C. Marquardt

Grip force control during object manipulation in cerebral stroke

OBJECTIVE To analyze impairments of manipulative grip force control in patients with chronic cerebral stroke and relate deficits to more elementary aspects of force and grip control. METHODS Nineteen chronic stroke patients with fine motor deficits after unilateral cerebral lesions were examined when performing 3 manipulative tasks consisting of stationary holding, transport, and vertical cyclic movements of an instrumented object. Technical sensors measured the grip force used to stabilize the object in the hand and the object accelerations, from which the dynamic loads were calculated. RESULTS Many patients produced exaggerated grip forces with their affected hand in all types of manipulations. The amount of finger displacement in a grip perturbation task emerged as a highly sensitive measure for predicting the force increases. Measures of grip strength and maximum speed of force changes could not account for the impairments with comparable accuracy. In addition to force economy, the precision of the coupling between grip and load forces was impaired. However, no temporal delays were typically observed between the grip and load force profiles during cyclic movements. CONCLUSIONS Impaired sensibility and sensorimotor processing, evident by delayed reactions in the perturbation task, lead to an excessive increase of the safety margin between the actual grip force and the minimum force necessary to prevent object slipping. In addition to grip force scaling, cortical sensorimotor areas are responsible for smoothly and precisely adjusting grip forces to loads according to predictions about movement-induced loads and sensory experiences. However, the basic feedforward mechanism of grip force control by internal models appears to be preserved, and thus may not be a cortical but rather a subcortical or cerebellar function, as has been suggested previously.

Grip and load force coupling during discrete vertical arm movements with a grasped object in cerebellar atrophy

Control of isometric grip forces during manipulation of objects is an essential feature of all skilled manual performances. Recent studies suggested that the anticipation of movement-induced loads may be a cerebellar function. We analysed grip force adjustments to fluctuations of inertial loads during discrete vertical movements with a grasped object in five patients with cerebellar atrophy and five healthy control subjects. Normally grip force is precisely adapted to the load fluctuations, in particular to the maximum load force, which occurs early in upward and late in downward movements. Both groups produced similar accelerations of the grasped object and consequently similar maximum loads. However, cerebellar patients established increased static grip forces during stationary holding of the object and increased force ratios between grip and load force at the time of maximum acceleration. These findings are congruent with earlier studies analysing grip and load force coupling in patients with cerebellar lesions. In contrast to earlier studies, we found no significant differences in the timing of grip force onset and grip force maximum relative to the onset of movement and maximum acceleration, respectively, between normal controls and four of five cerebellar patients. However, a regression analysis between grip and load forces during the load increase and decrease phases of the movement suggested deficits in the close temporospatial coupling between the two forces in all cerebellar patients. Our findings give further support to the notion that the cerebellum plays a crucial role in the forward control of grip force magnitude and timing during voluntary object manipulation. Compared to earlier studies, the increase in grip forces may be interpreted as a general control strategy to compensate for motor deficits, whereas impairments of temporal grip force regulation may occur at different degrees of dysfunction during the progression of cerebellar atrophy.

Prehension With the Ipsilesional Hand After Unilateral Brain Damage

Sensorimotor deficits in the hand ipsilateral to a brain lesion have been reported in different motor tasks. We evaluated performance of the ipsilesional hand in 12 patients with either left (LBD) or right brain damage (RBD) by kinematic analysis in order to precisely characterize possible deficits in the two components of prehension (transport and grasp). Both patient groups exhibited performance deficits in the main kinematic parameters, e.g., reduced velocity of the transport component and prolonged movement time. However, while LBD patients showed a more general slowing, RBD patients prolonged in particular the last phase of the movement toward the object. We suggest that relevant visuospatial representations and the adequate mapping of motor processes may be impaired after RBD. In contrast, LBD caused a more unspecific disturbance pattern, supporting the view that the precise parameterization of motor programs is impaired. Maximum grip aperture was normal in both patient groups. However, since aperture could be biased by slowed movement, the notion that the grasp component was preserved remains speculative. The patients ability to scale the maximum velocity of the transport component to adapt to changes in movement amplitude and to scale the maximum hand aperture of the grasp component to adapt to object size was preserved in both groups. Thus both hemispheres can have competence for this scaling mechanism.

