Amplitude changes in the 8-12, 20-25, and 40 Hz oscillations in finger tremor.

OBJECTIVE: The study examined the amplitude and frequency modulation of the 8-12, 20-25, and 40 Hz frequencies of tremor to determine the degree to which increments of load affect the amplitude of these neural rhythms. METHODS: Finger acceleration from the middle phalange and electromyographic (EMG) activity of the extensor digitorum communis (EDC) muscle were recorded on 10 normal adult subjects. Two experiments are reported that manipulated loads ranging from 0 to 40 and 0 to 200 g that were attached to the distal portion of the outstretched middle phalange. RESULTS: There were 8-12, 20-25, and 40 Hz oscillations in the EMG recording but only the 8-12 and 20-25 Hz rhythms were present in the tremor and tremor-EMG coherence. Adding load to the finger reliably decreased the 20-25 Hz band of acceleration, reduced the relative power within the 20-25 Hz EMG band, increased the relative power of the 40 Hz band, but had no effect on the relative power within the 8-12 Hz EMG frequency band. The tremor-EMG coherence in the 8-12 and 40 Hz regions was independent of load, but was markedly reduced with load in the 20-25 Hz band. CONCLUSIONS: The 8-12, 20-25, and 40 Hz neural rhythms of physiological tremor have a stable frequency consistent with central oscillations. There is an increase in the relative power of the 40 Hz EMG band with force, but only the amplitude of the 20-25 Hz band is modulated by mechanical-reflex feedback.     

Characteristics of physiologic tremor in young and elderly adults.

OBJECTIVE: To determine the prevalence of tremor-related motor-unit entrainment in young and elderly adults. METHODS: Postural hand tremor in neurologically healthy people, age ranges 20-42 (59 women and 41 men) and 70-92 (50 women and 50 men) years, was studied with accelerometry and forearm electromyogram (EMG). Tremor was recorded with and without a 300 g load distributed over the distal half of the horizontally extended hand and was analyzed with Fourier spectral techniques. RESULTS: No tremor-related spectral peak was found in the EMG of 59 young and 65 elderly controls, and inconsistent EMG peaks were observed in 29 young and 21 elderly. Twelve young and 14 elderly people exhibited a well-defined tremor-coherent EMG peak with and without 300 g loading, and the frequency of the EMG peak decreased less than 1 Hz in 8 young and 7 elderly. The EMG peak frequency was 9-12 Hz during mass loading in all 8 young adults but in only two elderly adults. The other 5 elderly people had peak frequencies at 5-7 Hz. Age had no significant effect on the frequency and amplitude of the mechanical-resonant component of hand tremor. CONCLUSIONS: Approximately 8% of young and elderly adult controls have an EMG-acceleration pattern that is indistinguishable from mild essential tremor. SIGNIFICANCE: These results provide a framework for the interpretation of electrophysiologic studies in patients with suspected essential tremor.     

Subdivision of essential tremor patients according to physiologic characteristics

OBJECTIVE: To test the effect of different forearm postures on tremor characteristics. We hypothesized that changes in tremor characteristics in relation to posture can subclassify essential tremor (ET) patients. METHODS: Fourteen ET patients were tested while seated and holding a full cup in three well-defined arm postures: 15 cm above the armrest, elbow at 90 degrees flexion and when the cup was near the mouth. Hand movements were recorded using a triaxial accelerometer. Concomitantly, we recorded surface electromyogram (EMG) signals from the wrist extensor muscles. Hand acceleration and the primary tremor frequency from the power spectrum were calculated for each posture in two independent trials. The coherence at the primary tremor frequency between the EMG and the accelerometry signals was calculated. RESULTS: ET patients could be classified into two groups: ET-1 (nine patients) had position-dependent peak frequencies while ET-2 (five patients) had position-independent peak frequencies. The latter group had significantly higher coherencies between EMG and accelerometry of tremor and insignificantly higher tremor amplitudes compared with ET-1. In both groups tremor amplitude increased when the hand was near the mouth. CONCLUSION: The results suggest that ET patients can be classified based on the position dependence of frequency and other physiologic properties.     

