We investigated the control of movement in 12 patients with Parkinson's disease (PD) after they received surgically implanted high‐frequency stimulating electrodes in the subthalamic nucleus (STN). The experiment studied ankle strength, movement velocity, and the associated electromyographic patterns in PD patients, six of whom had tremor at the ankle. The patients were studied off treatment, ON STN deep brain stimulation (DBS), on medication, and on medication plus STN DBS. Twelve matched control subjects were also examined. Medication alone and STN DBS alone increased patients' ankle strength, ankle velocity, agonist muscle burst amplitude, and agonist burst duration, while reducing the number of agonist bursts during movement. These findings were similar for PD patients with and without tremor. The combination of medication plus STN DBS normalized maximal strength at the ankle joint, but ankle movement velocity and electromyographic patterns were not normalized. The findings are the first to demonstrate that STN DBS and medication increase strength and movement velocity at the ankle joint. © 2005 Movement Disorder Society

The cerebellum, parietal cortex, and premotor cortex are integral to visuomotor processing. The parameters of visual information that modulate their role in visuomotor control are less clear. From motor psychophysics, the relation between the frequency of visual feedback and force variability has been identified as nonlinear. Thus we hypothesized that visual feedback frequency will differentially modulate the neural activation in the cerebellum, parietal cortex, and premotor cortex related to visuomotor processing. We used functional magnetic resonance imaging at 3 Tesla to examine visually guided grip force control under frequent and infrequent visual feedback conditions. Control conditions with intermittent visual feedback alone and a control force condition without visual feedback were examined. As expected, force variability was reduced in the frequent compared with the infrequent condition. Three novel findings were identified. First, infrequent (0.4 Hz) visual feedback did not result in visuomotor activation in lateral cerebellum (lobule VI/ Crus I), whereas frequent (25 Hz) intermittent visual feedback did. This is in contrast to the anterior intermediate cerebellum (lobule V/ VI), which was consistently active across all force conditions compared with rest. Second, confirming previous observations, the parietal and premotor cortices were active during grip force with frequent visual feedback. The novel finding was that the parietal and premotor cortex were also active during grip force with infrequent visual feedback. Third, right inferior parietal lobule, dorsal premotor cortex, and ventral premotor cortex had greater activation in the frequent compared with the infrequent grip force condition. These findings demonstrate that the frequency of visual information reduces motor error and differentially modulates the neural activation related to visuomotor processing in the cerebellum, parietal cortex, and premotor cortex.

Several measures of isometric contractions reflect motor impairments in subjects with Parkinson's disease (PD), including long relaxation times and greater power in the 5 to 15 Hz electromyographic (EMG) bandwidth during the holding phase of contractions compared to those measures in healthy subjects. We sought to determine whether the impairments observed in subjects with PD in the performance of isometric contractions reflect disease severity. Twenty-eight subjects with PD performed isometric contractions at a torque level equal to 50% of the torque generated during a maximum voluntary contraction while off medication. Subjects were instructed to reach the target torque as fast as possible upon hearing the auditory "go" signal and to relax their muscles when a second auditory cue signaled the end of the hold phase. There was a significant positive correlation between torque relaxation time and Unified Parkinson's Disease Rating Scale (UPDRS)-Motor score. A significant positive correlation was also observed between the proportion of power in the 5 to 15 Hz frequency bin of the agonist EMG signal and UPDRS-Motor score, and a significant negative correlation between the proportion of power in the 15 to 30 Hz frequency bin and UPDRS-Motor score. These measures provide objective quantification of the severity of motor impairment that can be used to investigate the efficacy of different interventions in individuals with PD.

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.

