Increased gravitational force reveals the mechanical,resonant nature of physiological tremor

Key points

• Physiological hand tremor has a clear peak between 6 and 12 Hz, which has been attributed to both neural and resonant causes.
• A reduction in tremor frequency produced by adding an inertial mass to the limb has usually been taken as a method to identify the resonant component.
• However, adding mass to a limb also inevitably increases the muscular force required to maintain the limb''s position against gravity, so ambiguous results have been reported.
• Here we measure hand tremor at different levels of gravitational field strength using a human centrifuge, thereby increasing the required muscular force to preserve limb position without changing the limb''s inertia.
• By comparing the effect of added mass (inertia + force) versus solely added force upon hand acceleration, we conclude that tremor frequency can be almost completely explained by a resonant mechanical system.

Abstract

Human physiological hand tremor has a resonant component. Proof of this is that its frequency can be modified by adding mass. However, adding mass also increases the load which must be supported. The necessary force requires muscular contraction which will change motor output and is likely to increase limb stiffness. The increased stiffness will partly offset the effect of the increased mass and this can lead to the erroneous conclusion that factors other than resonance are involved in determining tremor frequency. Using a human centrifuge to increase head‐to‐foot gravitational field strength, we were able to control for the increased effort by increasing force without changing mass. This revealed that the peak frequency of human hand tremor is 99% predictable on the basis of a resonant mechanism. We ask what, if anything, the peak frequency of physiological tremor can reveal about the operation of the nervous system.

Abbreviations

EMG

electromyography

g

gravitational field strength

RoG

radius of gyration

RF

resonant frequency

     

Independence between the amount and structure of variability at low force levels

The purpose of the current experiment was to investigate the amount (standard deviation (S.D.) and coefficient of variation (CV)) and structure (approximate entropy (ApEn)) of force variability at very low force levels. Participants produced isometric force output of index finger abduction at five levels (0.4, 0.8, 1.0, 2.0, and 4.0 N) with high and low visual feedback gain. The findings showed that: subjects scaled their force output to the targets; S.D. increased non-linearly with force level and decreased with visual gain; and CV decreased with force level as well as visual gain. ApEn of the force output did not change as a function of force level, although the high gain increased ApEn in contrast to low gain. It is proposed that the recruitment of additional motor units at very low force levels does not significantly alter the structure of the force output, although it does increase the magnitude of force and its amount of variability. Overall, the findings provide evidence that the amount and structure of motor variability can be influenced by separate control processes at low force levels.     

Frequency peaks of tremor, muscle vibration and electromyographic activity at 10 Hz, 20 Hz and 40 Hz during human finger muscle contraction may reflect rhythmicities of central neural firing

 The output from the central nervous system to muscles may be rhythmic in nature. Previous recordings investigating peripheral manifestations of such rhythmic activity are conflicting. This study attempts to resolve these conflicts by employing a novel arrangement to measure and correlate rhythms in tremor, electromyographic (EMG) activity and muscle vibration sounds during steady index finger abduction. An elastic attachment of the index finger to a strain gauge allowed a strong but relatively unfixed abducting contraction of the first dorsal interosseous (1DI). An accelerometer attached to the end of the finger recorded tremor, surface electrodes over 1DI recorded EMG signals and a heart-sounds monitor placed over 1DI recorded vibration. This arrangement enabled maintenance of a constant overall muscle contraction strength while still allowing measurement of the occurrence of tremulous movements of the finger. Ten normal subjects were studied with the index finger first extended at rest and then contracting 1DI to abduct the index finger against three different steady forces up to 50% of maximal voluntary contraction (MVC). Power spectral analysis of tremor, EMG activity and muscle vibration signals each revealed three frequency peaks occurring together at around 10 Hz, 20 Hz and 40 Hz. Coherence analysis showed that the same three peaks were present in the three signals. Phase analysis indicated a fixed time lag of tremor behind EMG of around 6.5 ms. This is compared with previous measurements of electromechanical delay. Other experiments indicated that the three peaks were of central nervous origin. Introducing mechanical perturbations or extra loading to the finger and making recordings under partial anaesthesia of the hand and forearm demonstrated preservation of all the peaks, suggesting that they did not originate from mechanical resonances or peripheral feedback loop resonances. It is concluded that, at least for a small hand muscle, there exist not one but a number of separate peak frequencies of oscillation during active contraction, and that these oscillations reflect synchronization of motor units at frequencies determined within the central nervous system. It is proposed that the multiple oscillations may be a means of frequency coding of motor commands. Received: 23 April 1996 / Accepted: 8 October 1996     

