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.     

Stabilization of the total force in multi-finger pressing tasks studied with the 'inverse piano' technique

When one finger changes its force, other fingers of the hand can show unintended force changes in the same direction (enslaving) and in the opposite direction (error compensation). We tested a hypothesis that externally imposed changes in finger force predominantly lead to error compensation effects in other fingers thus stabilizing the total force. A novel device, the “inverse piano”, was used to impose controlled displacements to one of the fingers over different magnitudes and at different rates. Subjects (n = 10) pressed with four fingers at a constant force level and then one of the fingers was unexpectedly raised. The subjects were instructed not to interfere with possible changes in the finger forces. Raising a finger caused an increase in its force and a drop in the force of the other three fingers. Overall, total force showed a small increase. Larger force drops were seen in neighbors of the raised finger (proximity effect). The results showed that multi-finger force stabilizing synergies dominate during involuntary reactions to externally imposed finger force changes. Within the referent configuration hypothesis, the data suggest that the instruction “not to interfere” leads to adjustments of the referent coordinates of all the individual fingers.     

Grasp force control in older adults

Diminished tactile sensibility and impaired hand dexterity have been reported for elderly individuals. Reports that younger adults with severely impaired tactile sensibility use excessive grasp force during routine grasp and manipulation tasks raise the possibility that elderly persons likewise produce large grasp forces that may contribute to impaired dexterity. Impaired pseudomotor functioning also occurs in elderly subjects and may yield a slipperier skin surface that enhances the possibility for excessive grasp force. The present study measured grasp force in 10 elderly and 9 young adult individuals, during grasp and vertical lift of a small object, using a precision (pinch) grip of the thumb and index finger. The slipperiness of the object's gripped surfaces was unexpectedly varied. Skin slipperiness was estimated by also measuring the grasp force at which the object slipped from grasp. The older subjects employed grasp forces that were, on average, twice as large as those of the young subjects, with some producing forces many times greater than the young subjects' average grip force. Grip forces also were significantly more variable across trials in older subjects. This increased variability was not caused simply by the elderly subjects' increased grip force. A portion of the increased force was due to increased skin slipperiness. The grip force that the elderly subjects produced in excess of the slip force (the "margin of safety" against object slippage) was larger than would have been predicted from their skin slipperiness, however. It is suggested that, in part, the excessive grasp forces represent a strategic response to tactile sensibility impairment. Twopoint discrimination limina in the older subjects averaged about four times greater than in the younger subjects. Increased grasp forces in elderly persons may result from other factors, such as increased variability in grip force production. The contributions of excessive grasp forces to impaired dexterity in older persons still need to be addressed experimentally.     

Neuromotor synergies as a basis for coordinated intentional action

Although neurally based units of action (neuromotor synergies) have often been proposed as a possible basis for coordinated intentional as well as automatic actions, the idea has rarely been translated into sets of testable hypotheses. This essay examines four issues which should facilitate the development of such hypotheses: (a) definitions of neuromotor synergies, (b) criteria for recognizing and comparing synergies in automatic and intentional actions, (c) problems in representing systems of synergies, and (d) models for generating intentional actions from sets of neuromotor synergies. Limitations of, and support for the neuromotor synergy hypothesis are discussed, both in general and for the specific cases of postural synergies and cervico-spinal reflexes. Although current data do not provide conclusive support for or against the neuromotor synergy hypothesis, the problem can be formulated in ways open to experimental investigation.     

Quantifying synergies in two-versus-one situations in team sports: An example from Rugby Union

Collective behaviors in team sports result in players forming interpersonal synergies that contribute to performance goals. Because of the huge amount of variables that continuously constrain players’ behavior during a game, the way that these synergies are formed remain unclear. Our aim was to quantify interpersonal synergies in the team sport of Rugby Union. For that purpose we used the Uncontrolled Manifold Hypothesis (UCM) to identify interpersonal synergies that are formed between ball carrier and support player in two-versus-one situations in Rugby Union. The inter-player angle close to the moment of the pass was used as a performance variable and players running lines velocities as task-relevant elements. Interpersonal synergies (UCM values above 1) were found in 19 out of 55 trials under analysis, which means that on 34% of the trials, the players’ running line velocities contribute to stabilizing the inter-player angle close the moment of the pass. The strength of the synergy fluctuates over time indicating the existence of a location effect during attack phases in Rugby Union. UCM analysis shows considerable promise as a performance analysis tool in team sports to discriminate between skilled sub-groups of players.     

