Physiological and Functional Effects of Acute

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Journal of Strength and Conditioning Research, 2003, 17(4), 686–693 q 2003 National Strength & Conditioning Association

Physiological and Functional Effects of Acute Low-Frequency Hand-Arm Vibration ANA L. GO´ MEZ, JEFF S. VOLEK, MARTYN R. RUBIN, DUNCAN N. FRENCH, NICHOLAS A. RATAMESS, MATTHEW J. SHARMAN, AND WILLIAM J. KRAEMER Human Performance Laboratory, Department of Kinesiology, University of Connecticut, Storrs, Connecticut 06269.

ABSTRACT The effects of low frequency of vibration have not been widely studied in the scientific literature, yet humans are exposed to such environmental stress everyday. The purpose of this investigation was to examine the physiological responses to low-frequency upper-body limb vibration. Fourteen healthy men were exposed to 1 hour of bilateral hand-arm vibration and control (no vibration) conditions in a counter-balanced, cross-over design separated by 2 days. Subjects gripped handles that were coupled to a vibrating device, which oscillated in an anterior to posterior direction at a constant frequency of 7.5 Hz and a displacement of 0.38 cm. A series of tests were performed prior to and following the vibration to assess cardiovascular response, visual acuity, tremor of the hand and fingers, grip strength, anticipation response, limb girths, and a movement repositioning task. There were significantly (p # 0.05) more visual errors postvibration compared with postcontrol on a standardized vision chart. Tremor was significantly reduced during the vibration compared with the control condition. There were no significant changes in grip strength. Mean anticipation response time was significantly increased during the control condition (13.3%) but not after vibration (11.0%). There was a significant improvement in the movement repositioning task after vibration compared with control. Heart rates during the vibration protocol were not significantly higher than the control condition. No significant increases in limb size representative of swelling were observed. These data indicate that exposure to 1 hour of low-frequency hand-arm vibration has only minor effects on physiological function

Key Words: dexterity, oscillation response, shaking, jolt, bilateral arm vibration, energy absorption Reference Data: Go´mez, A.L., J.S. Volek, M.R. Rubin, D.N. French, N.A. Ratamess, M.J. Sharman, and W.J. Kraemer. Physiological and functional effects of acute low-frequency hand-arm vibration. J. Strength Cond. Res. 17(4):686–693. 2003.

Introduction

V

ibration is common to everyday life and can be described as rapid quick movements back and

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forth, or oscillation of an object. Vibration can be transmitted from a variety of sources and impact people at work and at home. Hand-arm vibration is typically encountered when grasping or holding a vibrating object and primarily affects the upper limbs, wrists, and fingers (2). The effects of hand-arm vibration has focused on individuals exposed to moderate to high frequency of vibration at work, who report greater difficulty performing activities of daily living (3). Exposure to lower frequency of vibration also occurs at home. For example, hand-held mixers/blenders, steering wheels, vacuums, hairdryers, mowers, and hedge cutters/ trimmers all transmit low-frequency vibrations. Lowfrequency vibration (,20 Hz) is probably the most common form of exposure, yet few data exist to describe any of its acute effects on humans. Because vibration exposure is more commonly transmitted through objects that are held, there is a need to examine the acute impact of hand-arm vibration on various physiological functions and performance tasks. Prior studies have demonstrated that chronic vibration exposure is related to a variety of symptoms and pathological disorders (e.g., hand-arm vibration syndrome, Raynaud’s phenomenon and vibration-induced white finger) (2, 20). Few data exist to demonstrate the acute physiological stress associated with hand-arm vibration exposure and we are aware of no data examining vibration at a frequency ,10 Hz. Prior studies involving high-frequency hand-arm vibration have shown effects on tactile sensitivity (21, 12), vibration perception threshold, ratings of discomfort, and electromyographic activity (9). Despite these alterations in neurosensory/neuromuscular performance, manipulative skill does not seem to be impaired (21). The effects of acute hand-arm vibration on other measures of motor control are unknown. An improved understanding of the effects of low-frequency hand-arm vibration may help to prevent accidents that could result from adverse vibration-induced effects on coordination, dexterity, clumsiness, and performance of intri-

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Table 1. Physical characteristics of subjects.* Age (y) Height (cm) Mass (kg) Strength/mass ratio† (baseline control condition)

23 6 3.9 179.19 6 5.53 79.67 6 8.46 0.64 6 0.11

* Values are mean 6 SD; n 5 14. † Strength/mass ratio 5 average handgrip strength/total body mass.

cate tasks. The purpose of this investigation was to examine the acute physiological responses and functional performance after low-frequency hand-arm vibration in order to provide more information on performance of tasks requiring muscular, visual, movement repositioning, and anticipatory tasks.