Parallel neuronal mechanisms underlying physiological force tremor in steady muscle contractions of humans

We present results from a study of the 6-to 12-Hz force tremor in relation to motor unit (MU) firing synchrony. Our experimental observations from 32 subjects, 321 contractions, and 427 recorded MUs reveal that tremor is accompanied by corresponding, in-phase MU rhythms that are additional to the ones at the MU intrinsic firing rates. This rhythmical synchrony is widespread and has a uniform strength that ranges from near zero to very large (MU/MU coherence > 0.50) in different contractions. Both the synchrony and the tremor are suppressed during ischemia, and this strongly suggests an involvement of spindle feedback in their generation. Furthermore, in the presence of substantial synchrony, the tremor enhancement, relative to the minimal tremor of ischemia, reflects the strength of the synchrony. Theoretical considerations based on these observations indicate that the muscle force signal is expected to show 1) frequency components in the band of the firing rates of the last-recruited, large MUs, and 2) because of the synchronized MU rhythms, an additional, distinct component with a size reflecting the strength of synchrony. Furthermore, synchronized MU rhythms, with frequencies in the 6- to 12-Hz range, are expected to arise from self-oscillations in the monosynaptic stretch reflex loop, due primarily to the associated muscle delay (several tens of milliseconds). Our results therefore reveal the parallel action of two tremor mechanisms, one of which involves MU synchrony probably caused by loop action. Clearly, the results on the synchrony and its impact also apply to other possible generators of tremor synchrony, including supraspinal ones.     

A methodological approach for the characterization of cardiac pacemaker immunity to low frequency interferences: case of 50 Hz, 60 Hz, 10 kHz and 25 kHz led disruptions

The goal of this work is the implementation of a metrological set up dedicated to the characterization of cardiac pacemakers' immunity at low frequency electromagnetic disruptions. The studied frequencies are 50 Hz, 60 Hz, 10 kHz and 25 kHz. The assessment methodology that is applied is in accord with the electromagnetic compatibility. The tests are carried out on single-chamber pacemakers. The first approach, consisting of the application of the interfering signal directly between the housing and the electrode, enabled us to point out the influence of a preset detection sensitivity on the signal levels induced between the terminal and the pacemaker's housing. These attempts are completed by in vitro tests using an electromagnetic model which allows one to take into account the interface which constitutes the human body, and thus to get closer to a real life situation. The visualization of the pacemaker generated signal illustrates the performance of the pacemaker according to different test configurations. It is an initial approach meant to create a base for the creation of a metrological protocol.     

A methodological approach for the characterization of cardiac pacemaker immunity to low frequency interferences: case of 50 Hz, 60 Hz, 10 kHz and 25 kHz led disruptions

The goal of this work is the implementation of a metrological set up dedicated to the characterization of cardiac pacemakers' immunity at low frequency electromagnetic disruptions. The studied frequencies are 50 Hz, 60 Hz, 10 kHz and 25 kHz. The assessment methodology that is applied is in accord with the electromagnetic compatibility. The tests are carried out on single-chamber pacemakers. The first approach, consisting of the application of the interfering signal directly between the housing and the electrode, enabled us to point out the influence of a preset detection sensitivity on the signal levels induced between the terminal and the pacemaker's housing. These attempts are completed by in vitro tests using an electromagnetic model which allows one to take into account the interface which constitutes the human body, and thus to get closer to a real life situation. The visualization of the pacemaker generated signal illustrates the performance of the pacemaker according to different test configurations. It is an initial approach meant to create a base for the creation of a metrological protocol.     

