Trajectories of target reaching arm movements in individuals with spinal cord injury: effect of external trunk support

Test-retest reliability and validity of the Sitting Balance Measure-Korean in individuals with incomplete spinal cord injury

STUDY DESIGN

Cross-cultural reliability and validity.

OBJECTIVES

To develop and validate the Korean version of the Sitting Balance Measure (SBM-K) in Korean persons with incomplete spinal cord injury (ISCI).

SETTING

Tertiary care center.

METHODS

Twenty-nine persons with ISCI were evaluated using SBM-K, which was validated using the kappa coefficient and intraclass coefficient (ICC). The correlation between SBM-K individual items and total score was analyzed using Spearman's correlation, and the internal consistency of test items was measured using Cronbach's alpha. Additionally, the standard error measurement (SEM) and minimal detectable change (MDC) were measured. For the clinical validity of SBM-K, the correlation of SBM-K with the modified Sitting Balance Scale (mSBS) and the Korean-Spinal Cord Independence Measure-III (KSCIM-III) was determined via Spearman's correlation. Linear regression was performed to determine whether SBM-K could predict KSCIM-III.

RESULTS

The weighted kappa score of the SBM-K individual items and ICC of SBM-K total score were 0.76-0.83 (good-very good) and 0.98 (0.95-0.99), respectively. The correlation between the SBM-K total score and individual items was notable (r = 0.78-0.98). Cronbach's alpha, SEM, and MDC of SBM-K were 0.98, 0.59, and 1.64, respectively. The clinical validity of SBM-K correlated with mSBS (r = 0.88) and KSCIM-III (r = 0.65-0.89). SBM-K accounted for 17-72% of the variance in predicting KSCIM-III.

CONCLUSIONS

SBM-K showed sufficient test-retest reliability, validity, and marginal measurement errors. SBM-K can serve as an optimal clinical assessment tool for Korean ISCI patients and may provide clinicians with reliable sitting balance assessment in Korean clinical settings.

Quantifying an Upper Extremity Everyday Task With 3D Kinematic Analysis in People With Spinal Cord Injury and Non-disabled Controls

Objectives: Upper extremity function after spinal cord injury (SCI) is an important factor for performance of activities of daily living. An objective assessment of upper extremity function preferably in purposeful daily tasks is essential in understanding its impact on real-life activities. This study aimed to identify which movement parameters of upper extremity, measured by kinematic analysis during a purposeful daily task, are impaired in people with cervical or thoracic SCI.

Materials and Methods: The study included 29 adults (mean 59.5 years, 9 women and 20 men) with cervical (n = 19) or thoracic (n = 10) established complete (n = 15) or incomplete (n = 14) SCI, and 54 non-disabled controls with commensurable age and sex (mean 59 years, 15 women, 39 men). The 3D kinematic data were captured with a five-camera system during a standardized unilateral daily task (drinking from a glass). In SCI, the upper extremity functioning of each arm was assessed with Action Research Arm Test (ARAT). Having a full score in ARAT indicated full functioning; a score of <57 points indicated limited functioning. Kinematic data from full functioning arms (n = 27) and limited functioning arms (n = 30) in SCI were compared with the non-dominant arms (n = 54) in controls.

Results: In the limited upper extremity functioning group, movement time, smoothness, arm abduction, wrist angle, trunk displacement, and inter-joint coordination, but not peak velocity of the hand, angular velocity of elbow, and relative time to peak velocity, all differed from controls. In the full upper extremity functioning group, arm abduction alone was significantly different from controls.

Conclusions: The findings demonstrate that apart from measures of peak velocity, kinematic measures of movement quality including movement time, smoothness, trunk displacement, and joint angles are impaired in people with limited upper extremity functioning after SCI. The study provides robust results applicable to a representative population of individuals with established cervical or thoracic SCI. The results suggest that kinematic analysis might be useful for those with limited functioning in order to get a better understanding of the specific movement impairments in daily tasks after SCI.

