The acute effect of whole-body vibration on the hoffmann reflex
Power Plate Studies
ABSTRACT
 
The extent to which motoneuron pool excitability, as measured by the Hoffmann reflex (H-reflex), is affected by an acute bout of whole-body vibration (WBV) was recorded in 19 college-aged subjects (8 male and 11 female; mean age 19 ± 1 years) after tibial nerve stimulation. H/M recruitment curves were mapped for the soleus muscle by increasing stimulus intensity in 0.2- to 1.0-volt increments with 10-second rest intervals between stimuli, until the maximal M-wave and H-reflex were obtained. After determination of Hmax and Mmax, the intensity necessary to generate an H-reflex approximately 30% ofMmax (mean 31.5%6 4.1%) was determined and used for all subsequent measurements. Fatigue was then induced by 1 minute of WBV at 40 Hz and low amplitude (2–4 mm). Successive measurements of the H-reflex were recorded at the test intensity every 30 seconds for 30 minutes post fatigue. All subjects displayed a significant suppression of the H-reflex during the first minute post-WBV; however, four distinct recovery patterns were observed among the participants (œ = 0.50). There were no significant differences between genders across time (P = 0.401). The differences observed in this study cannot be explained by level or type training. One plausible interpretation of these data is that the multiple patterns of recovery may display variation of muscle fiber content among subjects. Future investigation should consider factors such as training specificity and muscle fiber type that might contribute to the differing H-reflex response, and the effect of WBV on specific performance measures should be interpreted with the understanding that there may be considerable variability among individuals. Recovery times and sample size should be adjusted accordingly.
 
KEY WORDS H-reflex, potentiation
 
INTRODUCTION
 
Whole-body vibration (WBV) is being examined for use in rehabilitation and sport training (6,7,9,23,30–32). Investigators have noted improvements in response to WBV training in such physiological measures as neuromuscular performance (2), force output (10,12,22,33), flexibility (36), and hormone concentrations (2). Not all investigators, however, have noted positive benefits (9,13,14,30), particularly in the short term. Rittweger and co-workers (31) examined the effects of squatting with and without vibration (26 Hz). They concluded that WBV may enhance neuromuscular excitability. This was measured, however, using the patellar reflex, and the duration of the effect was not determined. Still, little is known about the exact physiological adaptations responsible for these changes or what is the most appropriate combination of frequency (of vibration) and amplitude.
 
The Hoffmann reflex (H-reflex) is widely established as a measure of motoneuron excitability (18,20,21,28,29,34). The H-reflex is analogous to the spinal stretch reflex induced via electrical stimulation, thus bypassing the muscle spindle. Progressive stimulation of a percutaneous mixed nerve, such as the tibial nerve, reveals the H-reflex, which appears first on the electromyographic trace (EMG) as the threshold of the Ia afferents is attained, and is followed by the muscle response (M-wave) as the œ-motoneurons (œMNs) reach threshold. The peak-to-peak amplitude of the H-reflex increases until maximum and subsequently diminishes. As more aMNs are recruited, the M-wave increases until it plateaus at Mmax. The disappearance of the H-reflex as the M-wave approaches Mmax results from the collision of the reflexive action potential traveling down the motoneuron (orthodromic) with the action potential traveling up the motoneuron toward the spinal cord (antidromic), which blocks the H-reflex. For a review of the H-reflex and its application to sports medicine, the reader is referred to Palmieri et al. (29).
 
The H-reflex may be normalized using a stimulus intensity that produces a percentage of Mmax (29). Theoretically, the H-reflex represents a given percentage of the total motoneuron (MN) pool (i.e., a peak-to-peak amplitude equal to 30% of Mmax represents 30% of the MN pool). This method permits an investigator to measure changes in theMN pool after some intervention. Thus, changes in the peak-to-peak amplitude of the H-wave after a bout ofWBVmay be used to determine the extent to which WBV may affect neuromuscular activity and indicate the degree to which fatigue, recovery, and, possibly, potentiation of the reflex response occur.
 
METHODS
 
Experimental Approach to the Problem
 
The present study is the first in a series of studies involving WBVconducted in our laboratory. Exploratory in design, this study was designed to determine the time course of changes in the H-reflex after a single 1-minute bout of WBV at a moderate frequency (40 Hz) and low amplitude (2–4 mm). In particular, it was hoped that the results would allow the investigators to determine whether random order of treatment (e.g., frequency and amplitude levels) may be assigned and, if so, how much recover time is required between treatments.
 
