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.     

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.     

Regulation of grasping forces during bimanual in-phase and anti-phase coordination

When a hand-held object is moved, grip force is adapted in an anticipatory manner to load force due to a dynamic coupling between both forces. The present study addressed the issue of grip-load force regulation when moving rhythmically two hand-held objects in the vertical dimension, and more specifically the divergence of force control when performing according to the in-phase versus anti-phase mode. Results revealed that grip-load force ratio profiles were similar in both bimanual conditions. That is, force ratio was not constant throughout the movement cycles but followed a fairly regular pattern with maxima and minima, attained at upward and downward hand positions, respectively. However, anti-phase patterns showed an increased maximum grip-load force ratio as compared to in-phase patterns, whereas the latter did not differ from unimanual movements. The magnification of maximum force ratio during anti-phase movements suggests that rescaling occurred. This is likely due to the complexity of the anti-phase mode that necessitates increased monitoring and attention relative to the other performance conditions, creating a coordinative situation that imposes an additional degree of uncertainty. Therefore, the safety margin is amplified during anti-phase movements, probably as a strategy to prevent a potential destabilization of the grip during an asymmetrical load condition. Accordingly, these findings also demonstrate that grip-load force regulation is more proficiently controlled during bimanual in-phase than anti-phase movements. Herewith, the data add content to earlier work illustrating kinematic dissimilarities between both coordination modes.     

Grip force behavior in Gilles de la Tourette syndrome.

We analyzed predictive and reactive grip force behavior in 15 patients with Gilles de la Tourette syndrome (GTS) and 15 sex- and age-matched healthy control subjects. Nine patients were without medication; six patients were on medication. In a first experiment, participants lifted and held instrumented objects of different weight. In a second experiment, participants performed vertical point-to-point and continuous arm movements at different frequencies with a hand-held object. In a third experiment, preparatory and reactive grip force responses to sudden load perturbations were analyzed when a weight was dropped into a hand-held cup either by the subject or unexpectedly by the experimenter. Compared to the healthy subjects, GTS patients had increased grip forces relative to the load force in all tasks. Despite this finding, they adjusted the grip force to changes in load force (due to either a change in the mass lifted or accelerating the mass during continuous movements) in the same way as healthy subjects. The temporal coupling between grip and load force profiles was also similar in patients and healthy controls, and they displayed normal anticipation of impact forces when they dropped a weight into a hand-held cup. We found no significant effect of medication on the performance of GTS patients, regardless of the task performed. These results are consistent with deficient sensory-motor processing in Gilles de la Tourette syndrome.     

Different modes of grip force control: voluntary and externally guided arm movements with a hand-held load.

OBJECTIVE: When we move hand-held objects that exhibit stable physical properties grip force is regulated in anticipation of movement-induced inertial loads. In contrast, when the object's behaviour is unpredictable, grip force is adjusted in response to sensory feedback with the consequence that grip tends to lag behind load. Previous studies analysed reactive and predictive grip force behaviour by systematically varying the predictability of the physical object properties. METHODS: This study examines if anticipatory force control also depends on the predictability of the limb dynamics interfering with external objects. The coupling between grip and load force profiles was comparatively analysed during voluntary and externally guided vertical arm movements with an instrumented hand-held object. Voluntary and externally guided movements were performed with and without visual feedback. RESULTS: During voluntary arm movements grip force was precisely regulated in anticipation of movement-induced inertial load fluctuations with grip force increasing in parallel with load force without an obvious time delay. In contrast, during externally guided movements grip force was regulated in reaction to the imposed load fluctuations. However, the reflex-mediated grip force responses were still flexible to account for the differential loading requirements of movement direction. There was no difference of grip force performance between movements performed with and without visual feedback. CONCLUSIONS AND SIGNIFICANCE: The results suggest that predictability of both the external object and the dynamics of the own body is essential to establish an anticipatory mode of grip force regulation. Unpredictability of the own limb dynamics results in a reactive mode of grip force control. Reactive grip force control appears to be both highly automatised and flexible reflecting differential loading requirements of movement direction.     

