Sensorimotor processing in the grip-lift task: the impact of maximum wrist flexion/extension on force scaling

Effects of Hand Configuration on the Grasping, Holding, and Placement of an Instrumented Object in Patients With Hemiparesis

Objective: Limitations with manual dexterity are an important problem for patients suffering from hemiparesis post stroke. Sensorimotor deficits, compensatory strategies and the use of alternative grasping configurations may influence the efficiency of prehensile motor behavior. The aim of the present study is to examine how different grasp configurations affect patient ability to regulate both grip forces and object orientation when lifting, holding and placing an object.

Methods: Twelve stroke patients with mild to moderate hemiparesis were recruited. Each was required to lift, hold and replace an instrumented object. Four different grasp configurations were tested on both the hemiparetic and less affected arms. Load cells from each of the 6 faces of the instrumented object and an integrated inertial measurement unit were used to extract data regarding the timing of unloading/loading phases, regulation of grip forces, and object orientation throughout the task.

Results: Grip forces were greatest when using a palmar-digital grasp and lowest when using a top grasp. The time delay between peak acceleration and maximum grip force was also greatest for palmar-digital grasp and lowest for the top grasp. Use of the hemiparetic arm was associated with increased duration of the unloading phase and greater difficulty with maintaining the vertical orientation of the object at the transitions to object lifting and object placement. The occurrence of touch and push errors at the onset of grasp varied according to both grasp configuration and use of the hemiparetic arm.

Conclusion: Stroke patients exhibit impairments in the scale and temporal precision of grip force adjustments and reduced ability to maintain object orientation with various grasp configurations using the hemiparetic arm. Nonetheless, the timing and magnitude of grip force adjustments may be facilitated using a top grasp configuration. Conversely, whole hand prehension strategies compound difficulties with grip force scaling and inhibit the synchrony of grasp onset and object release.

Somatosensory feedback refines the perception of hand shape with respect to external constraints

Motor commands issued by the CNS are based upon memories of past experiences with similar objects, the current state of the hand and arm postures, and sensory input. Thus widespread somatosensory information is available to form precise representations of hand shape on which to base motor commands to match a desired posture or movement. The aim of this study was to examine the extent to which somatosensory information reflecting external influences on independent finger movement is incorporated into the perception of hand shape driving the motor command. To address this issue, a matching task was performed while pairs of fingers in the grasping hand were constrained to move in tandem when grasping familiar objects. The hypothesis was that motor commands would be driven by comparison of the online sensory information from the matching hand to a desired somatosensory state determined by the current somatosensory input from the grasping hand. The results demonstrated that multi-muscle patterns of activation and hand postures were altered with respect to the external constraint on independent finger movement. A secondary aim of this study was to examine the influence of sensory information on the structure of the multi-muscle patterns. The hypothesis was that the same synergies (patterns of activation across muscles) would be used to complete the task but would be rescaled with respect to condition. The results demonstrated that rescaling the patterns of multi-muscle activity from the unconstrained condition could not equivalently represent those from the constrained conditions. Thus it appears that external restriction of independent finger movement was signaled by somatosensory feedback and incorporated into the desired state driving the motor command resulting in selective activation of groups of muscles.

Influence of vision and posture on grip-lift task parameters in healthy adults

The grip-lift task enables a quantitative assessment of grasping ability. Patients are regularly assessed in a supine position, which offers a different view of the grasped object from that in the sitting position. To our knowledge, no data are currently available on the influence of posture and vision on grip-lift task parameters. We therefore aimed to determine the effects of posture and vision on these parameters. Twenty-six healthy right-handed adults performed grip-lift tasks with a manipulandum that measured different temporal and dynamic parameters in four conditions: sitting eyes open, sitting blindfolded, lying down eyes open and lying down blindfolded. A repeated-measures analysis of variance with two factors (vision and position) showed that the absence of vision affected all the parameters measured. The lying down position increased the time between the first contact with the object and the modification of the vertical force as well as the delay between the first increase of the horizontal force and the increase of the vertical force. In addition, there was a lower adaption of the horizontal force, required to squeeze the object, to the vertical force. Finally, the interaction of position and vision was associated with significant differences in the delay between the contact of each digit with the object, the maximum horizontal force and the ratio between the horizontal and vertical force during a static holding period. Both position and vision appear to affect the grip-lift task. Consequently, sequential assessments should be performed in the same condition to obtain reliable data.

Limited persistence of the sensorimotor memory when transferred across prehension tasks

A recent study has shown that the sensorimotor memory for the fingertip forces used to grasp and lift an object can be shared across two prehension tasks. However, the persistence (or decay) of these memory resources is not known. Reports of within-task sensorimotor memory indicate persistence of lifting forces, with evidence for reduced persistence of grip forces. Here we investigated the temporal dynamics of the transfer of memory related to vertical lifting forces across prehension tasks. Young adult participants in two separate experimental groups first held the object placed on their palm and 'hefted' it (moved it up and down) followed by lifting the object using a precision grip (thumb-finger) with the dominant hand after a delay of 10 s, or 20 min. The Control group lifted the object with the dominant hand using a precision grip and then did so again 20 min later. The Control group used higher load force rates (LFR) for their first lift compared to subsequent lifts, both before and after the 20-min delay. This suggests that the Control group initially overestimated the weight of the object, corrected their LFRs, and then was able to retain this corrected force scaling after the 20-min delay. The Experimental 10-second delay group accurately scaled their LFRs upon their first lift, indicating that they obtained an accurate memory for LFR scaling during hefting, and transferred it to the lift task. In contrast, the Experimental 20-minute delay group was unable to scale their LFRs upon their first lift, as indicated by high LFRs that were no different than those of the Control group. Thus, the memory related to the production of LFR remained stable over 20 min when obtained from the same task, while that obtained from a different task decayed completely within 20 min. This decay may indicate weakened sensorimotor memories related to prehension forces due to its dependence not only on the memory for object mechanical properties, but also on sensory signals generated during the prehension act, along with strong visual prior estimates of a size-weight relationship.

Intermanual transfer of sensorimotor memory for grip force when lifting objects: the role of wrist angulation

OBJECTIVE

To investigate the mechanisms underlying the intermanual transfer of sensorimotor memory when lifting an object.

METHODS

Twenty healthy subjects grasped and lifted an object with constant mechanical properties with the right hand (RH) first and then with the left hand (LH). Ten of the subjects lifted the object with the RH in a regular wrist angulation (WA), followed by lifts with the LH in a regular WA. The remaining 10 subjects lifted the object with the RH in a hyper-flexed WA, followed by lifts with the LH in a regular WA.

RESULTS

Subjects generated greater peak grip force (GF) rates, grip and lift forces when lifting the object with the wrist in a regular WA compared to lifts with the wrist in hyper-flexion. Importantly, subjects transferred the predictive scaling of GF from the RH to the LH, regardless of the WA.

CONCLUSIONS

Biomechanical properties of the object do not seem to be used by the CNS as a first line information to evaluate GF when handling an object or transferring information about the grasp to the opposite hemisphere.

SIGNIFICANCE

The predictive scaling of GF rather reflects an internal sense of effort than an internal representation of the mechanical object properties.