Efficient control of arm movements in advanced age

Control of arm movements in Friedreich’s ataxia patients: role of sensory feedback

Friedreich’s ataxia (FA) is a hereditary system degeneration, which progressively affects sensory functions such as proprioceptive feedback, which causes progressive ataxia in FA patients. While major clinical features of movement disorders in FA patients have been identified, the underlying impaired neural control is not sufficiently understood. To elucidate the underlying control mechanism, we investigated single-joint movements of the upper limb in FA patients. Small, tolerable force perturbations were induced during voluntary single-joint arm movements to examine the compensatory reaction of the FA patient’s motor system. Movement kinematics were measured, and muscle torques were quantified. We first found that as in healthy subjects, unperturbed single-joint movements in FA patients preserved similar temporal profiles of hand velocity and muscle torques, however, scaled in duration and amplitude. In addition, the small perturbations were compensated for efficiently in both groups, with the endpoint error < 0.5° (maximum displacement of 5–15°). We further quantified the differences in movement time, torque response, and displacement between patients and controls. To distinguish whether these differences were caused by a malfunction of top-down control or a malfunction of feedback control, the responses were fitted with a detailed model of the stretch reflex. The model simulations revealed that the feedback delay, but not the feedback gain was affected in FA patients. They also showed that the descending control signal was scaled in time and amplitude and co-contraction was smaller in FA patients. Thus, our study explains how the motor deficits of FA patients result from pathological alterations of both top-down and feedback control.

Adaptation of reach action to a novel force-field is not predicted by acuity of dynamic proprioception in either older or younger adults

Healthy ageing involves degeneration of the neuromuscular system which impacts movement control and proprioception. Yet the relationship between these sensory and motor deficits in upper limb reaching has not been examined in detail. Recently, we reported that age-related proprioceptive deficits were unrelated to accuracy in rapid arm movements, but whether this applied in motor tasks more heavily dependent on proprioceptive feedback was not clear. To address this, we have tested groups of younger and older adults on a force-field adaptation task under either full or limited visual feedback conditions and examined how performance was related to dynamic proprioceptive acuity. Adaptive performance was similar between the age groups, regardless of visual feedback condition, although older adults showed increased after-effects. Physically inactive individuals made larger systematic (but not variable) proprioceptive errors, irrespective of age. However, dynamic proprioceptive acuity was unrelated to adaptation and there was no consistent evidence of proprioceptive recalibration with adaptation to the force-field for any group. Finally, in spite of clear age-dependent loss of spatial working memory capacity, we found no relationship between memory capacity and adaptive performance or proprioceptive acuity. Thus, non-clinical levels of deficit in dynamic proprioception, due to age or physical inactivity, do not affect force-field adaptation, even under conditions of limited visual feedback that might require greater proprioceptive control.

Movement Kinematics and Interjoint Coordination Are Influenced by Target Location and Arm in 6-Year-Old Children

Rapid aiming movements are typically used to study upper limb motor control and development. Despite the large corpus of work in this area, few studies have examined kinematic manual asymmetries in children who have just started formal schooling and until now, none have characterized how children coordinate their joints to complete these movements (i.e., interjoint coordination). In the present study, manual asymmetries in kinematics and interjoint coordination in strongly right-handed 6-year-old children were investigated when reaching for ipsilateral and contralateral targets with their dominant right arm and the non-dominant left arm. Overall, manual asymmetries in interjoint coordination are apparent for both 6-year-old children and young adults, although young children completed the task by adopting a different strategy than adults. Also, control strategies employed by 6-year-old children were influenced by both the location of the target as well as the arm used to perform the task. Specifically, compared to all other conditions, children’s trajectories were more curved when performing contralateral movements with the non-dominant left arm, which were driven by smaller shoulder excursions combined with larger elbow excursions for this condition. Based on these results, we argue that the differences in interjoint coordination reflect the stage of development of 6-year-old children, the origin of which derives from maturational (e.g., hand dominance) and environmental factors (e.g., school-based experience).

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

Purpose

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

Methods

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

Results

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

Conclusions

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

Proprioceptive deficits in inactive older adults are not reflected in fast targeted reaching movements

During normal healthy ageing there is a decline in the ability to control simple movements, characterised by increased reaction times, movement durations and variability. There is also growing evidence of age-related proprioceptive loss which may contribute to these impairments. However, this relationship has not been studied in detail for the upper limb. We recruited 20 younger adults (YAs) and 31 older adults (OAs) who each performed 2 tasks on a 2D robotic manipulandum. The first assessed dynamic proprioceptive acuity using active, multi-joint movements constrained by the robot to a pre-defined path. Participants made perceptual judgements of the lateral position of the unseen arm. The second task required fast, accurate and discrete movements to the same targets in the absence of visual feedback of the hand, and without robotic intervention. We predicted that the variable proprioceptive error (uncertainty range) assessed in Task 1 would be increased in physically inactive OAs and would predict increased movement variability in Task 2. Instead we found that physically inactive OAs had larger systematic proprioceptive errors (bias) than YAs (t[33] = 2.8, p = 0.009), and neither proprioceptive uncertainty nor bias was related to motor performance in either age group (all regression model R2 ≤ 0.06). We suggest that previously reported estimates of proprioceptive decline with ageing may be exaggerated by task demands and that the extent of these deficits is unrelated to control of discrete, rapid movement. The relationship between dynamic proprioceptive acuity and movement control in other tasks with greater emphasis on online feedback is still unclear and warrants further investigation.