Impaired grip force modulation in the ipsilesional hand after unilateral middle cerebral artery stroke

Understanding grasping control after stroke is important for relearning motor skills. The authors examined 10 individuals (5 males; 5 females; ages 32-86) with chronic unilateral middle cerebral artery (MCA) stroke (4 right lesions; 6 left lesions) when lifting a novel test object using skilled precision grip with their ipsilesional ("unaffected") hand compared to healthy controls (n = 14; 6 males; 8 females; ages 19-86). All subjects possessed normal range of motion, cutaneous sensation, and proprioception in the hand tested and had no apraxia or cognitive deficits. Subjects lifted the object 10 times at each object weight (260 g, 500 g, 780 g) using a moderately paced self-selected lifting speed. The normal horizontal ("grip") force and vertical tangential ("lift") force were separately measured at the thumb and index finger. Regardless of the object weight or stroke location, the stroke group generated greater grip forces at liftoff of the object (> or =39%; P < or = 0.05) and across the dynamic (P < or = 0.05) and static portions (P < or = 0.05) of the lifts compared to the healthy group. Peak lift forces were equivalent between groups, suggesting accurate load force information processing occurred. These results warrant further investigation of altered sensorimotor processing or compensatory biomechanical strategies that may lead to inaccurate grip force execution after strokes.     

Neurocase,Volume 18, Guest reviewers

Abstract

In adults, moving an object using precision grip involves anticipatory adjustment of grip force for fluctuations in inertial load force. These adjustments suggest that motion planning is based on an internal model of the effect or system and the environment. In the current study, we evaluate the coordination of grip force with load force in a child with developmental coordination disorder (DCD) and a matched, normally developing, control child. The children completed five tasks: (i) lifting an object; (ii) moving an object upwards; (iii) moving an object downwards; (iv) holding an object subject to unpredictable perturbation; (v) a time production (tapping) task. A number of differences were observed between the children. In particular, compared to the control, the child with DCD showed an earlier rise in grip force when making both upward and downward movements. We discuss this result in relation to the greater variability in explicit timing and longer reflex delays observed in the child with DCD. We conclude that this paradigm offers insight into the motion planning difficulties seen in DCD, providing a useful new methodology for the investigation of the observed coordination difficulties.     

The effects of digital anaesthesia on predictive grip force adjustments during vertical movements of a grasped object

Grip force adjustments to fluctuations of inertial loads induced by vertical arm movements with a grasped object were analysed during normal and impaired finger sensibility. Normally grip force is modulated in a highly economical way in parallel with fluctuations of load force. Two subjects performed vertical up and down movements of a grasped object, both with normal finger sensibility and then cutaneously anaesthetized finger sensibility. Short breaks were taken in between single movements, during which the object was held stationary. After digital anaesthesia was applied to the grasping fingers, both subjects substantially increased the grip force. The grip force amplitude and timing still anticipated changes in load force, although the established grip force had already overcome movement-induced load force peaks. This implies that the increase of grip force and consequently the elevated force ratio between maximum grip and maximum load force are not processed to alter the feedforward system of grip force control. Cutaneous afferent information from the grasping digits appears to be necessary for economic scaling of the grip force level, but it plays a subordinate role in the precise anticipatory temporal coupling of grip and load forces during voluntary object manipulation.     

