Associations among strength, steadiness, and hand function across the adult life span

Motor unit discharge characteristics and walking performance of individuals with multiple sclerosis

Walking performance of persons with multiple sclerosis (MS) is strongly influenced by the activation signals received by lower leg muscles. We examined the associations between force steadiness and motor unit discharge characteristics of lower leg muscles during submaximal isometric contractions with tests of walking performance and disability status in individuals who self-reported walking difficulties due to MS. We expected that worse walking performance would be associated with weaker plantar flexor muscles, worse force steadiness, and slower motor unit discharge times. Twenty-three individuals with relapsing-remitting MS (56 ± 7 yr) participated in the study. Participants completed one to three evaluation sessions that involved two walking tests (25-ft walk and 6-min walk), a manual dexterity test (grooved pegboard), health-related questionnaires, and measurement of strength, force steadiness, and motor unit discharge characteristics of lower leg muscles. Multiple regression analyses were used to construct models to explain the variance in measures of walking performance. There were statistically significant differences (effect sizes: 0.21-0.60) between the three muscles in mean interspike interval (ISI) and ISI distributions during steady submaximal contractions with the plantar flexor and dorsiflexor muscles. The regression models explained 40% of the variance in 6-min walk distance and 47% of the variance in 25-ft walk time with two or three variables that included mean ISI for one of the plantar flexor muscles, dorsiflexor strength, and force steadiness. Walking speed and endurance in persons with relapsing-remitting MS were reduced in individuals with longer ISIs, weaker dorsiflexors, and worse plantar flexor force steadiness. NEW & NOTEWORTHY The walking endurance and gait speed of persons with relapsing-remitting multiple sclerosis (MS) were worse in individuals who had weaker dorsiflexor muscles and greater force fluctuations and longer times between action potentials discharged by motor units in plantar flexor muscles during steady isometric contractions. These findings indicate that the control of motor unit activity in lower leg muscles of individuals with MS is associated with their walking ability.

Robotic finger perturbation training improves finger postural steadiness and hand dexterity

The purpose of the study was to understand the effect of robotic finger perturbation training on steadiness in finger posture and hand dexterity in healthy young adults. A mobile robotic finger training system was designed to have the functions of high-speed mechanical response, two degrees of freedom, and adjustable loading amplitude and direction. Healthy young adults were assigned to one of the three groups: random perturbation training (RPT), constant force training (CFT), and control. Subjects in RPT and CFT performed steady posture training with their index finger using the robot in different modes: random force in RPT and constant force in CFT. After the 2-week intervention period, fluctuations of the index finger posture decreased only in RPT during steady position-matching tasks with an inertial load. Purdue pegboard test score improved also in RPT only. The relative change in finger postural fluctuations was negatively correlated with the relative change in the number of completed pegs in the pegboard test in RPT. The results indicate that finger posture training with random mechanical perturbations of varying amplitudes and directions of force is effective in improving finger postural steadiness and hand dexterity in healthy young adults.

Voluntary activation and variability during maximal dynamic contractions with aging

Whether reduced supraspinal activation contributes to age-related reductions in maximal torque during dynamic contractions is not known. The purpose was to determine whether there are age differences in voluntary activation and its variability when assessed with stimulation at the motor cortex and the muscle during maximal isometric, concentric, and eccentric contractions. Thirty young (23.6 ± 4.1 years) and 31 old (69.0 ± 5.2 years) adults performed maximal isometric, shortening (concentric) and lengthening (eccentric) contractions with the elbow flexor muscles. Maximal isometric contractions were performed at 90° elbow flexion and dynamic contractions at a velocity of 60°/s. Voluntary activation was assessed by superimposing an evoked contraction with transcranial magnetic stimulation (TMS) or with electrical stimulation over the muscle during maximal voluntary contractions (MVCs). Old adults had lower MVC torque during isometric (- 17.9%), concentric (- 19.7%), and eccentric (- 9.9%) contractions than young adults, with less of an age difference for eccentric contractions. Voluntary activation was similar between the three contraction types when assessed with TMS and electrical stimulation, with no age group differences. Old adults, however, were more variable in voluntary activation than young (standard deviation 0.99 ± 0.47% vs. 0.73 ± 0.43%, respectively) to both the motor cortex and muscle, and had greater coactivation of the antagonist muscles during dynamic contractions. Thus, the average voluntary activation to the motor cortex and muscle did not differ with aging; however, supraspinal activation was more variable during maximal dynamic and isometric contractions in the old adults. Lower predictability of voluntary activation may indicate subclinical changes in the central nervous system with advanced aging.

A framework for identifying the adaptations responsible for differences in pegboard times between middle-aged and older adults

Time to complete two tests of manual dexterity, the 9-hole Peg Test and Grooved Pegboard Test, increases with advancing age. However, the adaptations responsible for the differences in pegboard times between middle-aged and older adults are largely unknown. Potential mechanisms include neuromuscular characteristics, cognitive function, and cutaneous sensation. To provide a tractable framework to address these gaps in knowledge, the purpose of the current study was to identify the latent variables underlying age-associated differences in time to complete the 9-hole and grooved pegboard tests. The approach involved an independent component analysis that identified associations between the two pegboard times for the two groups of participants with two to six secondary outcomes. The common association across three of the four conditions (two groups and two pegboard tests) was features derived from force-matching tasks requiring submaximal isometric contraction. In addition, there were significant associations for older adults between age, measures of cognitive function, and pegboard times. Nonetheless, the significant associations were unique for each age group and pegboard test. The results provide a framework for subsequent mechanistic studies to identify the adaptations underlying age-associated declines in manual dexterity.

Peg-manipulation capabilities during a test of manual dexterity differ for persons with multiple sclerosis and healthy individuals

Manual dexterity declines with advancing age and the development of neurological disorders. Changes in manual dexterity are frequently quantified as the time it takes to complete the grooved pegboard test, which requires individuals to manipulate 25 pegs. The manipulation of each peg involves four phases: selection, transport, insertion, and return. The purpose of the study was to compare the times to complete the four phases of manipulating each peg and the forces applied to the pegboard during peg selection and insertion in persons with multiple sclerosis (MS) and age- and sex-matched healthy adults. Multiple-regression models that could explain the variance in pegboard times for each group of participants were compared to assess the relative significance of the peg-manipulation attributes. The performance of 17 persons with MS (52.2 ± 8.3 years) was compared with 17 control subjects (52.2 ± 11.5 years). The grooved pegboard test was performed on a force plate. Pegboard times for the MS group (104 ± 40 s) were longer than those for the Control group (61 ± 15 s). Regression analysis indicated that the pegboard times for the MS group could be predicted by the time for the peg-selection phase (R ² = 0.78), whereas the predictors for Control group (R ² = 0.77) were the times for the peg-transport (partial r = 0.80) and selection (partial r = 0.58) phases. The variance in the time it took the MS participants to complete the grooved pegboard test was strongly related to the time required to select each peg, whereas the pegboard times for the Control subjects depended mostly on the duration of the transport phase but also on the time to select each peg.

