The amplitude of physiological tremor can be voluntarily modulated

The Effects of Hand Tremors on the Shooting Performance of Air Pistol Shooters with Different Skill Levels

Physiologic hand tremors are a critical factor affecting the aim of air pistol shooters. However, the extent of the effect of hand tremors on shooting performance is unclear. In this study, we aim to explore the relationship between hand tremors and shooting performance scores as well as investigate potential links between muscle activation and hand tremors. In this study, 17 male air pistol shooters from China's national team and the Air Pistol Sports Center were divided into two groups: the elite group and the sub-elite group. Each participant completed 40 shots during the experiment, with shooters' hand tremors recorded using three-axis digital accelerometers affixed to their right hands. Muscle activation was recorded using surface electromyography on the right anterior deltoid, posterior deltoid, biceps brachii (short head), triceps brachii (long head), flexor carpi radialis, and extensor carpi radialis. Our analysis revealed weak correlations between shooting scores and hand tremor amplitude in multiple directions (middle-lateral, ML: r2 = -0.22, p < 0.001; vertical, VT: r2 = -0.25, p < 0.001), as well as between shooting scores and hand tremor complexity (ML: r2 = -0.26, p < 0.001; VT: r2 = -0.28, p < 0.001), across all participants. Notably, weak correlations between shooting scores and hand tremor amplitude (ML: r2 = -0.27, p < 0.001; VT: r2 = -0.33, p < 0.001) and complexity (ML: r2 = -0.31, p < 0.001) were observed in the elite group but not in the sub-elite group. Moderate correlation were found between the biceps brachii (short head) RMS and hand tremor amplitude in the VT and ML directions (ML: r2 = 0.49, p = 0.010; VT: r2 = 0.44, p = 0.025) in all shooters, with a moderate correlation in the ML direction in elite shooters (ML: r2 = 0.49, p = 0.034). Our results suggest that hand tremors in air pistol shooters are associated with the skill of the shooters, and muscle activation of the biceps brachii (long head) might be a factor affecting hand tremors. By balancing the agonist and antagonist muscles of the shoulder joint, shooters might potentially reduce hand tremors and improve their shooting scores.

Misdirected attentional focus in functional tremor

A characteristic and intriguing feature of functional neurological disorder is that symptoms typically manifest with attention and improve or disappear with distraction. Attentional phenomena are therefore likely to be important in functional neurological disorder, but exactly how this manifests is unknown. The aim of the study was to establish whether in functional tremor the attentional focus is misdirected, and if this misdirection is detrimental to the movement, or rather reflects a beneficial compensatory strategy. Patients with a functional action tremor, between the ages of 21-75, were compared to two age and gender matched control groups: healthy controls and patients with an organic action tremor. The groups included between 17 and 28 participants. First, we compared the natural attentional focus on different aspects of a reaching movement (target, ongoing visual feedback, proprioceptive-motor aspect). This revealed that the attentional focus in the functional tremor group, in contrast to both control groups, was directed to ongoing visual feedback from the movement. Next, we established that all groups were able to shift their attentional focus to different aspects of the reaching movement when instructed. Subsequently, the impact of attentional focus on the ongoing visual feedback on movement performance was evaluated under several conditions: the reaching movement was performed with direct, or indirect visual feedback, without any visual feedback, under three different instruction conditions (as accurately as possible/very slowly/very quickly), and finally as a preparatory movement that was supposedly of no importance. Low trajectory length and low movement duration were taken as measures of good motor performance. For all three groups, motor performance deteriorated with attention to indirect visual feedback, to accuracy, and when instructed to move slowly. It improved without visual feedback and when instructed to move fast. Motor performance improved, in participants with functional tremor only, when the movement was performed as a preparatory movement without any apparent importance. In addition to providing experimental evidence for improvement with distraction, we found that the normal allocation of attention during aimed movement is altered in functional tremor. Attention is disproportionately directed towards the ongoing visual feedback from the moving hand. This altered attentional focus may be partly responsible for the tremor, since it also worsens motor performance in healthy controls and patients with an organic action tremor. It may have its detrimental impact through interference with automatic movement processes, due to a maladaptive shift from lower- to higher-level motor control circuitry.

Volitional Suppression of Parkinsonian Resting Tremor

BACKGROUND

We have observed in the clinic that a number of patients with Parkinson's disease (PD) can suppress their tremor at will for brief periods, by conscious mental processes. To our knowledge, the ability to consciously diminish one's resting tremor has not yet been reported nor assessed quantitatively.