Grip force control in patients with neck and upper extremity pain and healthy controls

OBJECTIVE: To investigate whether sensory and motor problems in patients with non-specific neck and upper extremity pain can be ascribed to a deficit of sensory-motor integration. METHODS: Grip force control and adaptation were measured in 81 cases, 32 former cases and 39 healthy controls, during repetitive lifting and holding of an object. The object (300 g) was lifted vertically over 20 cm and held for 5s, using the dominant arm (the affected arm in all cases). The object was novel to the subjects when lifted for the first time, and was lifted five times consecutively. Grip forces orthogonal to the object's surface and its vertical acceleration were measured. RESULTS: Cases used significantly higher grip forces than both other groups, while vertical acceleration was not different. After the initial lift, all groups significantly reduced the maximum grip force. CONCLUSIONS: Subjects with neck and upper extremity pain consistently use higher grip forces than controls, but adjust grip forces by a similar amount after the first lift. Compensation of impaired sensory information rather than a general deficit in sensory-motor integration seems to account for these findings. SIGNIFICANCE: Non-specific neck and upper extremity pain coincides with objectifiable changes in control of grip force.     

Disturbances of grip force behaviour in focal hand dystonia: evidence for a generalised impairment of sensory-motor integration?

BACKGROUND: Focal task specific dystonia occurs preferentially during performance of a specific task. There may be an inefficiently high grip force when doing manipulative tasks other than the trigger task, possibly reflecting a generalised impairment of sensory-motor integration. OBJECTIVE: To examine how well subjects with writer's cramp (n = 4) or musician's cramp (n = 5) adapted their grip force when lifting a new object or catching a weight. METHODS: Nine patients with focal hand dystonia and 10 controls were studied. Experiments addressed different motor behaviours: (A) lifting and holding an object; (B) adjusting grip force in anticipation of or in reaction to a change in load force by catching a small weight dropped expectedly or unexpectedly into a hand held receptacle. RESULTS: In (A), patients produced a grip force overshoot during the initial lifts; force overflow was most pronounced in those with writer's cramp. Patients and controls adjusted their grip force to object weight within one or two lifts, though patients settled to a steady force level above normal. In (B), patients with focal hand dystonia and normal controls showed similar predictive grip force adjustments to expected changes in object load, suggesting that this aspect of sensory-motor integration was normal. Patients had a shorter latency of grip force response than controls after an unexpected load increase, reflecting either a greater level of preparatory motor activity or a disinhibited spinal reflex response. CONCLUSIONS: The overall increased grip force in patients with focal hand dystonia is likely to be a prelearned phenomenon rather than a primary disorder of sensory-motor integration.     

Coordination of fingertip forces during precision grasping in multiple system atrophy

While the pathology and autonomic nervous system components of multiple system atrophy (MSA) have been well described, little is known about the associated motor dysfunction. One prominent feature of MSA is parkinsonism, although ataxias and pyramidal tract signs are frequently present. To investigate the nature of motor deficits in MSA, a natural grip-lift movement requiring a precision grasp was used to examine force coordination. Subjects were asked to grasp an instrumented object using the fingertips of the thumb and index finger and lift it 10 cm above the table surface. Subjects with MSA demonstrated a prolonged duration between object contact and initiation of the lifting drive that increased with the weight of the object. During this period these subjects produced large grasping forces generating a significant portion of the eventual grip force employed to hold the object. In contrast, control subjects generated grip and load forces in parallel after establishing contact with the object. Therefore, subjects with MSA showed a disrupted performance on both the sequential (grasp, then lift) and simultaneous (grip and load force development) portions of this task. Only after initiation of the vertical lifting drive did subjects with MSA generate forces in a similar manner to control subjects. These findings demonstrate that subjects with MSA exhibit a disrupted coordination of grasp and could suggest a general deficit in motor control resulting from multi-focal neural degeneration.     

