Yaw postural perturbation through robotic platform: aging effects on muscle synergies

Aging causes a worsening in muscle system, which could cause balance impairments, increasing the risk of falls. The study aims at evaluating the effects of aging on muscle activation in response to a yaw rotation imposed by the RotoBiT 1D . Eight younger and eight older adults were enrolled in the study. A right sigmoidal rotation of 55° around the yaw axis was imposed to the subject by means of the RotoBiT 1D platform in two velocity conditions, characterized by an angular velocity peak equal to 80¼/s and 100 °/s, respectively. The activations of 16 bilateral muscles of upper body were recorded through wireless surface electromyography. A Non-Negative Matrix Factorization was performed to extract the muscle synergies. The number of muscle synergies was selected by using the Variability Account For. The cosine of similarity was computed for the quantification of intra-group and inter-group similarity related to the muscle synergy vectors. The number of muscle synergies ranged from 4 to 6 in younger and from 3 to 6 in older, even though no statistical difference was found between groups or velocity conditions. As regards intra-group similarity, younger adults showed values always above the similarity threshold; while a lower similarity was observed in older adults, confirming the heterogeneity of postural response. The overall structure of muscle synergy vectors was not similar between groups and the inter-group similarity decreased with the increase of the velocity. The differences were greater in synergies involving head and upper limb muscles. Findings unveiled a different muscle synergy organization in terms of muscle synergy vectors. Such a different organization calls for a deeper investigation towards the aim of identifying causes of fall in elderly.