The role of intersegmental dynamics in coordination of the forelimb joints during unperturbed and perturbed skilled locomotion

Joint coordination during locomotion and how this coordination changes in response to perturbations remains poorly understood. We investigated coordination among forelimb joints during the swing phase of skilled locomotion in the cat. While cats walked on a horizontal ladder, one of the cross-pieces moved before the cat reached it, requiring the cat to alter step size. Direction and timing of the cross-piece displacement were manipulated. We found that the paw was transported in space through body translation and shoulder and elbow rotations, whereas the wrist provided paw orientation required to step on cross-pieces. Kinetic analysis revealed a consistent joint control pattern in all conditions. Although passive interaction and gravitational torques were the main sources of shoulder and elbow motions for most of the movement time, shoulder muscle torque influenced movement of the entire limb at the end of the swing phase, accelerating the shoulder and causing interaction torque that determined elbow motion. At the wrist, muscle and passive torques predominantly compensated for each other. In all perturbed conditions, although all joints and the body slightly contributed to changes in the step length throughout the entire movement, the major adjustment was produced by the shoulder at the movement end. We conclude that joint coordination during the swing phase is produced mainly passively, by exploiting gravity and the limb's intersegmental dynamics, which may simplify the neural control of locomotion. The use of shoulder musculature at the movement end enables flexible responses to environmental disturbances. NEW & NOTEWORTHY This is the first study to investigate joint control during the swing phase of skilled, accuracy-dependent locomotion in the cat and how this control is altered to adapt to known and unexpected perturbations. We demonstrate that a pattern of joint control that exploits gravitational and interaction torques is used in all conditions and that movement modifications are produced mainly by shoulder muscle torque during the last portion of the movement.

Methodological Infrastructure in Surgical Ergonomics: A Review of Tasks, Models, and Measurement Systems

Though in its infancy, the discipline of surgical ergonomics is increasingly valued. Still, little has been written regarding this field's tasks, models, and measurement systems. These 3 critical experimental components are crucial in objectively and accurately assessing joint and postural control as exhibited by expert laparoscopic surgeons. Such assessments will establish characteristic patterns important for surgical training. In addition, risk factors associated with both minimally invasive surgical instruments and the operating room environment can be identified and minimized. Our review focuses on evidence-based experimental ergonomic studies undertaken in the field of laparoscopic surgery. Publications were located through PubMed and other database and library searches. This article describes tasks, models, and measurement systems and considers their specific applications and the types of data obtainable with the use of each. Advantages and limitations, especially those of measurement systems, are compared and discussed. Future trends and directions believed necessary for optimal investigation and results are also addressed.

Age-related differentiation of sensorimotor control strategies during pursuit and compensatory tracking

Motor control deficits during aging have been well-documented. Various causes of neuromotor decline, including both peripheral and central neurological deficits, have been hypothesized. Here, we use a model of closed-loop sensorimotor control to examine the functional causes of motor control deficits during aging. We recruited 14 subjects aged 19-61 years old to participate in a study in which they performed single-joint compensatory and pursuit tracking tasks with their dominant hand. We found that visual response delay and visual noise increased with age, while reliance on visual feedback, especially during compensatory tracking decreased. Increases in visual noise were also positively correlated with increases in movement error during a reach and hold task. The results suggest an increase in noise within the visuomotor control system may contribute to the decline in motor performance during early aging.

A preferred pattern of joint coordination during arm movements with redundant degrees of freedom

Redundancy of degrees of freedom (DOFs) during natural human movements is a central problem of motor control research. This study tests a novel interpretation that during arm movements, the DOF redundancy is used to support a preferred, simplified joint control pattern that consists of rotating either the shoulder or elbow actively and the other (trailing) joint predominantly passively by interaction and gravitational torques. We previously revealed the preference for this control pattern during nonredundant horizontal arm movements. Here, we studied whether this preference persists during movements with redundant DOFs and the redundancy is used to enlarge the range of directions in which this control pattern can be utilized. A free-stroke drawing task was performed that involved production of series of horizontal center-out strokes in randomly selected directions. Two conditions were used, with the arm's joints unconstrained (U) and constrained (C) to the horizontal plane. In both conditions, directional preferences were revealed and the simplified control pattern was used in the preferred and not in nonpreferred directions. The directional preferences were weaker and the range of preferred directions was wider in the U condition, with higher percentage of strokes performed with the simplified control pattern. This advantage was related to the usage of additional DOFs. We discuss that the simplified pattern may represent a feedforward control strategy that reduces the challenge of joint coordination caused by signal-dependent noise during movement execution. The results suggest a possibility that the simplified pattern is used during the majority of natural, seemingly complex arm movements.

