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PubMed1996

The temporary threshold shift of vibratory sensation induced by composite-band vibration exposure.

Eight healthy subjects were exposed to three 1/3 octave-band vibrations (63, 200, and 500 Hz) by hand clasping a vibrated handle in a soundproof and thermoregulated room. The vibratory sensation threshold at 125 Hz was measured before and after the vibration exposure at an exposed fingertip. According to a preceding study, we first determined the relationship between the acceleration of the vibration and the temporary threshold shift of vibratory sensation immediately after the vibratory exposure (TTSv,0) induced by 1/3 octave-band vibration. We then measured TTSv after the exposure to a composite vibration composed of two 1/3 octave-band vibrations that might induce an equal magnitude of TTSv,0 on the basis of the above relationship. The TTSv,0 induced by the composite vibration was not larger than the TTSv,0 induced by the component vibrations. This result suggests that the component of the vibration inducing the largest TTSv,0 determines the TTSv,0 by broad-band random vibration.

Adult↗
PubMed2003

Frequency-dependent effects of muscle tendon vibration on corticospinal excitability: a TMS study.

The aim of the present study was to investigate the effects of muscle tendon vibration at different frequencies on corticospinal excitability by means of transcranial magnetic stimulation (TMS). A second objective was to describe whether the observed modulations in motor evoked potentials (MEPs), as a function of vibration frequency, reflect the behavior of Ia afferents during and after vibration. In ten subjects, muscle tendon vibration (duration 30 s) was applied to the flexor carpi radialis (FCR) muscle at three different frequencies (20, 75 and 120 Hz). MEPs following single-pulse TMS were recorded from the targeted muscle during a previbration, vibration, and postvibration period. Muscle tendon vibration at 75 Hz increased the MEP amplitude significantly during vibration, whereas a smaller but still significant effect was observed during 120 Hz vibration. No significant MEP changes could be observed during 20 Hz vibration and during the postvibration period for each frequency. Our findings indicate that muscle tendon vibration exerts a frequency-dependent effect on corticospinal excitability. Furthermore, evidence is provided for the notion that the excitatory effect of muscle tendon vibration on the primary motor cortex is mediated by Ia afferent input.

Adult↗
PubMed2005

Effect of slow, small movement on the vibration-evoked kinesthetic illusion.

The study reported in this paper investigated how vibration-evoked illusions of joint rotation are influenced by slow (0.3 degrees /s), small (2-4 degrees ) passive rotation of the joint. Normal human adults (n=15) matched the perceived position of the left ("reference") arm with the right ("matching") arm while vibration (50 pps, 0.5 mm) was applied for 30 s to the relaxed triceps brachii of the reference arm. Both arms were constrained to rotate horizontally at the elbow. Three experimental conditions were investigated: (1) vibration of the stationary reference arm, (2) slow, small passive extension or flexion of the reference arm during vibration, and (3) slow, small passive extension or flexion of the reference arm without vibration. Triceps brachii vibration at 50 pps induced an illusion of elbow flexion. The movement illusion began after several seconds, relatively fast to begin with and gradually slowing down to a stop. On average, triceps vibration produced illusory motion at an average latency of 6.3 s, amplitude of 9.7 degrees , velocity of 0.6 degrees /s, and duration of 16.4 s. During vibration, slow, small ( approximately 0.3 degrees /s, 1.3 degrees ) passive rotations of the joint dramatically enhanced, stopped, or reversed the direction of illusory movement, depending on the direction of the passive joint rotation. However, the subjects' perceptions of these passive elbow rotations were exaggerated: 2-3 times the size of the actual movement. In the absence of vibration, the subjects accurately reproduced these passive joint rotations. We discuss whether the exaggerated perception of slow, small movement during vibration is better explained by contributions of non muscle spindle Ia afferents or by changes in the mechanical transmission of vibration to the receptor.

Adult↗
PubMed1999

Antagonist motor responses correlate with kinesthetic illusions induced by tendon vibration.