Subjects
 
Nineteen subjects (8 male and 11 female, mean age 19 ±1 years [range: 18–21 years]) with measurable H-reflexes were recruited from the student population at Hope College by word of mouth. No effort was made to control for training specificity, but current physical activity was self-reported on a medical history questionnaire. Subjects ranged from sedentary to NCAA Division III athletes (football, soccer, swimming, and track sprinters) and were overall heterogeneous. Subjects had no indicated neurological defects, no history of lower extremity surgery, and no lower extremity injury for 12 months before the start of the study as reported by the medical history questionnaire. Informed consent was obtained, and all procedures were approved by the Hope College human subjects review board.
 
Procedures
 
H-reflex and M-wave measurements were collected at a rate of 2000 Hz using surface EMG (MP150; Biopac Systems Inc., Santa Barbara, CA). Signals were amplified (EMG100C; Biopac Systems Inc.) from disposable, 10-mm pregelled Ag- AgCl electrodes (EL503; Biopac Systems Inc.). A stimulator module (STM100C; Biopac Systems Inc.) with a 200-V (maximum) stimulus adaptor (STMISOC; Biopac Systems Inc.) using a disc electrode (EL548; Biopac Systems Inc.) and a 4-cm dispersive pad were used to stimulate the muscle contraction.
 
Subjects reported to the Athletic Training Room at Hope College’s DeVos Fieldhouse after at least 12 hours’ rest from exercise and 12-hour abstention from alcohol, caffeine, and any medication that affects the central nervous system.With the subject lying prone, Subject position for H-reflex. Insert, Stimulating electrode in posterior popliteal fossa.two areas were abraded with fine sandpaper and cleaned with alcohol for placement of the EMG electrodes. Surface electrodes were placed on the posterior leg, centered on the soleus muscle, approximately 2 cm distal to the medial head of gastrocnemius and spaced 2 cmapart, parallel to the muscle fibers. The ground electrode was placed on the lateral malleolus of the ipsilateral leg.
 
The stimulating electrode was fixed in the medial portion of the popliteal fossa behind the knee over the tibial branch of the sciatic nerve (Figure 1). The corresponding anode (dispersive pad) was placed on the anterior thigh superior to the patella. The stimulating electrodes were then wrapped with 2-inch PowerFlexTM tape (Andover, MA) to maintain constant pressure on the electrodes. Subjects were subsequently positioned supine with the hands positioned at the sides, knees supported at approximately 10–15 degrees, and the feet secured against a footplate to maintain neutral foot position. Participants wore noise-canceling headphones (Phillips HN110) and listened to ocean sounds to minimize the effects of extraneous sound on the H-reflex. Subjects were asked to remain still, and the same body positioning was used throughout testing.
 
H/M recruitment curves were mapped for the soleus muscle by increasing the stimulus intensity in 0.2- to 1.0-V increments, with a 10-second rest interval between stimuli, until the Mmax was obtained. Peak-to-peak amplitudes of the H-reflex and M-wave were determined for all test stimulations. Five measurements of the H-reflex were made using a stimulus sufficient to produce an H-wave 30% of Mmax pre- WBV (T0), and a single measurement was made immediately post-WBV (approximately 1 minute post-WBV; T010) and every 30 seconds for 30 minutes. In addition, because previous studies have indicated a conditioning effect of the electrical stimulation on the H-response, three conditioning stimuli (approximately 100 V) were applied at onset of the recruitment curve. H-reflex measurements using the described protocol have been found to be reliable (r = 0.9953, 0.9514, and 0.9747 for Hmax, Mmax, and H/M ratio, respectively) (28).

 
Whole-body vibration (Next Generation Power Plate; Power Plate North America, Inc., Northbrook, Ill.).
Subjects stood with the feet shoulder-width apart and the knees flexed approximately 10 degrees on a Next Generation Power Plate (Power Plate North America, Inc., Northbrook, IL) for 1 minute with the frequency and amplitude settings at 40 Hz and 2–4 mm, respectively (Figure 2).
 