Predicting hand function in older adults: evaluations of grip strength, arm curl strength, and manual dexterity

BACKGROUND

Hand function is critical for independence in activities of daily living for older adults.

AIMS

The purpose of this study was to examine how grip strength, arm curl strength, and manual dexterous coordination contributed to time-based versus self-report assessment of hand function in community-dwelling older adults.

METHODS

Adults aged ≥60 years without low vision or neurological disorders were recruited. Purdue Pegboard Test, Jamar hand dynamometer, 30-second arm curl test, Jebsen-Taylor Hand Function Test, and the Late-Life Function and Disability Instrument were administered to assess manual dexterous coordination, grip strength, arm curl strength, time-based hand function, and self-report of hand function, respectively.

RESULTS

Eighty-four adults (mean age = 72 years) completed the study. Hierarchical multiple regressions show that older adults with better arm curl strength (β = -.25, p < .01) and manual dexterous coordination (β = -.52, p < .01) performed better on the time-based hand function test. In comparison, older adults with better grip strength (β = .40, p < .01), arm curl strength (β = .23, p < .05), and manual dexterous coordination (β = .23, p < .05) were associated with better self-report of upper extremity function.

CONCLUSIONS

The relationship between grip strength and hand function may be test-specific. Grip strength becomes a significant factor when the test requires grip strength to successfully complete the test tasks. Arm curl strength independently contributed to hand function in both time-based and self-report assessments, indicating that strength of extrinsic muscles of the hand are essential for hand function.

Altered visual strategies and attention are related to increased force fluctuations during a pinch grip task in older adults

The purpose of the study was to determine the visual strategies used by older adults during a pinch grip task and to assess the relations between visual strategy, deficits in attention, and increased force fluctuations in older adults. Eye movements of 23 older adults (>65 yr) were monitored during a low-force pinch grip task while subjects viewed three common visual feedback displays. Performance on the Grooved Pegboard test and an attention task (which required no concurrent hand movements) was also measured. Visual strategies varied across subjects and depended on the type of visual feedback provided to the subjects. First, while viewing a high-gain compensatory feedback display (horizontal bar moving up and down with force), 9 of 23 older subjects adopted a strategy of performing saccades during the task, which resulted in 2.5 times greater force fluctuations in those that exhibited saccades compared with those who maintained fixation near the target line. Second, during pursuit feedback displays (force trace moving left to right across screen and up and down with force), all subjects exhibited multiple saccades, and increased force fluctuations were associated (r_s = 0.6; P = 0.002) with fewer saccades during the pursuit task. Also, decreased low-frequency (<4 Hz) force fluctuations and Grooved Pegboard times were significantly related (P = 0.033 and P = 0.005, respectively) with higher (i.e., better) attention z scores. Comparison of these results with our previously published results in young subjects indicates that saccadic eye movements and attention are related to force control in older adults.NEW & NOTEWORTHY The significant contributions of the study are the addition of eye movement data and an attention task to explain differences in hand motor control across different visual displays in older adults. Older participants used different visual strategies across varying feedback displays, and saccadic eye movements were related with motor performance. In addition, those older individuals with deficits in attention had impaired motor performance on two different hand motor control tasks, including the Grooved Pegboard test.

Effects of age on force steadiness: A literature review and meta-analysis

The variability of force is indicative of the biological variability inherent in the human motor system. Previous literature showed inconsistent findings of the effect of age on the variability of force and hence a systematic review was performed. Twenty studies were included in this systematic review, of which twelve provided sufficient data to determine effect sizes for the effect of age. After determining the pooled effect size, the effect of sample size on dichotomized effect sizes (significant vs. non-significant) was determined. Also, the effect of possible determinants, age difference between age groups, dominance of investigated limb, muscle group, muscle location (proximal vs. distal and upper vs. lower extremity) and target force level on effect size (categorized as small, medium, or large) were investigated. A large pooled effect size of age was found (r_total=0.67, 95% CI [0.61; 0.72]). No relation between sample size and effect size significance was found, indicative of no lack of power in the studies reviewed. No relations were found of associations between age difference, upper vs. lower extremity muscle location, and dominance and effect size. Significant relations of effect size with muscle group, proximal vs. distal muscle location and target force level were found. Also, an interaction effect of muscle group and target force level was suggested. The meta-analysis results are in line with motor unit loss as the main cause of the effect of ageing on force steadiness and this effect can partially explain decreased motor performance associated with ageing.

Alterations in Neural Control of Constant Isometric Contraction with the Size of Error Feedback

Discharge patterns from a population of motor units (MUs) were estimated with multi-channel surface electromyogram and signal processing techniques to investigate parametric differences in low-frequency force fluctuations, MU discharges, and force-discharge relation during static force-tracking with varying sizes of execution error presented via visual feedback. Fourteen healthy adults produced isometric force at 10% of maximal voluntary contraction through index abduction under three visual conditions that scaled execution errors with different amplification factors. Error-augmentation feedback that used a high amplification factor (HAF) to potentiate visualized error size resulted in higher sample entropy, mean frequency, ratio of high-frequency components, and spectral dispersion of force fluctuations than those of error-reducing feedback using a low amplification factor (LAF). In the HAF condition, MUs with relatively high recruitment thresholds in the dorsal interosseous muscle exhibited a larger coefficient of variation for inter-spike intervals and a greater spectral peak of the pooled MU coherence at 13-35 Hz than did those in the LAF condition. Manipulation of the size of error feedback altered the force-discharge relation, which was characterized with non-linear approaches such as mutual information and cross sample entropy. The association of force fluctuations and global discharge trace decreased with increasing error amplification factor. Our findings provide direct neurophysiological evidence that favors motor training using error-augmentation feedback. Amplification of the visualized error size of visual feedback could enrich force gradation strategies during static force-tracking, pertaining to selective increases in the discharge variability of higher-threshold MUs that receive greater common oscillatory inputs in the β-band.

