Effects of age and gender on finger coordination in MVC and submaximal force-matching tasks

Reference data for the hand grip and palmar pinch force sense errors and the relationship between school-entry-age in young adults

BACKGROUND

Hands execute intricate tasks vital for everyday life and professions such as cooking, tailoring, and craftsmanship.

OBJECTIVE

This study aimed to establish reference data for hand grip and palmar pinch force sense in young adults, accounting for gender differences, and to determine the correlation between these data and school-entry ages.

METHODS

The cross-sectional observational study comprised 284 participants (156 females and 128 males). Demographic details, including age, gender, weight, and height, were recorded. Participant ages ranged from 18 to 29, representing the youth workforce population as defined by the International Labour Organization. Factors like hand dominance and school-entry age were ascertained based on participants' self-reports. Hand grip and palmar pinch force senses were separately assessed in the dominant and non-dominant hands of 130 randomly chosen participants to evaluate test-retest reliability.

RESULTS

Hand grip (dominant: p < 0.001, non-dominant: p = 0.002) and palmar pinch force sense errors were significantly lower in male participants compared to females. Palmar pinch force sense error for the dominant hand was reduced in males (p = 0.002), but no significant disparity existed between genders for the non-dominant hand (p = 0.222). Healthy adults who began school at age five or earlier exhibited a decreased force sense error rate (p < 0.05).

CONCLUSIONS

Force sense error reference values vary based on gender and school-entry age. This reference data will aid rehabilitation specialists working with young adults in physiotherapy and occupational therapy fields in identifying potential impairments.

Age-related differences in finger interdependence during complex hand movements

The well-known decrease in finger dexterity during healthy aging leads to a significant reduction in quality of life. Still, the exact patterns of altered finger kinematics of older adults in daily life are fairly unexplored. Finger interdependence is the unintentional comovement of fingers that are not intended to move, and it is known to vary across the lifespan. Nevertheless, the magnitude and direction of age-related differences in finger interdependence are ambiguous across studies and tasks and have not been explored in the context of daily life finger movements. We investigated five different free and daily-life-inspired finger movements of the right, dominant hand as well as a sequential finger tapping task of the thumb against the other fingers, in 17 younger (22-37 yr) and 17 older (62-80 yr) adults using an exoskeleton data glove for data recording. Using inferential statistics, we found that the unintentional comovement of fingers generally decreases with age in all performed daily-life-inspired movements. Finger tapping, however, showed a trend towards higher finger interdependence for older compared with younger adults. Using machine learning, we predicted the age group of a person from finger interdependence features of single movement trials significantly better than chance level for the daily-life-inspired movements, but not for finger tapping. Taken together, we show that for specific tasks, decreased finger interdependence (i.e., less comovement) could potentially act as a marker of human aging that specifically characterizes older adults' complex finger movements in daily life.NEW & NOTEWORTHY Kinematic finger movement data were analyzed with regard to age-related differences. Extensive analyses of complex and daily-life-inspired movements reveal that the direction of age effects is not uniform but task-dependent: Although older adults generally show more finger interdependence than younger adults in a simple finger tapping task, this effect is reversed for daily-life-inspired movement tasks. For these tasks, finger interdependence indices offer potential new markers to predict the age group of an individual using machine learning approaches.

Deficits in force production during multifinger tasks demarcate cognitive dysfunction

BACKGROUND

The multifinger force deficit (MFFD) is the decline in force generated by each finger as the number of fingers contributing to an action is increased. It has been shown to associate with cognitive status.

AIMS

The aim was to establish whether a particularly challenging form of multifinger grip dynamometry, that provides minimal tactile feedback via cutaneous receptors and requires active compensation for reaction forces, will yield an MFFD that is more sensitive to cognitive status.

METHODS

Associations between measures of motor function, and cognitive status (Montreal Cognitive Assessment [MoCA]) and latent components of cognitive function (derived from 11 tests using principal component analysis), were estimated cross-sectionally using generalized partial rank correlations. The participants (n  =  62) were community dwelling, aged 65-87.

RESULTS

Approximately half the participants were unable to complete the dynamometry task successfully. Cognitive status demarcated individuals who could perform the task from those who could not. Among those who complied with the task requirements, the MFFD was negatively correlated with MoCA scores-those with the highest MoCA scores tended to exhibit the smallest deficits, and vice versa. There were corresponding associations with latent components of cognitive function.

DISCUSSION

The results support the view that neurodegenerative processes that are a feature of normal and pathological aging exert corresponding effects on expressions of motor coordination-in multifinger tasks, and cognitive sufficiency, due to their dependence on shared neural systems.

CONCLUSIONS

The outcomes add weight to the assertion that deficits in force production during multifinger tasks are sensitive to cognitive dysfunction.

Muscle Recruitment Strategies in a Redundant Task: Age Differences Through Network Analyses

There are numerous studies comparing young and old adults in terms of muscle coordination in standard tasks (e.g., walking, reaching) and small variations of them. These tasks might hide differences: individuals would converge to similar behavior as they practice these throughout life. Also, we are unaware of studies that considered the muscle recruitment nested dynamics. For this reason, our study evaluated how young and old women coordinate and control the movement system while performing an unusual redundant motor control task through the network physiology approach. We acquired electromyographic signals from nine leg muscles of the dominant and non-dominant limbs during maximum voluntary isometric contractions (knee extension and flexion) and co-contraction bouts. Our results showed that young participants presented higher peak torque output, with similar EMG variability, compared to older participants. Considering firing rate frequencies, old and young women demonstrated different traits for network clustering and efficiency for the task. Age seems to affect muscle coordination at higher frequencies, even with a similar number of muscle synergies, indicating that younger women might have more integrated synergies than older women. The findings also point to differential muscle coordination adaptability.

Effects of wrist posture and stabilization on precision grip force production and muscle activation patterns

Most of the power for generating forces in the fingers arises from muscles located in the forearm. This configuration maximizes finger joint range of motion while minimizing finger mass and inertia. The resulting multiarticular arrangement of the tendons, however, complicates independent control of the wrist and the digits. Actuating the wrist impacts sensorimotor control of the fingers and vice versa. The goal of this study was to systematically investigate interactions between isometric wrist and digit control. Specifically, we examined how the need to maintain a specified wrist posture influences precision grip. Fifteen healthy adults produced maximum precision grip force at 11 different wrist flexion/extension angles, with the arm supported, under two conditions: 1) the participant maintained the desired wrist angle while performing the precision grip and 2) a robot maintained the specified wrist angle. Wrist flexion/extension posture significantly impacted maximum precision grip force (P < 0.001), with the greatest grip force achieved when the wrist was extended 30° from neutral. External wrist stabilization by the robot led to a 20% increase in precision grip force across wrist postures. Increased force was accompanied by increased muscle activation but with an activation pattern similar to the one used when the participant had to stabilize their wrist. Thus, simultaneous wrist and finger requirements impacted performance of an isometric finger task. External wrist stabilization can promote increased precision grip force resulting from increased muscle activation. These findings have potential clinical significance for individuals with neurologically driven finger weakness, such as stroke survivors.NEW & NOTEWORTHY We explored the interdependence between wrist and fingers by assessing the influence of wrist posture and external stabilization on precision grip force generation. We found that maximum precision grip force occurred at an extended wrist posture and was 20% greater when the wrist was Externally Stabilized. The latter resulted from amplification of muscle activation patterns from the Self-Stabilized condition rather than adoption of new patterns exploiting external wrist stabilization.