 
Statistical Analyses
 
Multiple recovery patterns were observed among the subjects. Therefore, data were separated into four patterns according to the rate at which the subject returned to baseline and whether potentiation of the H-reflex was observed. Comparisons of changes in the H-reflex over time and between groups were made using repeated-measures ANOVA. One-way ANOVA was also used to compare differences between groups and between genders at key time points where notable changes in the H-reflex occurred, i.e., T0, T010, 3-min post-WBV(T030), 15-min post-WBV (T150), and 27.5-min post-WBV (T275), and Bonferroni post hoc comparisons were made to determine where significant differences between groups occurred. In addition, the intraclass correlation coefficient was determined the five pre-WBV H-reflex measurements at the test stimulus intensity. These values were used to determine internal consistency because these were the only multiple measurements taken at a given intensity. The level of significance was set at œ= 0.05.
 
RESULTS
 
Four distinct recovery patterns were observed among the participants (Figure 3). All subjects displayed a significant suppression of the H-reflex during the first minute post-WBV. Recovery to baseline occurred in three of the groups, whereas the fourth group (G4) showed a nearly complete suppression (peak-to-peak amplitude <10% of Mmax) for the duration of recovery. One group (G1) returned to baseline within 3 minutes, and the H-reflex subsequently increased to above baseline. Group 2 (G2) returned to and remained at baseline after 7 minutes. Group 3 (G3) was suppressed until approximately 15 minutes post-WBVand gradually increased to baseline by 30 minutes.

 
Figure 1. Subject position for H-reflex. Insert, Stimulating electrode in posterior popliteal fossa.
 
There was a significant effect for time for all groups (P < 0.001) and for group by time (P <0.001). There were no significant differences between genders across time (P = 0.410). There was not, however, sufficient gender representation in all groups to perform time xgroup xgender comparisons. Pre-WBV, G1 displayed a baseline H-reflex significantly higher than G3 and G4 (P = 0.014 and 0.019, respectively) but not significantly different from G2 (P = 0.07). At T010, there was no significant difference between groups on the H-reflex (P >0.34). At T030, there was a significant between-group interaction (P = 0.002). G1 was significantly higher than G3 and G4 (P >0.35), but G2 did not differ from the other groups (P >0.16). At T150 and T275, there was a significant between-group interaction (P , 0.001). At T150, G1 was higher but not significantly different than G2 (P = 0.083); G3 and G4 were similar (P >0.99); and G1 and G2 were significantly higher than G3 and G4 (P < 0.001). At T275, G1 was significantly higher than the other groups (P <0.04), and G2 and G3 were similar (P >0.99) and significantly greater than G4 (P <0.004). Group means for T0, T010, T030, T150, and T275 are reported in Table 1.
 
TABLE 1. H-reflex at 40 Hz and low amplitude (2–4 mm).
Intraclass correlations were performed on the five pre- WBV H-reflex measurements at the stimulus test intensity. The observed intraclass correlations value was r = 0.81.
 
DISCUSSION
 
The investigators sought to examine the recovery response of the H-reflex to a 1-minute bout of WBV exercise of moderately high intensity. The most significant finding of the present study is that the response of the H-reflex to a single bout of WBV is highly variable and may be associated with individual differences other than gender, training specificity, or conditioning status. One plausible explanation for these differences is muscle fiber type differences. The present study is limited in that the effects of training cannot be thoroughly eliminated. In addition, without information (i.e., biopsies of muscle samples) about muscle fiber content, these conclusions are speculative at best.
 
It has been reported that acute bouts of cycling or jogging exercise reduce the H-reflex (3–5,15,26,27). deVries and coworkers (15) indicated a ‘‘tranquilizer effect’’ of 20 minutes of moderate aerobic exercise on H/Mratio. These investigators examined H-M recruitment curves in the gastrocnemius of 10 volunteers (2 ‘‘elderly’’ [66 and 80 years] and 8 ‘‘younger’’ [mean age 27.3 ±4.8 years]) for which training levels and muscle composition are not reported. Suppression of the H/M ratio ranged from 6% to 44%. Motl and Dishman (26) reported an attenuated soleus H-reflex 10 minutes after moderate-intensity leg cycling (60% V_ o2peak) but observed no change in the flexor carpi radialis H-reflex. In a similar study, Motl et al. (27) observed suppressed soleus H-waves 10 and 30 minutes after active and passive cycling. The mechanism by which this occurred is unclear. Bulbulian (3) concluded that endogenous opiods play no role in the suppression of H and M after exercise. No study was found that detailed the pattern of H-reflex response over time after fatiguing exercise.
 