OBJECTIVE

To provide the first detailed systematic investigation of the phenomenon of voluntary tremor suppression in PD.

METHODS

We examined changes in tremor characteristics during voluntary tremor suppression in 37 PD patients (on medication) presenting with rest tremor in their upper limb. We measured tremor oscillations with a triaxis accelerometer on the index finger of the most-affected hand (n = 27). With surface electromyography (EMG), we measured changes in neuromuscular activity of the forearm flexor digitorum superficialis and extensor digitorum muscles (n = 15). Participants completed four 1-minute trials, consisting of alternating consecutive 30-second periods of resting tremor and 30-second periods of attempted tremor suppression.

RESULTS

Bayesian multilevel modeling revealed that attempted voluntary tremor suppression did indeed reduce tremor amplitude (peak power) of the acceleration signal and increased tremor frequency of the acceleration and EMG signals. Relative EMG power in the 3- to 8-Hz tremor band was also smaller. Tremor suppression was not by enhanced voluntary contraction of the relevant muscle pairs.

CONCLUSIONS

We present novel empirical evidence that PD resting tremor can be suppressed by an act of will, as evidenced by significant modulation of key neurophysiological tremor characteristics. These data highlight that it is possible to exert significant conscious control over parkinsonian resting tremor.

Validation of a precision tremor measurement system for multiple sclerosis

BACKGROUND

Tremor is a debilitating symptom of Multiple Sclerosis (MS). Little is known about its pathophysiology and treatments are limited. Clinical trials investigating new interventions often rely on subjective clinical rating scales to provide supporting evidence of efficacy.

NEW METHOD

We present a novel instrument (TREMBAL) which uses electromagnetic motion capture technology to quantify MS tremor. We aim to validate TREMBAL by comparison to clinical ratings using regression modelling with 310 samples of tremor captured from 13 MS participants who performed five different hand exercises during several follow-up visits. Minimum detectable change (MDC) and test-retest reliability were calculated and comparisons were made between MS tremor and data from 12 healthy volunteers.

RESULTS

Velocity of the index finger was most congruent with clinical observation. Regression modelling combining different features, sensor configurations, and labelling exercises did not improve results. TREMBAL MDC was 84% of its initial measurement compared to 91% for the clinical rating. Intra-class correlations for test-retest reliability were 0.781 for TREMBAL and 0.703 for clinical ratings. Tremor was lower (p =  0.002) in healthy subjects.

COMPARISON WITH EXISTING METHODS

Subjective scales have low sensitivity, suffer from ceiling effects, and mitigation against inter-rater variability is challenging. Inertial sensors are ubiquitous, however, their output is nonlinearly related to tremor frequency, compensation is required for gravitational artefacts, and their raw data cannot be intuitively comprehended.

CONCLUSIONS

TREMBAL, compared with clinical ratings, gave measures in agreement with clinical observation, had marginally lower MDC, and similar test-retest reliability.

Keeping still doesn't "make sense": examining a role for movement variability by stabilizing the arm during a postural control task

Small-amplitude, higher frequency oscillations of the body or limb are typically observed when humans attempt to maintain the position of a body or limb in space. Recent investigations have suggested that these involuntary movements of the body during stance could be used as an exploratory means of acquiring sensory information. In the present study, we wanted to determine whether a similar phenomenon would be observed in an upper limb postural task that does not involve whole body postural control. Participants were placed in a supine position with the arm pointing vertically and were asked to maintain the position of the limb in space with and without visual feedback. The wrist was attached to an apparatus that allowed the experimenter to stabilize or "lock" movements of the arm without the participants' awareness. When participants were "locked," the forces recorded predicted greater accelerations than those observed when the arm was freely moving with and without visual feedback. From unlocked to locked, angular accelerations increased in the eyes-closed condition and when participants were provided visual feedback of arm angular displacements. Irrespective of their origin, small displacements of the limb may be used as an exploratory means of acquiring sensory information from the surrounding environment.NEW & NOTEWORTHY The role of movement variability during a static limb position task is currently unknown. We tested whether variability remains in the absence of sensory-based error with an apparatus that stabilized the limb without the participant's knowledge during a static postural task. Increased forces observed during arm stabilization predicted movements greater than those observed when not externally stabilized. These results suggest movement variability during static postures could facilitate the gathering of sensory information from the surrounding environment.

