Fig. 4

Training setup and protocol. A Training setup consists of three components: a mechanical apparatus for implementing isokinetic reaching movements, training interfaces, and EMG sensors. B The mechanical apparatus features a force-sensing handle that moves along a linear actuator. The actuator was rotated in a three-dimensional space to implement reaching movements in the six orthogonal directions (anterior, posterior, medial, lateral, superior, and inferior directions). For each movement direction, the initial and final positions of the handle were aligned with the participant. C Two training interfaces were developed. The participant interface provided visual feedback on the three-dimensional endpoint force direction. The participant modulated a force cursor (e.g., represented as a crosshair) by controlling the endpoint force direction to hit a force target (e.g., a meteor). During training, the experimenter set parameters to configure a training task, monitored EMG and endpoint force signals in real-time, and provided verbal instructions. D EMG signals were collected from eight major muscles involved in shoulder and elbow movements. E A participant underwent 18 training sessions and two assessment sessions—pre- and post-training—for three weeks. Each training session was divided into three blocks, varying in training time (5–10 min), and reaching speed (20–40 mm/s). In subsequent blocks, the training was conducted at slower speeds to increase intensity, as participants had to maintain the endpoint force direction for a longer time while reaching the same distance. After completing each block, a five-minute break was given