Trunk use and co-contraction in cerebral palsy as regulatory mechanisms for accuracy control

Muscle coactivation during gait in children with and without cerebral palsy

BACKGROUND

Children with Cerebral Palsy (CP) walk with an uncoordinated gait compared to Typically Developing (TD) children. This behavior may reflect greater muscle co-activation in the lower limb; however, findings are inconsistent, and the determinants of this construct are unclear.

RESEARCH OBJECTIVES

(i) Compare lower-limb muscle co-activation during gait in children with, and without CP, and (ii) determine the extent to which muscle co-activation is influenced by electromyography normalization procedures and Gross Motor Function Classification System (GMFCS) class.

METHODS

An electromyography system measured muscle activity in the rectus femoris, semitendinosus, gastrocnemius, and tibialis anterior muscles during walking in 46 children (19 CP, 27 TD). Muscle co-activation was calculated for the tibialis anterior-gastrocnemius (TA-G), rectus femoris-gastrocnemius (RF-G), and rectus femoris-semitendinosus (RF-S) pairings, both using root mean squared (RMS)-averaged and dynamically normalized data, during stance and swing. Mann-Whitney U and independent t-tests examined differences in muscle co-activation by group (CP vs. TD) and GMFCS class (CP only), while mean difference 95% bootstrapped confidence intervals compared electromyography normalization procedures.

RESULTS

Using dynamically normalized data, the CP group had greater muscle co-activation for the TA-G and RF-G pairs during stance (p < 0.01). Using RMS-averaged data, the CP group had greater muscle co-activation for TA-G (stance and swing, p < 0.01), RF-G (stance, p < 0.05), and RF-S (swing, p < 0.01) pairings. Muscle co-activation calculated with dynamically normalized, compared to RMS-averaged data, were larger in the RF-S and RF-G (stance) pairs, but smaller during swing (RF-G). Children with CP classified as GMFCS II had greater muscle co-activation during stance in the TA-G pair (p < 0.05).

SIGNIFICANCE

Greater muscle co-activation observed in children with CP during stance may reflect a less robust gait strategy. Although data normalization procedures influence muscle co-activation ratios, this behavior was observed independent of normalization technique.

Muscle co-contraction and co-activation in cerebral palsy during gait: A scoping review

BACKGROUND

Cerebral palsy (CP) results from an injury to a developing brain. Muscle activation patterns during walking are disrupted in individuals with CP. Indeed, excessive muscle co-contraction or co-activation (MCo/MCa) is one of the characteristics of pathological gait. Although some researchers have studied MCo/MCa in individuals with CP during gait, inconsistent results limit our understanding of this literature. Increased knowledge of MCo/MCa patterns in individuals with CP may help the development of improved gait management approaches.

RESEARCH QUESTION

This review aims to summarize MCo/MCa patterns while walking in individuals with CP across the existing literature and compare them with their healthy peers.

METHODS

This study follows the Joanna Briggs Institute (JBI) guidelines and the recommendations presented in PRISMA Extension for Scoping Reviews (PRISMA-ScR). The recommendations of the Preferred Reporting Items for Systematic Reviews and Meta-Analyses Extension for scoping Reviews statement were respected. The following databases were searched: MEDLINE (Ovid), EMBASE (Ovid), CINAHL Plus with Full Text (Ebsco), SPORTDiscus with Full Text (Ebsco), and Web of Science.

RESULTS

Among 2545 identified studies, 21 studies remained after screening. In total, 337 participants with CP and 249 healthy participants were included. Both MCo and MCa terminologies are used for describing simultaneous muscle activation; however, when it is measured by electromyography (EMG), MCa terminology should be preferred to facilitate interpretation. A wide range of MCo/MCa patterns has been found across studies using different methodologies (e.g., gait protocol, computation methods). Finally, most of the included studies confirm that MCo/MCa is increased in individuals with CP during walking compared to controls.

SIGNIFICANCE

This review identified missing concepts and common limitations in the literature which could be addressed in future research such as the association between MCo/MCa and gait deviations, and the most appropriate MCo/MCa computation method.