CSF drainage ameliorates the motor deficit in normal pressure hydrocephalus

Apart from the classic triad of hypokinetic gait disorder, cognitive dysfunction and urinary incontinence, the clinical spectrum of normal pressure hydrocephalus has been found to affect the upper limbs. It is unclear if the motor deficit of hand and arm movements improves with CSF evacuation. The present study was designed to quantitatively assess the effect of CSF evacuation on the hypokinesia of grasping movements in normal pressure hydrocephalus. Eight subjects with normal pressure hydrocephalus grasped to lift an instrumented object prior to and following evacuation of 40 ml CSF. The build-up of fingertip forces and the kinematics of the lifting movement were slower for patients compared with healthy controls. Patients also generated excessive grasping forces when lifting and holding the object stationary prior to and following CSF evacuation. CSF evacuation significantly improved the gait disorder, the cognitive impairment and the urinary incontinence in normal pressure hydrocephalus. CSF evacuation accelerated the lifting movement and reduced the grip force overshoot. These data suggest that the measurement of grasping forces may provide an additional test to quantify the clinical response to CSF tapping in normal pressure hydrocephalus.     

Distal and proximal prehension is differentially affected by Parkinson‘s disease

Prehension movements consist of distal (grasp) and proximal (reach, lift) components. The proximal lifting movements (achieved at the wrist) of patients with Parkinson’s disease (PD) are characterized by bradykinesia. With respect to the distal component, PD patients show pathologically high grip forces (generated by the fingers) and slowing of force development indicative of disturbed sensorimotor adjustments during prehension. Combining kinematic and force analyses of prehension movements, we investigated whether PD differentially affects the adjustments of the distal or proximal prehension components to current load conditions. First, PD patients (n = 12) and healthy, age-matched control subjects grasped and lifted light and heavy objects without any load cues. Then, they were presented with cues that indicated changes in object load. These load cues were either consciously perceived or rendered subconscious through use of the technique of metacontrast masking. Consistent with the functional organization of the basal ganglia, patients with PD could adapt distal prehension components (grip force) to current load conditions using both types of cues. However, they were impaired in adjusting proximal prehension components (lift velocity). While controls were able to normalize lift velocity with the help of both conscious and subconscious load cues, the PD patients could use neither form of cue, and retained a pathological overshoot in lift velocity. Our results demonstrate that visuomotor integration during prehension movements differs at distal and more proximal joints and that deficits in this integration are pronounced for the latter in Parkinson’s disease.     

Precision grip deficits in cerebellar disorders in man.

OBJECTIVE: To investigate the effect of a variety of cerebellar pathologies on a functional motor task (lifting an object in a precision grip). METHODS: The study involved 8 patients with unilateral damage in the region of the posterior inferior cerebellar artery (PICA), 6 with damage in the region of the superior cerebellar artery (SUPCA), 12 patients with familiar or idiopathic cortical cerebellar degeneration, and 45 age-matched normal subjects. Subjects lifted an object of unpredictable load (internally guided task) or responded to a sudden load increase while holding the object steadily (externally guided task). RESULTS: Damage to the dentate nucleus (SUPCA) or its afferent input (cerebellar atrophy) resulted in disruption of the close coordination normally seen between proximal muscles (lifting the object) and the fingers (gripping the object) during a self-paced lift. Both the SUPCA group and, more markedly, the atrophy group, showed exaggerated levels of grip force. All patients showed a normal rate of grip force development. Damage in the PICA region had no significant effect on any of the measured lifting parameters. All patient groups retained the ability to scale grip force to different object loads. The automatic grip force response to unexpected load increase of a hand held object showed normal latency and time course in all patient groups. The response was modulated by the rate of the load change. Response magnitude was exaggerated in the atrophy patients at all 3 rates tested. CONCLUSIONS: Disturbances associated with cerebellar disorders differed from those seen following damage to the basal ganglia, with no evidence of slowed rates of grip force development. Disruption of temporal coordination between the proximal muscles (lifting) and the fingers (gripping) in a lift was apparent, supporting the role of the cerebellum in coordinating the timing of multi-joint movement sequences. Exaggeration of grip force levels was found in association with damage to the dentate nucleus or, in particular, to its afferent input. This could support a role or the cerebellum in sensorimotor processing, but might also represent a failure to time correctly the duration of grip force generation.     

Sensorimotor dysfunction of grasping in schizophrenia: a side effect of antipsychotic treatment?