Age differences in dynamic fatigability and variability of arm and leg muscles: Associations with physical function

INTRODUCTION

It is not known whether the age-related increase in fatigability of fast dynamic contractions in lower limb muscles also occurs in upper limb muscles. We compared age-related fatigability and variability of maximal-effort repeated dynamic contractions in the knee extensor and elbow flexor muscles; and determined associations between fatigability, variability of velocity between contractions and functional performance.

METHODS

35 young (16 males; 21.0±2.6years) and 32 old (18 males; 71.3±6.2years) adults performed a dynamic fatiguing task involving 90 maximal-effort, fast, concentric, isotonic contractions (1 contraction/3s) with a load equivalent to 20% maximal voluntary isometric contraction (MVIC) torque with the elbow flexor and knee extensor muscles on separate days. Old adults also performed tests of balance and walking endurance.

RESULTS

Old adults had greater fatigue-related reductions in peak velocity compared with young adults for both the elbow flexor and knee extensor muscles (P<0.05) with no sex differences (P>0.05). Old adults had greater variability of peak velocity during the knee extensor, but not during the elbow flexor fatiguing task. The age difference in fatigability was greater for the knee extensor muscles (35.9%) compared with elbow flexor muscles (9.7%, P<0.05). Less fatigability of the knee extensor muscles was associated with greater walking endurance (r=-0.34, P=0.048) and balance (r=-0.41, P=0.014) among old adults.

CONCLUSIONS

An age-related increase in fatigability of a dynamic fatiguing task was greater for the knee extensor compared with the elbow flexor muscles in males and females, and greater fatigability was associated with lesser walking endurance and balance.

The aging neuromuscular system and motor performance

Age-related changes in the basic functional unit of the neuromuscular system, the motor unit, and its neural inputs have a profound effect on motor function, especially among the expanding number of old (older than ∼60 yr) and very old (older than ∼80 yr) adults. This review presents evidence that age-related changes in motor unit morphology and properties lead to impaired motor performance that includes 1) reduced maximal strength and power, slower contractile velocity, and increased fatigability; and 2) increased variability during and between motor tasks, including decreased force steadiness and increased variability of contraction velocity and torque over repeat contractions. The age-related increase in variability of motor performance with aging appears to involve reduced and more variable synaptic inputs that drive motor neuron activation, fewer and larger motor units, less stable neuromuscular junctions, lower and more variable motor unit action potential discharge rates, and smaller and slower skeletal muscle fibers that coexpress different myosin heavy chain isoforms in the muscle of older adults. Physical activity may modify motor unit properties and function in old men and women, although the effects on variability of motor performance are largely unknown. Many studies are of cross-sectional design, so there is a tremendous opportunity to perform high-impact and longitudinal studies along the continuum of aging that determine 1) the influence and cause of the increased variability with aging on functional performance tasks, and 2) whether lifestyle factors such as physical exercise can minimize this age-related variability in motor performance in the rapidly expanding numbers of very old adults.

Improved motor and cognitive performance with sodium nitrite supplementation is related to small metabolite signatures: a pilot trial in middle-aged and older adults

Advancing age is associated with reductions in nitric oxide bioavailability and changes in metabolic activity, which are implicated in declines in motor and cognitive function. In preclinical models, sodium nitrite supplementation (SN) increases plasma nitrite and improves motor function, whereas other nitric oxide-boosting agents improve cognitive function. This pilot study was designed to translate these findings to middle-aged and older (MA/O) humans to provide proof-of-concept support for larger trials. SN (10 weeks, 80 or 160 mg/day capsules, TheraVasc, Inc.) acutely and chronically increased plasma nitrite and improved performance on measures of motor and cognitive outcomes (all p<0.05 or better) in healthy MA/O adults (62 ± 7 years). Untargeted metabolomics analysis revealed that SN significantly altered 33 (160 mg/day) to 45 (80 mg/day) different metabolites, 13 of which were related to changes in functional outcomes; baseline concentrations of 99 different metabolites predicted functional improvements with SN. This pilot study provides the first evidence that SN improves aspects of motor and cognitive function in healthy MA/O adults, and that these improvements are associated with, and predicted by, the plasma metabolome. Our findings provide the necessary support for larger clinical trials on this promising pharmacological strategy for preserving physiological function with aging.

Force Maintenance Accuracy Using a Tool: Effects of Magnitude and Feedback

The ability to precisely produce a force via a hand-held tool is crucial in fine manipulations. In this paper, we study the error in maintaining a target force ranging from 0.5 to 5 N under two concurrent feedback conditions: pure haptic feedback (H), and visual plus haptic feedback (V + H). The results show that absolute error (AE) increases along with the increasing force magnitudes under both feedback conditions. For target forces ranging from 1.5 to 5 N, the relative error (RE) is approximately constant under both feedback conditions, while the RE significantly increases for the small target forces of 0.5 and 1 N. The effect of force magnitude on the coefficient of variation (CoV) is not significant for target forces ranging from 1.5 to 5 N. For both the RE and the CoV, the values under the H condition are significantly larger than those under the V + H condition. The effect of manipulation mode (i.e., a hand-held tool or a fingertip) on force maintenance accuracy is complex, i.e., its effect on RE is not significant while its effect on CoV is significant. Only for the magnitude of 0.5 N, the RE of using the tool was significantly greater than that of using the fingertip under both feedback conditions. For both the RE and the CoV, no interaction effect exists between manipulation mode, force magnitude and feedback condition.