临床不同分期帕金森病患者震颤特点分析

目的研究不同临床分期帕金森病(PD)患者震颤的特点。方法收集2014-11—2015-05首都医科大学附属北京天坛医院神内病房临床确诊或临床诊断PD可能性大的75例患者。患者至少具有一侧上肢静止性或姿势性震颤,按照Hoehn-Yahr分期分为1~1.5期、2~2.5期、3期3组,分别检测各组患者静止、姿势及持物1000g震颤的优势频率、振幅及震颤节律形式。结果 (1)震颤优势频率:静止、姿势状态下为4~7 Hz,1~1.5期与2~2.5期、3期组间比较差异无统计学意义(P0.05)。持物1000g状态下,1~1.5期与2~2.5期、3期组间比较差异有统计学意义(P0.01),1~1.5期患者优势频率除了4~7Hz之外,还有7~10Hz、无规律及无震颤。(2)震颤振幅:随病情进展有下降趋势。(3)震颤节律形式:临床不同分期患者静止、姿势及持物1000g时震颤的节律形式均以交替形式为主,但随着病情进展,非交替节律比例有增加趋势。结论 PD患者震颤属于中枢性震颤,静止、姿势震颤优势频率为4~7Hz,不随病情进展而变化。疾病初期,持物1000g状态震颤优势频率可能受外周调节而表现多样。震颤节律不受病情进展及姿势影响。随病情进展,非交替节律比例有增加趋势,震颤振幅有下降趋势。     

Scaling analysis of bilateral hand tremor movements in essential tremor patients

Recent evidence suggests that the dynamic-scaling behavior of the time-series of signals extracted from separate peaks of tremor spectra may reveal existence of multiple independent sources of tremor. Here, we have studied dynamic characteristics of the time-series of hand tremor movements in essential tremor (ET) patients using the detrended fluctuation analysis method. Hand accelerometry was recorded with (500 g) and without weight loading under postural conditions in 25 ET patients and 20 normal subjects. The time-series comprising peak-to-peak (PtP) intervals were extracted from regions around the first three main frequency components of power spectra (PwS) of the recorded tremors. The data were compared between the load and no-load condition on dominant (related to tremor severity) and non-dominant tremor side and with the normal (physiological) oscillations in healthy subjects. Our analysis shows that, in ET, the dynamic characteristics of the main frequency component of recorded tremors exhibit scaling behavior. Furthermore, they show that the two main components of ET tremor frequency spectra, otherwise indistinguishable without load, become significantly different after inertial loading and that they differ between the tremor sides (related to tremor severity). These results show that scaling, a time-domain analysis, helps revealing tremor features previously not revealed by frequency-domain analysis and suggest that distinct oscillatory central circuits may generate the tremor in ET patients.     

The dynamics of resting and postural tremor in Parkinson's disease.

OBJECTIVE: The study examines the time and frequency structure of Parkinson's disease tremor in patients that exhibit no clinical signs of tremor. METHODS: Eight mild to moderate Parkinson's disease and 8 matched control subjects maintained their limb in a constant position (30 s) under a postural finger, postural hand and resting tremor condition. Finger acceleration from the middle phalange, electromyographic (EMG) activity from extensor digitorum communis and flexor digitorum superficialis (FDS) were recorded. RESULTS: The data confirmed that there were no differences in the amount of limb motion and the modal frequency was around 9 Hz for each subject group. The time-dependent organization of tremor was more regular (lower approximate entropy [ApEn]) in Parkinson's disease. Both time and frequency analyses between the acceleration and extensor EMG signals demonstrate a reduction in the 20-25 Hz tremor component and an increase in the 8-12 Hz region of tremor. CONCLUSIONS: The results are discussed in relation to the proposal that increased regularity results from an increase in motor unit synchronization at 8-12 Hz and a reduction in the amplitude of the 20-25 Hz tremor component. The time and frequency structure of tremor may be useful in assessing individuals with Parkinson's disease.     