Assessing articulatory speed performance as a potential factor of slowed speech in older adults

PURPOSE To improve our understanding about the underlying factors of aging-related speaking rate decline, the authors sought to determine if lip and jaw speeds are physiologically constrained in older adults. METHOD Thirty-six females-10 young adults (ages 22-27 years), 9 middle-aged adults (ages 45-55 years), 10 young-old adults (65-74 years), and 7 very old adults (ages 87-95 years)-completed metronome-paced syllable repetitions while moving the lower lip or jaw to a fixed target with each repetition. Metronome paces incrementally increased from 1.4 Hz to 6.7 Hz. Lip and jaw movements were tracked using a 3-dimensional motion capture system. RESULTS Older adults' maximum percent increase in lip and jaw peak speed was comparable to or tended to be even greater than that of middle-aged and young adults. By contrast, lip and jaw stiffness, indexed by peak speed-displacement ratios, tended to decrease with age during fast and very fast repetition rates and were associated with mildly prolonged movement durations. CONCLUSIONS The findings suggest that lip and jaw speeds are not constrained in older adults. The trend of reduced stiffness during fast rates, however, suggests that fine-force regulation becomes difficult for older adults. Thus, older adults may implement reduced habitual speaking rates as a behavioral strategy to compensate for diminished articulatory control.

Aging, visual information, and adaptation to task asymmetry in bimanual force coordination

This study investigated the coordination and control strategies that the elderly adopt during a redundant finger force coordination task and how the amount of visual information regulates the coordination patterns. Three age groups (20-24, 65-69, and 75-79 yr) performed a bimanual asymmetric force task. Task asymmetry was manipulated via imposing different coefficients on the finger forces such that the weighted sum of the two index finger forces equaled the total force. The amount of visual information was manipulated by changing the visual information gain of the total force output. Two hypotheses were tested: the reduced adaptability hypothesis predicts that the elderly show less degree of force asymmetry between hands compared with young adults in the asymmetric coefficient conditions, whereas the compensatory hypothesis predicts that the elderly exhibit more asymmetric force coordination patterns with asymmetric coefficients. Under the compensatory hypothesis, two contrasting directions of force sharing strategies (i.e., more efficient coordination strategy and minimum variance strategy) are expected. A deteriorated task performance (high performance error and force variability) was found in the two elderly groups, but enhanced visual information improved the task performance in all age groups. With low visual information gain, the elderly showed reduced adaptability (i.e., less asymmetric forces between hands) to the unequal weighting coefficients, which supported the reduced adaptability hypothesis; however, the elderly revealed the same degree of adaptation as the young group under high visual gain. The findings are consistent with the notion that the age-related reorganization of force coordination and control patterns is mediated by visual information and, more generally, the interactive influence of multiple categories of constraints.

Modeling constraints to redundancy in bimanual force coordination

This study investigated the interactive influence of organismic, environmental, and task constraints on the organization of redundant force coordination patterns and the hypothesis that each of the three categories of constraints is weighted based on their relative influence on coordination patterns and the realization of the task goal. In the bimanual isometric force experiment, the task constraint was manipulated via different coefficients imposed on the finger forces such that the weighted sum of the finger forces matched the target force. We examined three models of task constraints based on the criteria of task variance (minimum variance model) and efficiency of muscle force output (coefficient-independent and coefficient-dependent efficiency models). The environmental constraint was quantified by the perceived performance error, and the organismic constraint was quantified by the bilateral coupling effect (i.e., symmetric force production) between hands. The satisficing approach was used in the models to quantify the constraint weightings that reflect the interactive influence of different categories of constraints on force coordination. The findings showed that the coefficient-dependent efficiency model best predicted the redundant force coordination patterns across trials. However, the within-trial variability structure revealed that there was not a consistent coordination strategy in the online control of the individual trial. The experimental findings and model tests show that the force coordination patterns are adapted based on the principle of minimizing muscle force output that is coefficient dependent rather than on the principle of minimizing signal-dependent variance. Overall, the results support the proposition that redundant force coordination patterns are organized by the interactive influence of different categories of constraints.