Neural Mechanisms of Age-Related Loss of Muscle Performance and Physical Function

BACKGROUND

This article discusses the putative neural mechanisms of age-related muscle weakness within the broader context of the development of function-promoting therapies for sarcopenia and age-related mobility limitations. We discuss here the evolving definition of sarcopenia and its primary defining characteristic, weakness.

METHODS

This review explores the premise that impairments in the nervous system's ability to generate maximal force or power contribute to sarcopenia.

RESULTS

Impairments in neural activation are responsible for a substantial amount of age-related weakness. The neurophysiological mechanisms of weakness are multifactorial. The roles of supraspinal descending command mechanisms, spinal motor neuron firing responsivity, and neuromuscular junction transmission failure in sarcopenia are discussed. Research/clinical gaps and recommendations for future work are highlighted.

CONCLUSION

Further research is needed to map putative neural mechanisms, determine the clinical relevance of age-related changes in neural activation to sarcopenia, and evaluate the effectiveness of various neurotherapeutic approaches to enhancing physical function.

Unintentional force drifts in the lower extremities

We explored the phenomenon of unintentional force drift seen in the absence of visual feedback during knee extension contractions in isometric conditions. Based on the importance of knee extensors for the anti-gravity function, we hypothesized that such force drifts would be slower and smaller compared to those reported for the upper extremities. We also explored possible effects of foot dominance and gender on the force drifts. Young healthy persons produced isometric knee extension contractions to different levels, ranging from 15 to 25% of maximal voluntary contraction force, with the help of visual feedback, and then, the visual feedback was turned off. Force change over the time interval without visual feedback was quantified. In the absence of visual feedback, force drifted to smaller magnitudes. The drift magnitude expressed in percent of the initial force magnitude was smaller for smaller initial force levels, ranging between 8 and 15% of the initial force for the initial force magnitude of 15% and 25% of maximal voluntary contraction force. The time exponent of the force drift was independent of the initial force magnitude and was, on average, 6.45 s. There were no significant effects of foot dominance or gender, although the male subjects tended to show stronger scaling of the drift magnitude with the initial force level compared to the female subjects. The results show that unintentional force drift is a common phenomenon across limbs and muscle groups. This conclusion fits the theory of control with spatial referent coordinates and the general tendency of all natural systems to drift to states with lower potential energy.

Unintentional drifts in performance during one-hand and two-hand finger force production

We explored the phenomena of force drifts and unintentional finger force production (enslaving), and their dependence on visual feedback. Predictions have been drawn based on the theory of control with spatial referent coordinates for condition with feedback on instructed (master) finger force, enslaved finger force, and total force for one-hand and two-hand tasks. Subjects produced force under the different feedback conditions without their knowledge. No feedback condition was also used for the one-hand tasks. Overall, feedback of master finger force led to an increase in the enslaved force, feedback on the slave finger force led to a drop in the master force, feedback on the total force led to balanced drifts in the master force down and enslaved force up, and under the no-feedback condition, master and total force drifted down with large variability in the enslaved force drifts. The patterns were the same in both hands in the two-hand tasks when feedback was provided on the forces of one hand only (without subject's knowledge). The index of enslaving always drifted toward higher values. We interpret the findings as reflecting three main factors: drifts in the referent coordinates toward actual finger coordinates, spread of cortical excitation over representations of the fingers, and robust sharing of referent coordinates between the two hands in bimanual tasks. The large consistent drifts in enslaving toward higher values have to be considered in studies of multi-finger synergies.

Altered Bimanual Kinetic and Kinematic Motor Control Capabilities in Older Women

Older women may experience critical neuromuscular impairments interfering with controlling successful bimanual motor actions. Our study aimed to investigate altered bimanual motor performances in older women compared with younger women by focusing on kinetic and kinematic motor properties. Twenty-two older women and 22 younger women performed bimanual kinetic and kinematic motor tasks. To estimate bimanual kinetic functions, we calculated bimanual maximal voluntary contractions (i.e., MVC) and force control capabilities (i.e., mean force, accuracy, variability, and regularity of the total force produced by two hands) during bimanual hand-grip submaximal force control tasks. For bimanual kinematic performances, we assessed the scores of the Purdue Pegboard Test (i.e., PPT) in both hands and assembly tasks, respectively. For the bimanual MVC and PPT, we conducted an independent t-test between two groups. The bimanual force control capabilities were analyzed using two-way mixed ANOVAs (Group × Force Level; 2 × 2). Our findings revealed that the older women showed less bimanual MVC (p = 0.046) and submaximal force outputs (p = 0.036) and greater changes in bimanual force control capabilities as indicated by a greater force variability (p = 0.017) and regularity (p = 0.014). Further, the older women revealed lower scores of PPT in both the hands condition (p < 0.001) and assembly task condition (p < 0.001). The additional correlation analyses for the older women showed that lower levels of skeletal muscle mass were related to less bimanual MVC (r = 0.591; p = 0.004). Furthermore, a higher age was related to lower scores in the bimanual PPT assembly task (r = -0.427; p = 0.048). These findings suggested that older women experience greater changes in bimanual motor functions compared with younger women.

Effect of pinch types on pinch force sense in healthy adults

Pinch force sense plays an important role in the performance of daily finger movements, including tip, key, palmar pinch. The present study investigated the roles of pinch type in the sensation of pinch force among healthy participants in the ipsilateral force reproduction trial. This study instructed forty healthy adult subjects (20 women and 20 men) in producing reference forces at different levels [10, 30, 50% maximal voluntary isometric contraction (MVIC)] by adopting 3 pinch types (tip, key, and palmar pinches) and in reproducing the above force levels with the identical hand. Our study revealed that subjects are significantly more sensitive detecting alterations of pinching forces with tip pinch but not key or palmar pinch under high forces (30 and 50% MVIC) but not at lower force levels (10% MVIC).

Deficits in rate of force production during multifinger tasks are associated with cognitive status

OBJECTIVES

The multifinger force deficit (MFFD) is the decline in force generated by an individual finger as the number of fingers contributing to the action is increased. It has been proposed that as a measure of neural sufficiency rather than muscle status, it provides a means of detecting individuals at risk of cognitive decline. Age-related deficits in central neural drive exert a disproportionate impact on the rate at which force can be generated. We examined whether a MFFD derived from the maximum rate at which force is generated, is more sensitive to individual differences in cognitive status, than one calculated using the maximum level of force.

METHODS

Monotonic associations between each of two variants of the MFFD, and cognition (measured with the Montreal Cognitive Assessment), were estimated cross sectionally using generalized partial rank correlations, in which age, level of education and degree of handedness were included as covariates. The participants (n=26) were community dwelling adults aged 66-87.

RESULTS

The MFFD derived using the maximum rate of force development was negatively associated with cognitive status. The association for the MFFD based on the maximum level of force, was not statistically reliable. The associations with cognitive status obtained for both variants of the MFFD were of greater magnitude than those reported previously for standard grip strength dynamometry.

CONCLUSION

The sensitivity with which the MFFD detects risk of cognitive decline may be enhanced by using the maximum rate of force developed by each finger, rather than the maximum force generated by each finger.

