The hemiparalysis, which is a widely known side effect of the disease, requires consistent and accurate rehabilitation exercise treatment. The Grahamizer is a representative piece of equipment for such treatment. Because it consists of a double pivot system, under exercise, the irregularly moving pivots could create an unintended movement and malfunction. Using the Grahamizer, this study analyzed movement patterns based on changes in the armrest’s length and the central axis location for a stable and accurate upper limb rehabilitation exercise.

Eight healthy subjects participated in the study. The Grahamizer, which allows the length of the armrest and the central axis location to be adjusted, was used for experimental equipment. The length could be adjusted to five levels (280mm, 220mm, 160mm, 100mm, and 40mm), and the location ranged from 5 levels to 1 level based on the length of the arm support (280mm: 5positions, 220mm: 4positions, 160mm: 3positions, 100mm: 2positions, 40mm: 1position). A marker located above a knob traced the knob’s movements. The experiment was conducted three times. Each subject held the knob and implemented rotational movements passively, with an angular velocity of 30°/s. After integrating the subjects’ movement traces, the tendency was analyzed. A previous study provided an analysis of the elbow and shoulder joint movements under a normal upper-body rotation movement. This study on movement traces was conducted based on those results.

Longer armrest lengths and greater distances between the central axis location and knob showed greater stability for the rotation movements. Remarkable results were found for changes in the axis location. However, smaller distances between the axis location and knob revealed larger movement traces. A previous study found, when a counterclockwise rotation was defined as 0°∼360°, that an elbow motion of 0°∼180° is flexion and 180°∼360° is extension. The shoulder joint motions are similar: 0°∼90°: medial rotation, 90°∼180°: lateral rotation, 180°∼270° medial rotation, and 270°∼360°: lateral rotation. According to the previous study, with a larger trace (smaller distance between the axis location and knob), the rotation movements of the elbow and shoulder are more accurate from 0° to 180° than from 180° to 360°.

The experiment confirmed that the Grahamizer’s armrest axis location has an impact on movement traces, and changes in the traces prevented the intended joint exercise. Under the circumstances, the location is an important design variable when developing the Grahamizer.

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