Force steadiness as a predictor of time to complete a pegboard test of dexterity in young men and women

The purpose of the study was to evaluate the capacity of an expanded set of force steadiness tasks to explain the variance in the time it takes young men and women to complete the grooved pegboard test. In a single experimental session, 30 participants (mean ± SD) (24.2 ± 4.0 yr; 15 women) performed the grooved pegboard test, two tests of hand speed, measurements of muscle strength, and a set of submaximal, steady contractions. The steadiness tasks involved single and double actions requiring isometric contractions in the directions of wrist extension, a pinch between the index finger and thumb, and index finger abduction. Time to complete the grooved pegboard test ranged from 41.5 to 67.5 s. The pegboard times (53.9 ± 6.2 s) were not correlated with any of the strength measurements or the reaction time test of hand speed. A stepwise, multiple-regression analysis indicated that much of the variance (R(2) = 0.70) in pegboard times could be explained by a model that comprised two predictor variables derived from the steadiness tasks: time to match the target during a rapid force-matching task and force steadiness (coefficient of variation for force) during a single-action task. Moreover, the pegboard times were significantly faster for women (51.7 ± 6.8 s) than men (56.1 ± 4.9 s). Participants with slower pegboard times seemed to place a greater emphasis on accuracy than speed as they had longer times to match the target during the rapid force-matching task and exhibited superior force steadiness during the single-action task.

Force fluctuations while pressing and moving against high- and low-friction touch screen surfaces

The purpose of this study was to identify the influence of a high- and low-friction surface on the ability to maintain a steady downward force during an index finger pressing and moving task. Fifteen right-handed subjects (24-48 years) performed a static force pressing task and a hybrid pressing and moving task on the surface of an iPad mini while holding a steady 2-N force on high- and low-friction surfaces. Variability of force was quantified as the standard deviation (SD) of normal force (F z) and shear force (F xy) across friction conditions and tasks. The SD of F z was 227 % greater during the hybrid task as compared to the static task (p < .001) and was 19 % greater for the high- versus low-friction condition (p = .033). There were positive correlations between SD of F z and F xy during the hybrid force/motion tasks on the high- and low-friction conditions (r (2) = 0.5 and 0.86, respectively), suggesting significant associations between normal and shear forces for this hybrid task. The correlation between the SD of F z for static and hybrid tasks was r (2) = 0.44, indicating that the common practice of examining the control of static tasks may not sufficiently explain performance during hybrid tasks, at least for the young subjects tested in the current study. As activities of daily living frequently require hybrid force/motion tasks (e.g., writing, doing the dishes, and cleaning counters), the results of this study emphasize the need to study motor performance during hybrid tasks in addition to static force tasks.

The Association Between Knee Extensor Force Steadiness, Force Accuracy, and Mobility in Older Adults Who Have Fallen

BACKGROUND

Older adults often experience limited mobility, lower extremity muscle weakness, and increased fall risk. Furthermore, when older adults perform tasks that require control of submaximal force, impairments in their ability to maintain steady and accurate force output have been reported. Such problems may be related to deteriorating levels of mobility, particularly in older adults who have fallen.

PURPOSE

The purpose of this study was to determine whether an association exists between muscle force steadiness (MFS) or muscle force accuracy (MFA) of the knee extensors and mobility in older adults who have fallen.

METHODS

Twenty older adults ((Equation is included in full-text article.)= 77.5 ± 7 years, 5 males and 15 females) with 2 or more comorbid conditions and who experienced a fall in the past year underwent assessment of maximal voluntary isometric contraction of the knee extensors. A submaximal target force of 50% of their maximal voluntary isometric contraction was used to determine concentric and eccentric (ECC) steadiness (the fluctuations in force production) and accuracy (the average distance of the mean force from the target force) measures. Mobility was indicated by the 6-minute walk test, the Timed Up and Go, stair ascent, and stair descent tests. Correlation analysis was used to assess the relationship between measures of muscle force control and mobility.

RESULTS

The correlations between muscle force steadiness and mobility were not significant (P > .05) for either contraction type. However, MFA during ECC contractions only was correlated significantly with all measures of mobility-6 minute walk test (r = -0.48; P = .03), Timed Up and Go (r = 0.68; P = .01), stair ascent (r = 0.60; P = .01), and stair descent (r = 0.75; P < .01).

CONCLUSION

The identification of the relationship between ECC MFA and mobility in older adults who have fallen is novel. Although the correlations are not causal, these relationships suggest that inaccurate force output during ECC contractions of the knee extensors is linked to impaired mobility.

Visuomotor Correction is a Robust Contributor to Force Variability During Index Finger Abduction by Older Adults

We examined aging-related differences in the contribution of visuomotor correction to force fluctuations during index finger abduction via the analysis of two datasets from similar subjects. Study (1) Young (N = 27, 23 ± 8 years) and older adults (N = 14, 72 ± 9 years) underwent assessment of maximum voluntary contraction force (MVC) and force steadiness during constant-force (CF) index finger abduction (2.5, 30, 65% MVC). For each trial, visual feedback of the force (VIS) was provided for 8–10 s and removed for 8–10 s (NOVIS). Visual gain of the force feedback at 2.5% MVC was high; 12- and 26-fold greater than the 30 and 65% MVC targets. Mean force, standard deviation (SD) of force, and coefficient of variation (CV) of force was calculated for detrended (<0.5 Hz drift removed) VIS and NOVIS data segments. Study (2) A similar group of 14 older adults performed discrete, randomly-ordered VIS or NOVIS trials at low target forces (1–3% MVC) and high visual gain. Study (1) For young adults the CV of force was similar between VIS and NOVIS for the 2.5% (4.8 vs. 4.3%), 30% (3.2 vs. 3.2%) and 65% (3.5 vs. 4.2%) target forces. In contrast, for older adults the CV of force was greater for VIS than NOVIS for 2.5% MVC (6.6 vs. 4.2%, p < 0.001), but not for the 30% (2.4 vs. 2.4%) and 65% (3.1 vs. 3.3%) target forces. At 2.5% MVC, the increase in CV of force for VIS compared with NOVIS was significantly greater (age × visual condition p = 0.008) for older than young adults. Study (2) Similarly, for older adults performing discrete, randomly ordered trials the CV of force was greater for VIS than NOVIS (6.04 vs. 3.81%, p = 0.01). When visual force feedback was a dominant source of information at low forces, normalized force variability was ~58% greater for older adults, but only 11% greater for young adults. The significant effect of visual feedback for older adults was not dependent on the order of presentation of visual conditions. The results indicate that impaired processing of visuomotor information underlies the greater motor variability observed in older adults during lab-based isometric contractions of a hand muscle.