In humans, vibration applied to muscle tendons evokes illusory sensations of movement that are usually associated with an excitatory tonic response in muscles antagonistic to those vibrated (antagonist vibratory response or AVR). The aim of the present study was to investigate the neurophysiological mechanisms underlying such a motor response. For that purpose, we analyzed the relationships between the parameters of the tendon vibration (anatomical site and frequency) and those of the illusory movement perceived (direction and velocity), as well as the temporal, spatial, and quantitative characteristics of the corresponding AVRs (i.e., surface EMG, motor unit firing rates and activation latencies). Analogies were supposed between the characteristics of AVRs and voluntary contractions. The parameters of the AVR were thus compared with those of a voluntary contraction with similar temporal and mechanical characteristics, involving the same muscle groups as those activated by vibration. Wrist flexor muscles were vibrated either separately or simultaneously with wrist extensor muscles at frequencies between 30 and 80 Hz. The illusory movement sensations were quantified through contralateral hand-tracking movements. Electromyographic activity from the extensor carpi radialis muscles was recorded with surface and intramuscular microelectrodes. The results showed that vibration of the wrist flexor muscle group induced both a kinesthetic illusion of wrist extension and a motor response in the extensor carpi radialis muscles. Combined vibration of the two antagonistic muscle groups at the same frequency evoked neither kinesthetic illusion nor motor activity. In addition, vibrating the same two antagonistic muscle groups at different frequencies induced both a kinesthetic illusion and a motor response in the muscle vibrated at the lowest frequency. The surface EMG amplitude of the extensor carpi radialis as well as the motor unit activation latency and discharge frequency were clearly correlated to the parameters of the illusory movement evoked by the vibration. Indeed, the faster the illusory sensation of movement, the greater the surface EMG in these muscles during the AVRs and the sooner and the more intense the activation of the motor units of the wrist extensor muscles. Moreover, comparison of the AVR with voluntary contraction showed that all parameters were highly similar. Mainly slow motor units were recruited during the AVR and during its voluntary reproduction. That the AVR is observed only when a kinesthetic illusion is evoked, together with the similarities between voluntary contractions and AVRs, suggests that this vibration-induced motor response may result from a perceptual-to-motor transformation of proprioceptive information, rather than from spinal reflex mechanisms.

Adult↗
PubMed1997

Transmission of vibration energy to different parts of the human hand-arm system.

The aim of this study was to investigate the transmission of vibration energy to three selected points along the hand and arm (knuckle, wrist and elbow) and to compare the energy transmission for two different kinds of vibration exposures, i.e. random and sinusoidal. The transmission of vibration energy was estimated for ten subjects during exposure to random (within the frequency range 20-5000 Hz) and sinusoidal vibration at eight different frequencies (20, 40, 80, 160, 320, 630, 1250 and 1600 Hz). The random and sinusoidal vibrations had a frequency-weighted acceleration level of 3 m/s2. The energy transmission was determined by simultaneous vibration measurements at the vibrating handle and in the hand-arm system. The measurements were made with a laser-velocity transducer and specially constructed equipment. The grip and feed forces were held constant at 40 N. The results show that the energy transmission decreases with the distance from the source. The results also show that the energy transmission is dependent on the frequency for the random vibration exposures. No clear frequency dependence of the energy transmission could be found for the sinusoidal vibrations. It may also be concluded that there are differences in the energy transmission due to types of exposure, sinusoidal vibration showing higher transmission of energy to the hand-arm system than random vibration, especially at higher frequencies.

Absorption↗
PubMed1993

Force and displacement-controlled tendon vibration in humans.