低频共振对大鼠生理功能影响研究

目的研究持续低频共振条件下大鼠血压、心率、呼吸频率和体温等生理指标变化特点,为生物体器官共振损伤及其反应研究奠定基础。方法 SD大鼠32只,随机分为假振动组、3 Hz频率振动组、6 Hz频率振动组和21 Hz频率振动组1,.5%戊巴比妥钠麻醉后,行股动脉插管,然后将大鼠以仰卧姿固定在振动台上,静置30 min以后,振动组施加振幅(p-p)为5 mm正弦振动,观测振动之前和振动1,3,5,10,30,601,20,1802,40,3003,60 min的血压、心率、呼吸频率、体温等生理指标,假振动组观测相应时间点的上述生理指标。结果持续低频共振会引起大鼠的血压、心率、呼吸频率在振动1 min时迅速升高,1～3 min达到最大值,之后逐渐下降3,0 min后低于正常值,而在振动初期共振不会对大鼠体温造成影响,体温在振动10 min之后才缓慢降低。在振动初期6 Hz频率共振对大鼠血压、心率、呼吸频率的影响比较大,而在后期3 Hz频率共振影响比较大。但这2个频率的共振对大鼠体温的影响是一致的。结论持续低频共振会引起大鼠的血压、心率、呼吸频率先迅速升高后逐渐降低,还会造成大鼠体温调节能力下降。     

The effects of load and force on tremor at the normal human elbow joint

1. Spontaneous flexion-extension tremor of the elbow was recorded in normal human subjects while they exerted a flexing force against different loads.2. Increases in the flexing force up to (1/2) or (3/4) of the subject's maximum were accompanied by increases in the amplitude of the tremor. A further increase to very large forces was associated with a decrease in tremor.3. When the subject flexed against a spring, the frequency of the tremor was related to the properties of the spring; with increasingly stiff springs the tremor frequency increased up to about 12 Hz. With the stiffest springs, however, the tremor became irregular and very small in amplitude.4. Weights added to the limb at the wrist reduced the frequency of tremor.5. By using appropriate combinations of spring and mass, the principal tremor frequency could be adjusted between 2 and 12 Hz, higher in some subjects. Within this range the frequency(2) was approximately proportional to the spring stiffness/mass, just as it is in a spring-mass system, though the forearm provided some of this mass, and some of the spring-like resistance was in the flexor muscles.6. When by suitable loading the tremor frequency was brought between 8 and 12 Hz, and the subject forcibly flexed his elbow, the tremor became regular and large in amplitude. This was a form of limited instability in the stretch reflex which arose because a powerful reflex response to extension of the elbow acted back on the flexor muscles after a delay.7. When the limb was so loaded that its natural frequency was outside this 8-12 Hz range, it behaved as a filter, and in response to any disturbing noise it oscillated mainly at frequencies close to the natural frequencies of either the mechanical system or the stretch reflex.8. This filtering function of the limb with its stretch reflex probably plays an important part in the control of normal movement and tremor.9. The large flexing forces that were here associated with vigorous tremor normally occur when a limb supports a large mass. Such a mass would give the mechanical system within the limb a low natural frequency; inconveniently large oscillations in the 8-12 Hz range would therefore seldom occur.     

Effects of aging on the regularity of physiological tremor

The purpose of this investigation was to determine the effects of healthy aging on the regularity of physiological tremor under rest and postural conditions. Additionally, we examined the contribution of mechanical reflex factors to age-related changes in postural physiological tremor. Tremor regularity, tremor-electromyographic (EMG) coherence, tremor amplitude, and tremor modal frequency were calculated for 4 age groups (young: 20-30 yr, young-old: 60-69 yr, old: 70-79 yr, and old-old: 80-94 yr) under resting and loaded postural conditions. There were 6 important findings from this study: 1) there were no differences between the young and elderly subjects for any of the dependent variables measured under the rest condition; 2) postural physiological tremor regularity was increased in the elderly; 3) postural physiological tremor-EMG coherence was also increased in the elderly, and there was a strong linear relation between peak tremor-EMG coherence in the 1- to 8-Hz frequency band and regularity of tremor. This relation was primarily driven by the increased magnitude of tremor-EMG coherence at 5.85 and 6.83 Hz; 4) enhanced mechanical reflex properties were not responsible for the increased magnitude of tremor-EMG coherence in the elderly subjects; 5) tremor amplitude was not different between the 4 age groups, but there was a slight decline in tremor modal frequency in the oldest age group in the unloaded condition; and 6) despite the increases in postural physiological tremor regularity and the magnitude of low frequency tremor-EMG coherence with age, there was a clear demarcation between healthy aging and previously published findings related to tremor pathology.     