The impact of aging and reaching movements on grip stability control during manual precision tasks

Background

Operating an object by generating stable hand-grip force during static or dynamic posture control of the upper extremities simultaneously is an important daily activity. Older adults require different attentional resources during grip strength control and arm movements. However, the impact of aging and reaching movements on precise grip strength and stability control among older adults is not well understood. This study investigated the impact of aging and reaching movements on grip strength and stability control in both hands of the upper extremities.

Methods

Fifty healthy young adults (age: 28.8 ± 14.0 years) and 54 healthy older adults (73.6 ± 6.3 years) were recruited to perform isometric grip strength test at 20% maximal voluntary contraction as the target force during three manual precision tasks simultaneously: stationary task (without arm movements), forward-reach task, and backward-reach task. The average grip force (in kg) and coefficient of variation values (expressed as a percentage) during manual precision tasks were calculated to determine the quality of participants’ grip strength. The deviation error, absolute error, and force-stability index values were calculated to determine the strength control relative to the target force.

Results

For both the young and older groups, the force-stability index values in both hands were significantly higher during forward- and backward-reaching movements than in the stationary condition (p < 0.05). The older group exhibited a significantly lower hand-grip strength and stability of strength control in both hands than the young group (p < 0.05).

Conclusions

Aging and reaching task performance reduced the grip strength of participants and increased the variations in strength control of both hands relative to the target force, indicating that older adults exhibit poor grip strength and stability control when performing arm-reaching movements. These findings may help clinical therapists in establishing objective indexes for poor grip-stability control screening and developing appropriate rehabilitation programs or health-promotion exercises that can improve grip strength and stability control in older people.

The Nature of Finger Enslaving: New Results and Their Implications

We present a review on the phenomenon of unintentional finger action seen when other fingers of the hand act intentionally. This phenomenon (enslaving) has been viewed as a consequence of both peripheral (e.g., connective tissue links and multifinger muscles) and neural (e.g., projections of corticospinal pathways) factors. Recent studies have shown relatively large and fast drifts in enslaving toward higher magnitudes, which are not perceived by subjects. These and other results emphasize the defining role of neural factors in enslaving. We analyze enslaving within the framework of the theory of motor control with spatial referent coordinates. This analysis suggests that unintentional finger force changes result from drifts of referent coordinates, possibly reflecting the spread of cortical excitation.

Handgrip strength asymmetry is associated with future falls in older Americans

BACKGROUND

Examining handgrip strength (HGS) asymmetry could extend the utility of handgrip dynamometers for screening future falls.

AIMS

We sought to determine the associations of HGS asymmetry on future falls in older Americans.

METHODS

The analytic sample included 10,446 adults aged at least 65 years from the 2006-2016 waves of the Health and Retirement Study. Falls were self-reported. A handgrip dynamometer measured HGS. The highest HGS on each hand was used for determining HGS asymmetry ratio: (non-dominant HGS/dominant HGS). Those with HGS asymmetry ratio < 1.0 had their ratio inverted to make all HGS asymmetry ratios ≥ 1.0. Participants were categorized into asymmetry groups based on their inverted HGS asymmetry ratio: (1) 0.0-10.0%, (2) 10.1-20.0%, (3) 20.1-30.0%, and (4) > 30.0%. Generalized estimating equations were used for the analyses.

RESULTS

Every 0.10 increase in HGS asymmetry ratio was associated with 1.26 (95% confidence interval (CI) 1.07-1.48) greater odds for future falls. Relative to those with HGS asymmetry 0.0-10.0%, participants with HGS asymmetry > 30.0% had 1.15 (CI 1.01-1.33) greater odds for future falls; however, the associations were not significant for those with HGS asymmetry 10.1-20.0% (odds ratio: 1.06; CI 0.98-1.14) and 20.1-30.0% (odds ratio: 1.10; CI 0.99-1.22). Compared to those with HGS asymmetry 0.0-10.0%, participants with HGS asymmetry > 10.0% and > 20.0% had 1.07 (CI 1.01-1.16) and 1.12 (CI 1.02-1.22) greater odds for future falls, respectively.

DISCUSSION

Asymmetric HGS, as a possible biomarker of impaired neuromuscular function, may help predict falls.

CONCLUSIONS

We recommend that HGS asymmetry be considered in HGS protocols and fall risk assessments.

A Narrative Review of Handgrip Strength and Cognitive Functioning: Bringing a New Characteristic to Muscle Memory

BACKGROUND

Measures of handgrip strength have not only emerged as a clinically viable screening tool for determining risk for morbidity, functional disability, and early mortality, but also for helping to identify cognitive deficits. However, the phenomena that links low handgrip strength with cognitive decline remains unclear. The role of the muscular and neural systems, and their adaptations to muscle strengthening activities over the life course, may provide important information for how age-related changes to muscle mass, strength, and neural capacity influence cognition. Moreover, disentangling how handgrip strength and cognitive function are associated may help to inform healthcare providers working with aging adults and guide targeted interventions aiming to preserve muscle and cognitive functioning.

OBJECTIVE

To 1) highlight and summarize evidence examining the associations of handgrip strength and cognitive functioning, and 2) provide directions for future research in this area.

METHODS

Articles from the PubMed database were searched from November 2018-May 2019. The search term algorithm, inclusion and exclusion criteria were pre-specified by investigators.

RESULTS

Several cross-sectional and longitudinal studies have revealed that measures of handgrip strength were associated with cognitive declines regardless of age demographics and the presence of comorbidities.

CONCLUSION

Handgrip strength can be used in clinical and epidemiological settings for helping to determine the onset and progression of cognitive impairment. Future research should continue to examine how handgrip strength and cognitive function are linked.

Sarcopenia and Neuroscience: Learning to Communicate

In the 1990s and early 2000s, the common definition for sarcopenia was age-related loss of skeletal muscle, and low levels of muscle mass were central to sarcopenia diagnosis. In more recent consensus definitions, however, low muscle strength displaces low muscle mass as a defining feature of sarcopenia. The change stems from growing evidence that muscle weakness is a better predictor of adverse health outcomes (e.g., mobility limitations) than muscle mass. This evidence accompanies an emerging recognition that central neural mechanisms are critical determinants of age-related changes in strength and mobility that can occur independently of variations in muscle mass. However, strikingly little practical attention is typically given to the potential role of the central nervous system in the aetiology or remediation of sarcopenia (i.e., low muscle function). In this article, we provide an overview of some mechanisms that mediate neural regulation of muscle contraction and control, and highlight the specific contributions of neural hypoexcitability, dopaminergic dysfunction, and degradation of functional and structural brain connectivity in relation to sarcopenia. We aim to enhance the lines of communication between the domains of sarcopenia and neuroscience. We believe that appreciation of the neural regulation of muscle contraction and control is fundamental to understanding sarcopenia and to developing targeted therapeutic strategies for its treatment.