Methods Experimental Approach to the Problem Subjects participated in a vibration and a control (no vibration) condition separated by 2 days using a counter-balanced, cross-over design. A vibration stimulus was the only difference between the control and vibration conditions. A series of tests were performed prior to and following each condition. All participants were free of any neurological disorders and reported normal health status information. In addition, subjects had no prior vibration stress history. In order to standardize the physiological status of subjects prior to each condition, the time and composition of the meal consumed prior to the first experimental condition (.2 hours prior to testing) was recorded and reproduced during the subsequent condition. Based on pilot work indicating minimal physiological stress (i.e., muscle soreness, numbness, etc.) associated with the vibration protocol, a washout period of 2 days was estimated to be adequate for complete recovery to occur. Because few data exist examining the acute physiological responses and functional performance after low-frequency hand-arm vibration (to provide more information on performance of tasks requiring muscular, visual, movement repositioning, and anticipatory tasks), participants were instructed not to participate in any heavy or strenuous physical activities 2 days prior to their first condition and during the wash-out period (between conditions) to minimize any confounding results to these measures. Subjects Fourteen healthy men free of neurological, metabolic, and endocrine disorders volunteered to participate in this investigation. The characteristics of the subjects in this investigation were age 23 6 3.9 years; height 179.2 6 5.5 cm; and body mass 79.7 6 8.5 kg (see Table 1). The majority of subjects were students and none were

Figure 1. Vibration device used in experiments.

exposed to regular vibration in their workplace. All subjects were informed of the purpose and all possible risks of this investigation prior to signing an institutionally approved informed consent document. One hundred percent compliance was achieved for these instructions, thus eliminating any residual physical fatigue from exercise or other activities. Vibration Device The vibration apparatus (Figure 1) was designed and built specifically for the purpose of this investigation. The computer-controlled device driven by electrical solenoids was 28 3 15 3 10 cm. Two handgrips separated by 34 cm were mounted to the base of the vibrating machine. Each handgrip was 1.6 cm in diameter. The vibration device was mounted to a table (height 71.4 cm) and oscillated in an anterior to posterior direction at a constant frequency of 7.5 Hz and a displacement of 0.38 cm. Vibration Protocol Subjects remained seated during the entire protocol while holding the handgrips coupled to the vibration device while being exposed to 1 hour of bilateral handarm vibration and control (no vibration) condition. Elbows were flexed at 908 and body posture (e.g., straight back, head forward, feet on floor, etc.) was standardized and maintained throughout each condition. Subjects were instructed to grasp the device lightly as they would during casual automobile driving. Ear protectors (designed with a noise reduction rating of 21 decibels according to OSHA standards) were worn during the vibration protocol to dampen the noise and prevent potential auditory problems. During the control condition, subjects grasped the device in the exact same manner as described except there was no vibration.

688 Go´mez, Volek, Rubin, French, Ratamess, Sharman, and Kraemer

The battery of tests included assessment of heart rate, circumferences, visual acuity, tremor of the fingers and hands, grip strength, anticipation response, and a steering repositioning task. Subjects were familiarized with all testing protocols immediately prior to actual data collection. Heart rate was assessed at 5minute intervals during the vibration; all other tests were performed while the vibration was turned off, with the exception of visual acuity. There was exactly 40 minutes of continuous vibration prior to administration of the first test, visual acuity. After exactly 45 minutes, the vibration device was temporarily stopped and the tremor test administered. Immediately after this test, vibration commenced for 5 minutes and grip strength was assessed. Vibration again was initiated for 5 minutes and anticipation response was measured. After the last 5 minutes of vibration, a steering repositioning task was assessed. Thus, there was a total of 60 minutes of total vibration (i.e., 45 minutes continuously and three 5-minute intervals). The rationale for this approach was to re-expose subjects to the vibration for 5 minutes prior to testing because the recovery rates for these tests are unknown. Had all the tests been performed continuously (as was done during baseline testing), the last test would have been preceded by 15 minutes of no vibration. Experimental Procedures A POLAR Vantage XL Heart Rate Monitor (model 145900; Polar CIC, Inc., Port Washington, NY) was used to determine heart rate during the vibration protocol. Heart rate was monitored and recorded every 5 minutes during both conditions. Circumference measurements at the wrist, forearm, biceps, and shoulder girdle were recorded on both arms. A spring-loaded measuring tape (Gulick II; Country Technology, Inc., Gays Mills, WI) was utilized using standard anatomical landmarks as references points (11). Visual acuity was assessed using a Snellen Letter Eye Chart (Logan Basic Methods, Inc., Chesterfield, MO) consisting of rows of characters in decreasing sizes. Subjects were seated exactly 20 ft away, with both eyes open, during the test. Subjects with corrective lenses or contacts were required to wear these during the test. The total number of letters missed per line was calculated for later analysis. Tremor was assessed using a modified television touch screen controller (Intrepidus, Muncie, IN) mounted on a chair (43 cm in height) interfaced with a computer that collected data at 40 Hz. A screen 44 3 29 cm recorded finger movements while the arm was extended perpendicular to the floor. A cone (9.5 cm in length) was placed on the middle finger of the dominant hand to record only the movement of one digit for 15 seconds. A hand-grip dynamometer (Vital Signs, model