The current study examined the influence of aging on the oscillatory activity of a population of motor units during rhythmical force production. Previously, it has been shown that aging humans have greater low-frequency and less high-frequency electromyographic (EMG) activity during constant and slow ramp force contractions. We hypothesized that more rapid force contractions would reverse the established finding of reduced high- and greater low-frequency EMG activity to greater high- and reduced low-frequency EMG activity in older adults. Intramuscular EMG activity and effector force were recorded while 45 human subjects (20-31 and 60-88 yr of age) rhythmically produced force at four distinct frequencies (1-4 Hz) and two force levels (5 and 25% maximal voluntary contraction). Spectral and coherence analyses were performed on the force output and EMG activity. In the 3- and 4-Hz targets, the older adults had greater 35- to 50-Hz and reduced 0- to 5-Hz EMG activity compared with the young adults. There was greater EMG-force coherence in the 0- to 5-Hz bandwidth for the young subjects. No systematic age difference in the phase relationship between the EMG and force signals were found. Higher frequency force contractions reversed the previously established aging differences in the relative contribution of low- and high-frequency EMG activity. Thus the frequency properties of the task goals channel the relative contribution of low and high EMG activity. Furthermore, it is proposed that aging humans lose the adaptive capability to coordinate the excitatory and inhibitory activity of multiple neural oscillators.

Deep brain stimulation (DBS) of the subthalamic nucleus (STN) and antiparkinsonian medication have proved to be effective treatments for tremor in Parkinson's disease. To date it is not known how and to what extent STN DBS alone and in combination with antiparkinsonian medication alters the pathophysiology of resting and postural tremor in idiopathic Parkinson's disease. The purpose of this study was to examine the effects of STN DBS and antiparkinsonian medication on the neurophysiological characteristics of resting and postural hand tremor in Parkinson's disease. Resting and postural hand tremor were recorded using accelerometry and surface electromyography (EMG) from 10 Parkinson's disease patients and 10 matched control subjects. The Parkinson's disease subjects were examined under four treatment conditions: (i) off treatment; (ii) STN DBS; (iii) medication; and (iv) medication plus STN DBS. The amplitude, EMG frequency, regularity, and 1-8 Hz tremor-EMG coherence were analysed. Both STN DBS and medication reduced the amplitude, regularity and tremor-EMG coherence, and increased the EMG frequency of resting and postural tremor in Parkinson's disease. STN DBS was more effective than medication in reducing the amplitude and increasing the frequency of resting and postural tremor to healthy physiological levels. These findings provide strong evidence that effective STN DBS normalizes the amplitude and frequency of tremor. The findings suggest that neural activity in the STN is an important modulator of the neural network(s) responsible for both resting and postural tremor genesis in Parkinson's disease.

Glover postulates that the inferior parietal lobule (IPL), along with the frontal lobes and basal ganglia, mediates planning, while the superior parietal lobule (SPL), coupled with motor processes in the cerebellum, regulates the control process. We demonstrate that the control process extends beyond the cerebellum and SPL into regions hypothesized to represent planning.

Deep brain stimulation (DBS) of the subthalamic nucleus (STN) and antiparkinsonian medication (Meds) have proved to be effective therapies for treating bradykinesia in Parkinson's disease. However, it is not currently known how or to what extent STN stimulation alters the control signals to agonist and antagonist muscles to change movement speed. Our objective was to investigate movement speed along with the amplitude and temporal features of EMG activity to determine how and to what extent these parameters are changed by DBS and medication. Nine patients with Parkinson's disease were studied following neurosurgery that implanted high-frequency stimulating electrodes in the STN. The experiments for the patients were performed in each of four treatment conditions: (i) OFF treatment; (ii) STN DBS; (iii) Meds; and (iv) Meds plus STN DBS. Also, a group of age- and gender-matched control subjects were examined. Medication and DBS had similar effects in that both treatments increased movement speed, increased the amplitude of the first agonist burst, increased burst duration, reduced the number of agonist bursts, reduced cocontraction, increased the size of the antagonist EMG, and reduced the centroid time of the antagonist EMG. When DBS and medication were combined, only temporal measures of burst duration and the number of agonist bursts were different from the medication alone condition. There was a positive association between the level of bradykinesia OFF treatment and the level of bradykinesia following DBS and medication. The movement speed of neurologically normal control subjects' was over 40% higher during both flexion and extension movements when compared with the patients during Meds plus STN DBS. The changes in the muscle activation patterns provide a mechanism of action for the pharmacological and surgical interventions used to treat bradykinesia in Parkinson's disease. However, despite the success of medication and DBS at improving bradykinesia in patients with Parkinson's disease, patients' movement speed was not restored to normal due to limitations in the amplitude and temporal scaling of the agonist and antagonist bursting pattern. These findings suggest a link between basal ganglia function in scaling both the amplitude and temporal parameters of the input to the motor neuron pool.