Bilateral organization of physiological tremor in the upper limb

The bilateral patterns of physiological tremor in the upper limb of adults were examined under conditions where eight combinations of the elbow, wrist and index-finger joints of the right arm were braced using individually molded splints. The hypotheses tested were that: (a) coordination of upper-limb tremor involves (compensatory) coupling of intra- but not inter-limb segments, (b) splinting the respective joints of the right arm changes the organization of this synergy in both limbs, and (c) reducing the involvement of joint-space degrees of freedom through restricting their motion (by splinting) results in increased tremor in the distal segments. Under no-splinting conditions, significant relationships were only observed between adjacent (intra-limb) effector units, with the strength of the correlation increasing from proximal to distal. Splinting the right limb resulted in an increase in the strength and number of significant intra-limb relationships in both limbs. No inter-limb tremor relationships were found between any segment during this task, irrespective of the splinting condition. The frequency profile for the tremor in each limb segment showed two prominent frequency peaks (at 2–4?Hz and 8–12?Hz). A third, higher frequency peak (18–22?Hz) was observed in the index fingers only. Splinting the right limb produced a general increase in the amplitude and variability of tremor in the fingertip of both arms. This effect was particularly strong under conditions where the more proximal joints were splinted. The lack of any between-limb relationships, coupled with the fact that splinting one limb influenced both limbs, suggests that some form of linkage does exist between the limbs. It is unlikely that mechanical linkages can explain fully these relationships. It is proposed that the tremor observed in either limb represents the output of a central oscillatory mechanism(s), but that this output is subsequently independently filtered in a parallel fashion on its way to each respective limb. A common bilateral (compensatory) strategy is employed to minimize the tremor in either limb during this multiple-degrees-of-freedom task.     

Normal genetic variation at the low density lipoprotein receptor (LDLR) locus influences cholesterol levels in children

The population of Czechoslovakia is at high risk of premature atherosclerosis. Normal DNA polymorphism at the low density lipoprotein receptor (LDLR) locus detectable with the restriction enzyme PvuII was analyzed in Czech children with a high or a low concentration of total serum cholesterol. The PvuII restriction site was found significantly more often in the low cholesterol group than in the high cholesterol group. Thus, normal genetic variation at the LDLR locus contributes to the population variation in cholesterol in children in the population studied.     

Periodontal anaesthetisation decreases rhythmic synchrony between masseteric motor units at the frequency of jaw tremor

This study links the reduction in jaw physiological tremor around 8 Hz following periodontal mechanoreceptor (PMR) anaesthetisation to changes in coherence between masseteric motor unit discharges. We have recorded single motor unit activity from two separate sites in the right masseter muscle during a low level tonic contraction, both prior to and during anaesthetisation of the peri-incisal PMRs. Anaesthetisation of PMRs decreased coherent activity between motor units circa 8 Hz, and decreased synchrony between the same motor unit pairs. It is proposed that tremor-generating inputs that cause rhythmic synchronisation of masseteric motor units arise from, or are amplified by the PMRs.     

Oscillation in the stretch reflex arc and the origin of the rhythmical, 8-12 c/s component of physiological tremor

1. A brief downward, stepwise displacement applied to the outstretched finger gives rise to a train of approximately sinusoidal movements of it, lasting often more than 1 sec. The frequency of these waves is the same, in any one subject, as that of physiological tremor.2. The oscillations are regular in form, and bear a constant phase relation to the applied displacement; they can be summated using an averaging computer (Biomac 1000) triggered by the mechanical stimulus.3. The oscillations are altered in the same way as is physiological tremor by a number of factors. Cooling the arm before recording lowers the frequency, warming raises it, while the application of an arterial cuff decreases the amplitude and tends to elevate the frequency. These factors have effects of similar magnitude on both the oscillations and the tremor.It thus appears highly likely that the waves produced by a mechanical input and physiological tremor waves are due to the same process, namely oscillation in an underdamped servo-system.4. The oscillation is not due simply to the mechanical, die-away resonance of the finger, because bursts of muscle action potentials can be recorded in phase with the finger movements both in the wave train evoked by the mechanical displacement and during normal tremor.5. It is concluded that physiological tremor in the 8-12 c/s band is due to oscillation in the stretch reflex servo-loop.     