Quantitative assessment of trunk movements in functional reaching in children and adolescents with dyskinetic cerebral palsy

BACKGROUND

Trunk control and upper limb function are often disturbed in people with dyskinetic cerebral palsy. While trunk control is fundamental in upper limb activities, insights in trunk control in dyskinetic cerebral palsy are missing. This study aimed to determine trunk movement characteristics in individuals with dyskinetic cerebral palsy during reaching.

METHODS

Twenty individuals with dyskinetic cerebral palsy (MACS level I-III (16y6m)) and 20 typical developing peers (17y2m) were included. Participants performed three tasks: reach forward, reach sideways, and reach and grasp vertically, using a cross-sectional study design. Movements were analyzed using 3D motion capture and a sensor on the trunk. Trunk range of motion, joint angle at point of task achievement, peak and range of angular velocity and linear acceleration were compared between groups using Mann-Whitney U and independent t-tests.

FINDINGS

Participants with dyskinetic cerebral palsy showed higher trunk range of motion in all planes during reach forward and reach and grasp vertically, and in rotation and lateral flexion during reach sideways. During reach and grasp vertically, the joint angle at point of task achievement differed in the transversal plane. Ranges of angular velocity and linear acceleration were higher for all tasks and planes for participants with dyskinetic cerebral palsy, and for peak values in nearly all planes.

INTERPRETATION

Current results provide insights in trunk control at population level. This is a first step towards a better and individualized evaluation and treatment for trunk control, being an important factor in improving functional activities for individuals with dyskinetic cerebral palsy.

Improving the objectivity of the current World Para Swimming motor coordination test for swimmers with hypertonia, ataxia and athetosis using measures of movement smoothness, rhythm and accuracy

The current protocol for classifying Para swimmers with hypertonia, ataxia and athetosis involves a physical assessment where the individual's ability to coordinate their limbs is scored by subjective clinical judgment. The lack of objective measurement renders the current test unsuitable for evidence-based classification. This study evaluated a revised version of the Para swimming assessment for motor coordination, incorporating practical, objective measures of movement smoothness, rhythm error and accuracy. Nineteen Para athletes with hypertonia and 19 non-disabled participants performed 30 s trials of bilateral alternating shoulder flexion-extension at 30 bpm and 120 bpm. Accelerometry was used to quantify movement smoothness; rhythm error and accuracy were obtained from video. Para athletes presented significantly less smooth movement and higher rhythm error than the non-disabled participants (p < 0.05). Random forest algorithm successfully classified 89% of participants with hypertonia during out-of-bag predictions. The most important predictors in classifying participants were movement smoothness at both movement speeds, and rhythm error at 120 bpm. Our results suggest objective measures of movement smoothness and rhythm error included in the current motor coordination test protocols can be used to infer impairment in Para swimmers with hypertonia. Further research is merited to establish the relationship of these measures with swimming performance.

The use of a task through virtual reality in cerebral palsy using two different interaction devices (concrete and abstract) - a cross-sectional randomized study

BACKGROUND

Cerebral Palsy (CP) is characterised by variable difficulties in muscular action, resulting in inability of the individual to perform functional movement. An option to provide functionality to the individual with CP is the use of computer innovation. The aim of this paper was to verify if there was any performance improvement in a task performed in a virtual environment and if there was transfer to the task performed in the real environment and vice versa in this population.

METHODS

A computer program was developed comprising a motor task, but with two possibilities of user interaction: a) concrete interface (with physical contact): in which the individual touches the computer screen to finish the task and b) abstract interface (no physical contact): in which the individual performs a hand movement in front of the Kinect device. Participants were split into two groups. The experimental group consisted of 28 individuals with CP within the ages of 6 and 15 years old. The control group included 28 typically developing individuals mirroring the age and sex of the experimental group.

RESULTS

Individuals from both groups were able to improve task performance and retain acquired information. The CP group presented worse performance than the control group in all phases of the study. Further findings showed that the CP group presented better performance in the abstract interface than in the concrete interface, whereas, in the control group, the opposite occurred: their best performance was in the concrete.