Prognostic validity of a clinical trunk control test for independence and walking in individuals with spinal cord injury

Objective: The objective of the present work was to determine the prognostic validity of the trunk control test for walking and independence in individuals with SCI.Design: A cohort, prospective study was carried out in all individuals with sub-acute SCI.Setting: All inpatients at the Mexico City based National Rehabilitation Institute (INR).Participants: Ninety individuals with a clinical diagnosis of sub-acute SCI, American Spinal Injury Association Impairment Scale (AIS) A-D, and that have not participated in a rehabilitation program were included. Thirty-five individuals had good initial trunk control and the remaining 55 had poor trunk control. All individuals participated in a standard rehabilitation program subsequently.Interventions: N/AOutcome Measures: The trunk control test was performed at baseline. At 1, 3, 6, 9 and 12 months after the first evaluation, walking and independence were assessed.Results: Survival Analysis revealed that 62.5% and 100% individuals with good trunk control at baseline assessment were respectively walking and independent in ADL at 12 months and 14% and 48% individuals with poor trunk control were walking and independent in ADL. Cox regression analysis revealed that individuals with good trunk control were 4.6 times more likely to walk independently at 12 months and 2.9 times more likely to be independent in activities of daily living.Conclusion: The present study revealed that the trunk control test is useful for providing a prognosis of independence and walking at 1 year in individuals with SCI, independently of the neurologic level and the severity of the injury.

Trunk Stability Enabled by Noninvasive Spinal Electrical Stimulation after Spinal Cord Injury

Electrical neuromodulation of spinal networks improves the control of movement of the paralyzed limbs after spinal cord injury (SCI). However, the potential of noninvasive spinal stimulation to facilitate postural trunk control during sitting in humans with SCI has not been investigated. We hypothesized that transcutaneous electrical stimulation of the lumbosacral enlargement can improve trunk posture. Eight participants with non-progressive SCI at C3-T9, American Spinal Injury Association Impairment Scale (AIS) A or C, performed different motor tasks during sitting. Electromyography of the trunk muscles, three-dimensional kinematics, and force plate data were acquired. Spinal stimulation improved trunk control during sitting in all tested individuals. Stimulation resulted in elevated activity of the erector spinae, rectus abdominis, and external obliques, contributing to improved trunk control, more natural anterior pelvic tilt and lordotic curve, and greater multi-directional seated stability. During spinal stimulation, the center of pressure (COP) displacements decreased to 1.36 ± 0.98 mm compared with 4.74 ± 5.41 mm without stimulation (p = 0.0156) in quiet sitting, and the limits of stable displacement increased by 46.92 ± 35.66% (p = 0.0156), 36.92 ± 30.48% (p = 0.0156), 54.67 ± 77.99% (p = 0.0234), and 22.70 ± 26.09% (p = 0.0391) in the forward, backward, right, and left directions, respectively. During self-initiated perturbations, the correlation between anteroposterior arm velocity and the COP displacement decreased from r = 0.5821 (p = 0.0007) without to r = 0.5115 (p = 0.0039) with stimulation, indicating improved trunk stability. These data demonstrate that the spinal networks can be modulated transcutaneously with tonic electrical spinal stimulation to physiological states sufficient to generate a more stable, erect sitting posture after chronic paralysis.

Trunk involvement in performing upper extremity activities while seated in neurological patients with a flaccid trunk - A review

BACKGROUND

Trunk control is essential during seated activities. The trunk interacts with the upper extremities (UE) and head by being part of a kinematic chain and by providing a stable basis. When trunk control becomes impaired, it may have consequences for the execution of UE tasks.

AIM

To review trunk involvement in body movement and stability when performing seated activities and its relation with UE and head movements in neurological patients with a flaccid trunk, with a focus on childhood and development with age.

METHODS AND PROCEDURES

A search using PubMed was conducted and 32 out of 188 potentially eligible articles were included.

OUTCOMES AND RESULTS

Patients with a flaccid trunk (e.g. with spinal cord injury or cerebral palsy) tend to involve the trunk earlier while reaching than healthy persons. Different balance strategies are observed in different types of patients, like using the contralateral arm as counterweight, eliminating degrees of freedom, or reducing movement speed.

CONCLUSIONS AND IMPLICATIONS

The key role of the trunk in performing activities should be kept in mind when developing interventions to improve seated task performance in neurological patients with a flaccid trunk.