Differential effects of changes in mechanical limb properties on physiological and pathological tremor.

The effect of changes in mechanical limb properties on the peak frequency of different tremor forms was analysed. Wrist tremor was recorded by an accelerometer fixed to the dorsum of the hand and demodulated surface EMG was recorded from the wrist extensors, while the extended hand was loaded with successively heavier weights. Physiological tremor was characterised by flat EMG spectra and a gradual decrease in tremor peak frequency with increasing load, as would be expected from the properties of a passive spring-mass-system. Also the peak frequency of activated physiological tremor characterised by increased synchronisation between motor units decreased in frequency with increasing loads. EMG spectra showed clear peaks of activity at the various mechanically determined tremor frequencies. In contrast, in two pathological tremor forms, the postural tremor in Parkinsonian patients and essential tremor, peak frequency tended to remain stable irrespective of changes in load. The method therefore allows a simple distinction between physiological and these two pathological tremors.     

Regularity of force tremor in Parkinson's disease.

OBJECTIVES: The study examines the time-dependent structure of force tremor to investigate two hypotheses: (1), the regularity of tremor can help in discriminating normal aging from that of Parkinson's disease (PD); and (2), there is increased tremor regularity with increases in the severity of PD. METHODS: Eight young (21-29 years), eight elderly (68-80 years), and eight PD (68-80 years) subjects produced constant grip force at 5, 25 and 50% of their maximal voluntary contraction by squeezing two load cells with their index finger and thumb under a vision and no vision condition. Spectral analysis and approximate entropy (ApEn) were used, respectively, to analyze the frequency and time-dependent structure of tremor. RESULTS: The analyses showed that there were no differences in the amplitude and modal frequency of force tremor between groups. The ApEn was significantly lower in the PD group compared with the controls. For the PD group, the linear relations between the total scores taken from the Unified Parkinson's Disease Rating Scale-motor section and the dependent variables were r(2)=0.71 (P<0.01) for ApEn, r(2)=0.20 (P>0.05) for the modal frequency, and r(2)=0.23 (P>0.05) for the standard deviation. Surrogate analyses revealed that the time-dependent structure of tremor provided additional information beyond that of amplitude and modal frequency analyses. CONCLUSIONS: These findings indicate that tremor analyses should not be limited to just the frequency and amplitude of the oscillation, and that the time-dependent structure of tremor is useful in differentiating tremor in healthy people from those with PD. The hypothesis that more regular tremor in PD is due to a loss of multiple neuronal oscillators contributing to the tremor output is discussed.     

Amitriptyline enhances the central component of physiological tremor

OBJECTIVES: Postural tremor is a regularly encountered side effect of amitriptyline which can be strong enough to cause discontinuation of therapy. The aim was to characterise amitriptyline induced tremor and to assess if the central or reflex component of physiological tremor was modulated by this drug. METHODS: The postural hand tremor was measured in 15 patients on a clinical rating scale, by power spectral analysis of accelerometer, forearm flexor, and extensor EMG before and after the beginning of amitriptyline treatment for major depression or chronic pain syndrome. A coherence analysis between flexor and extensor muscles on the same side was performed. RESULTS: There was a clinically visible increase in postural tremor in a third of these patients. The tremor amplitude measured by accelerometer total power increased in every patient under amitriptyline. The EMG synchronisation as reflected by significant peaks in the flexor or extensor spectrum generally occurring at higher frequencies (8-18 Hz) than the accelerometric tremor frequencies (6-11 Hz) did not change. The number of patients with a significant flexor-extensor coherence in the 7-15 Hz range increased significantly under amitriptyline, the frequency bands of significant coherence corresponded with the EMG frequencies, and both were independent of changes to the hand's resonant frequency by added inertia. CONCLUSIONS: An enhancement of postural tremor under amitriptyline is a common phenomenon although not always clinically apparent. The increase in EMG-EMG coherence indicates an increased common central drive to the motor units as its frequency is not influenced by peripheral resonance or reflex mechanisms. This is the first account of a drug induced enhancement of the central component of physiological tremor.     