CONCLUSIONS

Motor tasks performed by individuals with CP through an interface with a more virtual environment feature (abstract interface: Kinect) provided better performance when compared to an interface with a more real characteristic (concrete interface: Touchscreen).

TRIAL REGISTRATION

ClinicalTrials.gov Identifier - NCT03352440; Date of registration - November 17, 2017.

A lower-extremity exoskeleton improves knee extension in children with crouch gait from cerebral palsy

The ability to walk contributes considerably to physical health and overall well-being, particularly in children with motor disability, and is therefore prioritized as a rehabilitation goal. However, half of ambulatory children with cerebral palsy (CP), the most prevalent childhood movement disorder, cease to walk in adulthood. Robotic gait trainers have shown positive outcomes in initial studies, but these clinic-based systems are limited to short-term programs of insufficient length to maintain improved function in a lifelong disability such as CP. Sophisticated wearable exoskeletons are now available, but their utility in treating childhood movement disorders remains unknown. We evaluated an exoskeleton for the treatment of crouch (or flexed-knee) gait, one of the most debilitating pathologies in CP. We show that the exoskeleton reduced crouch in a cohort of ambulatory children with CP during overground walking. The exoskeleton was safe and well tolerated, and all children were able to walk independently with the device. Rather than guiding the lower limbs, the exoskeleton dynamically changed the posture by introducing bursts of knee extension assistance during discrete portions of the walking cycle, a perturbation that resulted in maintained or increased knee extensor muscle activity during exoskeleton use. Six of seven participants exhibited postural improvements equivalent to outcomes reported from invasive orthopedic surgery. We also demonstrate that improvements in crouch increased over the course of our multiweek exploratory trial. Together, these results provide evidence supporting the use of wearable exoskeletons as a treatment strategy to improve walking in children with CP.

Classification of upper limb disability levels of children with spastic unilateral cerebral palsy using K-means algorithm

Treatment for cerebral palsy depends upon the severity of the child's condition and requires knowledge about upper limb disability. The aim of this study was to develop a systematic quantitative classification method of the upper limb disability levels for children with spastic unilateral cerebral palsy based on upper limb movements and muscle activation. Thirteen children with spastic unilateral cerebral palsy and six typically developing children participated in this study. Patients were matched on age and manual ability classification system levels I to III. Twenty-three kinematic and electromyographic variables were collected from two tasks. Discriminative analysis and K-means clustering algorithm were applied using 23 kinematic and EMG variables of each participant. Among the 23 kinematic and electromyographic variables, only two variables containing the most relevant information for the prediction of the four levels of severity of spastic unilateral cerebral palsy, which are fixed by manual ability classification system, were identified by discriminant analysis: (1) the Falconer index (CAI _E ) which represents the ratio of biceps to triceps brachii activity during extension and (2) the maximal angle extension (θ _{Extension,max}). A good correlation (Kendall Rank correlation coefficient = -0.53, p = 0.01) was found between levels fixed by manual ability classification system and the obtained classes. These findings suggest that the cost and effort needed to assess and characterize the disability level of a child can be further reduced.

Impact of muscle activation on ranges of motion during active elbow movement in children with spastic hemiplegic cerebral palsy

BACKGROUND

Children with spastic hemiplegic cerebral palsy are restricted in their daily activities due to limited active ranges of motion of their involved upper limb, specifically at the elbow. Their impaired muscles are frequently targeted by anti-spastic treatments that reduce muscle tone. But these treatments do not necessarily improve the limb function. There is a lack of comprehensive knowledge of the quantitative relations between muscle activation and joint active ranges of motion. Consequently, the objective of this study is to quantify the impact of muscle activation on the elbow active ranges of motion.

METHODS

During voluntary elbow pronation/supination and extension/flexion movements, kinematic and electromyographic measurements were collected from the involved upper limb of 15 children with spastic hemiplegic cerebral palsy (mean age=8.7 years, standard deviation=2.2) and the dominant upper limb of 15 age-matched children who are typically developing. Representative indicators of the muscle activation, such as the muscle co-activation, were extracted from the electromyographic measurements.

FINDINGS

Muscle co-activation in the involved upper limb accounted for 78% and 59% of the explained variance of the supination and extension limited active ranges of motion respectively. The agonist and antagonist muscle activations were both longer in the involved upper limb.