Phase-dependent deficits during reach-to-grasp after human spinal cord injury

Most cervical spinal cord injuries result in asymmetrical functional impairments in hand and arm function. However, the extent to which reach-to-grasp movements are affected in humans with incomplete cervical spinal cord injury (SCI) remains poorly understood. Using kinematics and electromyographic (EMG) recordings in hand and arm muscles we studied the different phases of unilateral self-paced reach-to-grasp movements (arm acceleration, hand opening and closing) to a small cylinder in the more and less affected arms of individuals with cervical SCI and in age-matched controls. We found that SCI subjects showed prolonged movement duration in both arms during arm acceleration, and hand opening and closing compared with controls. Notably, the more affected arm showed an additional increase in movement duration at the time to close the hand compared with the less affected arm. Also, the time at which the index finger and thumb contacted the object and the variability of finger movement trajectory were increased in the more compared with the less affected arm of SCI participants. Participants with prolonged movement duration during hand closing were those with more pronounced deficits in sensory function. The muscle activation ratio between the first dorsal interosseous and abductor pollicis brevis muscles decreased during hand closing in the more compared with the less affected arm of SCI participants. Our results suggest that deficits in movement kinematics during reach-to-grasp movements are more pronounced at the time to close the hand in the more affected arm of SCI participants, likely related to deficits in EMG muscle activation and sensory function. NEW & NOTEWORTHY Humans with cervical spinal cord injury usually present asymmetrical functional impairments in hand and arm function. Here, we demonstrate for the first time that deficits in movement kinematics during reaching and grasping movements are more pronounced at the time to close the hand in the more affected arm of spinal cord injury. We suggest that this is in part related to deficits in muscle activation ratios between hand muscles and a decrease in sensory function.

Kinematic Outcome Measures using Target-Reaching Arm Movement in Stroke

This study aimed to quantitatively investigate upper extremity motor performance and disclose the abnormality of motor control induced by stroke. Ten patients and ten healthy subjects were instructed to perform target-reaching tasks at nine difficulty levels, and coordinates of the shoulder, elbow and tip of the index finger were recorded. Age-matched control performed significantly better than patients, as indicated by lower movement time (MT) and normalized jerk score (NJS) and higher peak velocity (V _peak), percentage time to peak velocity (PTPV), fuzzy approximate entropy (fApEn) and relative joint angles correlation (RJAC); also, significant effects of difficulty on all parameters except RJAC and fApEn, were observed in two groups. There were significant correlations between PTPV and Fugl-Meyer assessment for upper extremity (FMA-UE) and between RJAC and FMA-UE at certain difficulty levels. The stroke-related differences could be explained by the increase in intrinsic neuromotor noise, and the difficulty-related differences may be related to extrinsic neuromotor noise. The increase in either noises could result in a degradation in motor control. The significant linear relationships between some kinematic parameters and the clinical score suggested that the kinematic parameters could be applied as quantitative outcome measures in the clinic in the future.

Bimanual reach to grasp movements after cervical spinal cord injury

Injury to the cervical spinal cord results in bilateral deficits in arm/hand function reducing functional independence and quality of life. To date little research has been undertaken to investigate control strategies of arm/hand movements following cervical spinal cord injury (cSCI). This study aimed to investigate unimanual and bimanual coordination in patients with acute cSCI using 3D kinematic analysis as they performed naturalistic reach to grasp actions with one hand, or with both hands together (symmetrical task), and compare this to the movement patterns of uninjured younger and older adults. Eighteen adults with a cSCI (mean 61.61 years) with lesions at C4-C8, with an American Spinal Injury Association (ASIA) grade B to D and 16 uninjured younger adults (mean 23.68 years) and sixteen uninjured older adults (mean 70.92 years) were recruited. Participants with a cSCI produced reach-to-grasp actions which took longer, were slower, and had longer deceleration phases than uninjured participants. These differences were exacerbated during bimanual reach-to-grasp tasks. Maximal grasp aperture was no different between groups, but reached earlier by people with cSCI. Participants with a cSCI were less synchronous than younger and older adults but all groups used the deceleration phase for error correction to end the movement in a synchronous fashion. Overall, this study suggests that after cSCI a level of bimanual coordination is retained. While there seems to be a greater reliance on feedback to produce both the reach to grasp, we observed minimal disruption of the more impaired limb on the less impaired limb. This suggests that bimanual movements should be integrated into therapy.

