Precision grip responses to unexpected rotational perturbations scale with axis of rotation

Designing Feedback Controllers for Human-Prosthetic Systems Using H∞ Model Matching

Prosthetic hands are important tools for improving the lives of upper limb amputees, yet most devices lack the ability to provide a sense of touch back to the user. Recent improvements have been made in electromyography (EMG) prosthesis control as well as in biologically relevant tactile sensors to provide sensory feedback to amputees through nerve stimulation. However, sensory feedback has been designed heuristically, which can lead to either unnatural sensations or to excessive feedback that bothers the user. In this study, we apply optimal control techniques to synthesize sensory feedback to the user, and to synthesize the conversion from EMG to an actuation command to the prosthesis. Specifically, we construct a feedback control system architecture and solve the $H_{\infty }$ model matching problem to make the closed-loop user-prosthetic system to behave like a pre-specified ideal system in response to elemental inputs (e.g. impulse, step, etc). We design feedback controllers assuming that human and prosthetic components behave in a linear fashion as a proof-of-concept, and the closed-loop system is able to match ideal systems that are slow, fast and that have both slow and fast dynamics (like healthy humans).

Haptic Orientation Guidance Using Two Parallel Double-Gimbal Control Moment Gyroscopes

This paper presents a system of two double-gimbal control moment gyroscopes (CMGs) for providing ungrounded kinesthetic haptic feedback. By spinning a second flywheel opposite the first, and rotating them through opposite trajectories, undesired gyroscopic effects can be eliminated, isolating a single torque axis. This produces a moment pulse proportional to the flywheel spin speed and rotation speed. Rotating the CMG gimbals quickly in one direction, then resetting them more slowly generates repeated torque pulses indicating a clear direction cue. We present the mathematical model for moments produced by this system and verify that the performance of our device matches this model. Using these asymmetric moment pulses, we provide haptic cues to participants in two studies. In the first study, users simply identify the direction of torque cues. In the second study, we use the torque pulses to guide users to target orientations. Performance in both studies shows that this system has the potential to provide useful guidance for applications where ungrounded haptic feedback is desired.

Dynamical Coordination of Hand Intrinsic Muscles for Precision Grip in Diabetes Mellitus

This study investigated the effects of diabetes mellitus (DM) on dynamical coordination of hand intrinsic muscles during precision grip. Precision grip was tested using a custom designed apparatus with stable and unstable loads, during which the surface electromyographic (sEMG) signals of the abductor pollicis brevis (APB) and first dorsal interosseous (FDI) were recorded simultaneously. Recurrence quantification analysis (RQA) was applied to quantify the dynamical structure of sEMG signals of the APB and FDI; and cross recurrence quantification analysis (CRQA) was used to assess the intermuscular coupling between the two intrinsic muscles. This study revealed that the DM altered the dynamical structure of muscle activation for the FDI and the dynamical intermuscular coordination between the APB and FDI during precision grip. A reinforced feedforward mechanism that compensates the loss of sensory feedbacks in DM may be responsible for the stronger intermuscular coupling between the APB and FDI muscles. Sensory deficits in DM remarkably decreased the capacity of online motor adjustment based on sensory feedback, rendering a lower adaptability to the uncertainty of environment. This study shed light on inherent dynamical properties underlying the intrinsic muscle activation and intermuscular coordination for precision grip and the effects of DM on hand sensorimotor function.

Inertial torque during reaching directly impacts grip-force adaptation to weightless objects

A hallmark of movement control expressed by healthy humans is the ability to gradually improve motor performance through learning. In the context of object manipulation, previous work has shown that the presence of a torque load has a direct impact on grip-force control, characterized by a significantly slower grip-force adjustment across lifting movements. The origin of this slower adaptation rate remains unclear. On the one hand, information about tangential constraints during stationary holding may be difficult to extract in the presence of a torque. On the other hand, inertial torque experienced during movement may also potentially disrupt the grip-force adjustments, as the dynamical constraints clearly differ from the situation when no torque load is present. To address the influence of inertial torque loads, we instructed healthy adults to perform visually guided reaching movements in weightlessness while holding an unbalanced object relative to the grip axis. Weightlessness offered the possibility to remove gravitational constraints and isolate the effect of movement-related feedback on grip force adjustments. Grip-force adaptation rates were compared with a control group who manipulated a balanced object without any torque load and also in weightlessness. Our results clearly show that grip-force adaptation in the presence of a torque load is significantly slower, which suggests that the presence of torque loads experienced during movement may alter our internal estimates of how much force is required to hold an unbalanced object stable. This observation may explain why grasping objects around the expected location of the center of mass is such an important component of planning and control of manipulation tasks.

Independent digit contributions to rotational manipulation in a three-digit pouring task requiring dynamic stability

Many activities of daily living involve multi-digit, voluntary rotational manipulations of grasped objects. Yet, only a few studies have focused on coordination of individual fingertip forces during such tasks. The objective of this study was to investigate individual digit contributions to a three-digit task in which an object was rotationally manipulated against gravity. Center of mass was varied through the use of containers shaped like a water bottle, pint glass, and cocktail glass, from which subjects poured fluid carefully into a nearby receptacle. The center of mass of the grasped object changed continuously as fluid was poured out. Self-selected digit placement and contributions of fingertip forces to rotational manipulation were dependent upon anticipated center of mass location associated with container shape. The thumb resisted the rotation of the top-heavy, cocktail glass container until 79 % of the pouring phase had elapsed, but actively assisted the rotation of the less challenging containers. More directly opposing the thumb, the index finger contributed more to grasp stability. The middle finger contributed more to rotation of the container for pouring. It was found that the thumb, index, and middle fingers acted in unison temporally, but contributed independently to the grip forces and stabilizing moments throughout the dynamic, rotational manipulation task.