LETTERS TO THE EDITORAge-related changes in complexity depend on task dynamicsDavid E. Vaillancourt, Jacob J. Sosnoff, and Karl M. NewellDavid E. Vaillancourt, Jacob J. Sosnoff, and Karl M. NewellPublished Online:01 Jul 2004https://doi.org/10.1152/japplphysiol.00244.2004MoreSectionsPDF (42 KB)Download PDF ToolsExport citationAdd to favoritesGet permissionsTrack citations ShareShare onFacebookTwitterLinkedInWeChat The following is the abstract of the article discussed in the subsequent letter:The present study examined the time and frequency structure of force output in adult humans to determine whether the changes in complexity with age are dependent on external task demands. Healthy young (20–24 yr), old (60–69 yr), and older-old (75–90 yr) humans produced isometric force contractions to constant and sine wave targets that also varied in force level. First, force variability on each force task increased with advancing age. Second, both time and frequency analysis showed that the structure of the force output in the old and older-old adults was less complex in the constant-force level task and more complex in the sine wave force task. Third, the alterations in force output with aging were primarily due to low-frequency bands <4 Hz. These results support the postulation that the observed increase or decrease in physiological complexity with aging is influenced by the relatively fast time scale of external task demands (Vaillancourt DE and Newell KM. Neurobiol Aging 23: 1–11, 2002).Age-related changes in complexity depend on task dynamicsTo the Editor: The changes in complexity most frequently cited in both aging and diseased systems have physiological output consistent with a loss of complexity (3). However, across the cardiovascular, nervous, and endocrine systems, both increases and decreases in complexity with aging and disease occur, collectively termed the bidirectional complexity hypothesis (7). Our recent study tested the bidirectional complexity hypothesis in young (21–35 yr), old (60–75 yr), and older-old (75–90 yr) participants during constant and sinusoidal isometric force production tasks (6). The findings were consistent across all dynamical metrics in showing bidirectional changes in complexity with age that were dependent on the intrinsic dynamics of the experimental task.The bidirectional complexity hypothesis has been criticized for relying on entropy-based metrics that ignore sequential properties across multiple time scales, thereby misinterpreting the changes of reduced complexity with aging and disease (1, 2). The critique is based on the notion that greater entropy values are not always associated with an increase in physiological complexity. For instance, a white Gaussian noise time series returns a high entropy value even though there is no underlying dynamical structure in the time series. An aggregate analysis method, multiscale entropy analysis (MSE), has been suggested to overcome the limitations of single-scale entropy measures by calculating entropy over different variations of the same data set following a moving average window that varies in length or scale (1).The purpose of this commentary is to reexamine our previous findings (6) using the MSE method to determine whether the finding of age-related bidirectional changes in complexity was due to biological variation or because of the use of dynamical measures that do not account for scales other than the shortest one. Figure 1 depicts the results from the MSE analysis, in which sample entropy (SampEn) (5) was calculated over different time scales during constant (Fig. 1A) and sinewave (Fig. 1B) force production tasks. ANOVA statistics were used to examine the SampEn (m = 2; r = 0.2) changes across age (3-between subject factor), task (2-repeated factor), and time scale (20-repeated factor) with a Type I α-level set to 0.05. In both constant and sinewave force production tasks, the SampEn values increase from between 0.25 and 0.50 at the shortest time scale, to greater than 1.25 at the highest MSE time scale (time scale effect, P < 0.05). Although the directionality of age differences in SampEn values was maintained across all time scales, there was a bidirectional change in SampEn with age, depending on the force production task (age by task interaction, P < 0.05). We further explored the age-by-task interaction by conducting two separate two-way ANOVAs for age and time scale. The constant force task (see Fig. 1A) supports the loss of complexity viewpoint, that is, that SampEn was reduced with aging at each time scale (age effect, P < 0.05). In contrast, Fig. 1B demonstrates the opposite finding during the sinewave task, in which the older-old adults had greater SampEn values compared with the young subjects (age effect, P < 0.05).Fig. 1.Multiscale entropy analysis (MSE) across time scales. The dependent variable, sample entropy (SampEn), was calculated on the force output data set across each time scale. We report sample entropy instead of approximate entropy because sample entropy is robust across a different number of time series data points. However, approximate entropy (4) did produce similar findings as sample entropy (5). Each data point represents the average across sample entropy values for 10 subjects at each time scale in the young (21–35 yr; •), old (60–75 yr; ○), and older-old (75–90 yr; ▾) groups. The task required participants to abduct their index finger against the compressive force load cell, matching the visually determined constant (A) and 1-Hz sinewave (B) force targets.Download figureDownload PowerPointThus the MSE analysis produced a consistent pattern at short time scales as in our previous work (6), and the findings also demonstrated a similar pattern of age-related changes in complexity at long time scales. The findings demonstrate that the bidirectional change in complexity with advanced age is not an artifact of the dynamical measures used (2). In conclusion, the age-by-task interaction supports the primary thesis of the bidirectional complexity hypothesis that aging physiological systems are deficit in their ability to adapt to environmental stressors and directional changes in complexity depend on the nature of the task and dynamics of the specific physiological system (7).GRANTSThis work was supported by grants from the National Institutes of Health (F32-NS-44727, T32-AG-00048, and R01-HD-46918).We thank Dr. Randall