68812; Country Technology, Inc., Gays Mills, WI) was used to determine hand-grip strength. Each subject was familiarized with the dynamometer and allowed to practice twice and adjust the hand settings if necessary. Three trials were performed on each hand and were alternated between trials to allow for adequate rest between each trial. Each trial was performed with the same hand setting and seated position to ensure reliable results. The highest score of 3 trials was used for analysis. The Lafayette Instrument’s Bassin Anticipation Timer (models 50575 and 50575R; Lafayette Instrument Co., Lafayette, IN), previously described (7), was used to determine anticipation response. The Bassin Anticipation Timer consists of 80 red light-emitting diodes positioned on a runway 367 cm in length. The device was positioned in front of the subject with a series of lights moving sequentially at 10 mph in a line toward the subject. A yellow light at the beginning of the runway of lights farthest from the subject served as ready signal. A control box set away from the subject started the forewarning period, which was 3 seconds before the red lights illuminated sequentially from the start, simulating a moving object. A response button (1.27 3 1.27 cm), connected to a stop clock, was pressed with the left index finger and measured the subjects’ attempt to anticipate the last light illuminating on the runway nearest the subject. Feedback in the form of a digital readout indicating response time was provided after each trial. Each subject performed 10 trials. A familiarization period of approximately 10–15 test trials was allowed to each subject before actual data was collected. The means of 10 trials were calculated for each subject and used for further analysis. A steering repositioning task was performed using a small driving wheel (diameter 29 3 29 cm) mounted to a 37 3 14 cm board clamped to a table (height 71.4 cm). At the distal end of the steering wheel column, a 25-cm pointer was attached and positioned to brush against an oversized protractor (measurement in degrees), which measured movements to the left and to the right from the 08 center point. Each subject was blind folded and then manually taken to an angle, returned to 08 and then verbally instructed to return to the angle. Subjects were asked to reproduce the following angles in order: 208 right, 408 right, 258 left, 508 right, and 408 left. The error (68) was recorded for each attempt. To assess side effects, subjects were asked to rate the following symptoms on a scale from 1 to 5 (1 5 none, 2 5 minimal, 3 5 occasional, 4 5 frequently, 5 5 constant/continuous): dizziness, headache, ringing in ears, numbness, nausea, muscle soreness, fatigue, blurred vision, tingling, cramping, and other in that order.

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Table 2. Heart rate (beats per minute) changes during vibration and control conditions.† Time (min)

Vibration

Control

Pre 0 5 10 15 20 25 30 35 40 45 50 55 60 Post

70 69 71 72 73 71 74 72 72 74 70 70 71 68 71

6 6 6 6 6 6 6 6 6 6 6 6 6 6 6

73 6 13 68 6 10 69 6 7 72 6 9 68 6 10* 71 6 6 70 6 9 67 6 8 68 6 9 66 6 7* 68 6 10 69 6 8 66 6 9 68 6 11 67 6 10

11 11 10 11 10 7 8 9 10 10 10 9 11 9 9

Figure 2. Mean (6SE) change in circumferences before (Pre) and after (Post) vibration and control conditions. * p # 0.05 from the corresponding previbration value.