Intention to be force efficient improves high-level anticipatory coordination of finger positions and forces in young and elderly adults

Successful object manipulation requires anticipatory high-level control of finger positions and forces to prevent object slip and tilt. Unlike young adults, who efficiently scale grip forces (GFs) according to surface conditions, old adults were reported to exert excessive grip forces. In this study, we theoretically show how grip force economy depends on the modulation of the centers of pressure on opposing grip surfaces (ΔCoP) according to object properties. In a grasp-to-lift study with young and elderly participants, we investigated how the instruction to lift the object with efficient GF influences the anticipation of torques, ΔCoP and GF control during complex variations of mass distributions and surface properties. Provision of the explicit instruction to strive for force efficiency prompted both age groups to optimize their ΔCoP modulation, although to a lesser degree in the elderly, and also led to a refinement of torque anticipation for a right-sided weight distribution in the young, but not the elderly participants. Consequently, marked drops in GF levels resulted. Furthermore, participants enhanced ΔCoP modulation and lowered GF safety ratios in challenging surface conditions. Higher GF in the elderly was due to decreased skin-surface friction but also worse ΔCoP modulation for lateralized mass distributions when trying to be force efficient. In contrast, safety margins were not elevated in the elderly, suggesting preserved GF control. Our findings demonstrate how task goals influence high-level motor control of object manipulation differentially in young and elderly participants and highlight the necessity to control for both instructions and friction when investigating GF control.NEW & NOTEWORTHY Previous studies have shown that forces are covaried as a function of centers of pressure (CoPs) to exert adequate torques. Here, we demonstrate that force-efficient object manipulation requires the modulation of CoPs and show that providing the instruction to be force efficient and challenging surface conditions elicits a GF safety ratio reduction as well as an optimization of anticipatory CoP modulation and torques in the young and, to a lesser degree, in the elderly.

Effects of Daily Hand Activities on Age-Related Declines of Dynamic Motor Function in Individual Fingers

The present study examined the effects of daily activities of the hands on finger motor function in older adults. Maximum tapping frequency with each finger during single-finger tapping and alternate movements of index-middle, middle-ring, and ring-little finger pairs during double-finger tapping were compared between older adults who used their hands actively in their daily lives and those who did not. The active participants had significantly faster tapping rates for the ring finger in the single-finger tapping and the middle-ring finger pair in the double-finger tapping than did the inactive participants. Thus, daily activity of the hands in older adults could be effective at preventing the loss of dynamic motor function in individual fingers, especially with greater difficulty in movement, resulting from the degeneration with age.

The effects of aging on the distribution and strength of correlated neural inputs to postural muscles during unperturbed bipedal stance

The present study investigated the effects of aging on the distribution of common descending neural drives to main postural muscles acting on the ankle, knee, hip, and lower trunk. The presence, distribution, and strength of these drives were assessed using intermuscular coherence estimations at a low-frequency band (0-55 Hz). Ten healthy older adults (68.7 ± 3.5 years) with no recent history of falls and ten healthy younger adults (26.8 ± 2.7 years) performed bipedal stances with eyes either opened or closed. Electromyographic (EMG) signals of six postural muscles were recorded. Estimations of intermuscular coherence were obtained from fifteen muscle pairs and four muscle groups. In general, single-pair and pooled coherence analyzes revealed significant levels of signal synchronization within 1-10 Hz. Significant common drives to anterior, posterior, and antagonist muscle groups were observed for both cohorts of participants. However, older participants showed significantly stronger EMG-EMG synchronization in the frequency domain compared to younger participants. It seems that age-related sarcopenia, visual-vestibular-proprioceptive decline, cortical activation increase, presynaptic inhibition modulation decrease, and co-contraction increase had a major impact on strengthening the common drives to the aforementioned muscle groups. Differently from young adults, the absence of visual inputs did not reduce the magnitude of signal synchronization in older adults. These results suggest that the aging central nervous system seems to organize similar arrangements of common drives to postural antagonist muscles at different joints, and to postural muscles pushing the body either forward or backward when visual information is not available.

Validating the Capability for Measuring Age-Related Changes in Grip-Force Strength Using a Digital Hand-Held Dynamometer in Healthy Young and Elderly Adults

Background

Grip-force performance can be affected by aging, and hand-grip weakness is associated with functional limitations of dasily living. However, using an appropriate digital hand-held dynamometer with continuous hand-grip force data collection shows age-related changes in the quality of hand-grip force control may provide more valuable information for clinical diagnoses rather than merely recording instantaneous maximal hand-grip force in frail elderly adults or people with a disability. Therefore, the purpose of this study was to indicate the construct validity of the digital MicroFET3 dynamometer with Jamar values for maximal grip-force assessments in elderly and young adults and confirmed age-related changes in the maximal and the quality of grip-force performance using the MicroFET3 dynamometer in elderly people.

Methods

Sixty-five healthy young (23.3 ± 4.5 years) and 50 elderly (69.5 ± 5.8 years) adults were recruited and asked to perform a validity test of the grip-force maximum voluntary contraction (MVC) using both the dominant and nondominant hands with a Jamar dynamometer and a MicroFET3 dynamometer.

Results

A strong correlation of maximal grip-force measurements was found between the MicroFET3 dynamometer and Jamar standard dynamometer for both hands in all participants (p < 0.05). Although, the results showed that a lower grip force was measured in both hands by the MicroFET3 dynamometer than with the Jamar dynamometer by 49.9%~57% (p < 0.05), but confidently conversion formulae were also developed to convert MicroFET3 dynamometer values to equivalent Jamar values for both hands. Both dynamometers indicated age-related declines in the maximum grip-force performance by 36.7%~44.3% (p < 0.05). We also found that the maximal hand-grip force values generated in both hand by the elderly adults were slower and more inconsistent than those of the young adults when using the MicroFET3 dynamometer.

Conclusions

This study demonstrated that the digital MicroFET3 dynamometer has good validity when used to measure the maximal grip force of both hands, and conversion formulae were also developed to convert MicroFET3 dynamometer force values to Jamar values in both hands. Comparing with the Jamar dynamometer for measuring grip force, the MicroFET3 dynamometer not only indicated age-related declines in the maximum grip-force performance but also showed slower and more inconsistent maximal hand-grip strength generation by the elderly.

What are the association patterns between handgrip strength and adverse health conditions? A topical review

Measures of handgrip strength can be used to conveniently assess overall muscle strength capacity. Although stand-alone measures of handgrip strength provide robust health information, the clinical meaningfulness to determine prevention and treatment options for weakness remains limited because the etiology of muscle weakness remains unclear. Moreover, clinical outcomes associated with handgrip strength are wide-ranging. Therefore, disentangling how handgrip strength is associated with health conditions that are metabolically or neurologically driven may improve our understanding of the factors linked to handgrip strength. The purpose of this topical review was to highlight and summarize evidence examining the associations of handgrip strength with certain health outcomes that are metabolically and neurologically driven. From this perusal of the literature, we posit that stand-alone handgrip strength be considered an umbrella assessment of the body systems that contribute to strength capacity, and a panoptic measurement of muscle strength that is representative of overall health status, not a specific health condition. Recommendations for future strength capacity–related research are also provided.

Digit Force Controls and Corresponding Brain Activities in Finger Pressing Performance: A Comparison Between Older Adults and Young Individuals

This study aims toward an investigation and comparison of the digital force control and the brain activities of older adults and young groups during digital pressing tasks. A total of 15 young and 15 older adults were asked to perform force ramp tasks at different force levels with a custom pressing system. Near-infrared spectroscopy was used to collect the brain activities in the prefrontal cortex and primary motor area. The results showed that the force independence and hand function of the older adults were worse than that of the young adults. The cortical activations in the older adults were higher than those in the young group during the tasks. A significant hemodynamic between-group response and mild negative correlations between brain activation and force independence ability were found. Older adults showed poor force independence ability and manual dexterity and required additional brain activity to compensate for the degeneration.