Finger interdependence and unintentional force drifts: Lessons from manipulations of visual feedback

We explored the phenomenon of unintentional finger force drift by using visual feedback on the force produced either by explicitly instructed (master) finger pairs or by non-instructed (enslaved) finger pairs. In particular, we drew contrasting predictions from two hypotheses: that force drifts represented consequences of drifts in effector referent coordinates at the level of individual fingers vs. at the level of finger modes (hypothetical variables accounting for the finger force interdependence). Subjects performed accurate force production with two fingers of a hand, index-ring or middle-little. They received visual feedback on the force produced either by the master fingers or by the other two, enslaved, fingers. The feedback scale was adjusted to ensure that the subjects did not know the difference between these two, randomly presented, conditions. Under feedback on the master finger force, enslaved force showed a consistent drift upward. Under feedback on the enslaved finger force, master force showed a consistent drift downward. The subjects were unaware of the force drifts, which could reach over 35% of the initial force magnitude. The data support the hypothesis on drifts in the referent coordinate at the level of individual digits, not finger modes, as the origin of unintentional force drifts. The consistent increase in the relative amount of force produced by the enslaved fingers suggests that the commonly used methods to quantify enslaving should include relatively brief force production tasks.     

Synergies and Motor Equivalence in Voluntary Sway Tasks: The Effects of Visual and Mechanical Constraints

The authors used two analyses developed within the framework of the uncontrolled manifold hypothesis to quantify multimuscle synergies during voluntary body sway: analysis of intertrial variance and analysis of motor equivalence with respect to the center of pressure (COP) trajectory. Participants performed voluntary sway tasks in the anteroposterior direction at 0.33 and 0.66 Hz. Muscle groups were identified in the space of muscle activations and used as elemental variables in the synergy analyses. Changing mechanical and vision feedback–based constraints led to significant changes in indices of sway performance such as COP deviations in the uninstructed, mediolateral direction and indices of spontaneous postural sway. In contrast, there were no significant effects on synergy indices. These findings show that the neural control of performance and of its stability may involve different control variables and neurophysiological structures. There were strong correlations between the indices of motor equivalence and those computed using the intercycle variance analysis. This result is potentially important for studies of patients with movement disorders who may be unable to perform multiple trials (cycles) at any given task, making analysis of motor equivalence of single trials a viable alternative to explore changes in stability of actions.     

Age-related changes in multi-finger interactions in adults during maximum voluntary finger force production tasks

This study aimed to continue our characterization of finger strength and multi-finger interactions across the lifespan to include those in their 60s and older. Building on our previous study of children, we examined young and elderly adults during isometric finger flexion and extension tasks. Sixteen young and 16 elderly, gender-matched participants produced maximum force using either a single finger or all four fingers in flexion and extension. The maximum voluntary finger force (MVF), the percentage contributions of individual finger forces to the sum of individual finger forces during four-finger MVF task (force sharing), and the non-task finger forces during a task finger MVF task (force enslaving), were computed as dependent variables. Force enslaving during finger extension was greater than during flexion in both young and elderly groups. The flexion-extension difference was greater in the elderly than the young adult group. The greater independency in flexion may result from more frequent use of finger flexion in everyday manipulation tasks. The non-task fingers closer to a task finger produced greater enslaving force than non-task fingers farther from the task finger. The force sharing pattern was not different between age groups. Our findings suggest that finger strength decreases over the aging process, finger independency for flexion increases throughout development, and force sharing pattern remains constant across the lifespan.     

