Neural processes underlying human balance control

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The ability to maintain balance is essential for the execution of many daily activities. When voluntarily initiating a movement or in anticipation of a predictable external perturbation to the body, individuals must rely on feedforward mechanisms to generate an anticipatory postural adjustment (APA) prior to movement onset. This involves a rapid change in corticospinal excitability to facilitate an early increase in postural muscle activity. This causes a shift in the body's center of mass so that destabilization from the upcoming movement is minimized. Since APAs act to stabilize the body, the timing and magnitude of APAs are dependent on the force and speed of the upcoming movement, whereby larger and earlier APAs are observed when larger or faster movements are expected. Interestingly however, older adults have a reduced ability to scale their anticipatory responses, such that they demonstrate more homogenous-sized APAs across varying levels of force. This deficit in APA scaling leads to greater postural instability prior to movement initiation and may explain why falls in daily life often occur as a result of an incorrect shifting of body weight during movement preparation. To better understand the neural mechanisms underlying the poor scaling and generation of APAs, the proposed research program will investigate how the central nervous system contributes to anticipatory balance control across the lifespan. A series of experiments will be conducted whereby young and older adults will self-initiate a movement (e.g., rising onto their toes) or respond to a predictable perturbation to the body. Variations in task difficulty and individual constraints will be introduced to examine the adaptability of the central nervous system and whether it is associated with the requirements of the task. At various times during the preparatory phase of these movements (i.e., prior to movement onset), the excitability of the spinal pathways will be assessed by electrically stimulating a peripheral nerve to elicit Hoffmann reflexes in the lower limb postural muscles while the involvement of the supraspinal areas will be determined by eliciting motor-evoked potentials using transcranial magnetic stimulation. Balance performance, in the form of APA magnitude and onset, will be quantified through body kinematic and kinetic recordings. The proposed research program will expand our fundamental understanding of the neural mechanisms underlying human anticipatory balance control and provide insight into how and why these processes become altered with advanced age. Knowledge developed from the program may also help to improve our ability to determine why falls and balance deficits arise due to aging.