This study investigated how the mechanical characteristics of tendon vibration influence the responses of human muscle receptors. In this study, we used a tendon vibrator in which the force, displacement and frequency of vibration were precisely controlled. The tendon vibrator could produce large amplitude displacements, so it was also used to impose ramp-and-hold stretches to the tendon to help classify muscle spindle afferents. In normal human subjects, we recorded microneurographically from single muscle afferents during tendon vibration to determine how afferent responses are influenced by the force and the displacement applied to the tendon and how these influences of force and displacement change with vibration frequency. Our results indicate that the sensitivity of muscle spindle afferents to tendon vibration is enhanced by increasing force and displacement and decreased by increasing frequency. It is concluded that, in order to predict the afferent response to vibration, the mechanical characteristics of tendon vibration must be controlled. Controlling the mechanical characteristics of tendon vibration and understanding the effect of vibration on afferent discharge will be useful for furthering our understanding of the peripheral control of movement.

Adult↗
PubMed1988

Tachykinins enhance the depression of spinal nociceptive neurons caused by cutaneously applied vibration in the cat.

The present investigation was prompted by previous studies in our laboratory which have indicated that tachykinins enhance depressant effects of purines and that the purine adenosine mediates a vibration-induced depression of nociceptive dorsal horn neurons. Extracellular recordings were made from single nociceptive neurons in the lower lumbar segments of anaesthetized cats. Vibration (80 Hz; 2.5-3.5 s every 20-25 s) was applied to the hindlimb using a feedback-controlled mechanical stimulator. The tachykinins physalaemin, substance P and neurokinin A were administered by iontophoresis. Physalaemin was tested on vibration-induced responses of 29 neurons; each neuron was excited by this tachykinin. To control for possible changes in the response to vibration caused by the excitation per se, statistical comparisons were made of the vibration-induced responses during excitation by tachykinins and during excitation by glutamate. In 16 cases the magnitude of the vibration-induced depression was significantly greater during the excitation caused by physalaemin. With the remaining neurons the response to vibration failed to differ during the excitation by physalaemin compared with the excitation by glutamate. In four of the 16 cases subthreshold applications of vibration caused depression after administration of physalaemin. The P1-purinergic (adenosine) antagonist, caffeine, was administered in three cases where vibration caused depression only with application of physalaemin. In each of these cases the depression was reversibly blocked by caffeine (10 or 40 mg kg-1 i.v.). The magnitude of vibration-induced depression was significantly increased during excitation by neurokinin A (5/14 neurons) or by substance P (1/9 neurons). From the results of the present study we suggest that tachykinins enhance the vibration-induced depression. This enhancement may be due to enhanced depression by adenosine.(ABSTRACT TRUNCATED AT 250 WORDS)

Action Potentials↗
PubMed2006

The apparent mass of the seated human exposed to single-axis and multi-axis whole-body vibration.

Most workplaces where workers are exposed to whole-body vibration involves simultaneous motion in the fore-and-aft (x-), lateral (y-) and vertical (z-) directions. Previous studies reporting the biomechanical response of people exposed to vibration have almost always used single-axis vibration stimuli. This paper reports a study where apparent masses of 15 subjects were measured whilst exposed to single-axis and tri-axial whole-body vibration. Each subject was exposed to 28 vibration conditions comprising every combination of single-axis and tri-axial vibration with magnitudes of 0.4 and 0.8 ms(-2) r.m.s. in each direction, once with backrest contact and once without backrest contact. Results show that increasing the magnitude of vibration in directions orthogonal to that being measured affects the apparent mass, causing a reduction in the resonance frequency as the total magnitude of vibration increases. It is demonstrated that the apparent mass resonance frequency is a function of the total vibration magnitude in all axes rather than a function of the vibration magnitude in the direction being measured. It is also shown that, for individuals, the frequency of the peak in the apparent mass in one direction is not related to the frequency of the peak in another direction. It is concluded that more complex biomechanical models are required in order to simulate human response to multi-axis vibration.

Adult↗
PubMed2006

Theoretical study of the changes in the vibrational characteristics arising from the hydrogen bonding between Vitamin C (L-ascorbic acid) and H2O.

