Author(s): MOLLO, C., GROSPRÊTRE, S.1,2, DE PAUW, K.3,4, DURY, J.1, Institution: UNIVERSITÉ MARIE ET LOUIS PASTEUR, Country: FRANCE, Abstract-ID: 2329

INTRODUCTION:
Shoulder exoskeletons aim to reduce muscular effort during overhead tasks, but their effects on postural control remain unclear. By modifying body segment mass, joint angles and limb acceleration (1), exoskeletons may also affect anticipatory postural adjustments (APAs), characterized by an activation of postural muscles prior to the initiation of a focal movement (2). Investigating APAs in the context of exoskeleton use can reveal how the postural system adjusts to incorporate the assistance provided by the exoskeleton. Therefore, the main objective of the study was to determine the immediate effects of wearing a passive shoulder exoskeleton (PSE) on APAs. We hypothesized that wearing the exoskeleton will increase arm-raising velocity, leading to larger APAs.
METHODS:
Healthy young participants (age = 21.7 years) performed a rapid bilateral arm raise from a resting position (arms along the body) to a 90° shoulder flexion. It was performed with and without a PSE (10 trials per condition). Arm velocity was analyzed through a motion capture system analysis, while postural oscillations were quantified using a force. Electromyography signals (EMG) of several postural and focal muscles (soleus, biceps femoris, tibialis anterior, longissimus, deltoid anterior, triceps brachial) were also assessed. The window used for EMG analysis was 100ms before to 50ms after the start of arm movement. The mean EMG along this window was then compared between the conditions. Each EMG activity was expressed as a percentage of a predetermined maximal EMG activity during a maximal voluntary contraction (MVC).
RESULTS:
Data from 3 participants has been processed so far, using descriptive statistics. The results indicated a higher arm velocity (+272.2 mm/s) in the PSE condition compared to the no-PSE condition. Mean EMG activity of the biceps femoris was 3.7 ± 1.2 % MVC without the PSE, and 3.8 ± 1.4 % MVC with the PSE.
CONCLUSION:
The preliminary results showed an increase in arm movement velocity with the PSE, not surprising given the passive assistance provided in arm raising. Previous study has shown that APAs typically increase with movement speed (3). However, the activity of postural muscle implied in APAs regulation showed no difference between the movement executed with and without the PSE, despite higher arm velocity. This suggests that the PSE may not be adequately integrated into the participant’s postural control strategies that could be characterized by a center of pressure displacement impairment during movement. Further data analysis (center of pressure and activity of other muscles) and increase in sample size (ongoing) will help confirm and refine those results.
References
1- Casu et al., 2024 Applied Ergonomics, 121, 104357.
2- Massion (1992). Progress in Neurobiology, 38(1), 35–56
3- Lee et al. (1987). Experimental Brain Research, 66(2).