Micro movements of the upper limb in fibromyalgia: The relation to proprioceptive accuracy and visual feedback

The purpose of this study was to explore the role of visual and proprioceptive feedback in upper limb posture control in fibromyalgia (FM) and to assess the coherence between acceleration measurements of upper limb micro movements and surface electromyography (sEMG) of shoulder muscle activity (upper trapezius and deltoid). Twenty-five female FM patients and 25 age- and sex-matched healthy controls (HCs) performed three precision motor tasks: (1) maintain a steady shoulder abduction angle of 45° while receiving visual feedback about upper arm position and supporting external loads (0.5, 1, or 2kg), (2) maintain the same shoulder abduction angle without visual feedback (eyes closed) and no external loading, and (3) a joint position sense test (i.e., assessment of proprioceptive accuracy). Patients had more extensive increase in movement variance than HCs when visual feedback was removed (P<0.03). Proprioceptive accuracy was related to movement variance in HCs (R⩾0.59, P⩽0.002), but not in patients (R⩽0.25, P⩾0.24). There was no difference between patients and HCs in coherence between sEMG and acceleration data. These results may indicate that FM patients are more dependent on visual feedback and less reliant on proprioceptive information for upper limb posture control compared to HCs.

Measuring tremor with a smartphone

Tremor is the most common movement disorder. However; characterizing it in large populations is not easily accomplished since current methodologies are not adapted to large-scale field studies. To overcome this challenge, a smartphone application was developed as a stand-alone platform to assess tremor. The current book chapter details the steps taken to validate this mobile application. Data recorded with the smartphone was analyzed online and offline as well as compared to laboratory equipment and a clinical scale. This allowed for the identification of the tremor properties that could reliably be characterized with the smartphone as well as the limits of the hardware. It also allowed for the identification of tasks that could be performed with the smartphone when tremor was being assessed. Finally, we confirmed the clinical relevance of the results provided by the smartphone application.

Suppression of enhanced physiological tremor via stochastic noise: initial observations

Enhanced physiological tremor is a disabling condition that arises because of unstable interactions between central tremor generators and the biomechanics of the spinal stretch reflex. Previous work has shown that peripheral input may push the tremor-related spinal and cortical systems closer to anti-phase firing, potentially leading to a reduction in tremor through phase cancellation. The aim of the present study was to investigate whether peripherally applied mechanical stochastic noise can attenuate enhanced physiological tremor and improve motor performance. Eight subjects with enhanced physiological tremor performed a visuomotor task requiring the right index finger to compensate a static force generated by a manipulandum to which Gaussian noise (3–35 Hz) was applied. The finger position was displayed on-line on a monitor as a small white dot which the subjects had to maintain in the center of a larger green circle. Electromyogram (EMG) from the active hand muscles and finger position were recorded. Performance was measured by the mean absolute deviation of the white dot from the zero position. Tremor was identified by the acceleration in the frequency range 7–12 Hz. Two different conditions were compared: with and without superimposed noise at optimal amplitude (determined at the beginning of the experiment). The application of optimum noise reduced tremor (accelerometric amplitude and EMG activity) and improved the motor performance (reduced mean absolute deviation from zero). These data provide the first evidence of a significant reduction of enhanced physiological tremor in the human sensorimotor system due to application of external stochastic noise.

Load-induced changes in older individual's hand-finger tremor are ameliorated with targeting

The purpose of this study was to investigate hand-finger tremor dynamics when a load was applied to the finger in a group of healthy older adults. Moreover, we sought to determine if projecting a representation of the subject's finger tremor on a target was capable of overcoming the effects of loading so that hand-finger interactions returned to a state that was similar to normal tremor. Eight healthy older males (67 ± 1 year) performed a postural pointing task, where tremor was assessed using lightweight accelerometers attached to the hand and finger. Tremor was then assessed when a laser pointer was attached to the finger and switched off (the load), and then with the laser pointer attached and switched on pointing at targets of 40 mm and 20mm in diameter. The main findings of this study were that 1) loading the finger resulted in a reduction in finger tremor amplitude and increased finger tremor regularity, but no change in hand tremor, 2) loading caused increased hand-finger 8-12 Hz cross wavelet coherence and phase synchrony, and 3) pointing at different targets while the finger was loaded resulted in an increase in finger tremor amplitude, and changes in inter-segmental coupling to the extent that hand-finger dynamics reflected normal unloaded conditions. Overall, these results illustrate that the damping effects of limb loading can be offset, in part, by altering the accuracy demands of the task to make the pointing action more challenging.

Upper limb position control in fibromyalgia

BACKGROUND

Motor problems are reported by patients with fibromyalgia (FM). However, the mechanisms leading to alterations in motor performance are not well understood. In this study, upper limb position control during sustained isometric contractions was investigated in patients with FM and in healthy controls (HCs).