The vibrational characteristics (vibrational frequencies, infrared intensities and Raman activities) for the hydrogen-bonded system of Vitamin C (L-ascorbic acid) with five water molecules have been predicted using ab initio SCF/6-31G(d,p) calculations and DFT (BLYP) calculations with 6-31G(d,p) and 6-31++G(d,p) basis sets. The changes in the vibrational characteristics from free monomers to a complex have been calculated. The ab initio and BLYP calculations show that the complexation between Vitamin C and five water molecules leads to large red shifts of the stretching vibrations for the monomer bonds involved in the hydrogen bonding and very strong increase in their IR intensity. The predicted frequency shifts for the stretching vibrations from Vitamin C taking part in the hydrogen bonding are up to -508 cm(-1). The magnitude of the wavenumber shifts is indicative of relatively strong OH...H hydrogen-bonded interactions. In the same time the IR intensity and Raman activity of these vibrations increase upon complexation. The IR intensity increases dramatically (up to 12 times) and Raman activity increases up to three times. The ab initio and BLYP calculations show, that the symmetric OH vibrations of water molecules are more sensitive to the complexation. The hydrogen bonding leads to very large red shifts of these vibrations and very strong increase in their IR intensity. The asymmetric OH stretching vibrations of water, free from hydrogen bonding are less sensitive to the complexation than the hydrogen-bonded symmetric OH stretching vibrations. The increases of the IR intensities for these vibrations are lower and red shifts are negligible.

Ascorbic Acid↗
PubMed2006

Vibration and its effect on the respiratory system.

Vibration is a manual technique used widely to assist with the removal of pulmonary secretions. Little is known about how vibration is applied or its effect on the respiratory system. The purpose of this study was to describe mechanical consequences of vibration on the chest wall of a normal subject and the effects of vibration on expiratory flow rates and volumes. The effects of vibration were compared to other interventions of chest wall compression, chest wall oscillation, cough, huff from high lung volume, inspiration to total lung capacity with relaxed expiration, tidal breathing, and sham. Sixteen physiotherapists applied vibration and other interventions in a randomised order to the chest wall of a healthy adult female subject. The magnitude and direction of the force and the frequency of vibration were measured by an instrumented bed with seven load cells. Inductive plethsysmography measured the change in chest wall circumference with vibration. A heated pneumotachometer measured inspiratory and expiratory flow rates, which were integrated to provide volumes. Vibration was applied with a mean resultant force of 74.4 N (SD 47.1). The mean (SD) change in chest wall circumference and frequency of vibration were 0.8 cm (SD 0.4) and 5.5 Hz (SD 0.8) respectively. The mean peak expiratory flow rate was 0.97 l/s (SD 0.27). Peak expiratory flow rates with vibration were less than 20% of those achieved with cough or huff from high lung volume but greater than with chest wall compression, chest wall oscillation, relaxed expiration from total lung capacity, sham treatment or tidal breathing.

Adult↗
PubMed1999

Biological reaction to vibration--implications for sport.

In many situations of everyday life, vibration load occurs. Here whole body vibration in vehicles, such as boats, cars, helicopters and others as well as hand-transmitted vibration (motor saws etc.) can be named. As vibration is assumed liable to cause various threats to human health, a great number of studies in work science focussed on dose-effect relations and concepts for prevention. Although in many sports remarkable vibration load also occurs, there is very little research on the potential dangers and benefits of vibration stimuli, e.g. on whole body vibration and the implications for muscular activity and neuromuscular control in sport. In personal studies the damping behaviour and training effects under whole body vibration were investigated. Various research areas have been studied in order to approach the relevant topics: neuromuscular and posture control, energy metabolism in terms of oxygen uptake under whole body vibration and local concentration of phosphates by means of 31P-MRS. Furthermore the effects of a strength training under whole body vibration were analysed. The results underline that vibration is a neglected research topic in sport science from the preventive point of view as well as from the one focussing on the improvement of sport performance.