BACKGROUND: Antipsychotic treatment in schizophrenia is frequently associated with extrapyramidal side effects. Objective behavioural measures to evaluate the severity of extrapyramidal side effects in the clinical setting do not exist. OBJECTIVES: This study was designed to investigate grasping movements in five drug naive and 13 medicated subjects with schizophrenia and to compare their performance with that of 18 healthy control subjects. Deficits of grip force performance were correlated with clinical scores of both parkinson-like motor disability and psychiatric symptom severity METHODS: Participants performed vertical arm movements with a handheld instrumented object and caught a weight that was dropped into a handheld cup either expectedly from the opposite hand or unexpectedly from the experimenter's hand. The scaling of grip force and the temporospatial coupling between grip and load force profiles was analysed. The psychiatric symptom severity was assessed by the positive and negative symptom score of schizophrenia and the brief psychiatric rating scale. Extrapyramidal symptoms were assessed by the unified Parkinson's disease rating scale. RESULTS: Drug naive subjects with schizophrenia performed similar to healthy controls. In contrast, medicated subjects with schizophrenia exhibited excessive grip force scaling and impaired coupling between grip and load force profiles. These performance deficits were strongly correlated with the severity of both extrapyramidal side effects related to antipsychotic therapy and negative symptoms related to the underlying pathology. CONCLUSIONS: These data provide preliminary evidence that deficits of sensorimotor performance in schizophrenia are, at least in part, related to the side effects of antipsychotic treatment. The investigation of grasping movements may provide a sensitive measure to objectively evaluate extrapyramidal side effects related to antipsychotic therapy.     

Coordination of fingertip forces during precision grip in premanifest Huntington's disease

Precision grip control is important for accurate object manipulation and requires coordination between horizontal (grip) and vertical (load) fingertip forces. Manifest Huntington's disease (HD) subjects demonstrate excessive and highly variable grip force and delayed coordination between grip and load forces. Because the onset of these impairments is unknown, we examined precision grip control in premanifest HD (pre‐HD) subjects. Fifteen pre‐HD and 15 age‐ and sex‐matched controls performed the precision grip task in a seated position. Subjects grasped and lifted an object instrumented with a force transducer that measured horizontal grip and vertical load forces. Outcomes were preload time, loading time, maximum grip force, mean static grip force, and variability for all measures. We compared outcomes across groups and correlated grip measures with the Unified Huntington's Disease Rating Scale and predicted age of onset. Variability of maximum grip force (P < .0001) and variability of static grip force (P < .00001) were higher for pre‐HD subjects. Preload time (P < .007) and variability of preload time (P < .006) were higher in pre‐HD subjects. No differences were seen in loading time across groups. Variability of static grip force (r² = 0.23) and variability of preload time (r² = 0.59) increased with predicted onset and were correlated with tests of cognitive function. Our results indicate that pre‐HD patients have poor regulation of the transition between reach and grasp and higher variability in force application and temporal coordination during the precision grip task. Force and temporal variability may be good markers of disease severity because they were correlated with predicted onset of disease. © 2011 Movement Disorder Society     

Coordination of Manipulative Forces in Parkinson''s Disease

The coordination of manipulative forces was examined in 10 subjects with Parkinson's disease (PD) both OFF and ON medication while they grasped and lifted a small object using the precision grip. The development of grip (squeeze) force and load (vertical lifting) force was recorded and compared to a group of age-matched control subjects. Subjects with PD often exhibited a prolonged delay between the first digit contact with the object and initiation of the lifting drive. These subjects also exhibited stepwise increases in force, with regular oscillations in the force rates. However, once the vertical drive began, the main increase in grip and load force generally was in parallel and most other temporal aspects of the force coordination were similar to those of the control subjects. The extent to which the movement initiation was delayed was related to the stage of the disease, and most subjects improved ON medication. When the object was held in the air, subjects with PD used a grip force level which was similar to that of the control subjects, and all subjects adjusted their grip force according to the surface texture. Furthermore, they exhibited proper reflexive corrections to sudden changes in load (object perturbations), suggesting intact sensorimotor integration. We conclude that the most obvious impairments in the coordination of this task were delayed initiation of the grip–lift sequence and tremor-like oscillations superimposed on otherwise normal force.     