METHODS

Fifteen female FM patients and 13 HCs were asked to keep a constant upper limb position during sustained elbow flexion and shoulder abduction, respectively. Subjects received real-time visual feedback on limb position and both tasks were performed unloaded and while supporting loads (1, 2, and 3 kg). Accelerations of the dominant upper limb were recorded, with variance (SD of mean position) and power spectrum analysis used to characterize limb position control. Normalized power of the acceleration signal was extracted for three frequency bands: 1-3 Hz, 4-7 Hz, and 8-12 Hz.

RESULTS

Variance increased with load in both tasks (P < 0.001) but did not differ significantly between patients and HCs (P > 0.17). Power spectrum analysis showed that the FM patients had a higher proportion of normalized power in the 1-3 Hz band, and a lower proportion of normalized power in the 8-12 Hz band compared to HCs (P < 0.05). The results were consistent for all load conditions and for both elbow flexion and shoulder abduction.

CONCLUSION

FM patients exhibit an altered neuromuscular strategy for upper limb position control compared to HCs. The predominance of low-frequency limb oscillations among FM patients may indicate a sensory deficit.

Aging, neuromuscular decline, and the change in physiological and behavioral complexity of upper-limb movement dynamics

Aging is characterized by a general decline in physiological and behavioral function that has been widely interpreted within the context of the loss of complexity hypothesis. In this paper, we examine the relation between aging, neuromuscular function and physiological-behavioral complexity in the arm-hand effector system, specifically with reference to physiological tremor and isometric force production. Experimental findings reveal that the adaptive behavioral consequences of the aging-related functional decline in neurophysiological processes are less pronounced in simple motor tasks which provides support for the proposition that the motor output is influenced by both extrinsic (e.g., task related) and intrinsic (e.g., coordination, weakness) factors. Moreover, the aging-related change in complexity can be bidirectional (increase or decrease) according to the influence of task constraints on the adaptation required of the intrinsic properties of the effector system.

The effect of changes in joint angle on the characteristics of physiological tremor

INTRODUCTION

Physiological tremor, as a whole, can be influenced by changes in muscle activity. However, the origin of low-frequency physiological tremor oscillations has yet to be conclusively determined. It is possible that by experimentally manipulating muscular activity, a better determination of the origin of those low-frequency oscillations can be achieved. It was demonstrated that changes in joint angle modify characteristics of muscular activity. As such, we hypothesize that changes in wrist-joint angle will alter the characteristics of low-frequency physiological tremor oscillations.

OBJECTIVE

Assess the influence of changes in joint angle of the wrist on characteristics of physiological finger tremor.

METHODS

Physiological finger tremor was recorded (n = 25) using a laser displacement system while the arm and hand were supported. The relative angle between the dorsum of the hand and the forearm was altered between conditions (135°, 180°, 225° and 270°), while the hand and the finger remained parallel to the ground. EMG of the extensors and flexors were also recorded.

RESULTS

Tremor amplitude was significantly altered by changes in wrist-joint angle. This was especially the case for lower frequency oscillations. In addition, electromyography properties of forearm muscles were also significantly modified by changes in wrist-joint angles.

CONCLUSIONS

This study demonstrates that changes in wrist-joint angle modify the characteristics of physiological finger tremor. This should be taken into account when interpreting tremor data as well as when developing tools to minimize tremor.

Changes in tremor as a function of type of augmented visual information

The dynamics of postural finger tremor is typically investigated under conditions of natural vision of the finger. Here we investigated the effect of different types of augmented visual information feedback on finger tremor and on inter-limb tremor coordination. Four visual information conditions of postural finger tremor from either the dominant hand only or from both hands were investigated. The visual conditions were: (1) no vision, (2) natural vision, (3) augmented vision with instantaneous acceleration on a computer display, and (4) augmented vision with instantaneous and past acceleration on a computer display. Acceleration was measured with a 3D accelerometer on the distal phalanx of the index finger(s). The amount of tremor variability did not change as a function of visual information conditions. However, removing visual feedback increased tremor predictability and reduced small, random deviations of tremor acceleration output in the one finger condition. In the two-finger condition, augmented visual information increased the irregularity of the combined tremor variability. The no vision condition showed a stronger coupling between fingers than natural vision or augmented vision with past information. The findings revealed that augmented visual information increased tremor irregularity and facilitated coupling in two-hand tremor dynamics.