Energy Metabolism↗
PubMed2002

Effect of 4-min vertical whole body vibration on muscle performance and body balance: a randomized cross-over study.

The purpose of this randomized cross-over study was to investigate the effects of a 4-min long, 2-mm vertically-vibrating vibration-exercise on muscle performance and body balance in healthy subjects. Sixteen volunteers (eight men and women aged 18-35 years) underwent both the 4-min vibration- and sham-interventions in a randomized order on different days. Performance- and balance-tests (stability platform, grip strength, extension strength of lower extremities, tandem-walk, vertical jump and shuttle-run) were done 10 minutes before (baseline) and 2 and 60 minutes after the intervention. In addition, the effect of vibration on the surface electromyography (EMG) of soleus, vastus lateralis, gluteus medius, and paravertebralis muscles was investigated during the vibration. The 4-min vibration-loading did not induce any statistically significant change in the performance- or balance-tests at the 2- or 60-min tests. Interestingly, however, the mean power frequency of the EMG in the vastus lateralis and gluteus medius muscles decreased during the vibration-intervention, indicating muscle fatigue, particularly in the hip region. It was concluded that a 4-min long, 2-mm vertically-vibrating vibration-stimulus did not induce changes in the performance and balance tests. Future studies should focus on evaluating the effects of different kinds of vibration-regimens, as well as the long-term effects of vibration-training, on body balance and muscle performance, and, as a broader objective, on bone.

Adolescent↗
PubMed1999

Post-effects of long-term hand vibration on visuo-manual performance in a tracking task.

Movement precision and performance time were evaluated through a visuo-manual tracking task performed before and after 10-min hand vibration exposure. Constant displacement amplitude vibration of 0.2 and 0.3 mm peak to peak at 90, 150, 300 Hz were applied to the hand z-axis by a vertical handle. During exposure a grip force of 5% MVC was exerted for 5 s and then relaxed for 25 s while maintaining fingers-handle contact. The tracking task consisted in moving a ring (phi = 9 mm) attached to a thin rod held between the index finger and thumb along a zig-zagged wire (phi = 3.7 mm). Alterations of tracking errors (ring-wire contact) and tracking time were analysed as a function of the vibration parameters. The tasks were performed by ten healthy participants. Vibration induced a significant increase in tracking errors (ring-wire contact) and a significant decrease in tracking time. These impairments decayed with time after vibration exposure. The recovery period was > 5 min but < 10 min with the exception of 90 Hz vibration, for which recovery could be > 10 min. The number of tracking errors was neither influenced by vibration frequency nor by amplitude. The tracking time decreased as frequency increased and recovery was related to the displacement amplitude. The subjective rating of the performance on a visual analogue scale indicated that the subjects tended to perceive the task as being easier after vibration exposure. Vibration applied to the non-dominant hand while the participant performed the tracking task had no effect. These results show that vibration similar to hand-tool vibration affects precision and velocity control of visually guided hand movements. Furthermore, these performance decrements were not consciously perceived.

Adult↗
PubMedDate not supplied

Whole-body vibration exposure study in U.S. railroad locomotives--an ergonomic risk assessment.