Moorman and Dr. Douglas Lake for supplying the algorithm to calculate sample entropy.REFERENCES1 Costa M, Goldberger AL, and Peng CK. Multiscale entropy analysis of complex physiologic time series. Physiol Rev Lett 89: 068102, 2002.Crossref | PubMed | ISI | Google Scholar2 Kyriazis M. Practical applications of chaos theory to the modulation of human ageing: nature prefers chaos to regularity. Biogerontology 4: 75–90, 2003.Crossref | ISI | Google Scholar3 Lipsitz LA and Goldberger AL. Loss of “complexity” and aging. Potential applications of fractals and chaos theory to senescence. JAMA 267: 1806–1809, 1992.Crossref | PubMed | ISI | Google Scholar4 Pincus SM. Approximate entropy as a measure of system complexity. Proc Natl Acad Sci USA 88: 2297–2301, 1991.Crossref | PubMed | ISI | Google Scholar5 Richman JS and Moorman JR. Physiological time-series analysis using approximate entropy and sample entropy. Am J Physiol Heart Circ Physiol 278: H2039–H2049, 2000.Link | ISI | Google Scholar6 Vaillancourt DE and Newell KM. Aging and the time and frequency structure of force output variability. J Appl Physiol 94: 903–912, 2003.Link | ISI | Google Scholar7 Vaillancourt DE and Newell KM. Changing complexity in human behavior and physiology through aging and disease. Neurobiol Aging 23: 1–11, 2002.Crossref | PubMed | ISI | Google Scholar Download PDF Previous Back to Top FiguresReferencesRelatedInformation Cited ByNovel Behavioral and Neural Evidences for Age-Related changes in Force complexity17 February 2018 | The Journals of Gerontology: Series A, Vol. 73, No. 8Does motor expertise facilitate amplitude differentiation of lower limb-movements in an asymmetrical bipedal coordination task?Human Movement Science, Vol. 59Motor output complexity in Parkinson’s disease during quiet standing and walking: Analysis of short-term correlations using the entropic half-lifeHuman Movement Science, Vol. 58Gait stability, variability and complexity on inclined surfacesJournal of Biomechanics, Vol. 54Complexity Variability Assessment of Nonlinear Time-Varying Cardiovascular Control20 February 2017 | Scientific Reports, Vol. 7, No. 1Variations in task constraints shape emergent performance outcomes and complexity levels in balancing2 February 2016 | Experimental Brain Research, Vol. 234, No. 6Postural control deficits identify lingering post-concussion neurological deficitsJournal of Sport and Health Science, Vol. 5, No. 1Comparing dynamical systems concepts and techniques for biomechanical analysisJournal of Sport and Health Science, Vol. 5, No. 1Multi-Scale Entropy Analysis of Body Sway for Investigating Balance Ability During Exergame Play Under Different Parameter Settings4 November 2015 | Entropy, Vol. 17, No. 12The effect of hemodialysis session on postural strategies in older end-stage renal disease patients28 April 2015 | Hemodialysis International, Vol. 19, No. 4Respiration and heart rate complexity: Effects of age and gender assessed by band-limited transfer entropyRespiratory Physiology & Neurobiology, Vol. 189, No. 1Quality and structure of variability in children during motor development: A systematic reviewResearch in Developmental Disabilities, Vol. 34, No. 9External attentional focus enhances movement automatization: A comprehensive test of the constrained action hypothesisHuman Movement Science, Vol. 32, No. 4Effects of Practice on Magnitude and Structure of Force Variability during Periodic Unimanual Isometric Force Production1 December 2012 | Perceptual and Motor Skills, Vol. 115, No. 3Applying Multiscale Entropy to the Complexity Analysis of Rainfall-Runoff Relationships16 May 2012 | Entropy, Vol. 14, No. 5Human movement variability, nonlinear dynamics, and pathology: Is there a connection?Human Movement Science, Vol. 30, No. 5Changes in muscle fascicles of tibialis anterior during anisometric contractions are not associated with motor-output variability of the ankle dorsiflexors in young and old adults17 August 2010 | European Journal of Applied Physiology, Vol. 110, No. 6Unexpected heart rate variability complexity in the aging process of arrhythmic subjectsCommunications in Nonlinear Science and Numerical Simulation, Vol. 15, No. 7Adaptation to selective visual scaling of short time scale processes in isometric forceNeuroscience Letters, Vol. 469, No. 1Development of dynamic stability in children's rhythmic movement6 July 2009 | Developmental Psychobiology, Vol. 51, No. 6Complexity of force output during static exercise in individuals with Down syndromeKevin S. 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The purpose of this study was to examine the modulation of the motor neuronal pool as a function of task dynamics. Specifically, we investigated the effects of task frequency on the single motor unit discharge pattern, electromyogram (EMG) activity and effector force output. Myoelectric activity and effector force were recorded while young adults isometrically abducted their first dorsal interosseus at five sinusoidal targets (0.5Hz, 1Hz, 2Hz, 3Hz and 4Hz) and at two force levels (5% and 25% maximum voluntary contraction (MVC)). Individual motor unit spike trains were isolated from the EMG. Auto-spectral and coherence analyses were performed on the force output, EMG and motor unit spike trains. The frequency of maximal coherence between the EMG and force output closely corresponded to the target frequency in all conditions. There was a broadband distribution of power with multiple peaks in the EMG and motor unit spectrums in the 0.5Hz and 1Hz targets. However, the EMG and motor unit spectrums in the 2Hz, 3Hz and 4Hz targets were characterized by an increasingly narrower band of activity with one dominant peak that closely corresponded to the target. There is high coherence between EMG output and target force frequency, but the relative contribution of the fast and slow neuromuscular bands are differentially influenced by the task frequency. The rhythmical organization of neuromuscular output in the 0.5Hz task is relatively broadband and similar to that shown previously for constant level force output. The frequency structure of neuromuscular organization becomes increasingly more narrowband as the frequency of the target increases (2–4Hz). The modulation of the motor neuronal pool is adaptive and depends on the relative contribution of feedback and feedforward control processes, which are driven by the task demands.