Temporal and Force Characteristics of Rapid Single-Finger Tapping in Healthy Older Adults

The purpose of this study was to examine finger motor function in terms of temporal and force characteristics during rapid single-finger tapping in older adults. Ten older and 10 young males performed maximum frequency tapping by the index, middle, ring, or little finger. Nontapping fingers were maintained in contact with designated keys during tasks. Key-contact force for each of the fingers was monitored using four force transducers. The older subjects had slower tapping rates of all fingers during single-finger tapping than the young subjects. The average forces exerted by the nontapping fingers were larger for the older subjects than for the young subjects during tapping with the ring and little fingers. The ranges of the nontapping finger forces were larger for the older subjects during tapping by the middle, ring, and little fingers than for the young subjects. Thus, the motor abilities of the fingers evaluated by rapid single-finger tapping decline in older adults compared with young adults in terms of both movement speed and finger independence.

Age-related changes to motor synergies in multi-joint and multi-finger manipulative skills: a meta-analysis

Purpose

The aim of the current meta-analysis was to examine the extent to which there are differences in upper extremity motor synergies across different age groups in manipulative tasks.

Methods

The studies that used the uncontrolled manifold method to examine the effect of age on motor synergies in multi-joint and multi-finger tasks were selected. Sixteen relevant studies from 1154 articles were selected for the meta-analysis—4 and 12 studies considered multi-joint kinematics and multi-finger kinetic tasks respectively.

Results

The results of the meta-analysis suggested reduced strength of synergies in multi-finger task in older adults, but this was not the case for synergies in multi-joint task. Part of this age-related difference in finger function is related to the increased variability in total force in grasping tasks. However, reductions in the strength of multi-finger synergies in hand functions following ageing appear to depend on the characteristics of the task.

Conclusions

These findings indicate that the cooperation among fingers to stabilise the total required force to apply for grasping and other fine motor skills is less efficient in older adults that might affect the quality of manipulative tasks.

Bimanual coordination deficits in hands following stroke and their relationship with motor and functional performance

Background

Stroke can lead to movement disorders that affect interlimb coordination control of the bilateral upper extremities, especially the hands. However, few studies have investigated the influence of a stroke on bimanual force coordination control between the hands using a quantitative measurement tool, or the relationship of force coordination with paretic upper extremity motor and functional performance. We aimed to investigate these outcomes using a novel measurement device, and analyze the relationship of bimanual force coordination control deficits in both hands with motor and functional performances of the paretic upper extremity in stroke patients.

Methods

Sixteen healthy adults and 22 stroke patients were enrolled. A novel bilateral hand grip measurement device with two embedded dynamometers was used to evaluate the grip force during a bilateral hand grip-force coordination control task. The alternating time and force applied for coordination with the grip force of both hands were calculated to analyze control of bimanual grip force coordination. Motor and functional measurements included the upper-extremity portion of the Fugl-Meyer assessment (FMA-UE), Wolf Motor Function Test (WMFT), Motor Assessment Scale (MAS), and Barthel Index (BI).

Results

Compared with the healthy group, the alternating time from the non-paretic to the paretic hand was 27.6% shorter for stroke patients (p < 0.001). The grip force generated for coordination in the healthy group was significantly greater (30–59%) than that of the stroke group (p < 0.05), and the coefficients of variation of alternating time (p = 0.001) and force applied (p = 0.002) were significantly higher in the stroke group than the healthy group. The alternating time from the paretic to the non-paretic hand showed moderately significant correlations with the FMA-UE (r = − 0.533; p = 0.011), the WMFT (r = − 0.450; p = 0.036), and the BI (r = − 0.497; p = 0.019).

Conclusions

Stroke results in a decline in bimanual grip force generation and increases the alternating time for coordinating the two hands. A shorter alternating time is moderately to highly associated with enhanced motor and functional performances.

Case Studies in Neuroscience: The central and somatosensory contributions to finger interdependence and coordination: lessons from a study of a "deafferented person"

We tested finger force interdependence and multifinger force-stabilizing synergies in a patient with large-fiber peripheral neuropathy ("deafferented person"). The subject performed a range of tasks involving accurate force production with one finger and with four fingers. In one-finger tasks, nontask fingers showed unintentional force production (enslaving) with an atypical pattern: very large indices for the lateral (index and little) fingers and relatively small indices for the central (middle and ring) fingers. Indices of multifinger synergies stabilizing total force and of anticipatory synergy adjustments in preparation to quick force pulses were similar to those in age-matched control females. During constant force production, removing visual feedback led to a slow force drift to lower values (by ~25% over 15 s). The results support the idea of a neural origin of enslaving and suggest that the patterns observed in the deafferented person were reorganized based on everyday manipulation tasks. The lack of significant changes in the synergy index shows that synergic control can be organized in the absence of somatosensory feedback. We discuss the control of the hand in deafferented persons within the α-model of the equilibrium-point hypothesis and suggest that force drift results from an unintentional drift of the control variables to muscles toward zero values. NEW & NOTEWORTHY We demonstrate atypical patterns of finger enslaving and unchanged force-stabilizing synergies in a person with large-fiber peripheral neuropathy. The results speak strongly in favor of central origin of enslaving and its reorganization based on everyday manipulation tasks. The data show that synergic control can be implemented in the absence of somatosensory feedback. We discuss the control of the hand in deafferented persons within the α-model of the equilibrium-point hypothesis.

Single finger movements in the aging hand: changes in finger independence, muscle activation patterns and tendon displacement in older adults

With aging, hand mobility and manual dexterity decline, even under healthy circumstances. To assess how aging affects finger movement control, we compared elderly and young subjects with respect to (1) finger movement independence, (2) neural control of extrinsic finger muscles and (3) finger tendon displacements during single finger flexion. In twelve healthy older (age 68-84) and nine young (age 22-29) subjects, finger kinematics were measured to assess finger movement enslaving and the range of independent finger movement. Muscle activation was assessed using a multi-channel electrode grid placed over the flexor digitorum superficialis (FDS) and the extensor digitorum (ED). FDS tendon displacements of the index, middle and ring fingers were measured using ultrasound. In older subjects compared to the younger subjects, we found: (1) increased enslaving of the middle finger during index finger flexion (young: 25.6 ± 12.4%, elderly: 47.0 ± 25.1%; p = 0.018), (2) a lower range of independent movement of the index finger (young_middle = 74.0%, elderly_middle: 45.9%; p < 0.001), (3) a more evenly distributed muscle activation pattern over the finger-specific FDS and ED muscle regions and (4) a lower slope at the beginning of the finger movement to tendon displacement relationship, presenting a distinct period with little to no tendon displacement. Our study indicates that primarily the movement independence of the index finger is affected by aging. This can partly be attributed to a muscle activation pattern that is more evenly distributed over the finger-specific FDS and ED muscle regions in the elderly.

Get a grip: individual variations in grip strength are a marker of brain health

Demonstrations that grip strength has predictive power in relation to a range of health conditions-even when these are assessed decades later-has motivated claims that hand-grip dynamometry has the potential to serve as a "vital sign" for middle-aged and older adults. Central to this belief has been the assumption that grip strength is a simple measure of physical performance that provides a marker of muscle status in general, and sarcopenia in particular. It is now evident that while differences in grip strength between individuals are influenced by musculoskeletal factors, "lifespan" changes in grip strength within individuals are exquisitely sensitive to integrity of neural systems that mediate the control of coordinated movement. The close and pervasive relationships between age-related declines in maximum grip strength and expressions of cognitive dysfunction can therefore be understood in terms of the convergent functional and structural mediation of cognitive and motor processes by the human brain. In the context of aging, maximum grip strength is a discriminating measure of neurological function and brain health.

