Compensation Effect Amplification Control (CEAC): A movement-based approach for coordinated position and velocity control of the elbow of upper-limb prostheses

TL;DR

CEAC introduces a movement-based control paradigm that leverages trunk flexion as input to drive velocity-based control of the prosthetic elbow, using a dynamic, delayed reference to enable simultaneous positioning and velocity modulation. By simplifying the compensatory state to a single trunk angle and mapping it directly to elbow velocity, CEAC avoids complex inverse kinematics and supports a release-catch mechanism for intuitive control. Across drawing and reaching tasks with 12 able-bodied participants, CEAC yielded task performance and kinematic patterns closely resembling natural elbow movements, while redistributing effort toward trunk participation and maintaining ergonomic postures. The findings suggest CEAC as a practical, intuitive approach for intermediate joint control in UL prostheses, with potential for multi-DoF extension and integration with myoelectric hand control in future work.

Abstract

Despite advances in upper-limb (UL) prosthetic design, achieving intuitive control of intermediate joints - such as the wrist and elbow - remains challenging, particularly for continuous and velocity-modulated movements. We introduce a novel movement-based control paradigm entitled Compensation Effect Amplification Control (CEAC) that leverages users' trunk flexion and extension as input for controlling prosthetic elbow velocity. Considering that the trunk can be both a functional and compensatory joint when performing upper-limb actions, CEAC amplifies the natural coupling between trunk and prosthesis while introducing a controlled delay that allows users to modulate both the position and velocity of the prosthetic joint. We evaluated CEAC in a generic drawing task performed by twelve able-bodied participants using a supernumerary prosthesis with an active elbow. Additionally a multiple-target-reaching task was performed by a subset of ten participants. Results demonstrate task performances comparable to those obtained with natural arm movements, even when gesture velocity or drawing size were varied, while maintaining ergonomic trunk postures. Analysis revealed that CEAC effectively restores joint coordinated action, distributes movement effort between trunk and elbow, enabling intuitive trajectory control without requiring extreme compensatory movements. Overall, CEAC offers a promising control strategy for intermediate joints of UL prostheses, particularly in tasks requiring continuous and precise coordination.

Figures (10)

• Figure 1: Illustration of how users of Compensation Cancellation Control (CCC) and Compensation Effect Amplification Control (CEAC) execute a round-trip line-drawing task. Points A and B denote the nominal start and end points of the path, while B' is an extended target. Top row (A): The sequence with CCC, where the trunk reference $\phi_{\text{ref}}$ is fixed. Bottom row (B): The sequence with CEAC, where $\phi_{\text{ref}}$ is dynamic and follows the user's trunk with a delay. In both rows the movement is divided in five distinguished phases.
• Figure 2: Overview of the experimental setup. (A) Prosthesis configuration with an active elbow (1b), a prosthetic forearm (1c), and a blocking elbow orthosis (1a) attached to the natural limb. The prosthetic forearm is oriented at $35^\circ$ relatively to the natural forearm. OptiTrack markers are placed on the pen, the prosthetic forearm, above the elbow, and on the upper arm. (B) Natural-limb setup with markers (2) on the upper arm, elbow, and forearm, as well as a wrist orthosis to which the pen is rigidly attached. In both setups, a VIVE® Ultimate Tracker is mounted above the trapezius (3). (C) A participant performing the drawing task on a Plexiglas-covered screen (4). The top left of panel C shows two training paths (5), while the main experimental path is shown enlarged in the top right (6). The bottom left illustrates the table height relative to the participant (7). (D) A participant performing the reaching task using a Plexiglas-covered screen and 3D-printed cylinders as targets (8). The corresponding target positions are shown in screen coordinates (9), with numbers indicating the sequence in which the targets are to be reached. Targets 2, 3, and 9 are flush with the screen surface (0 cm depth), target 1 is placed at 5 cm, target 5 at 10 cm, targets 4 and 7 at 15 cm, and targets 6 and 8 at 20 cm from the screen.
• Figure 3: Drawing a 20 cm line with three control conditions. Columns compare Compensation Cancellation Control (CCC), Compensation Effect Amplification Control (CEAC), and the natural elbow. In the plots, labels A, B, and B' denote the start, nominal end, and extended/overshoot points along the path. Rows show: pen-tip trajectory in the $yz$-plane (top), pen-tip position and velocity over time (middle), and joint angles over time (bottom). The horizontal ribbon indicates when the elbow motor is active ($\epsilon^{*}>0$).
• Figure 4: Pen-tip and joint kinematics during line-drawing trials with the CEAC-controlled prosthesis at five different instructed speeds. Top row: pen-tip position ($p_y$) and velocity ($v_y$), where dashed lines label the start (A) and end (B') positions. Bottom row: trunk ($\phi$), shoulder ($\theta$), and elbow ($\beta$) joint angles. The horizontal ribbon indicates periods of active elbow control ($\epsilon^{*}>0$).
• Figure 5: Qualitative example of drawing the large racetrack path with the CEAC prosthesis. Left: Overlay of pen-tip trajectories for Slow, Medium, and Fast trials; the inset (a) provides a magnified view. Right: Joint-angle time series for the Medium-speed trial. The horizontal ribbon marks intervals of active elbow (green) control ($\epsilon^{*}>0$).