A repeated measures cross-over design was used with independent variables of vibration treatment or sham treatment. The dependent variables were bilateral maximal isometric hamstrings and quadriceps contractions measured by HHD, and SLH for maximal horizontal distance of each lower extremity. A flow chart of participants is depicted in .
Flow chart of participants' progression through testing protocol.
Thirty asymptomatic university students (19 female, 11 male) between the ages 22–32 years (average age 25.4 years, SD 2.7) were recruited by word of mouth to participate in this study. Inclusion criterion was that subjects had to be able to move their knee against resistance through a full, normal range without pain. Participants were excluded if they reported any of the following; taking prescription blood thinners, known pregnancy, history of hamstrings or quadriceps injury within the last year, disc problems, sciatica, osteoporosis, knee instability, or bleeding disorders. None of the potential participants had to be excluded. Before participating, subjects were informed of possible risks and signed an informed consent form approved by the University of Puget Sound Institutional Review Board.
Prior to the onset of this study, 30 university students volunteered to participate in a pilot study to establish the intrarater reliability of the researcher performing the HHD measurements. Subjects of the pilot study were tested in the same position, and with the same timing, as the subjects in the study.
At initial visit, participants were randomly assigned to one of two treatments: 1) localized vibration, 2) sham technique. Random assignment was initiated by having the first participant draw a piece of paper from a hat. The paper had either “A” or “B” written on it, with “A” correlating to the localized vibration, and “B” correlating to the sham technique. After the first subject drew, the treatments were alternated for the remaining participants. Participants then received the assigned intervention and the outcome measures of right and left hamstrings and quadriceps peak strength, and SLH were assessed. At the second visit, one week later, subjects received the alternate treatment and outcome measures were tested again.
The vibration treatment consisted of five minutes of localized vibration applied directly to the right hamstrings using the Thumper Versa Pro Massager (Thumper Massager Inc. Markham, Ontario) set to 30 Hz with 6 mm of amplitude. Subjects were in a seated position with the hips flexed at 908 for the duration of the treatment (). The sham treatment consisted of a mock neurological approach. For five minutes, subjects solved brain puzzles purchased at a grocery store newsstand that required processing of visual stimuli. Since this task predominantly involved the right cerebral hemisphere and the right hamstrings were being tested, direct neurologic influence on motor tasks would be minimized. Again, subjects were seated with the hips flexed at 908 for the duration of the sham treatment ().
Position of the subjects when receiving a five minute treatment of localized vibration to the right hamstring.
Position of the subjects when receiving a five minute sham treatment of solving visual puzzles.
Immediately following each intervention, subjects proceeded to isometric testing of quadriceps and hamstrings peak strength, and then to a SLH for distance. Peak quadriceps strength was tested first followed by peak hamstrings strength for each subject. Testers of all outcome measures were blinded to which treatment subjects had received. To determine maximal isometric muscle strength, the tester secured a handheld MicroFET 2 dynamometer (Hoggan Health Industries, 8020 South 1300 West, West Jordan, UT 84088) against the distal, anterior tibia for quadriceps testing and against the Achilles tendon for hamstrings testing. For quadriceps testing, subjects were seated at the edge of a treatment plinth positioned so that both feet were off the ground, hips and knees both flexed at 90°. Subjects were instructed to fold their arms across their chest and maximally extend their knee against the tester's resistance. Hamstring testing was performed with the subjects in a prone position on a treatment plinth, hips in neutral extension and knees flexed to 90°. Subjects were instructed to maximally flex their knee against the tester's resistance. Subjects attempted two submaximal trials in each position for familiarization, followed by two recorded maximal contractions. The duration of each maximal effort was five seconds, with a 30-second rest in between trials, and a one-minute rest before testing the other side.
Subjects then performed SLH for maximal horizontal distance with measurement being taken from initial heel position to heel contact at landing. Each subject was allowed two practice jumps before the recorded jump. Subjects were instructed to “stick the landing” so that the foot did not move after initial contact and the untested foot did not touch the ground. After a one-minute rest, the opposite leg was tested. The mean of 2 dynamometry trials for each muscle group on each leg and the SLH distances of each leg were used in analysis.