Task specific grip force control in writer’s cramp

ObjectiveWriter’s cramp is defined as a task specific focal dystonia generating hypertonic muscle co-contractions during handwriting resulting in impaired writing performance and exaggerated finger force. However, little is known about the generalisation of grip force across tasks others than writing. The aim of the study was to directly compare regulation of grip forces during handwriting with force regulation in other fine-motor tasks in patients and control subjects.MethodsHandwriting, lifting and cyclic movements of a grasped object were investigated in 21 patients and 14 controls. The applied forces were registered in all three tasks and compared between groups and tasks. In addition, task-specific measures of fine-motor skill were assessed.ResultsAs expected, patients generated exaggerated forces during handwriting compared to control subjects. However there were no statistically significant group differences during lifting and cyclic movements. The control group revealed a generalisation of grip forces across manual tasks whereas in patients there was no such correlation.ConclusionWe conclude that increased finger forces during handwriting are a task-specific phenomenon that does not necessarily generalise to other fine-motor tasks.SignificanceForce control of patients with writer’s cramp in handwriting and other fine-motor tasks is characterised by individualised control strategies.     

Die Analyse der Griffkraft bei der Manipulation von Objekten

The control of prehensile finger forces is an essential feature of all skilled manual performance. The basic aspects of healthy grip force control have been well documented within the past two decades. In healthy subjects, grip force is accurately adjusted to the mechanical object properties such as weight, surface friction, and shape: when we move a hand-held object, grip force is modulated in parallel with movement-induced load fluctuations without an apparent temporal delay. At all phases of the movement, grip force is always slightly higher than the minimum necessary to prevent the object from slipping. The absence of a temporal delay between grip and load force profiles implies that the central nervous system is able to predict load variations precisely before the intended manipulation, and consequently regulates grip force in anticipation. Feedback from the grasping fingertips is used to adjust the force level efficiently to the actual load requirements. Pathologic grip force control affects the efficiency of the forces produced and the predictive temporal coupling between grip and load force profiles. Here we demonstrate pathologic grip force control in various neurologic disorders. The analysis of grip force control is simple and highly sensitive for detecting healthy and pathologic motor behaviour. The examination of grip force control is well suited for an objective evaluation of therapy during sensorimotor rehabilitation of deficient hand function.     

The beneficial effects of subthalamic nucleus stimulation on manipulative finger force control in Parkinson's disease

We investigated the differential effects of levodopa medication and STN stimulation on finger force control in Parkinson subjects grasping to lift an object and performing vertical point-to-point movements of a hand-held object. The experiments were conducted in four treatment conditions: off-drug/off-stimulation, off-drug/on-stimulation, on-drug/off-stimulation and on-drug/on-stimulation. We found that the bradykinesia in Parkinsonian subjects improved by both levodopa medication and STN stimulation. As compared to healthy subjects, excessive grip force was observed in all Parkinson subjects, regardless of the treatment condition. This force excess was most pronounced in the on-drug condition and ameliorated by STN stimulation. We observed reliable correlations between the amount of force overflow and the severity of levodopa-induced dyskinesias in the on-drug condition. Despite some similarities regarding therapeutic effects on bradykinesia, our findings contrast with earlier observations with respect to the differential effects of levodopa and STN stimulation on the scaling of fingertip forces in Parkinson's disease. While levodopa causes an overshoot of fingertip forces, STN stimulation appears to be sufficient to alleviate, but not normalise the force excess. STN stimulation enables Parkinson subjects to scale grip force more accurately to the loads arising from voluntary manipulation of hand-held objects.