Frequency dependence of the cardiac threshold to alternating current between 10 Hz and 160 Hz

It is still unclear what fundamental criteria influence the ability of alternating current (AC) to induce ventricular fibrillation (VF) in vivo. As the VF threshold has a bowl-shaped relationship with frequency (showing a minimum threshold at some frequency), similar to the nervous system, one proposed model has assumed that the mechanisms underlying AC stimulation of nerves are at work for VF induction. More recent work has suggested a second approach, whereby a simple RC-like model is sufficient to understand the cardiac AC stimulation threshold's frequency dependence, suggesting that some unarticulated mechanism is at work for VF. The paper directly tests these two models. In 12 intact dogs and 20 intact guinea pigs, DC pulses were used to stimulate AC square and AC sine waves at 10, 20, 40, 80 and 160 Hz. All electrodes were endocardial, with the return electrode being on a paw or thorax. It was found that, for square and sine wave stimulation in both dogs and guinea pigs, the stimulation threshold increased monotonically with frequency from 10 Hz up to 160 Hz (p<0.01 for dogs and guinea pigs). Between 80 and 160 Hz, the AC stimulation threshold doubled, exactly as predicted by an RC model. It was concluded that the AC stimulation threshold is not bowl-shaped and is best understood with an RC model. As the VF threshold does exhibit a bowl-shape with frequency, as opposed to the stimulation threshold which does not, the VF induction frequency dependence must have different origins.     

Exploring the functional significance of physiological tremor: A biospectroscopic approach

The functional significance of physiological tremor - the high frequency (8-12 Hz), low amplitude oscillation that occurs during the maintenance of steady limb postures - is not known. Often tremor, perhaps because of its pathological manifestations, is considered a source of unwanted noise in the system, something to be damped out or controlled. An examination of the phase relationship between tremor and rapid voluntary finger movement in normal subjects suggests a very different view. In four experiments in which tremor displacement and accompanying electromyographic activity were simultaneously monitored, we show a clear and systematic relationship between tremor and movement initiation. Empirically obtained frequency distributions of tremor peak-to-movement initiation time were most closely aligned to a probability density function (derived via numerical integration techniques) that assumed movements were initiated when the muscle-joint system possessed peak momentum. This relationship - evaluated by Chi-square goodness-of-fit tests - was evident regardless of whether the movements were self-paced (Experiments 1 and 2) or in response to an auditory reaction time signal (Experiments 3 and 4). The addition of a load to the finger in Experiments 2 and 4, though tending to reduce tremor frequency, did not prove disruptive, nor did a fractionated reaction time analysis reveal any significant inertial contribution to the maintenance of the phase relationship. These data are consistent with an emerging view that the motor control system is sensitive to its own dynamics, and suggest that under certain conditions normal physiological tremor is a potentially exploitable oscillation intrinsic to the motor system.     

Age-related changes in the frequency profile of children's finger tremor

Little consensus exists as to the age-related pattern of change in the frequency characteristics of postural tremor through childhood. We investigated postural finger tremor of children (6 and 10 years) and adults (18-22 years) using accelerometers under dual and single limb conditions (10s trials). The postural tremor of the children exhibited proportionally more power below 10 Hz and less power above 20 Hz than that of the adults. It also showed a significantly lower peak frequency and lower proportion of power at the peak frequency than the adults in the 15-30 Hz frequency band but did not differ significantly from the adults in peak frequency or proportion of power at the peak frequency in the 5-15 Hz frequency band. The greater relative contribution of fast time scales over the 1-30 Hz frequency band in the organization of the postural tremor of the adults in comparison to the children may be a contributing factor to adult's typically observed reduced motor skill performance variability.