There were no significant group or time effects for any of the circumference measurements; however, there were significant interaction effects for the right wrist and for both the right and left forearm (Table 3). Dependent t-tests indicated significantly greater delta changes during vibration than control (Figure 2). Subjects were able to correctly identify all letters on lines 1 and 2 of the visual acuity test. A two-way ANOVA revealed a significant main condition effect but no time or interaction effects. There were significantly fewer errors previbration compared with precontrol on line 5. Compared with control, there were significantly more errors postvibration on lines 3 and 5 (Table 4). There were no significant main or interaction effects for tremor of the fingers and hands although the delta change was significantly greater during the control condition due to a lower starting value (Table 5). There were no significant main or interaction effects for grip strength of the dominant or nondominant hand (Table 6). Mean anticipation response time was significantly increased during the control condition (13.3%) but not after vibration (11.0%). The delta change during the

† Values are mean 6 SD; n 5 14. * p # 0.05 from corresponding vibration time point.

Statistical Analyses Circumference measurements, heart rate, visual acuity, tremor, grip strength, anticipation response, and steering repositioning performance were analyzed using a 2-way repeated analysis of variance (ANOVA). The factors were condition (vibration and control) and time (pre- and postvibration). When a significant F was achieved, the Fisher’s least significant difference (LSD) test was used to locate significant differences between paired means. Delta changes (post-pre) for all variables were calculated and analyzed via dependent ttests. The frequency of significance was set at p # 0.05.

Results Two-way ANOVA indicated no main effects of time or condition on heart rate (Table 2). Dependent t-tests between corresponding time points during vibration and control indicated a significant increase during vibration at the 15 and 40 minute time points. Table 3. Circumference measurements (cm).* Vibration Body part Wrist (R) Wrist (L) Forearm (R) Forearm (L) Bicep (R) Bicep (L) Shoulder

Pre 16.93 16.88 28.32 27.96 32.10 31.99 117.89

6 6 6 6 6 6 6

Control Post

0.75 0.63 2.00 1.80 2.91 2.58 6.21

* Values are mean 6 SD. R 5 right; L 5 left.

16.98 16.91 28.55 28.11 32.30 32.13 117.87

6 6 6 6 6 6 6

Pre 0.77 0.69 1.99 1.80 2.85 2.62 6.18

17.05 16.93 28.45 28.07 32.25 32.33 117.84

6 6 6 6 6 6 6

Post 0.77 0.64 1.98 1.83 2.87 2.68 6.27

16.98 16.89 28.38 28.04 32.21 32.46 117.81

6 6 6 6 6 6 6

0.78 0.64 1.90 1.80 2.86 2.99 6.24

690 Go´mez, Volek, Rubin, French, Ratamess, Sharman, and Kraemer Table 4. Visual acuity changes during vibration and control protocols.* Time point

Vibration

Control

Pre Line 3 Line 4 Line 5

1.00 6 1.10 1.43 6 2.31 4.07 6 3.15**

0.50 6 0.75 0.92 6 1.81 5.00 6 3.32

Post Line 3 Line 4 Line 5

0.71 6 0.99** 1.92 6 2.46 5.35 6 2.92**

0.07 6 0.26 0.92 6 2.16 4.64 6 3.60

* Values are mean 6 SD. Values represent letters missed per line. ** p # 0.05 from corresponding control value. Table 5. Tremor (mm) changes during vibration and control conditions.* Time point Pre Post Delta

Vibration 5.080 6 1.220 5.207 6 1.092 0.127 6 0.863

Figure 3. Mean (6SE) anticipation error time before (Pre) after (Post) vibration and control conditions. Time represents the difference between the actual arrival of an illuminated light and the anticipated arrival of the light by the subject. * p # 0.05 from corresponding previbration value.

Control 4.623 6 1.041 5.181 6 0.914 0.558 6 0.965**

* Values are mean 6 SD. ** p # 0.05 from change during vibration condition (t-test). Table 6. Grip strength (kg) changes in the dominant and nondominant hands during vibration and control protocols.† Time point

Vibration

Control

Dominant Pre Post

50.92 6 10.39 51.60 6 11.14

51.25 6 10.72 52.50 6 10.51

Nondominant Pre 48.50 6 9.86 Post 48.92 6 9.53

49.67 6 9.48 50.92 6 9.43*,**

† Values are mean 6 SD. * p # 0.05 from corresponding previbration value. ** p # 0.05 from corresponding vibration value.

control condition was significantly greater compared with the vibration condition (Figure 3). There were no significant main effects for any of the angle measurements during the repositioning task; however, the delta changes for angles left 258, right 508, and left 408 were significantly greater during the control condition (Figure 4). Subjective responses to vibration exposure varied; however, trends in the responses of the subjects were