Feedback-based training of grip force control in patients with brain damage

OBJECTIVE Feedback-based training of grip force control in patients with various brain lesions was evaluated. DESIGN Patients were instructed to hold a force transducer in a precision grip and to track with their grip force a moving target, which was presented together with the feedback signal on a monitor. Training performance was evaluated during a maximum of 10 sessions. Before and after the training, performance in two transfer tasks, which differed in target characteristics from the training task, was examined. PATIENTS Ten patients with impaired grip force control, after brain lesions of different origin, were selected on the basis of a clinical examination of hand function. MAIN OUTCOME MEASURES Tracking accuracy in training tasks and transfer tasks was evaluated by calculating the conventional root-mean-square error. RESULTS Nine out of the 10 patients reduced their tracking error considerably during a maximum of 10 subsequent sessions (t test, p < 0.05), and most of them reached normal or near-normal performance. In addition, they improved in both transfer tasks (t test, p < 0.05). Detailed analysis showed that impaired initial performance and improvement was not uniform among patients and could be attributed to individual aspects of force control. CONCLUSIONS In view of these results, a feedback-based training of grip force may be a useful enrichment of motor therapy.

Impairments of prehension kinematics and grasping forces in patients with cerebellar degeneration and the relationship to cerebellar atrophy

OBJECTIVE This study established the relationship between kinematic and grip force parameters in prehension tasks, disease severity and cerebellar atrophy in patients with cerebellar degeneration. METHODS Prehension was tested in a condition during which the hand reached out, grasped, and lifted an object. Task complexity was modified by limiting the transport component to a single-joint movement, and introducing a bimanual condition. RESULTS Compared to controls the cerebellar patients showed disturbances in hand transport, in hand shaping and the most pronounced in time to peak grip force and the grip/load force coupling. Task-dependent changes did not differ between groups. Ataxia scores revealed significant correlations with hand transport and shaping measures only. Ataxia subscores correlated with volume reduction of appropriate longitudinal cerebellar zones. Volume reduction of the intermediate zone was associated with grip force coordination deficits. CONCLUSIONS Results indicate that the cerebellum may have a more general role in motor control of grasping independent of task complexity. Temporal and coordinative measures of grip force appear to be most useful to assess the severity of grasping deficits in patients with cerebellar degeneration not detectable by clinical ataxia scales. SIGNIFICANCE To assess the severity and course of cerebellar disease grip force control in a standard prehension task is a sensitive quantitative measure.

Moving objects with clumsy fingers: how predictive is grip force control in patients with impaired manual sensibility?

OBJECTIVE Anticipatory grip force adjustments to movement-induced load fluctuations of a hand-held object suggest that motion planning is based on an internal forward model of both the external object properties and the dynamics of the own motor apparatus. However, the central nervous system also refers to real time sensory feedback from the grasping digits in order to achieve a highly economical coupling between grip force and the actual loading requirements. METHODS We analyzed grip force control during vertical point-to-point arm movements with a hand-held instrumented object in 9 patients with moderately impaired tactile sensibility of the grasping digits due to chronic median nerve compression (n = 3), axonal (n = 3) and demyelinating sensory polyneuropathy (n = 3) in comparison to 9 healthy age- and sex-matched control subjects. Point-to-point arm movements started and ended with the object being held stationary at rest. Load force changes arose from inertial loads related to the movement. A maximum of load force occurred early in upward and near the end of downward movements. RESULTS Compared to healthy controls, patients with impaired manual sensibility generated similar static grip forces during stationary holding of the object and similar force ratios between maximum grip and load force. These findings reflect effective grip force scaling in relation to the movement-induced loads despite reduced afferent feedback from the grasping digits. For both groups the maxima of grip and load force coincided very closely in time, indicating that the temporal regulation of the grip force profile with the load profile was processed with a similar high precision. In addition, linear regression analyses between grip and load forces during movement-related load increase and load decrease phases revealed a similar precise temporo-spatial coupling between grip and load forces for patients and controls. CONCLUSIONS Our results suggest that the precise and anticipatory adjustment of the grip force profile to the load force profile arising from voluntary arm movements with a hand-held object is centrally mediated and less under sensory feedback control. As suggested by previous investigations, the efficient scaling of the grip force magnitude in relation to the movement-induced loads may be intact when deficits of tactile sensibility from the grasping fingers are moderate.

Moving weightless objects. Grip force control during microgravity.