Whole-body vibration exposure of locomotive engineers and the vibration attenuation of seats in 22 U.S. locomotives (built between 1959 and 2000) was studied during normal revenue service and following international measurement guidelines. Triaxial vibration measurements (duration mean 155 min, range 84-383 min) on the seat and on the floor were compared. In addition to the basic vibration evaluation (aw rms), the vector sum (av), the maximum transient vibration value (MTVV/aw), the vibration dose value (VDV/(aw T1/4)), and the vibration seat effective transmissibility factor (SEAT) were calculated. The power spectral densities are also reported. The mean basic vibration level (aw rms) was for the fore-aft axis x = 0.18 m/sec2, the lateral axis y = 0.28 m/sec2, and the vertical axis z = 0.32 m/sec2. The mean vector sum was 0.59 m/sec2 (range 0.27 to 1.44). The crest factors were generally at or above 9 in the horizontal and vertical axis. The mean MTVV/aw was 5.3 (x), 5.1 (y), and 4.8 (z), and the VDV/(aw T1/4) values ranged from 1.32 to 2.3 (x-axis), 1.33 to 1.7 (y-axis), and 1.38 to 1.86 (z-axis), generally indicating high levels of shocks. The mean seat transmissibility factor (SEAT) was 1.4 (x) and 1.2 (y) and 1 (z), demonstrating a general ineffectiveness of any of the seat suspension systems. In conclusion, these data indicate that locomotive rides are characterized by relatively high shock content (acceleration peaks) of the vibration signal in all directions. Locomotive vertical and lateral vibrations are similar, which appears to be characteristic for rail vehicles compared with many road/off-road vehicles. Tested locomotive cab seats currently in use (new or old) appear inadequate to reduce potentially harmful vibration and shocks transmitted to the seated operator, and older seats particularly lack basic ergonomic features regarding adjustability and postural support.

Ergonomics↗
PubMed2003

Vibration-induced ocular torsion and nystagmus after unilateral vestibular deafferentation.

Vibration is an excitatory stimulus for both vestibular and proprioceptive afferents. Vibration applied either to the skull or to the neck muscles of subjects after unilateral vestibular deafferentation induces nystagmus and a shift of the subjective visual horizontal. Previous studies have ascribed these effects to vibratory stimulation of neck muscle proprioceptors. Using scleral search coils, we recorded three-dimensional eye movements during unilateral 92 Hz vibration of the mastoid bone or of the sternocleidomastoid (SCM) muscle in 18 subjects with chronic unilateral vestibular deficits after vestibular neurectomy or neuro-labyrinthitis. Nine subjects had lost function of all three semicircular canals (SSCs) on one side, and the other nine had lost function of only the anterior and lateral SSCs. Vibration of the mastoid bone or of the SCM muscle on either side induced an ipsilesional tonic shift of torsional eye position of up to 6.5 degrees during visual fixation, as well as a nystagmus with horizontal, vertical and torsional components in darkness. Subjects who had lost function of all three SSCs on one side showed a larger shift in ocular torsion in response to SCM vibration than did subjects who had lost function of only two SSCs. The difference between ocular torsion produced by ipsilesional muscle or bone vibration was not significantly different from that produced by contralesional bone or muscle vibration. The vibration-induced nystagmus rotation axis tended to align with the pitch (y) axis of the head in subjects who had lost only anterior and lateral SSC function, and with the roll (x) axis of the head in subjects who had lost function of all three SSCs. We propose that the previously described vibration-induced shift of the subjective visual horizontal can be explained by the vibration-induced ocular torsion, and that the magnitude of ocular torsion is related to the extent of the unilateral vestibular deficit. While altered proprioceptive inputs from neck muscles might be important in the mechanism of vibration-induced ocular torsion and nystagmus after unilateral vestibular deafferentation, vibratory stimulation of vestibular receptors in the intact labyrinth also appears to have an important role.

Adult↗
PubMed2002

Effects of mechanical vibration on seed germination of Arabidopsis thaliana (L.) Heynh.

The effects of sinusoidal vibration (40-120 Hz, amplitude equal to or smaller than 0.42 mm) on seed germination of Arabidopsis thaliana were examined. When the amplitude of vibration was fixed at 0.42 mm, vibration with frequencies higher than 70 Hz increased the rate of seed germination. When the frequency of vibration was fixed at 100 Hz, vibration with amplitudes larger than 0.33 mm also increased the rate of germination. The increase in the rate of germination appeared dependent on acceleration calculated from the frequency and amplitude of vibration. Vibration with a maximum acceleration of 70 m s(-2) increased the rate of germination, but the promotive effects leveled off at higher accelerations. Vibration had little effect on seed germination in a starch-deficient mutant, pgm. Thus, the amyloplasts appeared to act as a susceptor that senses mechanical vibrations. No vibration-induced promotion of germination was seen in an ethylene-insensitive mutant, etr1, or in the wild type in the presence of aminoethoxyvinylglycine, an inhibitor of ethylene synthesis, suggesting that vibration increased the rate of seed germination through the action of ethylene.