Resistance Training Reduces Force Tremor and Improves Manual Dexterity in Older Individuals With Essential Tremor

Although symptoms of Essential Tremor (ET) are typically controlled with medication, it is of interest to explore additional therapies to assist with functionality. The purpose of this study was to determine if a generalized upper limb resistance training (RT) program improves manual dexterity and reduces force tremor in older individuals with ET. Ten Essential Tremor and 9 controls were recruited into a dual group, pretest-posttest intervention study. Participants performed 6 weeks of upper-limb RT, and battery of manual dexterity and isometric force tremor assessments were performed before and after the RT to determine the benefits of the program. The six-week, high-load, RT program produced strength increases in each limb for the ET and healthy older group. These changes in strength aligned with improvements in manual dexterity and tremor-most notably for the ET group. The least affected limb and the most affected limb exhibited similar improvements in functional assessments of manual dexterity, whereas reductions in force tremor amplitude following the RT program were restricted to the most affected limb of the ET group. These findings suggest that generalized upper limb RT program has the potential to improve aspects of manual dexterity and reduce force tremor in older ET patients.

Energy balance and fitness in adult survivors of childhood acute lymphoblastic leukemia

There is limited information on body composition, energy balance, and fitness among survivors of childhood acute lymphoblastic leukemia (ALL), especially those treated without cranial radiation therapy (CRT). This analysis compares these metrics among 365 ALL survivors with a mean age of 28.6 ± 5.9 years (149 treated with and 216 without CRT) and 365 age-, sex-, and race-matched peers. We also report risk factors for outcomes among survivors treated without CRT. Male survivors not exposed to CRT had abnormal body composition when compared with peers (% body fat, 26.2 ± 8.2 vs 22.7 ± 7.1). Survivors without CRT had similar energy balance but had significantly impaired quadriceps strength (-21.9 ± 6.0 Newton-meters [Nm]/kg, 60°/s) and endurance (-11.4 ± 4.6 Nm/kg, 300°/s), exercise capacity (-2.0 ± 2.1 ml/kg per minute), low-back and hamstring flexibility (-4.7 ± 1.6 cm), and dorsiflexion range of motion (-3.1 ± 0.9°) and higher modified total neuropathy scores (+1.6 ± 1.1) than peers. Cumulative asparaginase dose ≥120,000 IU/m(2) was associated with impaired flexibility, vincristine dose ≥39 mg/m(2) with peripheral neuropathy, glucocorticoid (prednisone equivalent) dose ≥8000 mg/m(2) with hand weakness, and intrathecal methotrexate dose ≥225 mg with dorsiflexion weakness. Physical inactivity was associated with hand weakness and decreased exercise capacity. Smoking was associated with peripheral neuropathy. Elimination of CRT from ALL therapy has improved, but not eliminated, body-composition outcomes. Survivors remain at risk for impaired fitness.

Age and grip strength predict hand dexterity in adults

In the scientific literature, there is much evidence of a relationship between age and dexterity, where increased age is related to slower, less nimble and less smooth, less coordinated and less controlled performances. While some suggest that the relationship is a direct consequence of reduced muscle strength associated to increased age, there is a lack of research that has systematically investigated the relationships between age, strength and hand dexterity. Therefore, the aim of this study was to examine the associations between age, grip strength and dexterity. 107 adults (range 18-93 years) completed a series of hand dexterity tasks (i.e. steadiness, line tracking, aiming, and tapping) and a test of maximal grip strength. We performed three phases of analyses. Firstly, we evaluated the simple relationships between pairs of variables; replicating the existing literature; and found significant relationships of increased age and reduced strength; increased age and reduced dexterity, and; reduced strength and reduced dexterity. Secondly, we used standard Multiple Regression (MR) models to determine which of the age and strength factors accounted for the greater variance in dexterity. The results showed that both age and strength made significant contributions to the data variance, but that age explained more of the variance in steadiness and line tracking dexterity, whereas strength explained more of the variance in aiming and tapping dexterity. In a third phase of analysis, we used MR analyses to show an interaction between age and strength on steadiness hand dexterity. Simple Slopes post-hoc analyses showed that the interaction was explained by the middle to older aged adults showing a relationship between reduced strength and reduced hand steadiness, whereas younger aged adults showed no relationship between strength and steadiness hand dexterity. The results are discussed in terms of how age and grip strength predict different types of hand dexterity in adults.

Sleep-dependent motor memory consolidation in older adults depends on task demands

It is often suggested that sleep-dependent consolidation of motor learning is impaired in older adults. The current study challenges this view and suggests that the degree of motor consolidation seen with sleep in older age groups depends on the kinematic demands of the task. We show that, when tested with a classic sequence learning task, requiring individuated finger movements, older adults did not show sleep-dependent consolidation. By contrast, when tested with an adapted sequence learning task, in which movements were performed with the whole hand, sleep-dependent motor improvement was observed in older adults. We suggest that age-related decline in fine motor dexterity may in part be responsible for the previously described deficit in sleep-dependent motor consolidation with aging.

Investigating Age-related changes in fine motor control across different effectors and the impact of white matter integrity

Changes in fine motor control that eventually compromise dexterity accompany advanced age; however there is evidence that age-related decline in motor control may not be uniform across effectors. Particularly, the role of central mechanisms in effector-specific decline has not been examined but is relevant for placing age-related motor declines into the growing literature of age-related changes in brain function. We examined sub-maximal force control across three different effectors (fingers, lips, and tongue) in 18 young and 14 older adults. In parallel with the force variability measures we examined changes in white matter structural integrity in effector-specific pathways in the brain with diffusion tensor imaging (DTI). Motor pathways for each effector were identified by using an fMRI localizer task followed by tractography to identify the fiber tracts propagating to the midbrain. Increases in force control variability were found with age in all three effectors but the effectors showed different degrees of age-related variability. Motor control changes were accompanied by a decline in white matter structural integrity with age shown by measures of fractional anisotropy and radial diffusivity. The DTI metrics appear to mediate some of the age-related declines in motor control. Our findings indicate that the structural integrity of descending motor systems may play a significant role in age-related increases in motor performance variability, but that differential age-related declines in oral and manual effectors are not likely due to structural integrity of descending motor pathways in the brain.

Functional implications of impaired control of submaximal hip flexion following stroke

INTRODUCTION

We quantified submaximal torque regulation during low to moderate intensity isometric hip flexion contractions in individuals with stroke and the associations with leg function.

METHODS

Ten participants with chronic stroke and 10 controls performed isometric hip flexion contractions at 5%, 10%, 15%, 20%, and 40% of maximal voluntary contraction (MVC) in paretic, nonparetic, and control legs.