Single unit analysis of the human ventral thalamic nuclear group: correlation of thalamic "tremor cells" with the 3-6 Hz component of parkinsonian tremor

Although cells firing at tremor frequency, called "tremor cells" (Guiot et al., 1962), have often been recorded in the thalamus of parkinsonian patients, the extent of correlation between these spike trains and tremor has rarely been assessed quantitatively. This paper describes spectral cross-correlation functions calculated between the activity of "tremor cells" and electromyogram (EMG) signals recorded from several muscles in the contralateral arm. The power occurring in the spike train at tremor frequency was described in absolute terms by the spike autopower, and in relation to the average for all spectral components by the spike autopower signal-to-noise ratio (spike autopower SNR). The probability of significant cross-correlation between the thalamic spike train and EMG at tremor frequency was assessed by the coherence at tremor frequency. Autopower spectra of the activity of many of these cells exhibited a concentration of power at tremor frequency, indicated by spike autopower SNRs as high as 18. Of the EMG signals studied, signals recorded from finger flexors were most often significantly correlated at tremor frequency. Significant correlation between the thalamic spike train and finger flexor EMG activity was found in 34% of cells analyzed. Tremor frequency coherence was significantly correlated with tremor frequency spike autopower (r = 0.46, p less than 0.0001) and spike autopower SNR (r = 0.533, p less than 0.0001). The proportion of cells with a spike autopower SNR greater than 2 that were significantly correlated with finger flexor EMG activity was greater than that of cells with a spike autopower SNR of less than 2 (p less than 0.001; chi-square). Therefore, cells exhibiting a large amount of power at tremor frequency were those best correlated with EMG activity during tremor. Some of these cells may be involved in the generation of tremor.     

The impact of ventrolateral thalamotomy on high-frequency components of tremor.

OBJECTIVE: The present study assessed the impact of ventrolateral (VL) thalamotomy on the high-frequency components of tremor in patients with Parkinson's disease (PD). METHODS: Tremor was recorded prior to, and 7 days post-surgery using a laser displacement sensor. In addition, tremor was recorded in 10 age-matched patients with PD showing low amplitude tremor (named PD controls) and in 10 age-matched control subjects. Tremor recordings in patients were performed after a 12h withdrawal from anti-Parkinsonian drugs. Tremor characteristics such as amplitude, median power frequency (MPF) and power dispersion (a measure of concentration of power in the frequency domain) were assessed for all groups (i.e. controls, PD controls, pre-surgery and post-surgery). RESULTS: All tremor characteristics were similar between controls and PD controls. Tremor amplitude was significantly reduced post-surgery, to become statistically similar to that of controls and PD controls. However, MPF and power dispersion remained lower post-surgery, indicating that although there was normalization of tremor amplitude, tremor showed systematically slower oscillations after the surgical procedure. In order to eliminate amplitude as a possible confounding factor, epochs of post-surgical tremor (5s in duration) were paired for equal amplitude with 5s tremor epochs from matched controls. Results show once again that MPF and power dispersion were lower post-surgery compared to controls. In addition, when amplitude of power was compared within specific frequency bands (0-3.5, 3.5-7.5, 7.5-12.5, 12.5-16.5, 16.5-30 and 30-45), power regained normal values at frequencies below 7.5 Hz. Power within higher frequency bands was systematically lower, indicating that the surgical procedure had an impact on high-frequency components of tremor. CONCLUSIONS: Results from the present study showed that VL thalamotomy reduced tremor amplitude by selectively targeting centrally driven components of PD tremor. The high-frequency component of physiological tremor failed to emerge after amplitude normalization. SIGNIFICANCE: The thalamus should then be considered as an important component of the generation and/or propagation of high-frequency components of physiological tremor.     