Ageism and death: effects of mortality salience and perceived similarity to elders on reactions to elderly people

The present research investigated the hypotheses that elderly people can be reminders of our mortality and that concerns about our own mortality can therefore instigate ageism. In Study 1, college-age participants who saw photos of two elderly people subsequently showed more death accessibility than participants who saw photos of only younger people. In Study 2, making mortality salient for participants increased distancing from the average elderly person and decreased perceptions that the average elderly person possesses favorable attitudes. Mortality salience did not affect ratings of teenagers. In Study 3, these mortality salience effects were moderated by prior reported similarity to elderly people. Distancing from, and derogation of, elderly people after mortality salience occurred only in participants who, weeks before the study, rated their personalities as relatively similar to the average elderly person's. Discussion addresses distinguishing ageism from other forms of prejudice, as well as possibilities for reducing ageism.     

Muscle coordination patterns in regulation of medial gastrocnemius activation during walking

The ankle plantar flexion in the late stance phase is referred to as the ankle push-off. When the ankle push-off force is enhanced, compensatory adjustments occur in the adjacent phases. The muscle control that achieves these compensatory movements remains unknown, although they are expected to be coordinately regulated across multiple muscles and phases. Muscle synergy is used as a quantification technique for muscle coordination, and this analysis enables the comparison of synchronized activity between multiple muscles. Therefore, this study aimed to elucidate the tuning of muscle synergies in muscle activation adjustment of push-off. It is hypothesized that muscle activation adjustment of push-off is performed in the muscle synergy related to ankle push-off and in the muscle synergy that activates during the adjacent push-off phase. Eleven healthy men participated, and participants manipulated the activity of the medial gastrocnemius during walking through visual feedback. Two conditions were compared as experimental conditions: increasing the muscle activity to 1.6 times that during normal walking (High) and matching it with that during normal walking (Normal). Twelve muscle activities in the trunk and lower limb and kinematic data were recorded. Muscle synergies were extracted by the non-negative matrix factorization. No significant difference was observed in the number of synergies (High: 3.5 ± 0.8, Normal: 3.7 ± 0.9, p = 0.21) and muscle synergy activation timing and duration between the High and Normal conditions (p > 0.27). However, significant differences were observed in the peak muscle activity during the late stance phase of the rectus femoris (RF), biceps femoris (BF) between conditions (RF at High: 0.32 ± 0.21, RF at Normal: 0.45 ± 0.17, p = 0.02; BF at High: 0.16 ± 0.01, BF at Normal: 0.08 ± 0.06 p = 0.02). Although the quantification of force exertion has not been conducted, the modulation of RF and BF activation could have occurred due to the attempts to help knee flexion. Muscle synergies during normal walking are therefore maintained, and slight adjustments in the amplitude of muscle activity occurred for each muscle.     

Age-related EMG variables during maximum voluntary contraction

We studied the neuromuscular adaptation that occurs with aging by comparing changes in surface electromyography (EMG) variables from the tibialis anterior muscle in 12 young (21.4 +/- 1.7 yr. old) and 13 older subjects (70.8 +/- 3.1 yr. old). EMG variables such as the muscle fiber conduction velocity, median frequency, and averaged rectified value were calculated during maximum voluntary contraction for 5-sec. isometric contractions. The dorsiflexion force, muscle fiber conduction velocity, median frequency, and average rectified value during maximum voluntary contraction were significantly smaller in the older than in the younger group (p < .05). These results suggested that the neuromuscular system in older subjects is affected by the selective atrophy of fast twitch fibers and differences in motor unit firing statistics. Our results suggest the utility of applying the EMG observed during maximum voluntary contraction to the noninvasive evaluation of neuromuscular function in elderly persons.     