The basal ganglia, motor cortex, and cerebellum have been implicated as a circuit that codes for movement velocity. Since movement velocity covaries with the magnitude of force exerted and previous studies have shown that similar regions scale in activation for velocity and force, the scaling of neuronal activity with movement velocity could be due to the force exerted. The present study implemented a parametric functional magnetic resonance imaging (fMRI) design to determine which brain regions directly scale with the rate of change of force production, independent of the magnitude of force exerted. Nine healthy adults produced force with their right middle finger and thumb at 25% of their maximal voluntary contraction across four conditions: (1) fast pulse, (2) fast hold, (3) medium hold, and (4) slow hold. There were three primary findings: (i) the activation volume in multiple regions increased with the duration of the force contraction, (ii) only the activation volume in the bilateral internal globus pallidus and left subthalamic nucleus parametrically scaled with the rate of change of force production, and (iii) there was an inverse relation between the activation volume in the subthalamic nucleus and internal globus pallidus with the rate of change of force production. The current findings are the first to have used neuroimaging techniques in humans to segregate the functional anatomy of the internal globus pallidus from external globus pallidus, distinguish functional activation in the globus pallidus from the putamen, and demonstrate task-dependent scaling in the subthalamic nucleus and internal globus pallidus. We conclude that fast, ballistic force production is preprogrammed, requiring a small metabolic demand from the basal ganglia. In contrast, movements that require the internal regulation of the rate of change of force are associated with increased metabolic demand from the subthalamic nucleus and internal segment of the globus pallidus.

