Motor control of downward object-transport movements with precision grip by object weight

A systematic review of handover actions in human dyads

Introduction

Handover actions are joint actions in which an object is passed from one actor to another. In order to carry out a smooth handover action, precise coordination of both actors' movements is of critical importance. This requires the synchronization of both the kinematics of the reaching movement and the grip forces of the two actors during the interaction. Psychologists, for example, may be interested in studying handover actions in order to identify the cognitive mechanisms underlying the interaction of two partners. In addition, robotic engineers may utilize insights from sensorimotor information processing in human handover as models for the design controllers in robots in hybrid (human-robot) interaction scenarios. To date, there is little knowledge transfer between researchers in different disciplines and no common framework or language for the study of handover actions.

Methods

For this reason, we systematically reviewed the literature on human-human handover actions in which at least one of the two types of behavioral data, kinematics or grip force, was measured.

Results

Nine relevant studies were identified. The different methodologies and results of the individual studies are here described and contextualized.

Discussion

Based on these results, a common framework is suggested that, provides a distinct and straightforward language and systematics for use in future studies. We suggest to term the actors as giver and receiver, as well as to subdivide the whole action into four phases: (1) Reach and grasp, (2) object transport, (3) object transfer, and (4) end of handover to comprehensively and clearly describe the handover action. The framework aims to foster the necessary exchange between different scientific disciplines to promote research on handover actions. Overall, the results support the assumption that givers adapt their executions according to the receiver's intentions, that the start of the release of the object is processed feedforward and that the release process is feedback-controlled in the transfer phase. We identified the action planning of the receiver as a research gap.

Muscle effort is best minimized by the right-dominant arm in the gravity field

The central nervous system (CNS) develops motor strategies that minimize various hidden criteria, such as end-point variance or effort. A large body of literature suggests that the dominant arm is specialized for such open-loop optimization-like processes, whilst the non-dominant arm is specialized for closed-loop postural control. Building on recent results suggesting that the brain plans arm movements that take advantage of gravity effects to minimize muscle effort, the present study tests the hypothesized superiority of the dominant arm motor system for effort minimization. Thirty participants (22.5 ± 2.1 years old; all right-handed) performed vertical arm movements between two targets (40° amplitude), in two directions (upwards and downwards) with their two arms (dominant and non-dominant). We recorded the arm kinematics and electromyographic activities of the anterior and posterior deltoid to compare two motor signatures of the gravity-related optimization process; i.e., directional asymmetries and negative epochs on phasic muscular activity. We found that these motor signatures were still present during movements performed with the non-dominant arm, indicating that the effort-minimization process also occurs for the non-dominant motor system. However, these markers were reduced compared with movements performed with the dominant arm. This difference was especially prominent during downward movements, where the optimization of gravity effects occurs early in the movement. Assuming that the dominant arm is optimal to minimize muscle effort, as demonstrated by previous studies, the present results support the hypothesized superiority of the dominant arm motor system for effort-minimization.

Motor Planning of Vertical Arm Movements in Healthy Older Adults: Does Effort Minimization Persist With Aging?

Several sensorimotor modifications are known to occur with aging, possibly leading to adverse outcomes such as falls. Recently, some of those modifications have been proposed to emerge from motor planning deteriorations. Motor planning of vertical movements is thought to engage an internal model of gravity to anticipate its mechanical effects on the body-limbs and thus to genuinely produce movements that minimize muscle effort. This is supported, amongst other results, by direction-dependent kinematics where relative durations to peak accelerations and peak velocity are shorter for upward than for downward movements. The present study compares the motor planning of fast and slow vertical arm reaching movements between 18 young (24 ± 3 years old) and 17 older adults (70 ± 5 years old). We found that older participants still exhibit strong directional asymmetries (i.e., differences between upward and downward movements), indicating that optimization processes during motor planning persist with healthy aging. However, the size of these differences was increased in older participants, indicating that gravity-related motor planning changes with age. We discuss this increase as the possible result of an overestimation of gravity torque or increased weight of the effort cost in the optimization process. Overall, these results support the hypothesis that feedforward processes and, more precisely, optimal motor planning, remain active with healthy aging.