INTERPRETATIONS

This study succeeded in quantifying the impact of longer antagonist muscle activation on decreased elbow active ranges of motion in children with spastic hemiplegic cerebral palsy. Longer agonist muscle activation suggests that strengthening agonist muscles could increase the extension and supination ranges of motion, which constitutes a perspective of future clinical studies.

Kinematic changes in jaw and lip control of children with cerebral palsy following participation in a motor-speech (PROMPT) intervention

This study evaluates kinematic movements of the jaw and lips in six children (3-11 years) with moderate-to-severe speech impairment associated with cerebral palsy before, during, and after participation in a motor-speech (PROMPT) intervention program. An ABCA single subject research design was implemented. Subsequent to the baseline phase (A), phase B targeted each participant's first intervention priority on the PROMPT motor-speech hierarchy. Phase C then targeted one level higher. A reference group of 12 typically-developing peers, age- and sex-matched to each participant with CP, was recruited for comparison in the interpretation of the kinematic data. Jaw and lip measurements of distance, velocity, and duration, during the production of 11 untrained stimulus words, were obtained at the end of each study phase using 3D motion analysis (Vicon Motus 9.1). All participants showed significant changes in specific movement characteristics of the jaw and lips. Kinematic changes were associated with significant positive changes to speech intelligibility in five of the six participants. This study makes a contribution to providing evidence that supports the use of a treatment approach aligned with dynamic systems theory to improve the motor-speech movement patterns and speech intelligibility in children with cerebral palsy.

Biceps brachii can add to performance of tasks requiring supination in cerebral palsy patients

The aim of this study was to assess whether cerebral palsy patients can use biceps brachii for supination during movement tasks requiring supination and pronation. 3D upper extremity kinematic and EMG-data of 12 patients (mean age 13 y 8 mo ± 36 mo) were compared to 10 healthy age-matched controls. Significant difference in biceps brachii activation between maximal isolated pronation and supination in both groups showed that it is possible for CP patients to use biceps brachii for supination. Performance of reach-to-grasp with either pronation or supination showed similar activation patterns as during isolated tasks in both groups, although increased biceps brachii activation likely also hampered performance of reach-to-grasp in the patient group by causing increased, and possibly unwanted elbow flexion. However, the functional effect of this flexion for supination purposes cannot be ruled out. Therefore, one should be cautious with simply weakening biceps brachii when the purpose is to improve functional reach. Ideally treatment might focus more on changing the flexion moment/supination moment ratio of biceps toward a stronger supination function.

Agonistic and antagonistic interaction in speed/accuracy tradeoff: a delta-lognormal perspective

This paper reports the results of a model-based analysis of movements gathered in a 4×4 experimental design of speed/accuracy tradeoffs with variable target distances and width. Our study was performed on a large (120 participants) and varied sample (both genders, wide age range, various health conditions). The delta-lognormal equation was used for data modeling to investigate the interaction between the output of the agonist and the antagonist neuromuscular systems. Empirical observations show that the subjects must correlate more tightly the impulse commands sent to both neuromuscular systems in order to achieve good performances as the difficulty of the task increases whereas the correlation in the timing of the neuromuscular action co-varies with the size of the geometrical properties of the task. These new phenomena are discussed under the paradigm provided by the Kinematic Theory and new research hypotheses are proposed for further investigation of the speed/accuracy tradeoffs.

Postural adjustment of children with spastic diplegic cerebral palsy during seated hand reaching in different directions

OBJECTIVES

To examine the effect of reaching in different directions on postural adjustment in children with diplegic cerebral palsy (CP), and to examine the relationship between hand reach performance and postural adjustment, and between postural control ability and postural adjustment.

DESIGN

Cross-sectional study.

SETTING

A movement science laboratory at a medical university.

PARTICIPANTS

Children with CP (n=12) and typically developing (TD) children (n=16).

INTERVENTIONS

Not applicable.