Approaches to analysis of handwriting as a task of coordinating a redundant motor system

We consider problems of motor redundancy associated with handwriting using the framework of the uncontrolled manifold (UCM) hypothesis. Recent studies of finger coordination during force production tasks have demonstrated that the UCM-hypothesis provides a fruitful framework for analysis of multi-finger actions. In particular, it has been shown that during relatively fast force changes, finger force variance across trials is structured such that a time pattern of total moment produced by the fingers with respect to a point between the two most lateral fingers involved in the task is stabilized while the time pattern of total force may be destabilized. The findings of selective moment stabilization have been interpreted as being conditioned by the experience with everyday motor tasks that commonly pose more strict requirements to stabilization of total moment than to stabilization of total force. We discuss implications of these findings for certain features of handwriting seen in elderly, children, patients with neurological disorders, and forgers.     

Perceptions of an aggressive encounter as a function of the victim's salience and the perceiver's arousal.

Two studies were conducted to investigate the effects of a victim's physical salience and perceivers' arousal on perceptions of a verbally aggressive interaction. Based upon evidence that there is a tendency to attribute causality to salient stimulus persons and to form more evaluatively extreme impressions of such persons, it was predicted that an aggressor's behavior would be attributed more to causes in a physically salient than a nonsalient victim and that the behavior of a salient victim would be evaluated more extremely than that of a nonsalient victim. Based upon Easterbrook's hypothesis that arousal narrows the focus of attention to the most salient cues in the situation, it was further predicted that aroused perceivers would manifest both a stronger tendency to attribute causality to a physically salient victim of aggression and more extreme ratings of the stimulus persons than would nonaroused perceivers. The results supported all of the experimental hypotheses except one: The tendency to attribute the aggressor's behavior more to a physically salient than a nonsalient victim was not greater for aroused than for nonaroused perceivers.     

Learning a pointing task with a kinematically redundant limb: Emerging synergies and patterns of final position variability

The study tested a hypothesis that practice of arm pointing movement can lead to a reorganization of the joint coordination reflected in the emergence of several synergies based on the same set of joints. In particular, involvement of the wrist may represent a choice by the central nervous system and not be driven by the typical “freezing-to-freeing” sequence. The effects of practice on the kinematic patterns and variability of a “fast and accurate” pointing movement using a pointer were studied. An obstacle was placed between the initial position and the target to encourage a curvilinear trajectory and larger wrist involvement. Practice led to a decrease in variability indices accompanied by an increase in movement speed of the endpoint and of the elbow and the shoulder, but not of the wrist joint. Five out of six subjects decreased the peak-to-peak amplitude of wrist motion. Before practice, the variability along the line connecting the endpoint to the shoulder (extent) was similar to that in the direction orthogonal to this line. After practice, variability was reduced along the extent, but not along the orthogonal direction perpendicular to this line. Prior to practice, indices of variability of the endpoint were lower than those of the marker placed over the wrist; after practice, the endpoint showed higher variability indices than the wrist. We interpret the data as consequences of the emergence of two synergies: (a) Pointing with a non-redundant set of the elbow and shoulder joints; and (b) keeping wrist position constant. The former synergy is based on a structural unit involving the elbow and the shoulder, while the latter is based on a structural unit that includes all the major arm joints.     

The influence of speed and force on bimanual finger tapping patterns.

The current study investigated factors that affect the stability of anti-phase bimanual finger tapping. Past research employing the order parameter and control parameter concepts, has identified frequency of movement as a control parameter that affects the stability of finger movement patterns (the order parameter). The present study investigated the hypothesis that multiple movement related variables can interact to influence the stability of an order parameter. Specifically, the combined effect of the rate of movement and movement force on the stability of bimanual finger tapping was examined. Participants were required to initiate an anti-phase tapping pattern under three different movement rate conditions (600, 400, and 200 ms), and were required to increase the force of one finger at the onset of a randomly presented stimulus. The results indicate that an increase in the force parameter at lower tapping rates (600 ms) did not affect the phase relation of the fingers, however at higher rates (200 and 400 ms), the introduction of a force parameter resulted in fluctuations of the phase relation of the fingers, which were followed by pattern shifts from anti-phase to in-phase tapping. The results indicate that movement force and rate of movement interact to influence the outcome of the tapping pattern. Further research is required to investigate force as a control parameter.     