Figure 4. Mean (6SE) delta error (Post-Pre) during vibration and control conditions. Error represents the difference between the actual degree and the reproduction performed by the subject. * p # 0.05 from corresponding control change.

noted. After 5 minutes of exposure to vibration, numbness, muscle soreness, fatigue, tingling, and cramping were the most prevalent symptoms reported affecting the hands, arms, and shoulders. After 15 minutes of exposure to the vibration, the rate of occurrence of these symptoms increased to ‘‘frequently’’ or ‘‘constant/continuous.’’ Blurred vision was only observed during the visual acuity test performed at baseline and at 40 minutes during both the vibration and control conditions. During the control condition, almost no symptoms were observed.

Discussion Due to the natural occurrence of vibration, particularly the low frequency of upper-body vibration associated

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with daily living, further study is needed to better understand the effects of low-frequency hand-arm vibration on exposure response relationship and performance. Interestingly, study of the impact of vibration on physiological systems has only recently become more sophisticated (e.g., neurological studies, musculoskeletal) (5). While a significant amount of research has investigated vibration exposure to the hand-arm system, many questions still remain about the physiological effects of vibration specific to an exposureresponse relationship and performance. This investigation examined sensory and functional responses to acute low-frequency hand-arm limb vibration. In general, the findings of this study indicate that low frequency of vibration results in minimal adverse responses and may actually enhance certain aspects of motor performance related to tremor, anticipation response, and steering repositioning tasks. The vibration protocol resulted in increased circumference of the wrist, forearm, and biceps. Handarm vibration at a higher frequency (120 Hz) for a shorter duration (5 minutes) has been shown to increase local blood flow (19). Increased blood flow is the most likely explanation for the increased circumferences after vibration in the present study; however, it is also possible that vibration-induced muscle injury resulted in local edema. The low-frequency vibration and short duration of exposure may well reflect the limited amount of changes. If blood flow to the handarm system were increased, this was likely due to local vasodilation and redistribution of blood because heart rates during the vibration protocol were not significantly different from the control conditions. As hypothesized, there were slight decrements in visual acuity with the addition of a vibration exposure. Visual performance may be speculated to be affected by the frequency and duration of vibration exposure. Possible explanations for the increase in visual errors include the transduction of rotational head motions by the vestibular apparatus resulting in compensatory eye movements (vestibulo-ocular reflex) that will tend to stabilize the line of regard of the eye (1). Other factors, such as the relatively large displacements between the subject and the display could also cause oscillations in the perceived distance and consequent variations in the angle subtended at the eye (18). The ability of subjects to read and discern characters during exposure to vibration at low frequencies ranging from 0.5–5.0 Hz is impaired (18). Here again, one might hypothesize that visual impairments are related to the magnitude of the mechanical stress. Visual impairments induced by vibration alone or in combination with other motor tasks could compromise safety and increase the occurrence of accidents (e.g., automobile, shaving, slicing, etc.). Although it was hypothesized that tremor would increase during the vibration condition, subjects in this

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study actually demonstrated a decrease in tremor (improvement) of the hand and fingers during the vibration protocol. This appears to be a novel finding in this study. Previous literature has not directly evaluated these effects as it relates to vibration. The mechanisms remain at best highly speculative at this point in time. One might hypothesize a potential arousal of neurological systems, increased blood flow, and activation of more motor units prior to the test protocol. However, low-frequency vibration may not activate many of these mechanisms (21). The lack of an effect of hand-arm vibration on performance of intricate tasks (13), a skill that could be adversely affected by increased tremor, is consistent with our data showing no adverse effects on tremor. The mechanism(s) that explain enhancement of the steering repositioning task after low frequency of vibration remain unknown. It has been speculated that vibration exposure may disrupt continuous motorcontrol performance (15). Steering error may be related to neuromuscular activity through increased voluntary or involuntary response to motion (16). Kinesthetic sense may involve muscle receptors, especially the primary endings of the muscle spindles as they are predominantly sensitive to movement (14, 17). Increased errors during kinesthetic tasks have been noted after a vibration stimulus (4), possibly due to sustained Ia sensory inflow (23). Inglis et al. (8) observed an overestimation of limb position after a vibration load of 83 Hz and postulated that the muscle spindle information from the lengthening muscle was important for the accurate perception of limb movement and or position. Thonnard et al. (21) observed that exposure to vibration had no effect on proprioceptive skills after 30 minutes of vibration at 125 Hz, which may have been attributed to many years of vibration exposure. It is hypothesized that proprioceptive sense plays an important role in joint stabilization and that muscle fatigue may alter proprioceptive ability (22). As seen in this investigation, low frequency of vibration exposure did not fatigue the muscles enough to alter steering repositioning performance. Nevertheless, in healthy normal subjects, kinesthetic sense and intention tremor following low-frequency frequency actually improved relative to the control condition. There were no significant changes in maximal grip strength. Maximal force is dependent on the rate and number of motor units recruited. During the vibration and control protocols, subjects lightly gripped the handles of the vibration device, thus producing lowfrequency force demands and little, if any, recruitment of higher threshold motor units utilized to produce maximal force. Here again, the magnitude of vibration appears to be vital in eliciting physiological stress on maximal force production. In this study, low-frequency vibration appears to have little effect on much of the neuromuscular force-production motor unit pool.

