Grip force adjustments during rapid hand movements suggest that detailed movement kinematics are predicted

Handedness, laterality and the size-weight illusion

The goal of this study was to determine how handedness and lifting hand influence the way in which we lift objects and perceive their weights. To this end, we examined the fingertip forces and perceptual judgements of 30 left-handers and 30 right-handers during lifts of specially constructed 'size-weight illusion' (SWI) cubes with their left and right hands. All participants completed a series of lifts first with one hand and then the other, so we could additionally examine asymmetries in the retention and transfer of force information between the limbs. Right-handers experienced a larger illusion with their left hand than they did with their right hand, whereas left-handers showed no such asymmetry in their illusions. The perceptual illusion's independence from the application of fingertip force was highlighted by an unexpected lack of asymmetry in terms of fingertip force scaling. Left- and right-handers showed no dominant hand advantage in this task - they were no more skilled at correcting their fingertip force errors with their preferred hand than they were with their non-preferred hand. In addition, although no asymmetries were observed with regard to the most efficient direction of intermanual transfer, the right-handed individuals transferred force information between the hands more effectively than the left-handers. Overall, these findings indicate that hand dominance does not affect the control of the fingertip forces, suggesting that existing models of cerebral laterality must be re-visited to consider kinetic (i.e., related to forces), as well as kinematic (i.e., related to movement) variables.

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.

Efficiency of grip force adjustments for impulsive loading during imposed and actively produced collisions

During object manipulation, both predictive feedforward and reactive feedback mechanisms are available to adjust grip force (GF) levels to compensate for the destabilizing effects of load force changes. During collisions, load force increases impulsively (< 20 ms). Thus, only predictive control of GF can be used to ensure grasp stabilization. A collision paradigm is here used to investigate the effects of practice and vision on the efficiency of the predictive control of GF. Subjects actively produced or received an imposed collision with a pendulum. Subjects were more efficient (used smaller GF for identical loads) when producing than when receiving the collisions. Effects of practice were evident in the active producing task only, with GF levels reducing over repetitions, suggesting that sensorimotor memory for the task was used to adjust GF more efficiently. With imposed collisions, GF levels did not reduce with repetition, which suggests that a direct relation between motor action and sensory feedback may be necessary to improve efficiency. Nevertheless, in this condition GF was lower with visual feedback, indicating potential for more efficient grip possibly associated with subjects degree of confidence. We discuss the implications of these results for accounts of the predictive and the reactive control of movement.

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.