Regardless of whether a vibration platform was used before performing resistance exercises, no difference was evident between groups. However, an increase in rotator cuff strength was seen in both groups over the duration of this study, supporting the findings of a previous 6-week study.24
have demonstrated the effects of vibration training. Bosco et al18
found that electromyography of the elbow flexors revealed increased neural activity with the vibration stimulus that was up to more than twice the baseline values. We did not verify the stimulus with electromyographic (EMG) analysis in our study and do not know whether their findings are also applicable to the shoulder complex.
The size of intersubject SDs has not been addressed by authors of previous vibration studies but may be due to the normal variation in anatomical structures and joint laxity of the participants. Two groups26,27
have proposed theories of variation in joint structure and laxity that may be experienced bilaterally or unilaterally. Also, total work performed by 1 volunteer in the vibration and weight-training group declined approximately 23% during the study. Although we do not have a definitive explanation for this outcome, it did reduce the group mean, which was higher in the vibration and weight-training group, by 7%. Consequently, the unexpected decrease in total work by this or any other participant may have affected the statistical analysis.
We chose the experimental protocol based on a critical review of published literature and the capabilities of the vibration platform. The vibration platform was used for 2 sets of 60 seconds, with a rest of 30 seconds between sets, because previous researchers28
indicated that exposure to vibration lasting longer than 4 minutes may induce muscle fatigue. Other authors29
have also reported decreased EMG activity and maximal voluntary contractions as early as 2 minutes into prolonged vibration. Several authors22,30–,32
recommended that frequencies between 20 and 120 Hz and amplitudes between 1 and 4 mm be used to avoid possible tissue damage. In experiments conducted with nearly identical methods which differed only in amplitude, EMG activity and strength measurements increased, with a 4-mm amplitude compared with a 1-mm amplitude.28,30
Martin and Park20
demonstrated greater muscle activation at a 50-Hz frequency. Therefore, we set the vibration sessions at an amplitude of 4 mm and a frequency of 50 Hz. We believed that vibration exposure 15 minutes before training would result in persistent adaptations because investigators30
noted improvements in strength measurements up to 60 minutes postvibration. A 15-minute window between the vibration protocol and training sessions would also allow all participants adequate time to complete the protocol at the same time of day.
Typically volunteers are placed on a vibrating platform in positions such as standing, squatting, or sitting.7,18,22,33,34
We used a protocol of indirect WBV in a static, modified–push-up position. No previous authors have used this exact position, but some21,35
have safely transmitted vibration through the upper extremity via handles. We chose this position for several reasons. The magnitude of vibration stimulus decreases with distance from the point of origin of the vibration as a result of attenuation.30,32
Changes in joint stability with muscle contraction may play a role in the transmission of mechanical vibration.36
Furthermore, exposure to vibration during isometric contractions has resulted in greater strength gains than have been noted with concentric or eccentric contractions.17
The concept of proximal stability for distal mobility has long been used in rehabilitation programs, and the relationships among scapular and shoulder stabilization and shoulder pain have been well documented.37–,39
In the context of this study, muscular co-contraction was also emphasized to the participants while they were positioned on the vibration platform. Although metabolically costly, co-contraction can increase joint stability and motor unit recruitment.40
have documented increased joint stability during coactivation of antagonist muscles around a joint due to increased stiffness. We considered these principles when placing volunteers in the described position, which should have allowed the most effective exposure of the rotator cuff musculature to the stimulus. Despite the paucity of literature on vibration and the upper extremity, we found it plausible that the upper extremity might react similarly to the lower extremity.7,17,19
The isokinetic protocol of the modified neutral shoulder test position was used for several reasons. First, it has been reported43
that this position places the bony, ligamentous, and muscular components of the glenohumeral and scapulathoracic joints in relationships to optimize congruency, reduce impingement, and enhance length-tension forces. Second, we chose this position in an effort to accommodate our population. Participants were active in various sports, which did not all require overhead dominant activities. Therefore, we avoided the position of 90° of glenohumeral abduction because it may be more specific to muscular function during overhead activities.44
Similar, but not identical, protocols have been used by other researchers.45,46
Additionally, shoulder internal-external rotation strength may provide a better measurement of global shoulder strength development than does abduction-adduction or flexion-extension strength. In a recent isokinetic study, improvements were seen in all shoulder motions after strengthening only internal and external rotation, whereas strengthening of flexion, extension, abduction, and adduction resulted in improvements specific to the motions trained.47
We decided to augment a strength and conditioning program because it is an integral part of collegiate athletics in today's society, and this approach has not been documented in the previous literature. Programs comprising multijoint dynamic resistance exercises are among the most popular forms in the athletic setting. Examples of these exercises include the pull-up, seated overhead pull-down, dead lift, push-up, squat, and overhead press. Variations of these exercises, including push-up progressions, diagonal proprioceptive neuromuscular facilitation patterns, core strengthening exercises, and others, are often used in the rehabilitation process.
Previous authors have produced results that indicate the possible beneficial nature of vibration for bone density,14,48
and oxygen uptake,50
but we did not examine any of those areas. Our study was unique in comparing a complete resistance training program and an equivalent program using vibration as an additional tool just before training. Although the vibration intervention did not produce a statistical difference, we believe that the protocol as outlined and the use of a vibration platform in this real-life context deserve further investigation. It would be sensible to continue to explore this technology further to identify risks and rewards associated with its use as it becomes more widespread and popular in athletic facilities.