Control of human limb movements: the leading joint hypothesis and its practical applications

The leading joint hypothesis (LJH) offers a novel interpretation of control of human movements that involve multiple joints. The LJH makes control of each multijoint movement transparent. This review highlights effective applications of the LJH to learning of new motor skills and to analysis of movement changes caused by aging and motor disorders.

Limitations on coupling of bimanual movements caused by arm dominance: when the muscle homology principle fails

Studies of bimanual movements typically report interference between motions of the two arms and preference to perform mirror-symmetrical patterns. However, recent studies have demonstrated that the two arms differ in the ability to control interaction torque (INT). This predicts limitations in the capability to perform mirror-symmetrical movements. Here, two experiments were performed to test this prediction. The first experiment included bimanual symmetrical and asymmetrical circle drawing at two frequency levels. Unimanual circle drawing was also recorded. The increases in cycling frequency caused differences between the two arms in movement trajectories in both bimanual modes, although the differences were more pronounced in the asymmetrical compared with the symmetrical mode. Based on torque analysis, the differences were attributed to the nondominant arm's decreased capability to control INT. The intraarm differences during the symmetrical pattern of bimanual movements were similar (although more pronounced) to those during unimanual movements. This finding was verified in the second experiment for symmetrical bimanual oval drawing. Four oval orientations were used to provide variations in INT. Similar to the first experiment, increases in cycling frequency caused spontaneous deviations from perfect bimanual symmetry associated with inefficient INT control in the nondominant arm. This finding supports the limitations in performing mirror-symmetrical bimanual movements due to differences in joint control between the arms. Based on our results and previous research, we argue that bimanual interference occurs during specification of characteristics of required motion, whereas lower-level generation of muscle forces is independent between the arms. A hierarchical model of bimanual control is proposed.

Multicomponent control strategy underlying production of maximal hand velocity during horizontal arm swing

Movement control responsible for generation of maximal hand velocity was studied on the example of horizontal arm swing that is a component of various sports activities. The movement was performed with the nondominant arm in similarity with the baseball bat swing. The task was to generate maximum hand velocity at a target. The movement included trunk long-axis rotation and horizontal shoulder and elbow extension. Kinematics and torque analyses were performed to study the organization of fastest movements and to compare trials representing the best and worst performance in each subject. Results revealed complex control strategy, with the trunk, shoulder, and elbow playing unique roles in generation of maximal hand velocity. The trunk provided a crucial contribution, directly, rotating the entire arm, and indirectly, exerting interaction torque that caused swift elbow extension. The major role of the shoulder was to transfer the mechanical effect of trunk motion to the elbow. However, the shoulder became the primary motion generator when the trunk reached its limits of rotation, revealing sequential organization of control. The role of the elbow was to maximally comply with passive influence of proximal joints. The findings are discussed in light of the leading joint hypothesis that offers a straightforward interpretation of control of horizontal arm swing as well as practically efficient recommendations for increases in movement speed. The revealed role of intersegmental dynamics in production of high movement speed suggests that movement slowness characteristic for some motor disorders may be partially a compensatory strategy that facilitates regulation of interaction torque.

Characterization of age-related modifications of upper limb motor control strategies in a new dynamic environment

Background

In the past, several research groups have shown that when a velocity dependent force field is applied during upper limb movements subjects are able to deal with this external perturbation after some training. This adaptation is achieved by creating a new internal model which is included in the normal unperturbed motor commands to achieve good performance. The efficiency of this motor control mechanism can be compromised by pathological disorders or by muscular-skeletal modifications such as the ones due to the natural aging process. In this respect, the present study aimed at identifying the age-related modifications of upper limb motor control strategies during adaptation and de-adaptation processes in velocity dependent force fields.

Methods

Eight young and eight elderly healthy subjects were included in the experiment. Subjects were instructed to perform pointing movements in the horizontal plane both in a null field and in a velocity dependent force field. The evolution of smoothness and hand path were used to characterize the performance of the subjects. Furthermore, the ability of modulating the interactive torque has been used as a paradigm to explain the observed discoordinated patterns during the adaptation process.

Results

The evolution of the kinematics during the experiments highlights important behavioural differences between the two groups during the adaptation and de-adaptation processes. In young subjects the improvement of movement smoothness was in accordance with the expected learning trend related to the consolidation of the internal model. On the contrary, elders did not show a coherent learning process. The kinetic analysis pointed out the presence of different strategies for the compensation of the external perturbation: older people required an increased involvement of the shoulder with a different modulation of joint torque components during the evolution of the experiments.

Conclusion

The results obtained with the present study seem to confirm the presence of different adaptation mechanisms in young and senior subjects. The strategy adopted by young subjects was to first minimize hand path errors with a secondary process that is consistent with the optimization of the effort. Elderly subjects instead, seemed to shift the importance of the two processes involved in the control loop slowing the mechanism optimizing kinematic performance and enabling more the dynamic adaptation mechanism.