RESULTS

Participants with stroke had larger torque fluctuations (coefficient of variation, CV), for both the paretic and nonparetic legs, than controls (P < 0.05) with the largest CV at 5% MVC in the paretic leg (P < 0.05). The paretic CV correlated with walking speed (r2 = 0.54) and Berg Balance Score (r2 = 0.40). At 5% MVC, there were larger torque fluctuations in the contralateral leg during paretic contractions compared with the control leg.

CONCLUSIONS

Impaired low-force regulation of paretic leg hip flexion can be functionally relevant and related to control versus strength deficits poststroke.

Motor Variability during Sustained Contractions Increases with Cognitive Demand in Older Adults

To expose cortical involvement in age-related changes in motor performance, we compared steadiness (force fluctuations) and fatigability of submaximal isometric contractions with the ankle dorsiflexor muscles in older and young adults and with varying levels of cognitive demand imposed. Sixteen young (20.4 ± 2.1 year: 8 men, 9 women) and 17 older adults (68.8 ± 4.4 years: 9 men, 8 women) attended three sessions and performed a 40 s isometric contraction at 5% maximal voluntary contraction (MVC) force followed by an isometric contraction at 30% MVC until task failure. The cognitive demand required during the submaximal contractions in each session differed as follows: (1) high-cognitive demand session where difficult mental math was imposed (counting backward by 13 from a 4-digit number); (2) low-cognitive demand session which involved simple mental math (counting backward by 1); and (3) control session with no mental math. Anxiety was elevated during the high-cognitive demand session compared with other sessions for both age groups but more so for the older adults than young adults (p  < 0.05). Older adults had larger force fluctuations than young adults during: (1) the 5% MVC task as cognitive demand increased (p  = 0.007), and (2) the fatiguing contraction for all sessions (p  = 0.002). Time to task failure did not differ between sessions or age groups (p  > 0.05), but the variability between sessions (standard deviation of three sessions) was greater for older adults than young (2.02 ± 1.05 vs. 1.25 ± 0.51 min, p  < 0.05). Thus, variability in lower limb motor performance for low- and moderate-force isometric tasks increased with age and was exacerbated when cognitive demand was imposed, and may be related to modulation of synergist and antagonist muscles and an altered neural strategy with age originating from central sources. These data have significant implications for cognitively demanding low-force motor tasks that are relevant to functional and ergonomic in an aging workforce.

Use of objective psychomotor tests in health professionals

Evaluation of psychomotor skills is undertaken in a number of broad contexts. This includes testing of health professional populations as a measure of innate ability, to evaluate skill acquisition, or to compare professions. However, the use of psychomotor tests is frequently confounded by a lack of understanding of a particular tool's psychometric properties, strengths, and weaknesses. To identify and appraise the most commonly used tests on health professional populations, 86 articles were reviewed and the top nine tests identified. Few tests have had sufficient validity or reliability testing on health professionals. Based on the evidence available, use of the Grooved Pegboard Test, the Purdue Pegboard Test, or the Finger Tapping Test is recommended for the evaluation of dexterity in a health professional population; however, this choice may be dependent on the task(s) to which findings are generalised. More rigorous evaluation of validity and other psychometric properties is required.

Fatigability of the dorsiflexors and associations among multiple domains of motor function in young and old adults

Declines in neuromuscular function, including measures of mobility, muscle strength, steadiness, and patterns of muscle activation, accompany advancing age and are often associated with reduced quality of life and mortality. Paradoxically, older adults are less fatigable than young adults in some tasks. The purpose of this study was to determine the influence of age on fatigability of the dorsiflexors and to evaluate the ecological validity of this test by comparing it to motor function subdomains known to decline with advancing age. The community-dwelling older adults (n=52, 75.2±6.0years) were more fatigable than young adults (n=26, 22.2±3.7years), as assessed by endurance time for supporting a submaximal load (20% of one-repetition maximum; 1-RM) with an isometric contraction of the dorsiflexor muscles (8.9±0.6min and 15.5±0.9min, p<0.001), including participants matched for 1-RM load and sex (Y: 13.3±4.0min, O: 8.5±6.1min, n=11 pairs, 6 women, p<0.05). When the older adults were separated into two groups (65-75 and 76-90years), however, only endurance time for the oldest group was less than that for the other two groups (p<0.01). All measures of motor function were significantly correlated (all p<0.05) with dorsiflexor endurance time for the older adults, and multiple regression analysis revealed that the variance in endurance time was most closely associated with age, steadiness, and knee flexor strength (R(2)=0.50, p<0.001). These findings indicate that dorsiflexor fatigability provides a valid biomarker of motor function in older adults.

Effects of transcranial direct current stimulation in combination with motor practice on dexterous grasping and manipulation in healthy older adults

Abstract Transcranial anodal stimulation (tDCS) over primary motor cortex (M1) improves dexterous manipulation in healthy older adults. However, the beneficial effects of anodal tDCS in combination with motor practice on natural and clinically relevant functional manual tasks, and the associated changes in the digit contact forces are not known. To this end, we studied the effects of 20 min of tDCS applied over M1 for the dominant hand combined with motor practice (MP) in a sham-controlled crossover study. We monitored the forces applied to an object that healthy elderly individuals grasped and manipulated, and their performances on the Grooved Pegboard Test and the Key-slot task. Practice improved performance on the Pegboard test, and anodal tDCS + MP improved retention of this performance gain when tested 35 min later, whereas similar performance gains degraded in the sham group after 35 min. Interestingly, grip force variability on an isometric precision grip task performed with visual feedback of precision force increased following anodal tDCS + MP, but not sham tDCS + MP. This finding suggests that anodal tDCS over M1 might alter the descending drive to spinal motor neurons involved in the performance of isometric precision grip task under visual feedback leading to increased fluctuations in the grip force exerted on the object. Our results demonstrate that anodal stimulation in combination with motor practice helps older adults to retain their improved performance on a functionally relevant manual task in healthy older adults.

Dexterous manipulation is poorer at older ages and is dissociated from decline of hand strength

BACKGROUND

The ability to dynamically control fingertip force vector magnitude and direction is critical for dexterous manipulation. We quantified the dynamic control of fingertip forces to examine how dexterous manipulation declines with age.