Adapted sport effect on postural control after spinal cord injury

STUDY DESIGN

Cross-sectional study.

OBJECTIVE

The aim of this study was to compare trunk muscle activation during anterior and lateral reach in athletic and sedentary individuals with spinal cord injury (SCI) and able-bodied people.

SETTINGS

University Hospital-UNICAMP, Campinas, Brazil.

METHODS

Individuals with complete traumatic SCI and thoracic neurological level were separated into two groups: sedentary (SSCI: n=10) and physically active (PASCI: n=10). The control group (C: n=10) without SCI was assessed. Trunk muscle activation was recorded during reach and grasp tasks. The significant level was set at P<0.05.

RESULTS

The control group showed a highest mean activation for left longissimus muscle during all activities (P<0.05). The PASCI group presented significant highest activation for left iliocostalis muscles during all activities, except in the anterior reach task of 90% maximum reach (anterior reach (AR) 75: P=0.02; right lateral reach (RLR) 75: P=0.03; RLR90: P=0.01). The SSCI group presented highest activation for the left iliocostalis during the right lateral reach task of 75 and 90% maximum reach and right iliocostalis during the anterior reach task of 75% maximum reach (AR75: P=0.007; RLR75: P=0.02; RLR90: P=0.03). A different pattern of muscle activation between the control group and the groups with SCI was observed.

CONCLUSION

Our results indicated that sports practice did not affect the trunk muscle activation in people with paraplegia. However, the pattern muscle activation in individuals with SCI is different compared with people without SCI during anterior reach tasks.

Motor imagery reinforces brain compensation of reach-to-grasp movement after cervical spinal cord injury

Individuals with cervical spinal cord injury (SCI) that causes tetraplegia are challenged with dramatic sensorimotor deficits. However, certain rehabilitation techniques may significantly enhance their autonomy by restoring reach-to-grasp movements. Among others, evidence of motor imagery (MI) benefits for neurological rehabilitation of upper limb movements is growing. This literature review addresses MI effectiveness during reach-to-grasp rehabilitation after tetraplegia. Among articles from MEDLINE published between 1966 and 2015, we selected ten studies including 34 participants with C4 to C7 tetraplegia and 22 healthy controls published during the last 15 years. We found that MI of possible non-paralyzed movements improved reach-to-grasp performance by: (i) increasing both tenodesis grasp capabilities and muscle strength; (ii) decreasing movement time (MT), and trajectory variability; and (iii) reducing the abnormally increased brain activity. MI can also strengthen motor commands by potentiating recruitment and synchronization of motoneurons, which leads to improved recovery. These improvements reflect brain adaptations induced by MI. Furthermore, MI can be used to control brain-computer interfaces (BCI) that successfully restore grasp capabilities. These results highlight the growing interest for MI and its potential to recover functional grasping in individuals with tetraplegia, and motivate the need for further studies to substantiate it.

Upper limb kinematics after cervical spinal cord injury: a review

Although a number of upper limb kinematic studies have been conducted, no review actually addresses the key-features of open-chain upper limb movements after cervical spinal cord injury (SCI). The aim of this literature review is to provide a clear understanding of motor control and kinematic changes during open-chain upper limb reaching, reach-to-grasp, overhead movements, and fast elbow flexion movements after tetraplegia. Using data from MEDLINE between 1966 and December 2014, we examined temporal and spatial kinematic measures and when available electromyographic recordings. We included fifteen control case and three series case studies with a total of 164 SCI participants and 131 healthy control participants. SCI participants efficiently performed a broad range of tasks with their upper limb and movements were planned and executed with strong kinematic invariants like movement endpoint accuracy and minimal cost. Our review revealed that elbow extension without triceps brachii relies on increased scapulothoracic and glenohumeral movements providing a dynamic coupling between shoulder and elbow. Furthermore, contrary to normal grasping patterns where grasping is prepared during the transport phase, reaching and grasping are performed successively after SCI. The prolonged transport phase ensures correct hand placement while the grasping relies on wrist extension eliciting either whole hand or lateral grip. One of the main kinematic characteristics observed after tetraplegia is motor slowing attested by increased movement time. This could be caused by (i) decreased strength, (ii) triceps brachii paralysis which disrupts normal agonist-antagonist co-contractions, (iii) accuracy preservation at movement endpoint, and/or (iv) grasping relying on tenodesis. Another feature is a reduction of maximal superior reaching during overhead movements which could be caused by i) strength deficit in agonist muscles like pectoralis major, ii) strength deficit in proximal synergic muscles responsible for scapulothoracic and glenohumeral joint stability, iii) strength deficit in distal synergic muscles preventing the maintenance of elbow extension by shoulder elbow dynamic coupling, iv) shoulder joint ankyloses, and/or v) shoulder pain. Further studies on open chain movements are needed to identify the contribution of each of these factors in order to tailor upper limb rehabilitation programs for SCI individuals.