Abstract. When we move grasped objects, our grip force precisely anticipates gravitational and inertial loads. We analysed the control of grip forces during very substantial load changes induced by parabolic flights. During these flight manoeuvres, the gravity varies between hypergravity associated with a doubling of normal terrestrial gravity and a 20-s period of microgravity. Accordingly, the contribution of the objects weight to the load changed from being twice the normal value to being absent. Two subjects continuously performed vertical and horizontal movements of an object equipped with grip force and acceleration sensors. Whereas, during vertical movements performed under normal and hypergravity, a load force maximum occurred at the lower turning point and a minimum at the upper turning point, the load force pattern was completely changed under microgravity. In particular, the upper turning point was also associated with a load force maximum. Analysis of the grip forces produced by the two subjects revealed that the grip forces underwent the same characteristic changes as the load forces. Thus, subjects were able to adjust grip forces in anticipation of arm movement-induced fluctuations in load force under different and novel load conditions. Adaptation to changing levels of gravity was also obvious when the vertical and horizontal movements were compared: grip forces depended heavily on movement direction during normal and hypergravity but not during microgravity. The predictive coupling of grip force and load force was observed even during transitions between gravity levels, indicating rapid adaptation to changing load conditions. To account for the striking preservation of the normal characteristics of grip force control, we suggest that a highly automatized, extremely flexible sensorimotor mechanism firmly implemented within the central nervous system can cope with even massive changes in the environmental conditions.

Comparative analysis of diadochokinetic movements

Tests of diadochokinesia are an inherent part of a neurological examination. Various quantifying methods have been proposed to increase the objectivity, sensitivity, and reliability of such examinations. The methods used and analyses performed, however, differ substantially between tasks. We used a three-dimensional, ultrasound-based recording device to continuously record joint angles during three diadochokinetic movements, avoiding any external constraints of the movements. Alternate pronation and supination of the forearm, tapping with the whole hand and with the index finger in isolation were analyzed in a sample of 63 healthy control subjects. The most sensitive measure for capturing effects of gender, sex, and active hand was frequency. The right hand was faster than the left in all tasks, tapping performance declined with increasing age, and male subjects were faster than females in forearm diadochokinesia. Other measures that characterize speed of movement such as maximum angular velocities and accelerations did not yield comparable sensitivity in detecting the same statistical effects. However, angular velocity achieved the highest test-retest reliability for forearm diadochokinesia, while frequency was reproduced in the tapping tasks. Additional measures characterizing symmetry of the angular velocity profiles and intraindividual variability were shown to be largely independent of movement speed. Examples in neurological patients showed that the data define a valuable standard against which pathological performance can be precisely evaluated. In addition, the different measures captured dissociable aspects of motor performance that may further help to characterize the deficit and adjust therapy.

Visual background motion reduces size distortion in spatial neglect

Patients with visual neglect show deficits in horizontal size perception in their neglected hemispace, as previously reported. The present study examined whether this size distortion can be modulated by visual background motion to the left or right while the patient performs a visual size judgment task. Six neglect patients and six normal subjects were investigated with a psychophysical size judgment task. All neglect patients showed a significant perceptual underestimation of horizontal bars in their left hemispace expressed as an overestimation of horizontal object size in the baseline (no motion) condition. Slow visual motion of background stimuli towards the right, ipsilesional side aggravated the deficit slightly, but not significantly, whereas leftward background motion completely normalized the size distortion (in four cases) or even led to an overcompensation (in two cases). This facilitatory effect was specific as it was obvious for the constant errors in the size judgments, but not in their accuracy as reflected by unchanged difference thresholds. These results suggest that coherent background motion restores temporarily the disturbed perception and representation of horizontal object size in spatial neglect.

Circadian Variations in the Kinematics of Handwriting and Grip Strength

The present study determined whether the motor process of handwriting is influenced by a circadian rhythm during writing tasks of high everyday relevance and analyzed the relationship to the circadian rhythm of grip strength. Ten healthy young male subjects underwent a 40 h sleep‐deprivation protocol under constant routine conditions. Starting at 09:00 h, subjects performed three handwriting tasks of increasing perceptual‐motor complexity (writing a sentence, writing ones signature, and copying a text for 3 min) and assessed grip strength of both hands every 3 h. Handwriting performance was analyzed by writing speed, writing fluency, script size, break times, and pen pressure. The handwriting tasks revealed a coincident circadian rhythm for the frequency of handwriting as a measure of movement speed, with slowest writing speed at 03:16 h. A weak effect of task complexity was evident for the non‐writing episodes: while copying a text, break times were influenced by a circadian rhythm, whereas during sentence writing, the non‐writing episodes remained constant. The circadian rhythm of grip strength paralleled the time course of motivation ratings, with least motivation and weakest grip strength around 06:00 h concurrently for both hands. The rate of force production also displayed circadian rhythmicity and sharply decreased with the onset of melatonin secretion. Neither grip strength nor the kinematics of handwriting was influenced by sleep deprivation; only the level of the force rate was decreased the second day. The results show a clear circadian rhythm in the speed of handwriting and grip strength.