METHODS

The strength-dexterity (SD) test measures fingertip forces during compression of a slender spring prone to instability and buckling. The greatest sustained compression (designed to be under 3 N), and force dynamics therein, have been shown to be simple and quick measures of dynamic dexterous manipulation ability. We measured pinch strength and strength-dexterity test in a cross-sectional population of 98 people from 18 to 89 years of age.

RESULTS

Dexterous manipulation ability is poorer at older ages, beginning in middle age (p < .001), with greater decline past 65 years of age. Fingertip force dynamics during spring compression and stabilization show a deterioration of neuromuscular control with age. Importantly, this novel detection of decline in dynamic manipulation ability is not correlated with, and thus cannot be entirely explained by, the known decline in pinch strength. We also measured standardized tests of dexterity in participants older than 45, and discuss how the strength-dexterity test uniquely captures features of sensorimotor capabilities for dexterous manipulation in this adult population.

CONCLUSIONS

Starting in middle age, changes in the functional interactions among sensory, motor, and neural capabilities result in measurably poorer dynamic dexterous manipulation. This deterioration of neuromuscular control motivates and enables future studies to understand the physiological bases for this functional decline so critical to activities of daily living and quality of life.

Grasping the changes seen in older adults when reaching for objects of varied texture

Old age is associated with reduced mobility of the hand. To investigate age related decline when reaching-to-lift an object we used sophisticated kinematic apparatus to record reaches carried out by healthy older and younger participants. Three objects of different widths were placed at three different distances, with objects having either a high or low friction surface (i.e. rough or slippery). Older participants showed quantitative differences to their younger counterparts - movements were slower and peak speed did not scale with object distance. There were also qualitative differences with older adults showing a greater propensity to stop the hand and adjust finger position before lifting objects. The older participants particularly struggled to lift wide slippery objects, apparently due to an inability to manipulate their grasp to provide the level of precision necessary to functionally enclose the object. These data shed light on the nature of age related changes in reaching-to-grasp movements and establish a powerful technique for exploring how different product designs will impact on prehensile behavior.

Muscle weakness, fatigue, and torque variability: effects of age and mobility status

INTRODUCTION

Whereas deficits in muscle function, particularly power production, develop in old age and are risk factors for mobility impairment, a complete understanding of muscle fatigue during dynamic contractions is lacking. We tested hypotheses related to torque-producing capacity, fatigue resistance, and variability of torque production during repeated maximal contractions in healthy older, mobility-impaired older, and young women.

METHODS

Knee extensor fatigue (decline in torque) was measured during 4 min of dynamic contractions. Torque variability was characterized using a novel 4-component logistic regression model.

RESULTS

Young women produced more torque at baseline and during the protocol than older women (P < 0.001). Although fatigue did not differ between groups (P = 0.53), torque variability differed by group (P = 0.022) and was greater in older impaired compared with young women (P = 0.010).

CONCLUSIONS

These results suggest that increased torque variability may combine with baseline muscle weakness to limit function, particularly in older adults with mobility impairments.

Aging effects on sensorimotor integration: a comparison of effector systems and feedback modalities

Research on motor aging has focused on visuomotor effects in limb musculature, with few comparisons across effectors or feedback modalities. The authors examined steady fine force control in oral and manual effectors under visual and auditory feedback in 13 young (19-23 years old) and 13 older (60-77 years old) participants, hypothesizing that force variability would increase with aging (a) more in the finger than the lip and (b) for both feedback modalities. The magnitude of variability increased with age for both visuomotor and audiomotor tasks but age-related differences were greater in the lip than the finger. These results point to increased variability as a potential early marker of changing motor function (prior to loss of strength) that extends beyond the visuomotor system.

Motor unit activity when young and old adults perform steady contractions while supporting an inertial load

The purpose of the study was to compare the discharge characteristics of biceps brachii motor units of young and old adults when they performed steady, submaximal contractions while the arm supported different inertial loads. Young (28 ± 4 yr; n = 16) and old (75 ± 4 yr; n = 14) adults performed steady contractions with the elbow flexors at target forces set at either small (11.7 ± 4.4% maximum) or large (17.8 ± 6.5% maximum) differences below the recruitment threshold force of the motor unit (n = 40). The task was to maintain an elbow angle at 1.57 rad until the motor unit was recruited and discharged action potentials for ∼120 s. Time to recruitment was longer for the larger target force difference (187 ± 227 s vs. 23 ± 46 s, P < 0.001). Once recruited, motor units discharged action potentials either repetitively or intermittently, with a greater proportion of motor units exhibiting the repetitive pattern for old adults. Discharge rate at recruitment and during the steady contraction was similar for the two target force differences for old adults but was greater for the small target force difference for young adults. Discharge variability was similar at recruitment for the two age groups but less for the old adults during the steady contraction. The greatest difference between the present results and those reported previously when the arm pulled against a rigid restraint was that old adults modulated discharge rate less than young adults across the two contraction intensities for both load types.

Control of fingertip forces in young and older adults pressing against fixed low- and high-friction surfaces

Mobile computing devices (e.g., smartphones and tablets) that have low-friction surfaces require well-directed fingertip forces of sufficient and precise magnitudes for proper use. Although general impairments in manual dexterity are well-documented in older adults, it is unclear how these sensorimotor impairments influence the ability of older adults to dexterously manipulate fixed, low-friction surfaces in particular. 21 young and 18 older (65+ yrs) adults produced maximal voluntary contractions (MVCs) and steady submaximal forces (2.5 and 10% MVC) with the fingertip of the index finger. A Teflon covered custom-molded splint was placed on the fingertip. A three-axis force sensor was covered with either Teflon or sandpaper to create low- and high-friction surfaces, respectively. Maximal downward forces (F_z) were similar (p = .135) for young and older adults, and decreased by 15% (p<.001) while pressing on Teflon compared to sandpaper. Fluctuations in F_z during the submaximal force-matching tasks were 2.45× greater (p<.001) for older adults than in young adults, and reached a maximum when older adults pressed against the Teflon surface while receiving visual feedback. These age-associated changes in motor performance are explained, in part, by altered muscle activity from three hand muscles and out-of-plane forces. Quantifying the ability to produce steady fingertip forces against low-friction surfaces may be a better indicator of impairment and disability than the current practice of evaluating maximal forces with pinch meters. These age-associated impairments in dexterity while interacting with low-friction surfaces may limit the use of the current generation of computing interfaces by older adults.