震颤分析在帕金森病和原发性震颤鉴别诊断中的应用

目的 探讨震颤分析在帕金森病(parkinson's disease,PD)和原发性震颤(essential trem-or,ET)鉴别诊断中的应用价值.方法 选取2017年9月至2020年11月在福建省立金山医院门诊和住院确诊的PD患者27例(PD组)和ET患者23例(ET组),所有患者均至少有一侧上肢静止性或姿势性...     

Physiological tremor: does handedness make a difference?

The purpose of this study was to quantify with precision the characteristics of normal physiological tremor in the dominant and nondominant hand of a group of right-handed females. Twenty-two right-handed females (aged 20 to 40) gave informed consent to participate. Manual dominance determined with a questionnaire had to be equal to, or above 90%, for subjects to be accepted in the study. While the subjects kept their eyes closed, tremor was recorded simultaneously in the two extended index fingers during 40 s. Preceding the recording of postural tremor there was a period during which the subjects were asked to relax (control condition); to exert a pressure with the two index fingers on two strain gauges (static condition); or to press on and off with both index fingers on two strain gauges (dynamic condition). Each condition was recorded twice with two helium neon velocity lasers. This procedure was repeated after permutation of the lasers, thus a total of twelve sweeps of data were recorded. Seven parameters were used to analyze the tremor signal including measures of harmonicity, median frequency, proportional power in the 7-12 Hz range, amplitude, waveform morphology, entropy, and nonlinearity. Significant differences between hands were found in three parameters (median frequency, power in 7-12 Hz and entropy) but not in tremor amplitude (RMS). In addition, significant differences were found in five out of seven characteristics between conditions. These results suggest that neuromotor activity preceding the recording of tremor has a significant influence on tremor characteristics and that some of these characteristics are significantly different between the right and left hand in right-handed females.     

Postural and resting tremor in the upper limb.

OBJECTIVE: Tremor from multiple segments of the upper limb was recorded under postural and resting conditions. The aims of this study were to examine the nature of tremor within a single limb segment, intra- and inter-limb co-ordination of tremor, and the influence of cardiac mechanical events on physiological tremor. METHODS: Tremor was recorded from eight healthy adult subjects during a postural pointing task where the level of support for the upper limb segments was successively increased. The dynamics of tremor within a single segment were examined using power spectral, ApEn and amplitude analyses. Inter-segment tremor relations were determined using coherence and Cross-correlation analyses.RESULTS: Single segment analysis demonstrated that each (unsupported) limb segment contained two major frequency peaks (at 1-4 Hz and 8-12 Hz). Both peaks were still evident in the distal segments when the proximal segments were supported. External support of the more proximal limb segments also resulted in decreased finger tremor, but these changes were not simply additive over segments within a limb or equal across fingers. There were significant relations between adjacent proximal and distal limb segment pairs but no correlations between contralateral limb segments or between heart rate and limb tremor.CONCLUSIONS: These findings imply that: the low frequency component (1-4 Hz) of physiological tremor in the hand and finger could not be attributed to passive transmission of oscillations from the upper arm and forearm; and the contribution of proximal segments on tremor in the index finger tremor could not be predicted from mechanical principles alone. The minimization of finger tremor involved compensatory coupling of segments of the upper arm with particular emphasis upon active control of the wrist joint.     

Determinants of physiologic tremor in a large normal population.