OBJECTIVE:We studied the ability of patients with Parkinson's disease to improve their performance in a motor task requiring both speed and accuracy in the execution of elbow flexion movements. Our goal was to investigate the changes in electromyographic activity associated with the changes in movement performance.METHODS:Eleven patients on anti-Parkinsonian medication were tested. The patients were selected for being bradykinetic, having little or no resting tremor or dyskinesias, and being in stages II or III of the Hoehn and Yahr rating scale.RESULTS:The untrained patients displayed multiple bursts of agonist activity, characteristic of Parkinsonian EMG recordings. All patients improved their performance by increasing peak velocity while maintaining movement accuracy within strict boundaries. With practice, the patients' performance changed in a manner similar to that which has been previously observed for performance curves in neurologically normal subjects. As movement duration decreased (i.e. peak velocity increased), we observed a slight decrease in the number of agonist bursts and an increase in the average burst duration. However, the patients continued to generate a fractionated, multi-burst agonist pattern.CONCLUSIONS:We conclude that Parkinsonian patients benefit from practice by improving their performance but remain fundamentally impaired in the generation of muscle activation patterns. This study has shown that the generation of fractionated, multiple short bursts of EMG activity that is characteristic of movements made by Parkinsonian patients is not normalized by practice.

Background: Pharmacologic interventions (e.g., beta blockers) and thalamic lesions have failed to alter the pathophysiology of essential tremor (ET) beyond a reduction in tremor amplitude. Deep brain stimulation (DBS) of the ventral intermediate (VIM) nucleus of the thalamus successfully reduces tremor rating scores. It is unknown how VIM DBS alters the pathophysiologic characteristics of ET. Objective: To determine the effects of VIM DBS on the neurophysiologic characteristics of ET. Methods: Hand tremor and EMG activity of forearm extensor and flexor muscles were recorded in six patients with ET ON-DBS and OFF-DBS and from six age- and sex-matched control subjects. Hand tremor was assessed across different inertial loads. The amplitude, frequency, regularity, and tremor-EMG coherence were analyzed. Results: VIM DBS reduced the amplitude, increased the frequency, decreased the regularity, and reduced the 1 to 8 Hz tremor-EMG coherence of ET. ON-DBS, patients with ET had greater tremor amplitude, lower frequency, more regularity, and greater tremor-EMG coherence compared to control subjects. Conclusions: Whereas pharmacologic and thalamic lesions have previously failed to change characteristics of ET beyond amplitude reduction, VIM DBS modified multiple features of ET. The changes in ET after VIM DBS provide strong evidence for clinical efficacy.

The present study examined the time and frequency structure of force output in adult humans to determine whether the changes in complexity with age are dependent on external task demands. Healthy young (20-24 yr), old (60-69 yr), and older-old (75-90 yr) humans produced isometric force contractions to constant and sine wave targets that also varied in force level. First, force variability on each force task increased with advancing age. Second, both time and frequency analysis showed that the structure of the force output in the old and older-old adults was less complex in the constant-force level task and more complex in the sine wave force task. Third, the alterations in force output with aging were primarily due to low-frequency bands <4 Hz. These results support the postulation that the observed increase or decrease in physiological complexity with aging is influenced by the relatively fast time scale of external task demands (Vaillancourt DE and Newell KM. Neurobiol Aging 23: 1-11, 2002).

Despite an intricate understanding of the neural mechanisms underlying visual and motor systems, it is not completely understood in which brain regions humans transfer visual information into motor commands. Furthermore, in the absence of visual information, the retrieval process for motor memory information remains unclear. We report an investigation where visuomotor and motor memory processes were separated from only visual and only motor activation. Subjects produced precision grip force during a functional MRI (fMRI) study that included four conditions: rest, grip force with visual feedback, grip force without visual feedback, and visual feedback only. Statistical and subtractive logic analyses segregated the functional process maps. There were three important observations. First, along with the well-established parietal and premotor cortical network, the anterior prefrontal cortex, putamen, ventral thalamus, lateral cerebellum, intermediate cerebellum, and the dentate nucleus were directly involved in the visuomotor transformation process. This activation occurred despite controlling for the visual input and motor output. Second, a detailed topographic orientation of visuomotor to motor/sensory activity was mapped for the premotor cortex, parietal cortex, and the cerebellum. Third, the retrieval of motor memory information was isolated in the dorsolateral prefrontal cortex, ventral prefrontal cortex, and anterior cingulate. The motor memory process did not extend to the supplementary motor area (SMA) and the basal ganglia. These findings provide evidence in humans for a model where a distributed network extends over cortical and subcortical regions to control the visuomotor transformation process used during visually guided tasks. In contrast, a localized network in the prefrontal cortex retrieves force output from memory during internally guided actions.

