Fluctuations in motor output during steady contractions are weakly related across contraction types and between hands

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.

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.

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.

Movement trajectory smoothness is not associated with the endpoint accuracy of rapid multi-joint arm movements in young and older adults

The minimum variance theory proposes that motor commands are corrupted by signal-dependent noise and smooth trajectories with low noise levels are selected to minimize endpoint error and endpoint variability. The purpose of the study was to determine the contribution of trajectory smoothness to the endpoint accuracy and endpoint variability of rapid multi-joint arm movements. Young and older adults performed arm movements (4 blocks of 25 trials) as fast and as accurately as possible to a target with the right (dominant) arm. Endpoint accuracy and endpoint variability along with trajectory smoothness and error were quantified for each block of trials. Endpoint error and endpoint variance were greater in older adults compared with young adults, but decreased at a similar rate with practice for the two age groups. The greater endpoint error and endpoint variance exhibited by older adults were primarily due to impairments in movement extent control and not movement direction control. The normalized jerk was similar for the two age groups, but was not strongly associated with endpoint error or endpoint variance for either group. However, endpoint variance was strongly associated with endpoint error for both the young and older adults. Finally, trajectory error was similar for both groups and was weakly associated with endpoint error for the older adults. The findings are not consistent with the predictions of the minimum variance theory, but support and extend previous observations that movement trajectories and endpoints are planned independently.

Steadiness in plantar flexor muscles and its relation to postural sway in young and elderly adults

To investigate the functional significance of force fluctuations during voluntary contraction with a select muscle group, we examined the association between force fluctuations during voluntary contraction with plantar flexor muscles and postural sway during quiet standing in 20 young and 20 elderly adults. Young and elderly subjects maintained a quiet standing position on a force platform. They also performed a force-matching task with unilateral isometric plantar flexion. A positive correlation was found in young and elderly adults between the coefficient of variation (CV) of center of pressure during quiet standing and the CV of force during plantar flexion only at contraction intensities of < or =5% maximum voluntary contraction that corresponded to muscle activity during quiet standing. The electromyogram power in the medial gastrocnemius was greater in the elderly than in young adults by approximately 10 Hz during quiet standing and at low contraction intensities during plantar flexion. Fluctuations in motor output during low-intensity plantar flexion were associated with postural sway during quiet standing in both young and elderly adults.

Maximal voluntary fingertip force production is not limited by movement speed in combined motion and force tasks

Numerous studies of limbs and fingers propose that force-velocity properties of muscle limit maximal voluntary force production during anisometric tasks, i.e., when muscles are shortening or lengthening. Although this proposition appears logical, our study on the simultaneous production of fingertip motion and force disagrees with this commonly held notion. We asked eight consenting adults to use their dominant index fingertip to maximize voluntary downward force against a horizontal surface at specific postures (static trials), and also during an anisometric "scratching" task of rhythmically moving the fingertip along a 5.8 +/- 0.5 cm target line. The metronome-timed flexion-extension movement speed varied 36-fold from "slow" (1.0 +/- 0.5 cm/s) to "fast" (35.9 +/- 7.8 cm/s). As expected, maximal downward voluntary force diminished (44.8 +/- 15.6%; p = 0.001) when any motion (slow or fast) was added to the task. Surprisingly, however, a 36-fold increase in speed did not affect this reduction in force magnitude. These remarkable results for such an ordinary task challenge the dominant role often attributed to force-velocity properties of muscle and provide insight into neuromechanical interactions. We propose an explanation that the simultaneous enforcement of mechanical constraints for motion and force reduces the set of feasible motor commands sufficiently so that force-velocity properties cease to be the force-limiting factor. While additional work is necessary to reveal the governing mechanisms, the dramatic influence that the simultaneous enforcement of motion and force constraints has on force output begins to explain the vulnerability of dexterous function to development, aging, and even mild neuromuscular pathology.