Expectation of movement generates contrasting changes in multifinger synergies in young and older adults

Anticipatory synergy adjustment (ASA) is a feed-forward control mechanism that describes a continuous decrease in the stability of the current motor state beginning about 150 ms prior to a state transition. Recently, we described an associated phenomenon in which the system stability was reduced solely in response to a cue that generates an expectation of a state change, independent of whether the state change actually occurs. Both phenomena are of the same kind (stability reduction), but evoked by distinct antecedent conditions. Since, logically, cuing for movement must occur before the initiation of that movement, we named this new phenomenon 'Stage-1 ASA' and rechristened the well-established version 'Stage-2 ASA'. Here, we used a four-finger, isometric force production task to explore (1) the effect of healthy aging on Stage-1 ASA, and (2) if Stage-1 ASA resulted in a more rapid state change. Young and older adult participants produced 10% of their maximal force when they did not expect to produce any change in the force, and when they expected to change their force in an unknown direction and at an unknown time. In the latter condition, the 10% constant-force phase was followed by a choice reaction time task, in which the participants rapidly changed their force to follow a moving target presented on a computer monitor. Both young and older adults displayed equivalent amount of Stage-1 ASA. This was driven by a 42% reduction in finger-force variability in young adults. In contrast, it was driven by a 38% increase in finger-force variability in older adults. We speculate that the reduction in finger force variability assists the young adults in rapid state changes via two mechanisms: (1) the finger forces occupy a restricted set of states that are optimal for quick state transitions, and (2) lower variability during steady state translates into lower self-motion during state transition. Self-motion is the covariation between finger forces that fails to change the total force. The older adults are unable to adopt this strategy, and the increase in finger-force variability arises from (1) the adoption of an alternative strategy of destabilizing the attractor associated with the current state to facilitate state transitions and (2) the inability to coordinate multiple finger forces. Finally, older adults displayed longer reaction times than young adults, but a clear relation between Stage-1 ASA and consequent behavioral benefit in terms of reduced reaction time remained elusive.

Aging differentially affects online control and offline control in finger force production

Human central nervous system (CNS) undergoes neurological changes during the aging process, leading to declines in hand and finger functions. Previous studies have shown that the CNS can independently process multi-finger force control and moment of force control. However, if both force and moment control are simultaneously imposed by motor task constraints, the CNS needs to resolve competing interests of generating negative and positive covariances between fingers, respectively, which causes "conflict of interest or COI". Here, we investigated how aging affects the CNS's abilities to solve COI through a new experimental paradigm. Both elderly and young subjects performed a constant force production task using index and middle fingers under two conditions, multi-finger pressing with no COI and with COI. We found that the elderly increased variance of a virtual finger (VF: an imagined finger producing the same mechanical effect as both fingers together) in time-to-time basis (i.e. online control), while increasing covariance between individual fingers (IF) forces in trial-to-trial basis (i.e. offline control) with COI than no COI. Aging affects the CNS's abilities to solve COI by deteriorating VF actions in online control and IF actions in offline control.

Increased enslaving in elderly is associated with changes in neural control of the extrinsic finger muscles

Aging has consequences for hand motor control, among others affecting finger force enslaving during static pressing tasks. The aim of this study was to assess whether the extent of finger force enslaving changes with aging during a task that involves both static and dynamic phases. Ten right-handed young (22-30 years) and ten elderly subjects (67-79 years) were instructed to first exert a constant force (static phase) and then flex their index finger while counteracting constant resistance forces orthogonal to their fingertips (dynamic phase). The other fingers (non-instructed) were held in extension. EMG activities of the flexor digitorum superficialis (FDS) and extensor digitorum (ED) muscles in the regions corresponding to the index, middle and ring fingers together with their forces and position of index finger were measured. In both elderly and young, forces exerted by the non-instructed fingers increased (around 0.6 N for both young and elderly) during isotonic flexion of the index finger, but with a different delay of on average 100 ± 72 ms in elderly and 334 ± 101 ms in young subjects. Results also suggest different responses in activity of FDS and ED muscle regions of the non-instructed fingers to index finger flexion between elderly and young subjects. The enslaving effect was significantly higher in elderly than in young subjects both in the static (12% more) and dynamic (14% more) phases. These differences in enslaving can at least partly be explained by changes in neuromuscular control.

Spiral tracing on a touchscreen is influenced by age, hand, implement, and friction

Dexterity impairments are well documented in older adults, though it is unclear how these influence touchscreen manipulation. This study examined age-related differences while tracing on high- and low-friction touchscreens using the finger or stylus. 26 young and 24 older adults completed an Archimedes spiral tracing task on a touchscreen mounted on a force sensor. Root mean square error was calculated to quantify performance. Root mean square error increased by 29.9% for older vs. young adults using the fingertip, but was similar to young adults when using the stylus. Although other variables (e.g., touchscreen usage, sensation, and reaction time) differed between age groups, these variables were not related to increased error in older adults while using their fingertip. Root mean square error also increased on the low-friction surface for all subjects. These findings suggest that utilizing a stylus and increasing surface friction may improve touchscreen use in older adults.

Multi-Finger Interaction and Synergies in Finger Flexion and Extension Force Production

The aim of this study was to discover finger interaction indices during single-finger ramp tasks and multi-finger coordination during a steady state force production in two directions, flexion, and extension. Furthermore, the indices of anticipatory adjustment of elemental variables (i.e., finger forces) prior to a quick pulse force production were quantified. It is currently unknown whether the organization and anticipatory modulation of stability properties are affected by force directions and strengths of in multi-finger actions. We expected to observe a smaller finger independency and larger indices of multi-finger coordination during extension than during flexion due to both neural and peripheral differences between the finger flexion and extension actions. We also examined the indices of the anticipatory adjustment between different force direction conditions. The anticipatory adjustment could be a neural process, which may be affected by the properties of the muscles and by the direction of the motions. The maximal voluntary contraction (MVC) force was larger for flexion than for extension, which confirmed the fact that the strength of finger flexor muscles (e.g., flexor digitorum profundus) was larger than that of finger extensor (e.g., extensor digitorum). The analysis within the uncontrolled manifold (UCM) hypothesis was used to quantify the motor synergy of elemental variables by decomposing two sources of variances across repetitive trials, which identifies the variances in the uncontrolled manifold (V_UCM) and that are orthogonal to the UCM (V_ORT). The presence of motor synergy and its strength were quantified by the relative amount of V_UCM and V_ORT. The strength of motor synergies at the steady state was larger in the extension condition, which suggests that the stability property (i.e., multi-finger synergies) may be a direction specific quantity. However, the results for the existence of anticipatory adjustment; however, no difference between the directional conditions suggests that feed-forward synergy adjustment (changes in the stability property) may be at least independent of the magnitude of the task-specific apparent performance variables and its direction (e.g., flexion and extension forces).

Reference values of manual dexterity using Minnesota Rate of Manipulation Test for Indian adults

Introduction

Dexterity assessment is an integral component of hand function evaluation. The Minnesota Rate of Manipulation Test is a valid and reliable tool to evaluate manual dexterity and has been used to study prognosis of therapy. Information available on its reference values is limited. Reference values are required to interpret, set realistic treatment goals, and assess the ability to return to employment. Therefore, manual dexterity was studied in healthy Indian adults to generate reference values.

