The effects of an object’s height and weight on force calibration and kinematics when post-stroke and healthy individuals reach and grasp

Impairment in force regulation and motor control impedes the independence of individuals with stroke by limiting their ability to perform daily activities. There is, at present, incomplete information about how individuals with stroke regulate the application of force and control their movement when reaching, grasping, and lifting objects of different weights, located at different heights. In this study, we assess force regulation and kinematics when reaching, grasping, and lifting a cup of two different weights (empty and full), located at three different heights, in a total of 46 participants: 30 sub-acute stroke participants, and 16 healthy individuals. We found that the height of the reached target affects both force calibration and kinematics, while its weight affects only the force calibration when post-stroke and healthy individuals perform a reach-to-grasp task. There was no difference between the two groups in the mean and peak force values. The individuals with stroke had slower, jerkier, less efficient, and more variable movements compared to the control group. This difference was more pronounced with increasing stroke severity. With increasing stroke severity, post-stroke individuals demonstrated altered anticipation and preparation for lifting, which was evident for either cortical lesion side.


Time point calculations
In order to analyse the data, the movement was divided into three logical phase: Reaching for the cup, Grasp and Lift. The segments were determined using the following four time points: movement initiation (T1), the time at which the cup was grasped (T2), the time at which the cup was lifted (T3), and the end of the movement, once the cup was placed on the shelf (T4) 39 . The combinations of the time points produced the phases of movement: Phase 1 "Reach to grasp" (T1 to T2: from starting position until grasp of the cup), Phase 2 "Grasp" (T2 to T3), Phase 3 "Lift" (T3 to T4: from grasp until placing the cup on the shelf ), and the whole movement (T1 to T4). First T4 was calculated, then T3, T2 and finally T1.
They were calculated as follows: Movement initiation (T1): The speed trace of the wrist movement measured by the wrist sensor was scanned from the start of the recording to T2, when the cup was grasped. T1 was the time at which the wrist speed exceeded 10% of its maximal value during this phase.
• Cup grasp (T2): The maximal value of the force within the first 100 data points on the force trace was calculated (F baseline_max ). T2 was the time at which the force trace first reached a value of 0.2 N beyond F baseline_max .
• Cup lift (T3): The trace of the vertical position of the cup was scanned from the start of the recording to T4, and the maximum value was calculated -this was the highest cup location during the movement as measured by the Y component of the rigid body sensor (Y max ). The minimal cup height (Y min ) was similarly calculated. The vertical distance travelled by the cup Y range = Y max -Y min was then computed, and T3 was taken to be the time at which the cup reached a height equal to 10% of Y range (above Y min ).
• End of movement (T4): The first 100 data points on the force trace were averaged, to establish the average baseline force value, and this value was subtracted from the force trace, to remove measurement noise. Then, the maximum value of the force in that movement was calculated (F max ). The last time point where the force value was 20% of F max was identified, and the force trace was scanned from that point forward to find the first instance where the force value exceeded 5% of F max . This point was determined as T4. Figure S1. Force-to-time ratio (FTR; N/sec). FTR values for the Grasp phase (left), and the Lift phase (right). The top row shows the results by group (control/stroke). The middle row shows the results by height (low/medium/high), and by sub-group (control/mild impairment/moderate impairment/moderate-severe impairment). The bottom row shows the results by weight (empty/full cup, corresponding to light/heavy weight), and by sub-group. Asterisks denote the p-value: *p≤0.05, **p≤0.01, ***p≤0.001. Abbreviations: N-Newtons; sec-seconds; NS-not significant Figure S2. Number of Force Peaks (NFP). NFP values for the Grasp phase (left), and the Lift phase (right). The top row shows the results by group (control/stroke). The middle row shows the results by height (low/medium/high), and by sub-group (control/mild impairment/moderate impairment/moderate-severe impairment). The bottom row shows the results by weight (empty/full cup, corresponding to light/heavy weight), and by sub-group. Asterisks denote the p-value: *p≤0.05, **p≤0.01, ***p≤0.001. Abbreviations: NS-not significant.