MAIN OUTCOME MEASURES

Two force platforms were used to measure the ground reaction force (GRF) and center of pressure (COP) data. Absolute peak COP velocity, COP sway ratio (SR), and mean GRF in the anterior posterior direction during the acceleration and deceleration segments of a reaching task were the main outcome measures.

RESULTS

Children with CP showed a greater absolute peak COP velocity in the medial lateral direction, a smaller SR (wider COP pattern), and greater amplitude of force modulation (exaggerated postural adjustments) than TD children in lateral or medial reaches. There was a moderate correlation between SR and total Pediatric Reach Test score. The chair SR was also negatively correlated with the hand movement units.

CONCLUSIONS

Children with CP showed wider, more crooked, and less efficient COP patterns than TD children, especially on medial or lateral reaches. Reaching medially or laterally involves trunk rotation, which produces more postural challenges than reaching anteriorly to children with CP. The patterns of postural adjustments in children with CP were correlated with their postural control ability and hand-reach smoothness.

Bimanual force coordination in children with spastic unilateral cerebral palsy

In this study bimanual grip-force coordination was quantified using a novel "Gripper" system that records grip forces produced while holding a lower and upper unit, in combination with the lift force necessary to separate these units. Children with unilateral cerebral palsy (CP) (aged 5-14 years, n=12) were compared to age matched typically developing (TD) children (n=23). Compared to TD, the CP-group is much slower and takes 50% more time to generate grip and lift forces with more fixating force before lifting the upper unit. In addition the coordination between forces in both hands is reduced. The CP-group increases the lift force in the upper hand 2.5 times more than the holding force when pulling the two units apart, while this is only 1.5 times in TD. Moreover, the correlation between forces generated in both hands in the CP-group is lower. The lack of fine tuning of the forces, measured by the linearity error is increased, especially when the magnet load keeping the unit together is low. The results indicate an impaired pull-hold synergy between upper and lower hand and the lift force. Bimanual tasks evaluating bimanual grip and lift forces in children with CP and can give us new insights in the underlying force control mechanisms of the spastic hand.

Activity modulations of trunk and lower limb muscles during impact-absorbing landing

This study aimed to investigate the activity patterns of trunk and lower limb muscles during impact-absorbing landing. Electromyogram activities of the trunk and lower limb muscles along with kinematic and ground reaction forces were measured while subjects (n=17) performed 10 landings from a height of 35 cm. Landing motions were divided into three phases: 100 ms preceding ground contact (GC) (PRE phase), from GC through 100 ms (ABSORPTION phase), and from the end of the ABSORPTION phase until the vertical position of the center of mass was minimized (BRAKING phase). During the PRE phase, the rectus abdominis, external oblique, and medial gastrocnemius were highly activated. Upon GC, the hip and knee joints were in a flexed position; the ankle joints, in a plantarflexed position. After GC, peak timings of muscle activities and lower limb joint rotations were characterized by distal-to-proximal sequential patterns. The peak vertical ground reaction force in the ABSORPTION phase relative to body weight positively correlated with the activity levels of the vastus lateralis and gluteus maximus in the PRE phase and that of rectus abdominis in the ABSORPTION phase. These findings indicate that the intensities and peak timings of muscle activities in the trunk and lower limb are coordinated to absorb landing impact.

Intra- and inter-observer reliability of the Trunk Impairment Scale for children with cerebral palsy

Standardized scales to evaluate qualities of trunk movements in children with dysfunction are sparse. An examination of the reliability of scales that may be useful in the clinic is important. The aim of this study was to examine the reliability of the Trunk Impairment Scale (TIS) for children with cerebral palsy (CP). Standardized scales are useful for treatment planning and evaluation. This was an intra- and inter-observer reliability study. Video recordings of 25 children, 20 with CP and 5 with no motor impairment, in the age group 5-12 years of age, were analyzed by three observers on two occasions. Intraclass correlation coefficients (ICC [1,1] and [3,1]) with 95% confidence intervals, standard error of measurement, kappa values and percent agreement, and Bland-Altman Plots were calculated. The relative reliability (intra- and inter-observer reliability) was very high for the total score and subscale score of the TIS: ICC [1,1] and [3,1] varied between .94 and 1.00. Kappa values for the items ranged from .45 to 1.00. The absolute reliability values for the parameters are reported. The Bland-Altman analysis showed consistency of scores. This study indicates that TIS is a reliable measure of trunk control for children, 5-12 years of age, with CP.