692 Go´mez, Volek, Rubin, French, Ratamess, Sharman, and Kraemer

Anticipation responses were unchanged after vibration and adversely affected after the control condition (no vibration), perhaps due to lack of arousal (boredom). The underlying mechanisms remain speculative because no direct data were collected to address this question. Anticipation response involves the use of the eyes, the central nervous system, and muscle (10). McLeod and Griffin (16) speculated that vibration might interfere with cognitive processes, thus affecting task performances dependent on arousal, motivation, or anxiety. A gestalt of neurological mechanisms was presumably operational after the vibration to mediate enhanced anticipation response. Whether this was primarily due to central mechanisms related to the arousal (indicated in part by the slightly higher heart rate with vibration) or if local proprioceptive mechanisms (e.g., enhanced Golgi tendon, muscle spindle) predominate remains a topic for future investigations. There was variability among subjective ratings of adverse side effects to the vibration protocol. This could be due to differences in absorption of energy among subjects and/or the grip forces applied to the vibration device, which affects the response to vibration (6). Even though standardized instructions were given, the position of the subject’s shoulders while seated, length of the arms, and the size of the hands, could have contributed to variation in absorption of energy. Other internal (muscle soreness, headaches, hunger, etc.) and external (noise) sources may have also contributed to the symptomatic feelings a person might have that contributed to this variability. Participants were free of any neurological disorders and diet, activity frequency, and time of day were closely monitored and reproduced in an effort to eliminate these confounding variables. While these results are acutely consistent with a warm-up phenomenon and show no negative effects compared with control conditions, we have no data to demonstrate the veracity of these changes over a repetitive time course of exposure. Thus, before any conclusions are made as to the use of such a protocol for potential warm-up or facilitatory purposes, repetitive and chronic experimentation is needed. It appears that negative effects of hand-arm vibration occur when the frequency and duration are much greater than the experimental conditions addressed in this article. Nevertheless, millions of people experience low-frequency vibrations while in their everyday life (e.g., hairdryers, driving, vacuum cleaners, power tools; sanders, drills etc.), thus making the importance of this study relevant to everyday life. The relationships between different frequencies of acute hand-arm vibration and length of habitual exposure to vibration is an important topic for future research. This study investigated low-frequency vibration exposure in a controlled laboratory setting to examine the acute

physiological responses and functional performance after low-frequency hand-arm vibration in order to provide more information on performance of tasks requiring muscular, visual, movement repositioning, and anticipatory tasks. These data indicate that low frequency hand-arm vibration results in minimal adverse responses and may actually enhance certain aspects of physiological function.

Practical Applications The findings of this study provide a set of basic facts about the acute exposure (60 minutes) to low-level (7.5 Hz) hand-arm vibration. The results demonstrate that, under such conditions, hypothesized effects may not be easily predictable. Surprisingly, certain variables demonstrated significant positive improvements while all others were not significantly affected by the vibration treatment protocol. Thus, what mechanisms mediate such acute responses remain to be further elucidated. The practical applications at this point in time remain equivocal as one could theorize a potential warm-up effect. Caution must be taken when interpreting the use of such an acute vibration protocol because the long-term effects remain to be demonstrated. Furthermore, prior literature supports the concept that hand-arm vibration, even at such low frequency levels, can produce negative health outcomes with chronic exposure.

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Acknowledgments We would like to thank a dedicated group of test subjects for their efforts in this project.

Address correspondence to Dr. William J. Kraemer, [email protected].