The purpose of this investigation was to examine the discharge properties of single motor units and the structure of the rectified 10, 20, and 40 Hz electromyographic (EMG) activity to determine a physiological correlate for the greater force variability with aging. Young (n = 10; mean: 22 +/- 1 years), old (n = 10; mean: 67 +/- 2 years), and older-old (n = 10; mean: 82 +/- 5 years) adult humans produced isometric second finger abduction force in both constant and sine-wave tasks at 5, 10, 20, and 40% of their maximal voluntary contraction. Force and fine-wire intramuscular electromyography were recorded from the first dorsal interosseous muscle. The amount and time-dependent structure of the discharge rate variability of single motor units and Fourier analysis of the rectified intramuscular EMG was performed. Force output variability increased across the young, old, and older-old groups. The amount and time-dependent structure of the discharge rate variability of single motor units did not differ between the young and aging groups. There was a progressive decrease in the relative power of similar to40 Hz EMG activity from the young > old > older-old subjects across the 5, 10, 20, and 40% maximum voluntary contraction (MVC) force levels. There was also a progressive increase in the relative power of the similar to10 Hz EMG activity from young < old < older-old subjects at each target force level. The findings showed that a shift in the relative contribution of similar to40 Hz to similar to10 Hz neural activity is related to the reduced capacity of older adults to maintain optimal force control. (C) 2002 Elsevier Science Inc. All rights reserved.

Objectives: The aim of this study was to examine the influence of visual and motor processes on the deterministic and stochastic structure of force output and motor unit discharge variability.Methods: Young adult subjects produced continuous, isometric force at 3, 6, 12, and 24% of their maximal voluntary contraction at low and high visual gain levels through abduction of the index finger. Force and fine-wire intramuscular electromyography were recorded.Results: There was a linear increase in discharge irregularity with increases in the mean motor unit discharge rate (8-30 Hz). Recurrence analysis showed that the percentage of deterministic structure in discharge variability remained high, but decreased linearly with increased motor unit discharge rate. Surrogate analyses confirmed that the motor unit discharge variability was inconsistent with an uncorrelated and linearly correlated Gaussian noise process. Spectral analysis revealed that both the force output and the mean time-varying motor unit discharge time series had a dominant frequency of 0-2 Hz. Visual feedback gain did not affect the individual motor unit discharge patterns.Conclusions: The motor unit discharge rate has deterministic time-dependent structure. The motor unit discharge rate is modulated at multiple time scales likely by pre- and post-synaptic induced fluctuations from spinal level pathways impinging on the motor neuron. (C) 2002 Elsevier Science Ireland Ltd. All rights reserved.

We examine the force fluctuations in the control of grip force to determine if force variability increases or decreases in relation to the degree of inter-digit individuation. This relation was examined in young (n = 7) and elderly (n = 7) participants, and in participants diagnosed with Parkinson's disease (n = 7). Force was produced under different force levels (5%, 25%, 50% MVC) with and without visual feedback. Force variability was assessed using the standard deviation and root mean square error, and inter-digit individuation was examined using cross-approximate entropy. Force variability increased with the force level, the removal of visual feedback, and also in the Parkinson's disease compared to the young and elderly matched control participants. There was a reduction in the degree of inter-digit individuation, with increases in force level, the removal of visual feedback, and in Parkinson's disease participants compared to the matched controls. Overall, there was a negative correlation between the degree of inter-digit individuation and force variability. The force fluctuations in precision grip revealed a continuum for the degree of inter-digit individuation in which task constraints, aging, and Parkinson's disease alter the coupling between the digits in controlling grip force.