Motor variability: within-subject correlations during separate and simultaneous contractions

To determine the similarity of motor variability in proximal muscles, young and elderly adults performed steady elbow flexor (EF) and knee extensor (KE) contractions separately (SEP; at 2.5, 30, and 65% of maximum) and simultaneously (SIM; at 2.5 and 30% of maximum), with (VIS) and without (NVIS) visual feedback. Between-muscle correlations of fluctuation amplitude (SD, CV of force), time-based cross-correlations (CC), force power spectra, and frequency-based coherence (COH) values were computed from the concurrent force records. Correlations of fluctuation amplitude ranged from r = 0.34 to 0.86 (P < 0.05) across forces, SEP/SIM, and vision conditions, but were absent for 2.5% NVIS. The relatively low CC values for SIM (r = 0.22-0.33) were stronger for elderly than young adults. The vast majority of the power in the force fluctuations was <4 Hz for all records. Weak COH peaks were only observed <2 Hz for elderly and between 3 and 4 Hz for young, and COH was slightly stronger for elderly below 3 Hz for the 30% MVC target force. The correlations in force fluctuation amplitude suggest that the EF and KE motor neuron pools similarly transform the oscillating synaptic input and may influence each other. The cross-correlations suggest the remote motor neuron pools are influenced similarly in time by a common source of excitation, perhaps more coherently for elderly adults at low frequencies.

Fluctuations in motor output of a hand muscle can be altered by the mechanical properties of the position sensor

Fluctuations in motor output are typically quantified by the standard deviation (SD) of displacement or acceleration. The aim of the study was to determine the influence of a linear variable-displacement transducer (LVDT) on the SDs and spectral content of displacement and acceleration during steady isometric and anisometric contractions performed with the first dorsal interosseus muscle. Thirteen young adults supported six loads when performing position-holding and position-tracking tasks when the LVDT either was or was not attached to the index finger. The LVDT reduced the magnitude of the SDs in displacement and acceleration and disrupted the load-dependent modulation of the spectral properties of these signals. When the LVDT was not connected to the finger, the displacement SD was greatest during concentric contractions, the acceleration SD was greatest during eccentric contractions, and there were load-dependent changes in the power density spectra. Although the LVDT may be used for assessing relative changes in displacement, its ability to provide absolute measures of fluctuations in motor output is limited. The results provide baseline measures of the fluctuations in motor output during steady contractions with a hand muscle and how the method used to detect displacement alters these measures.

Does muscular weakness account for younger children's enhanced force variability?

This study examined the degree to which younger children's greater variability in force control is associated with muscular weakness. Children aged 6, 8, and 10 years and adults aged 18-22 years produced isometric force via index finger metacarpo-phalangeal joint flexion to varying force levels (5%, 15%, 25%, and 35% maximal voluntary contraction). The force output of the younger children was more variable and had greater time dependent structure compared to that of the adults at all force levels. However, the effect of age on variability was significantly reduced, but not eliminated when absolute muscular strength was taken into account. It is concluded that age-related changes in children's force control result from a multitude of developmental processes including increases in muscular strength.

The generalization of perceptual-motor intra-individual variability in young and old adults

We examined the generalizability of intra-individual perceptual-motor variability in young and old adults. Participants completed four motor tasks that all required the maintenance of a steady state: finger tremor, and single-digit, two-digit, and three-digit isometric force production. Older adults had a greater amount of variability and an increased time and frequency structure of variability, except in the tremor task. There was a moderate association between the magnitudes of variability across tasks in the oldest age group, but the structural measures of variability were more highly related across tasks and the strength of this relationship increased with age. These findings support the hypothesis that older adults are less able than younger adults to adapt the structure of their perceptual-motor variability to task demands.

Are age-related increases in force variability due to decrements in strength?

The purpose of this investigation was to examine the relationship between strength and the magnitude and time sequential structure of force variability. Young and old adults produced isometric force via index finger abduction to a visually presented target corresponding to a constant force level of 5 or 25% maximal voluntary contraction (MVC). Cluster analysis was used to divide subjects into groups based on age and strength. The variability of older adults was greater and showed more time dependent structure than their younger counterparts. The force output of weaker subjects was also more variable and had a stronger sequential structure. Indeed, when MVC was controlled for there was no significant age effect on force variability. The relationship between strength and variability remained significant, however, when chronological age was controlled for. The findings revealed that the established age-related changes in force variability are more fundamentally due to the association between strength and force variability and provide a further challenge to using chronological age as a marker of the biological aging process in studies of motor control.