OBJECTIVES: It has been well established that peripheral mechanical resonant factors as well as central mechanisms may play a role in the generation of physiological tremor (PT). Furthermore it has been postulated that subject's attributes like age and sex might influence PT. The present study was designed to quantify these influences on PT in a large normal population. METHODS: Physiological hand and finger tremors were measured in a group of 117 normal subjects between 20 and 94 years of age using accelerometry and surface EMG recordings from the forearm flexor and extensor muscles. The hand tremor was measured in a postural position with and without weight, and the finger tremor was recorded with the arm outstretched, forearm supported and hand supported. Hand volume and grip force were measured in each subject. RESULTS: Hand tremor frequency (mean 7.7 Hz) was reduced significantly by added inertia (mean 5.2 Hz) and it was negatively correlated with hand volume while there was no correlation with grip force. Finger tremor showed, subject to the arm position, maximally 3 and at least two distinct frequency bands (1-4, 6-11 and 15-30 Hz) reflecting the resonance frequencies of the whole arm, the hand and the finger, respectively. A significant EMG peak was found in 50-80% of the recordings. This EMG synchronization gave rise to a corresponding accelerometer peak or a significant EMG-EMG coherence in about one-third of the population indicating a central component of PT because its frequency was unaffected by mechanical changes in the periphery. We did not find a significant influence of age on the tremor frequency, while the sex of the subjects slightly but significantly changed the frequency range of hand tremor. Multiple partial correlations revealed, however, that the only direct influence on hand tremor frequency is the hand volume indicating that the influence of sex on hand tremor frequency is an indirect effect produced by the significantly larger hands of male subjects. CONCLUSIONS: In conclusion, the main determinants of PT are the mechanical properties of the oscillating limb. Apart from the dominating peripheral resonance mechanism we found indications of an additional central component of PT in about one-third of the normal population. There was no age dependence of tremor frequency and it was shown that the influence of the subjects' sex on tremor frequency only represents an indirect mechanical effect.     

The effect of contraction intensity on force fluctuations and motor unit entrainment in individuals with essential tremor

ObjectivesQuantify the effect of increasing contraction intensity on the amplitude of force fluctuations and neuromuscular and force tremor spectral power.MethodsTwenty-one subjects with essential tremor (ET) and 22 healthy controls applied isometric wrist extension contractions. Various sub-maximal contraction intensities were evaluated (5%-, 10%-, 20%- and 30%-MVC). Force fluctuations and wrist extensor neuromuscular activity were recorded using a load cell and electromyography (EMG).ResultsHigher contraction intensities were associated with larger amplitude force fluctuations and greater neuromuscular activation. However, spectral power associated with tremor peaks remained relatively constant (EMG) or decreased (force) with increasing contraction intensity.ConclusionsMotor unit entrainment associated with centrally generated oscillatory inputs does not increase with greater levels of muscle activation.SignificanceRather than influencing a constant proportion of active motor units, abnormal oscillatory drive influences a relative constant number of total motor units. When combined with the findings from our previous study on postural tremor, the present results provide preliminary evidence that abnormal stretch reflex activity may contribute to this motor unit entrainment.     

Evidence for a non-orthostatic origin of orthostatic tremor

OBJECTIVES: Orthostatic tremor was first described by Heilman in 1984. It usually occurs in the legs during stance and decreases markedly during sitting or walking. The aim of this study was to determine if orthostatic tremor is invariably associated with the orthostatic and weight bearing conditions in the arms and legs, and to investigate the features of orthostatic tremor under different levels of peripheral loading. METHODS: Multichannel surface EMG recordings were obtained under different conditions (body posture and peripheral loading) from the proximal arm and leg muscles of seven patients fulfilling the clinical and electrophysiological criteria of orthostatic tremor. RESULTS: In weight bearing positions (stance; weight bearing on the hands on all fours), all patients showed 13 Hz-16 Hz tremor activity, predominantly in the active limb. No tremor activity could be found in a supine position with muscles at rest. Isometric contraction of the limbs in the supine position led to synchronous 13 Hz-16 Hz rhythmic activity in five patients. No tremor was seen when the subjects were suspended in a harness with relaxed legs. Isometric contraction of the legs in this position produced tremor in two patients. A stepwise reduction of the body weight by a harness reduced the tremor activity. Additional loading (10 kg-20 kg) during stance led to an increase in tremor amplitude, but tremor frequency remained unchanged. CONCLUSIONS: Orthostatic tremor is invariably present during stance or other weight bearing positions. It is not, however, always associated with orthostasis. In at least some patients it can be classified as an orthostasis independent action tremor. The failure of peripheral loading to modify tremor frequency indicates that orthostatic tremor may have a central, rather than a peripheral, origin.     