The data in the present study support previous findings that there is a fatiguing effect of WBV and that, comparing intensities for research, a substantial recovery should be allowed between exercise bouts. At least some of the subjects showed a suppressed H-reflex for 30 minutes, whereas others showed some level of potentiation for this duration. In either case, not all subjects were fully recovered with the 30-minute period. Because there was a limit to the recovery time, there is no way to conclude how long a recovery is required. The minimal period of recovery may be anywhere between 3 hours (13) and 24 hours (4).
 
There were at least four clear response patterns revealed in the study. Similar patterns were observed in an earlier experiment in which the researchers studied the H-reflex for 15 minutes after two bouts of the Wingate test (1).
 
The role of training on the H-reflex has been clearly demonstrated (16,24). The differences observed in both the Wingate study (1) and the present study, however, cannot be explained by level or type of training. In the Wingate study (1), the sample size was too small to quantify significant difference in training. In the present study, observed variability was not consistent among individuals with similar training experience (e.g., similar sport team participation, endurance-trained vs. agility/power-trained). The sample in this study was heterogeneous, and no definitive analysis of training effects could be performed. Although there were similarities in the nature of training among some of the subjects, few subjects trained with the same specificity (i.e., whereas soccer goalies and football defensive backs both train for agility, the activities to accomplish this goal are not necessarily the same). Examination of individual differences should be performed using larger, homogeneous groups controlled for training specificity.
 
The soleus is composed largely of type I muscle fibers (25). The posture tested using WBVwould put stress on the soleus muscle to maintain balance, and these data confirm that the soleus was indeed fatigued by the protocol. One plausible interpretation of these data is that the multiple patterns of recovery may be a reflection of muscle fiber content variations among subjects. Postactivation potentiation may be affected by the muscle fiber characteristics (8,11,17–19, 35,37), although this hypothesis was not tested. Comparison of biopsied soleus muscle tissue with H-reflex recovery after fatiguing exercise is warranted. The postactivation response of the H-reflex may be dependent not only on fiber type and the nature of the stimulus (e.g., frequency and intensity), but also on the complex interaction between the muscle fiber fatigue characteristics and the nature of the fatiguing protocol (17,18). In addition, other muscles (e.g., flexor carpi radialis, quadriceps femoris, gastrocnemius) should be tested to compare the response of the H-reflex after vibration and other fatiguing exercise.
 
Intraclass correlations indicate that the H-reflex may have been more variable in the present study than noted by Palmieri and co-workers (28). This is likely the result of the prolonged time the subjects were required to lay motionless. H/M recruitment curves can take longer than 30 minutes, and finding a suitable testing stimulus intensity took more than 5 minutes in some individuals. The subjects were also asked to lie still for an additional 30 minutes post-WBV. Maintaining a stable level of alertness (i.e., keeping the subjects awake, focused, and in the same postural position) proved impossible. In addition, the rhythmicity of stimulus application may have caused some subjects to anticipate the stimulation, thereby affecting the H-reflex. Nevertheless, the frequency of measurements permitted the investigators to note observable patterns in the recovery H-reflex.
 
PRACTICAL APPLICATIONS
 
H-reflex is a viable measure ofMN excitability. After fatiguing exercise, this variable is notably suppressed. The rate at which the H-reflex recovers after an acute bout of WBVwas studied in the present study. The data indicate that the pattern of recovery is variable and may not be attributable to gender or type of training. Recovery from a single bout may be longer for some individuals. Therefore, when comparing the acute effects of WBV at varying intensities, investigators should consider designs that include subjects who are tested on multiple days rather than using designs that, although randomly ordered, may not permit adequate recovery of the motor unit (i.e., utilize short-duration recovery between tests).
 
These data may also indicate that a physiological difference other than training occurs among subjects. This, however, was not quantified in the present study. Future investigation should consider factors such as training specificity and muscle fiber type that might contribute to the differing H-reflex response. In addition, the effect of WBV on specific performance measures (e.g., flexibility, vertical jump, force output, balance) should be addressed and interpreted with the understanding that there may be considerable variability among individuals, and sample size should be adjusted accordingly.