Handedness but not dominance influences variability in endurance time for sustained, submaximal contractions

The purpose of this study was to compare endurance time and accompanying neuromuscular adjustments when left- and right-handed subjects used the dominant and nondominant arms to sustain submaximal contractions that required either force or position control. Ten left-handed and 10 right-handed healthy adults (21 ± 5 yr) participated in the study. Each subject exerted a similar net torque about the elbow joint during the force and position tasks to achieve a target force of 20% maximal voluntary contraction (MVC) force (56 ± 18 N). MVC force declined to a similar level immediately after task failure for left- and right-handed subjects (27 ± 13 vs. 25 ± 15%, P = 0.9). Endurance time for the position task was similar for the dominant and nondominant arms (task × dominance interaction, P = 0.17). Although the difference in endurance time between the two tasks was similar for left-handed (136 ± 165 s) and right-handed individuals (92 ± 73 s, task × handedness interaction, P = 0.38), there was greater variance in the ratio of the endurance times for the force and position tasks for left-handed (0.77) than right-handed subjects (0.13, P < 0.001; see Fig. 2 ). Furthermore, endurance time for the force and position tasks was significantly correlated for right-handed subjects ( r2 = 0.62, P < 0.001), but not for left-handed subjects ( r2 = 0.004, P = 0.79). Multiple regression analyses identified sets of predictor variables for each endurance time, and these differed with handedness and task. Hand dominance, however, did not influence endurance time for either group of subjects. These findings indicate that endurance times for the elbow flexors when performing submaximal isometric contractions that required either force or position control were not influenced by hand dominance but did depend on handedness.

Practicing a functional task improves steadiness with hand muscles in older adults

INTRODUCTION

Improvements in steadiness with practice have been associated with enhanced performance on a functional task in old adults.

PURPOSE

The aims of the study were to examine the specificity of the association between steadiness and a functional task and to assess the influence of practicing a functional task on force steadiness of hand muscles.

METHODS

Twenty-three older adults (≥70 yr) participated in the study and were assigned to either a practice group (n = 15) or a control group (n = 8). Subjects completed two testing sessions that were 2 wk apart. The practice group completed six additional sessions to practice a functional task (Grooved Pegboard). Tests included maximal voluntary contractions (MVC), force steadiness (precision pinch and index finger abduction) at three target forces (5%, 15%, and 25% MVC), and the Grooved Pegboard test. The associations between strength, steadiness, and the time needed to complete the Grooved Pegboard test were examined. In addition, MVC force, steadiness, and pegboard time were compared between the two testing sessions.

RESULTS

The time needed to complete the Grooved Pegboard test was associated with index finger abduction steadiness for two of the three target forces (15% and 25% MVC) but was not associated with pinch steadiness. Practice significantly reduced the time needed to complete the Grooved Pegboard test and improved steadiness in both tasks.

CONCLUSIONS

Force steadiness provides an appropriate index of hand function, especially when measured at low forces.

Low-frequency oscillations of the neural drive to the muscle are increased with experimental muscle pain

We investigated the influence of nociceptive stimulation on the accuracy of task execution and motor unit spike trains during low-force isometric contractions. Muscle pain was induced by infusion of hypertonic saline into the abductor digiti minimi muscle of 11 healthy men. Intramuscular EMG signals were recorded from the same muscle during four isometric contractions of 60-s duration at 10% of the maximal force [maximal voluntary contraction (MVC)] performed before injection (baseline), after injection of isotonic (control) or hypertonic saline (pain), and 15 min after pain was no longer reported. Each contraction was preceded by three 3-s ramp contractions from 0% to 10% MVC. The low-frequency oscillations of motor unit spike trains were analyzed by the first principal component of the low-pass filtered spike trains [first common component (FCC)], which represents the effective neural drive to the muscle. Pain decreased the accuracy of task performance [coefficient of variation (CoV) for force: baseline, 2.8 ± 1.8%, pain, 3.9 ± 1.8%; P < 0.05] and reduced motor unit discharge rates [11.6 ± 2.3 pulses per second (pps) vs. 10.7 ± 1.7 pps; P < 0.05]. Motor unit recruitment thresholds (2.2 ± 1.2% MVC vs. 2.4 ± 1.6% MVC), interspike interval variability (18.4 ± 4.9% vs. 19.1 ± 5.4%), strength of motor unit short-term synchronization [common input strength (CIS) 1.02 ± 0.44 vs. 0.83 ± 0.22], and strength of common drive (0.47 ± 0.08 vs. 0.47 ± 0.06) did not change across conditions. The FCC signal was correlated with force (R = 0.45 ± 0.06), and the CoV for FCC increased in the painful condition (5.69 ± 1.29% vs. 7.83 ± 2.61%; P < 0.05). These results indicate that nociceptive stimulation increased the low-frequency variability in synaptic input to motoneurons.

Aging and movement errors when lifting and lowering light loads

The purpose was to determine the influence of movement variability and level of muscle activation on the accuracy of targeted movements performed with the index finger by young and older adults. Twelve young (27.4 ± 4.4 years) and 12 older adults (74.5 ± 8.9 years) attempted to match the end position of an index finger movement to a target position when lifting and lowering a light load (10% of the maximum). Visual feedback was provided after each trial. Movement error was calculated as the absolute distance from the target. Movement variability was quantified as the standard deviation of finger acceleration and the variability of end position across trials. The EMG activity of first dorsal interosseus (FDI) and second palmar interosseus (SPI) muscles was measured with intramuscular electrodes. Older adults exhibited greater spatial and temporal errors and greater variability in finger acceleration and end position during both the lifting and lowering tasks. Older adults lifted the load by activating FDI less but SPI the same as young adults, whereas they lowered the load by activating SPI less and FDI the same as young adults. In addition, older adults exhibited lower variability across trials in SPI activation when lifting the load and lower variability for FDI activation when lowering the load. The findings demonstrate that the decrease in spatial and temporal accuracy observed in older adults when lifting and lowering a light load to a target position was due to greater movement variability and differences in antagonistic muscle activity.

Aging and variability of voluntary contractions

Older adults exhibit greater motor variability, which impairs their accuracy and function, compared with young adults. Low-intensity training that emphasizes muscle coordination reduces variability in older adults. Furthermore, a low amount of visual feedback minimizes age-associated differences in variability. We hypothesize that an intervention that combines muscle coordination and reduced visual feedback would be advantageous to improve motor control in older adults.