Height of chair seat and movement characteristics in sit-to-stand by young and elderly adults

The height of a chair seat affects the burden on the lower limbs during the sit-to-stand (STS). To develop an objective test to evaluate muscle function of the lower limbs using floor reaction force during a STS, the relationship between chair-seat height and the burden on the lower legs must be assessed. To examine the influence of the chair-seat height, floor reaction forces during a STS performed with 5 chair heights adjusted to each subject's lower leg length were compared. The force production and quickness of movement tended to decrease in the phases of trunk flexion and knee and hip joint extension when performing a STS from a lower chair, when the chair height differed by 20% (6.2 cm) from the lower leg length, and was marked when the difference between chair height and lower leg length became larger. In 52 elderly and 50 young adults, floor reaction forces during a STS performed from a chair of the same height as subjects' lower leg lengths were compared. Elderly persons were inferior in force production (strength) and quickness of movement, which decreased as elderly stood up from a chair of a lower height.     

An exploration of perception of body boundary, personal space, and body size in elderly persons.

A study of 108 elderly persons using the Body Distortion Questionnaire and the personal space simulation technique test did not support hypotheses that elderly persons with a large personal space will have a larger distortion of body boundary, a larger perception of large body size, a smaller perception of small body size, a larger distortion of body size, and a larger body distortion than elderly persons with a small personal space. The analyses with one-tail t tests showed elderly persons with a small personal space have a larger perception of large body size and a larger distortion of body size than elderly persons with a large personal space. When the extremes of personal space were used the results were the same. Males have a larger personal space and greater distortion of skin perceptions than females.     

Coordination among thigh muscles including the vastus intermedius and adductor magnus at different cycling intensities

Although many studies have been focused on muscle synergies in the lower limbs, synergies of the thigh muscles during cycling have not been investigated in detail. We examined synergies of the thigh muscles including the vastus intermedius (VI) and adductor magnus (AM) while cycling. Eight healthy men pedaled at 20%, 40%, 60%, 80% and 100% of maximal aerobic power output at a constant cadence of 60 rpm. Surface electromyography (EMG) recorded signals from the deep VI and the three superficial quadriceps femoris (QF) muscles, the two hamstrings and the AM. The root mean square of the EMG signal was averaged every 2° of crank rotation and normalized by the peak value for each muscle. We used factor analysis to assess normalized EMG recordings while cycling and to identify thigh muscle synergies. The VI, the superficial QF muscles and the AM dominated the first muscle synergy at all power output levels. The AM also formed a second synergy with the two hamstrings at all power output levels. These results suggest that the VI coordinates with the other QF and AM muscles, and that the AM coordinates with the QF and hamstring muscles while cycling.     

Are synergies continuously present in cyclical movements? An example with the basketball dribble task

In human movement, synergies occur when two or more variables co-vary to stabilize a performance goal. The concept of motor redundancy is associated with the existence of several strategies to complete the same task, which enables a movement system to adapt to an ever-changing environment. This feature provides the system with the ability of being flexible enough to produce adaptive movements, but also stable enough to produce acceptable outputs which is a key issue in motor performance. In a kinetic chain of movement, two proximal joints might reciprocally compensate to stabilize an end-effector (i.e., the most distal segment in the limb that interacts with the environment). End-effector variables are ‘controlled’, and directly linked to performance, whereas the task relevant elements are allowed by the system to have high variability, providing adaptability. In basketball dribbling, we hypothesized that shoulder and elbow variability contributes to stabilize the dribble height as an end-effector performance variable. A specific computational procedure based on the UCM (i.e., Uncontrolled Manifold) notion was used to capture synergies in two groups according to the experience level: amateurs and professionals. Results identified synergy presence during the basketball dribbling, which only occurred when the wrist reached its peak height. The control of the wrist peak height is achieved due to a reciprocal compensation between shoulder and elbow which stabilizes the dribbling height.