Methods

A descriptive study was conducted on 900 subjects divided into three groups: Group 1: 18–30 years ( n = 300): Group 2: 31–60 years (n = 300), and Group 3: ≥61 years ( n = 300). The Minnesota Rate of Manipulation Test was used to evaluate manual dexterity. All subjects performed three trials (one practice + two test trials) of the five subtests. The average of two test trials was used to compute the Minnesota Rate of Manipulation Test score.

Results

The total score of the Minnesota Rate of Manipulation Test in Groups 1, 2, and 3 was 275.7 ± 24.1 s, 299.3 ± 31.9 s, and 357.4 ± 48.6 s, respectively. Manual dexterity was moderately associated with age ( r = 0.707, p < 0.001); however, differences between males and females were nonsignificant ( p = 0.306).

Conclusion

Manual dexterity declined with advancing age; however, differences between genders were not significant. These reference values of manual hand dexterity for healthy adult Indian people will be relevant for clinical assessment of hand function.

Changes in Multidigit Synergies and Their Feed-Forward Adjustments in Multiple Sclerosis

The authors explored the changes in multidigit synergies in patients with multiple sclerosis (MS) within the framework of the uncontrolled manifold hypothesis. The specific hypotheses were that both synergy indices and anticipatory synergy adjustments prior to the initiation of a self-paced quick action would be diminished in the patients compared to age-matched controls. The MS patients and age-matched controls (n = 13 in both groups) performed one-finger and multifinger force production tasks involving both accurate steady-state force production and quick force pulses. The patients showed significantly lower maximal finger forces and a tendency toward slower force pulses. Enslaving was increased in MS, but only in the lateral fingers (index and little). Indices of multifinger synergies during steady-state force production were lower in MS, mainly due to the lower amount of intertrial variance that did not affect total force. Anticipatory synergy adjustments were significantly delayed in MS. The results show that MS leads to significant changes in multidigit synergies and feed-forward adjustments of the synergies prior to a quick action. The authors discuss possible contributions of subcortical structures to the impaired synergic control.

Towards physics of neural processes and behavior

Behavior of biological systems is based on basic physical laws, common across inanimate and living systems, and currently unknown physical laws that are specific for living systems. Living systems are able to unite basic laws of physics into chains and clusters leading to new stable and pervasive relations among variables (new physical laws) involving new parameters and to modify these parameters in a purposeful way. Examples of such laws are presented starting from the tonic stretch reflex. Further, the idea of control with referent coordinates is formulated and merged with the idea of hierarchical control and the principle of abundance. The notion of controlled stability of behaviors is linked to the idea of structured variability, which is a common feature across living systems and actions. The explanatory and predictive power of this approach is illustrated with respect to the control of both intentional and unintentional movements, the phenomena of equifinality and its violations, preparation to quick actions, development of motor skills, changes with aging and neurological disorders, and perception.

Effects of unilateral stroke on multi-finger synergies and their feed-forward adjustments

We explored the changes in multi-finger synergies in patients after a single cortical stroke with mild motor impairments. We hypothesized that both synergy indices and anticipatory synergy adjustments prior to the initiation of a self-paced quick action would be diminished in the patients compared to age-matched controls. The patients with history of cortical stroke, and age-matched controls (n=12 in each group) performed one-finger and multi-finger accurate force production tasks involving both steady-state and quick force pulse production. Finger interdependence (enslaving) and multi-finger synergies stabilizing total force were quantified. The stroke patients showed lower maximal finger forces, in particular in the contralesional hand, which also showed increased enslaving indices. Multi-finger synergies during steady-state force production were, however, unchanged after stroke. In contrast, a drop in the synergy index prior to the force pulse generation was significantly delayed in the stroke patients. Our results show that mild cortical stroke leads to no significant changes in multifinger synergies, but there is impairment in feed-forward adjustments of the synergies prior to a quick action, a drop in the maximal force production, and an increase in enslaving. We conclude that studies of synergies reveal two aspects of synergic control differentially affected by cortical stroke.

Influence of spatial accuracy constraints on reaction time and maximum speed of performance of unilateral movements

The goal was to study reaction time and maximal velocity of upper limbs of healthy young adults of both sexes during transition from a simple to a more involved task. Performance of dominant and non-dominant arms was recorded. Participants were 43 healthy, right-handed, untrained men (n=22) and women (n=21), 18-22 years old. The simple task required a single jerk-like movement. The involved task required both speed and accuracy where necessity for high speed of performance was emphasized. The effectiveness of transition between tasks was calculated for both reaction time and maximal velocity. No lateral differences were found. Men usually had a shorter reaction time on both tasks and a higher maximal velocity in the simple task. Women were more effective at modifying velocity.

Changes in sensory function and force production in adults with type II diabetes

INTRODUCTION

The purpose of this study was to evaluate the relationship among sensory function, disease severity, and upper extremity force production in adults with type II diabetes (T2D) as compared with healthy age- and gender-matched controls.

METHODS

Ten adults with T2D and 10 healthy age- and gender-matched control subjects underwent a battery of sensory and motor function evaluations. Data on disease severity and duration were also collected.

RESULTS

The T2D group exhibited sensory deficits and altered force production as compared with healthy controls. Sensory function correlated with disease severity, as did signal predictability of kinetic output during submaximal force production tasks. Maximal force production tasks were associated with altered output in T2D, but these data did not correlate with disease severity or sensory dysfunction.

CONCLUSIONS

Some, not all, motor performance deficits in T2D are associated with sensory dysfunction. Mechanisms responsible for these changes in adult-onset T2D are described.

Stability of multifinger action in different state spaces

We investigated stability of action by a multifinger system with three methods: analysis of intertrial variance, application of transient perturbations, and analysis of the system's motion in different state spaces. The "inverse piano" device was used to apply transient (lifting-and-lowering) perturbations to individual fingers during single- and two-finger accurate force production tasks. In each trial, the perturbation was applied either to a finger explicitly involved in the task or one that was not. We hypothesized that, in one-finger tasks, task-specific stability would be observed in the redundant space of finger forces but not in the nonredundant space of finger modes (commands to explicitly involved fingers). In two-finger tasks, we expected that perturbations applied to a nontask finger would not contribute to task-specific stability in mode space. In contrast to our expectations, analyses in both force and mode spaces showed lower stability in directions that did not change total force output compared with directions that did cause changes in total force. In addition, the transient perturbations led to a significant increase in the enslaving index. We consider these results within a theoretical scheme of control with referent body configurations organized hierarchically, using multiple few-to-many mappings organized in a synergic way. The observed volatility of enslaving, greater equifinality of total force compared with elemental variables, and large magnitude of motor equivalent motion in both force and mode spaces provide support for the concept of task-specific stability of performance and the existence of multiple neural loops, which ensure this stability.

Validity and reliability of a novel device for bilateral upper extremity functional measurements

BACKGROUND AND OBJECTIVE

This study was designed to establish the validity and reliability of a new device that measures bilateral shoulder and elbow range of motion (ROM) and grip force performance in vivo. A further aim was to investigate the control of inter-limb grip force coordination during isometric force-maintenance tasks. Validity of the ROM and grip force measurements was examined using a validated clinical goniometer and standard weights.

SUBJECTS

Twenty-one healthy adults (six female, 15 male; mean±standard deviation age=23.05±3.51) were recruited for this study.

DESIGN

All subjects were asked to perform tests to evaluate the validity and reliability of ROM, grip force maximum voluntary contraction (MVC) and coordination control measurements.