Impairment severity selectively affects the control of proximal and distal components of reaching movements in children with hemiplegic cerebral palsy

This study explored proximal-to-distal components during goal-directed reaching movements in children with mild or moderate hemiplegic cerebral palsy (HCP); [seven females, four males; mean age 8 y 6 mo; SD 27 mo], compared with age-matched, typically developing children (seven females, five males; mean age 8 y 3 mo [SD 25 mo]. Severity of HCP was assessed following the approach of Claeys et al. Optoelectronic registrations were made during unimanual reaching-to-grasp and reaching-to-hit movements with both the affected/non-preferred and unaffected/preferred side. Regardless of task, the children with HCP, particularly those with moderate impairment, displayed less optimal spatiotemporal organization of movements performed with the affected arm. Compared with the goal to hit, and increasingly with more severe impairment, children with HCP adapted to the goal to grasp by recruiting augmented shoulder movements when reaching with the affected side. A resulting impact on distal kinematics was found in shorter, straighter, and less segmented movement paths. Thus, depending on severity of hemispheric lesions and task complexity, unilateral brain injuries in HCP may selectively affect neural pathways underlying both proximal and distal arm movement control. Levels of both ipsi- and contralateral activation in relation to side and lesion severity should be considered in future studies on prehension movements in HCP.

Eye hand coordination in children with cerebral palsy

Reaching to grasp an object of interest requires complex sensorimotor coordination involving eye, head, hand and trunk. While numerous studies have demonstrated deficits in each of these systems individually, little is known about how children with cerebral palsy (CP) coordinate multiple motor systems for functional tasks. Here we used kinematics, remote eye tracking and a trunk support device to examine the functional coupling of the eye, head and hand and the extent to which it was constrained by trunk postural control in 10 children with CP (6-16 years). Eye movements in children with CP were similar to typically developing (TD) peers, while hand movements were significantly slower. Postural support influenced initiation of hand movements in the youngest children (TD & CP) and execution of hand movements in children with CP differentially depending on diagnosis. Across all diagnostic categories, the most robust distinction between TD children and children with CP was in their ability to isolate eye, head and hand movements. Results of this study suggest that deficits in motor coordination for accurate reaching in children with CP may reflect coupled eye, head, and hand movements. We have previously suggested that coupled activation of effectors may be the default output for the CNS during early development.

Co-activity during maximum voluntary contraction: a study of four lower-extremity muscles in children with and without cerebral palsy

This study was designed to determine whether children with cerebral palsy (CP) showed more co-activity than comparison children in non-prime mover muscles with regard to the prime mover during maximum voluntary isometric contraction (MVIC) of four lower-extremity muscles. Fourteen children with spastic diplegic CP (10 males, four females; age range 4-10y), seven children with spastic hemiplegic CP (five males, two females; age range 5-10y), and 14 comparison children (eight males, six females; age range 4-11y) participated in the study. Gross Motor Function Classification System levels of the children with CP were as follows: eight children at Level I, seven children at Level II, five children at Level III, and one child at Level I V. Surface electromyographic recordings were made simultaneously from the vastus lateralis (VL), medial hamstrings (MH), tibialis anterior, and lateral gastrocnemius (LG) muscles during maximal voluntary contraction. Children with CP showed higher co-activity than the comparison children in both antagonist and adjacent muscles. This was particularly true when VL, MH, or LG muscles were engaged in MVIC. These findings may contribute to the weakness and abnormal movement patterns seen in CP, and they have implications for treatment.