Three-dimensional motion analysis of arm-reaching movements in healthy and hemispinalized common marmosets

Spinal cord injury (SCI) is a devastating neurological injury. At present, pharmacological, regenerative, and rehabilitative approaches are widely studied as therapeutic interventions for motor recovery after SCI. Preclinical research has been performed on model animals with experimental SCI, and those studies often evaluate hand and arm motor function using various indices, such as the success rate of the single pellet reaching test and the grip force. However, compensatory movement strategies, involuntary muscle contraction, and the subject's motivation could affect the scores, resulting in failure to assess direct recovery from impairment. Identifying appropriate assessments of motor impairment is thus important for understanding the mechanisms of motor recovery. In this study, we developed a motion capture system capable of reconstructing three-dimensional hand positions with millimeter and millisecond accuracy and evaluated hand kinematics during food retrieval movement in both healthy and hemispinalized common marmosets. As a result, the endpoint jerk, representing the accuracy of hand motor control, was asserted to be an appropriate index of upper limb motor impairment by eliminating the influence of the subject's motivation, involuntary muscle contraction, and compensatory strategies. The result also suggested that the kinematics of the limb more consistently reflects motor restoration from deficit due to spinal cord injury than the performance in the single pellet reaching test. Because of recent attention devoted to the common marmoset as a nonhuman primate model for human diseases, the present study, which clarified arm-reaching movements in spinalized marmosets, provides fundamental knowledge for future therapeutic studies.

Habilidades motoras de cadeirantes influenciadas pelo controle de tronco

O objetivo deste estudo foi analisar a efetividade de um protocolo de treinamento de estabilização segmentar lombar (ESL) no controle de tronco e nas habilidades em cadeira de rodas (CR) em indivíduos paraplégicos. A amostra foi composta por 5 homens (31,2±12,9 anos) com lesão medular abaixo de T6. O deslocamento total (DT) e as amplitudes ântero-posterior (AP) e médio-lateral (ML) do centro de pressão (CP) foram analisados por meio de uma plataforma de força, em duas posições de teste (mãos no joelho e braços cruzados) e as habilidades na cadeira de rodas foram analisadas pelo Wheelchair Skills Test (WST), pré e pós protocolo de treinamento de ESL. A análise estatística foi realizada no software SPSS (15.0), utilizando o teste ANOVA para α ≤ 0,05. Na comparação pré e pós-treino, houve redução significativa do deslocamento total do CP na posição sentada com mãos nos joelhos (P<0,01) e braços cruzados (P<0,01). Não se observou mudança nas habilidades com CR do WST entre as avaliações pré e pós treinamento em nenhum dos níveis estudados. O protocolo de treinamento de ESL proposto foi efetivo para a melhora do controle de tronco, mas não modificou as habilidades no manejo de cadeira de rodas em indivíduos paraplégicos.

Kinematic characteristics of tenodesis grasp in C6 quadriplegia

STUDY DESIGN

Descriptive control case study.

OBJECTIVES

To analyze the kinematics of tenodesis grasp in participants with C6 quadriplegia and healthy control participants in a pointing task and two daily life tasks involving a whole hand grip (apple) or a lateral grip (floppy disk).

SETTING

France.