Arabidopsis↗
PubMed1983

Effect of unilateral hind limb vibration on blood rheology in rabbits.

The right hind limb of twelve rabbits was vibrated for 15 min. Blood samples were taken before vibration from the right hind limb and after vibration from both the vibrated and the non-vibrated hind limbs. A control series was also run. Estimations were made of: haematocrit, whole-blood viscosity, erythrocyte xi potential and plasma fibrinogen concentration. There was no significant change in whole-blood viscosity in either limb after vibration. Erythrocyte xi potential was significantly increased (in both limbs) after vibration. Fibrinogen concentration did not change (in either limb) after vibration. Yield stress was significantly reduced (in both limbs) after vibration. High Shear Casson Law slope decreased in the vibrated limb after vibration.

Animals↗
PubMed1981

Effect of vibrating agonist or antagonist muscle of the reflex response to sinusoidal displacement of the human forearm.

1. The mechanical resistance of the human forearm to imposed sinusoidal movements has been determined. By means of a visual monitor, subjects maintained a steady force (typically 100 N) by flexing the elbow so as to pull with the wrist against an isometric force transducer. This was mounted upon a stretcher which displaced the forearm sinusoidally at frequencies of 7-11 Hz with a peak-to-peak amplitude of movement of about 1 mm. The average mechanical resistance over 10-40 sec of stretching was analysed into its vector components at the fundamental of the stretching frequency. Observations were made of both the normal resistance and that obtained while applying continuous vibration at 100 Hz to the tendon of either the biceps (agonist) or triceps (antagonist).2. In confirmation of Joyce, Rack & Ross (1974), at frequencies around 10 Hz the normal (unvibrated) response sometimes showed a component of ;negative viscosity' (force increasing during muscle shortening), rather than the simple ;positive viscosity' attributable to muscle visco-elasticity; this effect is attributable to the stretch reflex being appropriately delayed and of sufficient magnitude to over-ride the inherent properties of muscle. Vibration of either agonist or antagonist usually increased the extent of the ;negative viscosity' (negative quadrature component of force), as well as changing the ;elastic' stiffness of the arm (in-phase component of force).3. More commonly, the component of viscosity was initially positive. It was then normally reduced by vibration; that is, the vibration had (in formal terms) again added a component of negative viscosity.4. The vibration did not produce these effects by acting directly upon the contractile system of muscle to reduce its ;visco-elasticity'. On increasing the frequency of stretching the effect of vibration systematically shifted from being the addition of a negative viscosity, as above, to being the addition of a positive viscosity. These effects may all be attributed to an action of vibration on the stretch reflex, with the precise action of the reflex determined by the relation between the cycle time and the delays round the reflex pathway.5. In some experiments the activity of the flexor muscles was sampled by surface electromyograms from biceps and from brachioradialis; these were rectified, smoothed and averaged. For biceps, the absolute depth of e.m.g. modulation in relation to the cycle of stretching was sometimes, but not always, increased by vibration; but for brachioradialis the modulation was always reduced. Thus vibration cannot invariably produce its effects on the mechanical resistance of the arm by increasing the size (gain) of the stretch reflex. However, in all subjects the phase of the electromyographic modulation of both muscles was significantly delayed during vibration, whether of biceps or of triceps. In comparison with the normal, vibration introduced a phase lag on average of 18 degrees . In qualitative terms, this can be shown to explain the typical augmentation of ;negative viscosity'.6. The findings are discussed in relation to the genesis of tremor and to the reflex regulation of muscle contraction. They support the classical idea that afferent activity from the antagonist is as crucially implicated as that from the agonist.

Adult↗