Effect of upper limb disturbance on balance control and adaptability during static and dynamic motor tasks in younger and older people

Abstract

Adequate control of reaching and grasping movements is essential to manipulate objects and prevent excessive forces from compromising postural stability. Disturbances in the upper limb generate corrective responses in the upper and lower limbs. However, it is not yet known how unexpected disturbances can affect the balance of standing posture or while walking in young and older individuals. Thus, the general purpose of this study is to analyze the organization of the reactive response and the adaptation of this response to successive perturbations when reaching and grasping an object with unexpected weights in static (standing posture) and dynamic (walking) tasks in younger and older people. Two studies will be developed. Study 1 will assess standing posture, and Study 2 will evaluate walking. Each study will have the participation of 30 individuals, 15 younger adults and 15 older people. Volunteers will initially be evaluated through the Mini-Mental State Examination (MMSE), International Physical Activity Questionnaire, Mini-BESTest, Trail Making Test - Parts A and B, and tactile sensitivity of feet and hands. Thirty-nine retroreflective markers will be affixed to anatomical points throughout the body, on the thumb, index finger and the top of the object. The markers will be tracked by 10 Vicon motion capture cameras. The activity of muscles in the upper and lower limbs and trunk will be assessed using surface electromyography (Delsys). Additionally, in Study 1, two Bertec force platforms (one for each foot) will be used to record the displacement of the center of pressure. In Study 2, a force platform will be positioned next to the object to record the displacement of the foot's center of pressure when the object is grasped and lifted. In both studies, individuals will perform 63 trials to reach and grasp an object, 56 of which have the reference weight. After every eight repetitions of the task with the reference weight, they will be disturbed by the unexpected reduction in the object's weight. With the perturbation, participants will apply an amount of force greater than necessary to lift the object, resulting in an unexpected acceleration of the body's center of mass and disturbing postural stability. The reference weight will be 500 g (4.90 N) for all participants, and during trials with perturbation, it will be reduced to 100 g (0.98 N) without the participant knowing. The onset of electromyographic, kinematic and kinetic responses will be calculated after the occurrence of the unexpected disturbance. Furthermore, the margin of stability will be analyzed from the calculation of the extrapolated center of mass to determine how the disturbance in the upper limb affects postural stability. In both studies, analysis of variance (ANOVA) will be performed. The significance level will be p<0.05.