Assessment of tremor: a new computerised tool for clinical practice.

The quantitative assessment of tremor represents the main difficulty in clinical evaluation. We developed a software package - T-Lab - for the biomechanical analysis of hyperkinetic movement disorders. This software can receive and elaborate data from an electronic device interfaced with a personal computer. The aim of our study was to verify the validity of the PC-aided method proposed in the routine assessment of tremor. We did this by a correlation (regression) analysis between the scores obtained by Webster's Amplitude Clinical Scale and the amplitude data by T-Lab and between EMG data and frequency measures of T-Lab. Forty-seven patients presenting with upper limb tremor were enrolled. Four series of data were obtained: two series for frequency and two for amplitude. A significant correlation between all sets of data compared was found. T-Lab represents a valid, objective and useful device of quantifying tremors in clinical practice.     

Using frequency domain characteristics to discriminate physiologic and parkinsonian tremors.

The manner in which characteristics of time series in the frequency domain can enhance discrimination between physiologic and parkinsonian tremor when tremor amplitude is low was examined. Rest tremor and postural tremor with and without visual feedback were recorded twice in the two hands of a group of patients with Parkinson's disease (PD) (n = 21) and a group of healthy control subjects (n = 30) using displacement laser systems. Recordings were analyzed quantitatively using amplitude and seven frequency domain characteristics. Postural tremor with no visual feedback allowed the most efficient discrimination between the two groups of subjects especially in velocity and acceleration (derived from displacement) and allowed identification of more patients with PD as separate from the range observed in the control group. Moreover, the frequency domain characteristics that were investigated identified the majority of the patients even when amplitude did not. After eliminating redundant (correlated) characteristics, it was found that the frequency composition of tremor in PD can be described adequately with four characteristics, which are the most reliable, independent, and discriminative elements for detecting early or subtle modifications in tremor. Also, a series of finger flexions was found to enhance physiologic tremor but not tremor in PD. Discrimination of low-amplitude tremor in PD from normal physiologic tremor is enhanced by examining the median frequency of oscillations, the concentration of power in the power spectrum, and the distribution of power in particular ranges. Tremor measurement should not be limited to acceleration data as some information is more visible in velocity time series.     

Postural tremor in X‐linked spinal and bulbar muscular atrophy

Postural tremor is a common initial symptom in spinal and bulbar muscular atrophy (SBMA), but its pathophysiological mechanisms remain to be studied. This study was undertaken to examine the physiological mechanisms underlying postural tremor in SBMA. For eight patients (36–63 years old) with genetically confirmed SBMA, we recorded surface electromyograms (EMGs) from the forearm muscles and hand movements with an accelerometer (ACC) while maintaining a posture with and without a weight load. We then analyzed their power spectra and coherence. The peak tremor frequency was 6–9 Hz in seven patients and 2–3 Hz in one patient. Oscillatory movements were associated with EMG activity in five patients, but not in three patients. Weight loads and postural changes affected the tremor frequency in all patients. Tremor was classified as “reflex tremor” in five patients and “mechanical tremor” in three patients. These results suggest that peripheral factors play important roles in tremor genesis in SBMA, although its clinical features resemble essential tremor. Subclinical sensory disturbance or a decrease of motor unit numbers might be candidates for such peripheral factors contributing to tremor genesis in SBMA. © 2009 Movement Disorder Society