METHODS

Four complete participants with C6 quadriplegia were age matched with four healthy control participants. All participants were right-handed. The measured kinematic parameters were the movement time (MT), the peak velocity (PV), the time of PV (TPV) and the wrist angle in the sagittal plane at movement onset, at the TPV and at the movement end point.

RESULTS

The participants with C6 quadriplegia had significantly longer MTs in both prehension tasks. No significant differences in TPV were found between the two groups. Unlike control participants, for both prehension tasks the wrist of participants with C6 quadriplegia was in a neutral position at movement onset, in flexion at the TPV, and in extension at the movement end point.

CONCLUSION

Two main kinematic parameters characterize tenodesis grasp movements in C6 quadriplegics: wrist flexion during reaching and wrist extension during the grasping phase, and increased MT reflecting the time required to adjust the wrist's position to achieve the tenodesis grasp. These characteristics were observed for two different grips (whole hand and lateral grip). These results suggest sequential planning of reaching and tenodesis grasp, and should be taken into account for prehension rehabilitation in patients with quadriplegia.

Adaptation of torso movement strategies in persons with spinal cord injury or low back pain

STUDY DESIGN

Controlled laboratory study. Statistical regression and between-group comparisons.

OBJECTIVE

To characterize functional limitation and adaptive strategies in seated manual transport tasks for spinal cord injury (SCI), low back pain (LBP), and control participants.

SUMMARY OF BACKGROUND DATA

People with SCI are known to have adapted electromyographic activities and slow hand movement velocity, while those with LBP have reduced range of motion and lumbar joint contribution. However, their resultant outcome in torso movements has not been systematically quantified.

METHODS

Seated participants performed either 2- or 1-handed loaded transports to 1 of 6 targets 49 cm above the hip-point, at 0 degrees, 45 degrees, and 90 degrees azimuths, at close and far distance. Three-dimensional torso movements were modeled by combinations of B-spine base functions.

RESULTS

The SCI and LBP participants exhibit smaller torso flexion and axial rotation than control participants. The SCI participants tend to move the torso away from the target to maintain upper body balance. These differences among groups are significantly reduced in the 1-handed transport condition and/or transports to the frontal target.

CONCLUSION

The movement patterns suggest that SCI participants may have adapted torso movement strategies to compensate for the limited control of upper body balance, while LBP participants may limit torso motion to avoid pain.

Seated reach distance and trunk excursion accurately reflect dynamic postural control in individuals with motor-incomplete spinal cord injury

Objective

Dynamic seated postural control is essential for individuals who perform their daily activities from a wheelchair. While seated reach distance is used as a proxy measure for postural control, it is unknown whether this is an accurate and reliable measure of the limits of stability in individuals with motor-incomplete spinal cord injury (MISCI).

Design

To determine whether kinematic measures of excursion are valid measures of postural control in individuals with MISCI, seated reach test (SRT; obtained from a wrist marker) and associated trunk excursion values (obtained from a C7 marker) were compared with center of pressure excursion (COPE). Data were obtained from individuals with MISCI and from non-disabled individuals for each of four directions. To assess the reliability of these measures in subjects with MISCI, these values were collected on two separate days.

Results

The SRT was correlated with the COPE in three directions of reaching (r≥0.71) with the exception being leftward reaching. Trunk excursion was correlated with COPE in all directions (r≥0.93). In ND individuals, both SRT (r≥0.56) and trunk excursion (r≥0.91) were correlated with COPE for all directions. In individuals with MISCI, there was significant intersession agreement for both reach distance (ICC≥0.78) and trunk excursion (ICC≥0.77) measured in all directions.

Conclusions

While both SRT and trunk excursion are reasonable reflections of COPE, measurement of postural control based on trunk excursion has advantages for individuals who may have difficulty maintaining arm position during reaching. Reach distance is highly reliable in individuals with MISCI in all directions of reaching.