Computational motor control: feedback and accuracy

Speed/accuracy trade-off is a ubiquitous phenomenon in motor behaviour, which has been ascribed to the presence of signal-dependent noise (SDN) in motor commands. Although this explanation can provide a quantitative account of many aspects of motor variability, including Fitts' law, the fact that this law is frequently violated, e.g. during the acquisition of new motor skills, remains unexplained. Here, we describe a principled approach to the influence of noise on motor behaviour, in which motor variability results from the interplay between sensory and motor execution noises in an optimal feedback-controlled system. In this framework, we first show that Fitts' law arises due to signal-dependent motor noise (SDN(m)) when sensory (proprioceptive) noise is low, e.g. under visual feedback. Then we show that the terminal variability of non-visually guided movement can be explained by the presence of signal-dependent proprioceptive noise. Finally, we show that movement accuracy can be controlled by opposite changes in signal-dependent sensory (SDN(s)) and SDN(m), a phenomenon that could be ascribed to muscular co-contraction. As the model also explains kinematics, kinetics, muscular and neural characteristics of reaching movements, it provides a unified framework to address motor variability.

Eye-hand coordination during manual object transport with the affected and less affected hand in adolescents with hemiparetic cerebral palsy

In the present study we investigated eye-hand coordination in adolescents with hemiparetic cerebral palsy (CP) and neurologically healthy controls. Using an object prehension and transport task, we addressed two hypotheses, motivated by the question whether early brain damage and the ensuing limitations of motor activity lead to general and/or effector-specific effects in visuomotor control of manual actions. We hypothesized that individuals with hemiparetic CP would more closely visually monitor actions with their affected hand, compared to both their less affected hand and to control participants without a sensorimotor impairment. A second, more speculative hypothesis was that, in relation to previously established deficits in prospective action control in individuals with hemiparetic CP, gaze patterns might be less anticipatory in general, also during actions performed with the less affected hand. Analysis of the gaze and hand movement data revealed the increased visual monitoring of participants with CP when using their affected hand at the beginning as well as during object transport. In contrast, no general deficit in anticipatory gaze control in the participants with hemiparetic CP could be observed. Collectively, these findings are the first to directly show that individuals with hemiparetic CP adapt eye-hand coordination to the specific constraints of the moving limb, presumably to compensate for sensorimotor deficits.

Severe aberrant glenohumeral motor patterns in a young female rower: A case report

Background

This case features an 18-year-old female with glenohumeral dysrhythmia and subluxation-relocation patterns. This unusual case highlights the need for careful examination and consideration to the anatomical structures involved.

Conventional approaches to shoulder examination include range of motion, orthopaedic tests and manual resistance tests. We also assessed the patient's cognitive ability to coordinate muscle function. With this type of assessment we found that co-contraction of local muscle groups seemed to initially improve the patients abnormal shoulder motion. With this information a rehabilitation method was instituted with a goal to maintain the improvement.

Case presentation

An 18-year-old female with no history of trauma, presented with painless kinesiopathology of the left shoulder (in abduction) consisting of dysrhythmia of the glenohumeral joint and early lateral rotation of the scapula. Examination also showed associated muscle atrophy of the lower trapezius and surrounding general muscle weakness. We used an untested functional assessment method in addition to more conventional methods.

Exercise rehabilitation interventions were subsequently prescribed and graduated in accordance with what is known as the General Physical Rehabilitation Pyramid.

Conclusion

This paper presents an unusual case of aberrant shoulder movement. It highlights the need for careful examination and thought regarding the anatomical structures and normal motor patterns associated with the manoeuvre being tested. It also emphasised the use of co-contraction during examination in an attempt to immediately improve a regional dysrythmia if there is suspicion of a regional aberrant motor pattern. Further research may be warranted to test this approach.

Impedance modulation and feedback corrections in tracking targets of variable size and frequency

Humans are able to adjust the accuracy of their movements to the demands posed by the task at hand. The variability in task execution caused by the inherent noisiness of the neuromuscular system can be tuned to task demands by both feedforward (e.g., impedance modulation) and feedback mechanisms. In this experiment, we studied both mechanisms, using mechanical perturbations to estimate stiffness and damping as indices of impedance modulation and submovement scaling as an index of feedback driven corrections. Eight subjects tracked three differently sized targets (0.0135, 0.0270, and 0.0405 rad) moving at three different frequencies (0.20, 0.25, and 0.33 Hz). Movement variability decreased with both decreasing target size and movement frequency, whereas stiffness and damping increased with decreasing target size, independent of movement frequency. These results are consistent with the theory that mechanical impedance acts as a filter of noisy neuromuscular signals but challenge stochastic theories of motor control that do not account for impedance modulation and only partially for feedback control. Submovements during unperturbed cycles were quantified in terms of their gain, i.e., the slope between their duration and amplitude in the speed profile. Submovement gain decreased with decreasing movement frequency and increasing target size. The results were interpreted to imply that submovement gain is related to observed tracking errors and that those tracking errors are expressed in units of target size. We conclude that impedance and submovement gain modulation contribute additively to tracking accuracy.