RESULTS

The ROM and grip force measurements were linearly correlated with criterion standards. For reliability testing, all of the intraclass correlation coefficient values were >0.99. The inter-limb grip force coordination control task showed that the force modulation timing during dominant-to-non-dominant hand transition was longer than the non-dominant-to-dominant hand transition (p<0.05).

CONCLUSIONS

These results demonstrate that this device is valid and reliable when used to measure shoulder and elbow ROM and grip force of both hands. Isometric force-maintenance tasks also indicated changes in inter-limb grip force control.

Quantification of dexterity as the dynamical regulation of instabilities: comparisons across gender, age, and disease

Dexterous manipulation depends on using the fingertips to stabilize unstable objects. The Strength-Dexterity paradigm consists of asking subjects to compress a slender and compliant spring prone to buckling. The maximal level of compression [requiring low fingertip forces <300 grams force (gf)] quantifies the neural control capability to dynamically regulate fingertip force vectors and motions for a dynamic manipulation task. We found that finger dexterity is significantly affected by age (p = 0.017) and gender (p = 0.021) in 147 healthy individuals (66F, 81M, 20-88 years). We then measured finger dexterity in 42 hands of patients following treatment for osteoarthritis of the base of the thumb (CMC OA, 33F, 65.8 ± 9.7 years), and 31 hands from patients being treated for Parkinson's disease (PD, 6F, 10M, 67.68 ± 8.5 years). Importantly, we found no differences in finger compression force among patients or controls. However, we did find stronger age-related declines in performance in the patients with PD (slope -2.7 gf/year, p = 0.002) than in those with CMC OA (slope -1.4 gf/year, p = 0.015), than in controls (slope -0.86 gf/year). In addition, the temporal variability of forces during spring compression shows clearly different dynamics in the clinical populations compared to the controls (p < 0.001). Lastly, we compared dexterity across extremities. We found stronger age (p = 0.005) and gender (p = 0.002) effects of leg compression force in 188 healthy subjects who compressed a larger spring with the foot of an isolated leg (73F, 115M, 14-92 years). In 81 subjects who performed the tests with all four limbs separately, we found finger and leg compression force to be significantly correlated (females ρ = 0.529, p = 0.004; males ρ = 0.403, p = 0.003; 28F, 53M, 20-85 years), but surprisingly found no differences between dominant and non-dominant limbs. These results have important clinical implications, and suggest the existence - and compel the investigation - of systemic versus limb-specific mechanisms for dexterity.

Mechanical properties of the human hand digits: age-related differences

BACKGROUND

Mechanical properties of human digits may have significant implications for the hand function. We quantified several mechanical characteristics of individual digits in young and older adults.

METHODS

Digit tip friction was measured at several normal force values using a method of induced relative motion between the digit tip and the object surface. A modified quick-release paradigm was used to estimate digit apparent stiffness, damping, and inertial parameters. The subjects grasped a vertical handle instrumented with force/moment sensors using a prismatic grasp with four digits; the handle was fixed to the table. Unexpectedly, one of the sensors yielded leading to a quick displacement of the corresponding digit. A second-order, linear model was used to fit the force/displacement data.

FINDINGS

Friction of the digit pads was significantly lower in older adults. The apparent stiffness coefficient values were higher while the damping coefficients were lower in older adults leading to lower damping ratio. The damping ratio was above unity for most data in young adults and below unity for older adults. Quick release of a digit led to force changes in other digits of the hand, likely due to inertial hand properties. These phenomena of "mechanical enslaving" were smaller in older adults although no significant difference was found in the inertial parameter in the two groups.

INTERPRETATIONS

The decreased friction and damping ratio present challenges for the control of everyday prehensile tasks. They may lead to excessive digit forces and low stability of the grasped object.

Finger enslaving in the dominant and non-dominant hand

During single-finger force production, the non-instructed fingers unintentionally produce force (finger enslaving). In this study, enslaving effects were compared between the dominant and non-dominant hands. The test consisted of a series of maximum voluntary contractions with different finger combinations. Enslaving matrices were calculated by means of training an artificial neural network. The dominant hand was found to be stronger, but there was found to be no difference between the overall enslaving effects in the dominant and non-dominant hands. There was no correlation between the magnitude of finger enslaving and the performance in such tests as the Edinburgh Handedness Inventory, the Grooved Pegboard test, and the Jebsen-Taylor Hand Function test. Each one of those three tests showed a significant difference between the dominant and non-dominant hand performances. Eleven subjects were retested after two months, and it was found that enslaving effects did not fluctuate significantly between the two testing sessions. While the dominant and non-dominant hands are involved differently in everyday tasks, e.g. in writing or eating, this practice does not cause significant differences in enslaving between the hands.

Enslaving in a serial chain: interactions between grip force and hand force in isometric tasks

This study was motivated by the double action of extrinsic hand muscles that produce grip force and also contribute to wrist torque. We explored interactions between grip force and wrist torque in isometric force production tasks. In particular, we tested a hypothesis that an intentional change in one of the two kinetic variables would produce an unintentional change in the other (enslaving). When young healthy subjects produced accurate changes in the grip force, only minor effects on the force produced by the hand (by wrist flexion/extension action) were observed. In contrast, a change in the hand force produced consistent changes in grip force in the same direction. The magnitude of such unintentional grip force change was stronger for intentional hand force decrease as compared to hand force increase. These effects increased with the magnitude of the initial grip force. When the subjects were asked to produce accurate total force computed as the sum of the hand and grip forces, strong negative covariation between the two forces was seen across trials interpreted as a synergy stabilizing the total force. An index of this synergy was higher in the space of "modes," hypothetical signals to the two effectors that could be changed by the controller one at a time. We interpret the complex enslaving effects (positive force covariation) as conditioned by typical everyday tasks. The presence of synergic effects (negative, task-specific force covariation) can be naturally interpreted within the referent configuration hypothesis.

Dopaminergic modulation of motor coordinaton in Parkinson's disease

BACKGROUND

We applied the idea of synergies and the framework of the uncontrolled manifold hypothesis to explore the effects of dopamine replacement therapy on finger interaction and coordination in patients with early-stage Parkinson's disease (PD).

METHODS

Eight patients performed single-finger and multi-finger force production tasks with both the dominant and non-dominant hand before (off-drug) and after (on-drug) taking their dopaminergic medications. Synergy indices were defined as co-varied adjustments of commands to fingers that stabilized the total force produced by the hand.

RESULTS

PD patients showed significantly lower maximal finger forces off-drug compared to the on-drug condition, while indices of finger individuation (enslaving) were unchanged. The synergy indices were weaker during steady-state force production off-drug compared to on-drug. Anticipatory adjustments of synergies prior to the quick force pulse initiation were delayed and reduced off-drug as compared to the on-drug condition. These drug effects were observed in both the symptomatic and asymptomatic hands of the patients whose symptoms were limited to one side of the body.

CONCLUSIONS

The study demonstrates dopaminergic modulation of motor coordination in PD and supports that the analysis of different components of multi-finger synergies offers a set of indices sensitive to the effects of dopamine replacement therapy in early-stage PD. The results suggest an important role of the basal ganglia in synergy formation and in feed-forward synergy adjustments. Future studies using these methods may yield more objective, quantitative biomarker(s) of motor coordination impairments in PD, and better understanding of subcortical involvement in the neural control of natural actions.