Trunk posture affects upper extremity function of adults

This study examined the effects of various seated trunk postures on upper extremity function. 59 adults were tested using the Jebsen Taylor Hand Function Test while in three different trunk postures. Significant mean differences between the neutral versus the flexed and laterally flexed trunk postures were noted during selected tasks. Specifically, dominant hand performance during the tasks of feeding and lifting heavy cans was significantly slower while the trunk was flexed and laterally flexed than when performed in the neutral trunk position. Performance of the nondomi nant hand during the tasks of picking up small objects, page turning, as well as the total score was slower while the trunk was flexed compared to performance in the neutral trunk position. These findings support the assumption that neutral trunk posture improves upper extremity performance during daily activities although the effect is not consistent across tasks. Findings are discussed along with limitations and recommendations for research.

How to extend the elbow with a weak or paralyzed triceps: control of arm kinematics for aiming in C6-C7 quadriplegic patients

This study aims to investigate how quadriplegic patients with a C6-C7 spinal lesion coordinate their upper limb to extend the elbow despite the paralysis or weakness of the triceps brachii, and what is the effect of a surgical musculotendinous transfer. For this purpose, aiming movements in a wide workspace were recorded in seven healthy subjects and in patients with incomplete (five cases) or complete (eight cases) triceps paralysis and after musculotendinous transfer (eight cases). We used four electromagnetic field sensors to quantify hand trajectory and to compute the angles describing the rotations at the scapula, glenohumeral joint, elbow and wrist (10 degrees of freedom). Extent and smoothness of the hand trajectories and hand velocity profiles were surprisingly similar between healthy subjects and quadriplegic patients. The reduction of elbow extension observed in patients was compensated by rotations distributed across several degrees of freedom including the scapula. Principal components analysis showed that the joint rotations could be summarized by an additive combination of two synergies, respectively orientating and stretching out the limb, which explained similar amounts of variance in healthy subjects and in patients. The participations of degrees of freedom in the synergies were roughly similar in the different groups of subjects, the main difference concerning scapular medial-lateral rotation, which seems to be critical in patients with a complete triceps paralysis. This demonstrates that elbow extension in quadriplegic patients is due to anticipated mechanical interaction coupling between upper limb segments. We propose that the persisting (incomplete paralysis) or restored (musculotendinous transfer) elbow extensor strength may act by stabilizing the elbow. This counterintuitive preservation of limb kinematics for horizontal aiming movements in quadriplegic patients despite the drastic changes in muscle action provoked by paralysis and/or by surgery strongly suggests that the motor system does not primarily control forces but the morphological aspects of movement, via joint rotation synergies.

Novel muscle patterns for reaching after cervical spinal cord injury: a case for motor redundancy

A fundamental issue in the neuromotor control of arm movements is whether the nervous system can use distinctly different muscle activity patterns to obtain similar kinematic outcomes. Although computer simulations have demonstrated several possible mechanical and torque solutions, there is little empirical evidence that the nervous system actually employs fundamentally different muscle patterns for the same movement, such as activating a muscle one time and not the next, or switching from a flexor to an extensor. Under typical conditions, subjects choose the same muscles for any given movement, which suggests that in order to see the capacity of the nervous system to make a different choice of muscles, the nervous system must be pushed beyond the normal circumstances. The purpose of this study, then, was to examine an atypical condition, reaching of cervical spinal cord injured (SCI) subjects who have a reduced repertoire of available distal arm muscles but otherwise a normal nervous system above the level of lesion. Electromyography and kinematics of the shoulder and elbow were examined in the SCI subjects performing a center-out task and then compared to neurologically normal control subjects. The findings showed that the SCI-injured subjects produced reaches with typical global kinematic features, such as straight finger paths, bell-shaped velocities, and joint excursions similar to control subjects. The SCI subjects, however, activated only the shoulder agonist muscle for all directions, unlike the control pattern that involved a reciprocal pattern at each joint (shoulder, elbow, and wrist). Nonetheless, the SCI subjects could activate their shoulder antagonist muscles, elbow flexors, and wrist extensor (extensor carpi radialis) for isometric tasks, but did not activate them during the reaching movements. These results demonstrate that for reaching movements, the SCI subjects used a strikingly different pattern of intact muscle activities than control subjects. Hence, the findings imply that the nervous system is capable of choosing either the control pattern or the SCI pattern. We would speculate that control subjects do not select the SCI pattern because the different choice of muscles results in kinematic features (reduced fingertip speed, multiple shoulder accelerations) other than the global features that are somehow less advantageous or efficient.