Deviations in upper-limb function of the less-affected side in congenital hemiparesis

In the present study we examined upper-limb function of the less-affected side in young adolescents with congenital hemiparesis (cerebral palsy: CP). Five participants with hemiparetic CP and five control participants performed a cyclical reach-and-grasp task with the less-affected hand towards targets placed at 60%, 100%, and 140% of the participant's arm-length. Trunk involvement, end-effector kinematics and activation of the biceps and triceps were examined together with several clinical measures. Movements at the less-affected side were slower and peak velocity was reached later in the experimental group. Even though total trunk involvement was identical in both groups, it was selectively limited to forward bending in participants with CP. Elbow amplitudes of these participants were smaller for the 60% and 100% arm-length target distances. Additionally, participants with CP showed weak positive correlations between agonist (triceps) activity and elbow amplitude, suggesting that deficient agonist rather than antagonist innervation was responsible for the decreased elbow involvement. Especially the more severely affected participants with CP proved to compensate their relatively small elbow amplitudes by increased forward bending. Collectively, the findings demonstrate deviations in upper-limb control of the less-affected body side in congenital hemiparesis.

Impedance is modulated to meet accuracy demands during goal-directed arm movements

The neuromuscular system is inherently noisy and joint impedance may serve to filter this noise. In the present experiment, we investigated whether individuals modulate joint impedance to meet spatial accuracy demands. Twelve subjects were instructed to make rapid, time constrained, elbow extensions to three differently sized targets. Some trials (20 out of 140 for each target, randomly assigned) were perturbed mechanically at 75% of movement amplitude. Inertia, damping and stiffness were estimated from the torque and angle deviation signal using a forward simulation and optimization routine. Increases in endpoint accuracy were not always reflected in a decrease in trajectory variability. Only in the final quarter of the trajectory the variability decreased as target width decreased. Stiffness estimates increased significantly with accuracy constraints. Damping estimates only increased for perturbations that were initially directed against the movement direction. We concluded that joint impedance modulation is one of the strategies used by the neuromuscular system to generate accurate movements, at least during the final part of the movement.

Differential effects of mental load on proximal and distal arm muscle activity

Work-related upper extremity disorders (WRUEDs) that result from keyboarding tasks are prevalent and costly. Although the precise mechanisms causing the disorder are not yet fully understood, several risk factors have been proposed. These include the repetitive nature of the motor task and the associated sustained static working postures, but also more psychological factors such as mental load. Epidemiological surveys have shown that WRUEDs are more prone to develop in the postural muscles of the neck/shoulder area than in the executive muscles controlling the hand. The present study investigated whether the activation patterns of these two muscle types are differentially affected by an additional mental load during the performance of a repetitive tapping task. Participants tapped various keying patterns with their dominant index finger at two prescribed tempi. Mental load was manipulated by means of an auditory short-term memory task. We recorded the EMG activity of two neck/shoulder muscles (trapezius and deltoid), two upper arm muscles (biceps and triceps), and four forearm muscles (flexor digitorum superficialis, extensor digitorum, extensor carpi radialis longus and extensor carpi ulnaris) and analyzed the kinematics and impact forces of the index finger. The results confirmed that the upper limb has two functions. Specifically, activity of the executive distal musculature was increased during tapping at the higher pace, while the activity of the postural upper limb musculature was elevated due to the memory task. We argue that continuously increased muscular activity can lead to fatigue and thus eventually cause musculoskeletal complaints. The results are discussed with respect to biomechanical adaptation strategies that deal with the consequences of increased noise in the neuromotor system due to enhanced mental processing.