat-foot condition resulted in a statistically significant decrease in ACMVJ height (p = .013).

Table 1. Pre and Post WBV Intervention ACMVJ and rGRF data presented as Mean ± SD. Pre

Post

Flat-Foot ACMVJ (cm)

48.46 ± 2.17

47.58 ± 2.17*

On-Toes ACMVJ (cm)

47.97 ± 2.02

48.13 ± 2.01

Flat-Foot rGRF (N/kg)

34.61 ± 2.40

34.53 ± 2.04

On-Toes rGRF (N/kg)

34.75 ± 2.04

35.64 ± 2.42

*statistically significant (p = .013) DISCUSSION The primary purpose of this study was to examine the acute effects of flat-foot vs. on-toes foot positioning during WBV therapy on vertical jump height and relative ground reaction force production in females. We hypothesized that the on-toes foot position would elicit a significant increase in both ACMVJ height and rGRF in comparison to the flat-foot position. Our results however did not indicate any significant difference between foot position and rGRF (Figure 3), but did reveal a significant decrease in ACMVJ performance immediately following five minutes of WBV treatment when utilizing the flat-foot position but not after on-toes position (Figure 4). 14.40 14.20

rGRF (N/kg)

14.00 Pre

13.80

Post

13.60 13.40 13.20 Flat-Foot

On-Toes Intervention

Figure 3: rGRF pre and post WBV intervention for the flat-foot vs. on-toes foot positions.


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48.80

*

48.60

ACMVJ Height (cm)

48.40 48.20 48.00

Pre

Post

47.80 47.60 47.40 47.20 47.00 Flat-Foot

On-Toes Intervention

Figure 4: ACMVJ height pre and post WBV intervention for the flat-foot vs. on-toes foot position. *Statistically significant (p < .05) interaction effect (pre-post).

These results although not what the authors had hypothesized were in agreement with previous research.9,10 Specifically with observations made by Bosco et al10 who also reported a decrease in ACMVJ following whole body vibration treatment. It is therefore surmised that our results provide evidence to suggest that a flat foot position is in fact detrimental to vertical jump performance as both a decrease in rGRF and jump height can be seen in the flat-foot versus the on-toes position. The observed increase in rGRF during the on-toes position although not statistically significant may suggest an important finding regarding the foot position parameter. With every subject performing both foot position interventions, and all other parameters held standard, the marked increase in rGRF during take-off suggests that the foot position does play a role in facilitating desired performance outcomes. However, this study did not aim to quantify the neurological response during the vertical jump, therefore the examination of muscle activation through electromyographic analysis was not examined in this study and consequently it is therefore not possible to deduce direct neurological effects in our study findings. Future research should include electromyographic analysis as part of their investigation to further the understanding of the neural response produced by WBV therapy. The intervention measures in this study were performed without the implementation of a warm-up, whereas previous research11 12 included a cycling warm-up exercise and showed a significant increase in vertical jump height. This increase in vertical jump height could be attributed to the ergogenic effect of the cycling exercise rather than the WBV therapy itself. Therefore the present study aimed to isolate the effects of the WBV therapy by excluding a warmup. Additionally, without previous research specifically defining the most effective combination Journal of Athletic Medicine • Volume 2 • Number 2 • June 2014 • Menefee et al.

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of WBV parameters,1 it is difficult to surmise whether the foot position investigated in our study was the solitary cause of the deleterious effects in vertical jump performance, or if frequency, amplitude, and time of treatment could have also played a confounding role. Without research addressing this, it can only be speculated that foot position was the main cause of the deleterious effect, and further investigation would be required to support this theory. CONCLUSION In summary, our findings indicate that foot positioning during WBV therapy does not have a significant acute effect on rGRF produced during take-off. However, our findings do indicate that assuming the flat-foot position during WBV therapy does acutely have a significant deleterious effect on vertical jump height. This deleterious effect observed in vertical jump height should be investigated further with a specific focus into the effectiveness of all WBV parameters, including foot position, in order to accurately recommend proper WBV treatment protocols for maximum results. REFERENCES 1. Cochrane, D., & Stannard, S. (2005). Acute whole body vibration training increases vertical jump and flexibility performance in elite female field hockey players. Br J Sports Med, 39, 860-865. 2. Rittweger, J. (2010). Vibration as an exercise modality: how it may work, and what its potential might be. Eur J Appl Physiol, 108, 877-904. 3. Wilcock, I., Whatman, C., Harris, N., & et al, (2009). Vibration training: could it enhance strength, power, or speed of athletes?. J Strength Cond Res, 23, 593-603. 4. Bosco, C., Colli, R., Introini, E., & et al, (1999). Adaptive responses of human skeletal muscle to vibration exposure. Clin Physiol, 19, 1837. 5. Cardinale, M., & Bosco, C. (2003). The use of vibration as an exercise intervention. Exerc Sport Sci Rev, 31, 3-7. 6. Jacobs, P., & Burns, P. (2009). Acute enhancement of lower-extremity dynamic strength and flexibility with whole-body vibration. J Strength and Cond Res, 23, 51-57. 7. Lythgo, N., Eser, P., de Groot, P., & Galea, M. (2009). Whole-body vibration dosage alters leg blood flow. Clin Physiol Funct Imaging, 29, 53-59. 8. Fagnani, F., Giombini, A., Di Cesare, A., Pigozzi, F., & Di Salvo, V. (2006). The effects of a whole-body vibration program on muscle performance and flexibility in female athletes. Am J Phys Med Rehabil, 85, 956-962. 9. Torvinen, S., Sievanen, H., Jayinen, T., & et al, (2002). Effect of 4-min vertical whole body vibration on muscle performance and body balance: a randomized cross-over study. Int J Sports Med, 23, 374-379. 10. Bosco, C., Colli, O., Cardinale, M., & et al, (1999). The effects of whole body vibration on mechanical behavior of skeletal muscle and hormonal profile.GP Lyritis: Musculoskeletal interactions; Basic and clinical aspects, 2, 67-76. 11. Bosco, C., Jacovelli, M., Tsarpela, O., & et al, (2000). Hormonal responses to whole-body vibration in men. Eur J Appl Physiol, 81, 449-454. 12. Torvinen, S., Kannus, P., Sievanen, H., & et al, (2002). Effect of a vibration exposure on muscular performance and body balance: randomized cross-over study. Clin Physiol Funct Imaging, 22, 145-152.


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13. Everett, A., & et al, (1990). The effects of arms and countermovement on vertical jumping. Medicine and Science in Sports and Exercise, 22(6), 825-833. 14. Leard, J., & et al, (2007). Validity of two alternative systems for measuring vertical jump height. J of Strength and Conditioning Res, 21(4), 1296-1299. 15. Cordova, M., & Armstrong, C. (1996). Reliability of ground reaction forces during vertical jump: implications for functional strength assessment. J of Athl Train, 31(4), 342-345. 16. Dowling JJ, Vamos L. Identification of kinetic and temporal factors related to vertical jump performance. J Appl Biomech. 1993;9:95-1 10. 17. Gerodimos, V., & et al, (2010). The acute effects of different whole-body vibration amplitudes and frequencies on flexibility and vertical jumping performance. Journal of Science and Medicine in Sport, 13, 438-443. 18. Parco, S., & et al, (2010). Immediate effects of 2 different whole-body vibration frequencies on muscle peak torque and stiffness. Arch Phys Med Rehabil, 91, 1608-1615.


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