Exertion dependent alternations in force fluctuation and limb acceleration during sustained fatiguing contraction

The study was conducted to contrast exertion-dependent changes in electromyography (EMG), force fluctuation (FF), and limb acceleration (LA) during isometric contraction to attain a versatile picture of muscle fatigue. Fifteen volunteers performed sustained index abduction at 25 and 75% maximal voluntary contractions (MVC) until failure at the tasks; meanwhile, changes in temporal/spectral features of force, muscle activity of the first dorsal interosseous (FDI), and acceleration of the index and hand were monitored. The results showed a manifest increase in all recorded signals for the 25% MVC paradigm, especially for LA, which demonstrated the largest increment in amplitude. In addition to progressive enhancement of the mechanical coupling of the metacarpophalangeal (MCP) joint, the 25% MVC paradigm added to EMG-FF and EMG-LA coherences (CohEMG-FF and CohEMG-LA) at 8-12 Hz and the shift of the spectral peak of the LA to higher frequencies. In contrast, the 75% MVC paradigm did not modulate significantly the spectral peak of LA. Also, CohEMG-FF, CohEMG-LA at 8-12 Hz, and the mechanical coupling of the MCP joint, were conversely undermined consequent to the high exertion paradigm. The present study suggests that LA was most susceptible to muscle fatigue following sustained contraction at a lower exertion level, and diverging alternations among various physiological signals ascribed to exertion-dependent contributions of central and peripheral origins to muscle fatigue.

The medial gastrocnemius muscle attenuates force fluctuations during plantar flexion

Force fluctuations during steady contractions of multiple agonist muscles may be influenced by the relative contribution of force by each muscle. The purpose of the study was to compare force fluctuations during steady contractions performed with the plantar flexor muscles in different knee positions. Nine men (25.8+/-5.1 years) performed steady contractions of the plantar flexor muscles in the knee-flexed and knee-extended (greater involvement of the gastrocnemii muscles) positions. The maximal voluntary contraction (MVC) force was 32% greater in the knee-extended position compared with the knee-flexed position. The target forces were 2.5-10% MVC force in the respective position. The amplitude of electromyogram in the medial gastrocnemius muscle was greater in the knee-extended position (10.50+/-9.80%) compared with the knee-flexed position (1.26+/-1.15%, P<0.01). The amplitude of electromyogram in the soleus muscle was not influenced by the knee position. The amplitude of electromyogram in the lateral gastrocnemius and tibialis anterior muscles was marginal and unaltered with knee position. At the same force (in Newtons), the standard deviation of force was lower in the knee-extended position compared with the knee-flexed position. These results indicate that force fluctuations during plantar flexion are attenuated with greater involvement of the medial gastrocnemius muscle.

Prolonged muscle vibration increases stretch reflex amplitude, motor unit discharge rate, and force fluctuations in a hand muscle

The purpose of this study was to compare the influence of prolonged vibration of a hand muscle on the amplitude of the stretch reflex, motor unit discharge rate, and force fluctuations during steady, submaximal contractions. Thirty-two young adults performed 10 isometric contractions at a constant force (5.0 +/- 2.3% of maximal force) with the first dorsal interosseus muscle. Each contraction was held steady for 10 s, and then stretch reflexes were evoked. Subsequently, 20 subjects had vibration applied to the relaxed muscle for 30 min, and 12 subjects received no vibration. The muscle vibration induced a tonic vibration reflex. The intervention (vibration or no vibration) was followed by 2 sets of 10 constant-force contractions with applied stretches (After and Recovery trials). The mean electromyogram amplitude of the short-latency component of the stretch reflex increased by 33% during the After trials (P < 0.01) and by 38% during the Recovery trials (P < 0.01). The standard deviation of force during the steady contractions increased by 21% during the After trials (P < 0.05) and by 28% during the Recovery trials (P < 0.01). The discharge rate of motor units increased from 10.3 +/- 2.7 pulses/s (pps) before vibration to 12.2 +/- 3.1 pps (P < 0.01) during the After trials and to 11.9 +/- 2.6 pps during the Recovery trials (P < 0.01). There was no change in force fluctuations or stretch reflex magnitude for the subjects in the Control group. The results indicate that prolonged vibration increased the short-latency component of the stretch reflex, the discharge rate of motor units, and the fluctuations in force during contractions by a hand muscle. These adjustments were necessary to achieve the target force due to the vibration-induced decrease in the force capacity of the muscle.