Biomechanical fundamentals of balance control across the lifespan

If someone is standing or walking without falling, the state of this person can be defined as stable. Maintaining, achieving or restoring this state of stability requires balance control. Balance control can be defined as controlling the state of the body center of mass (CoM) relative to the base of support (BoS), i.e., the area within an outline of all points on the body which are directly in contact with the support surface. Balance control is regulated by the sensorimotor control system; this integrates sensory input from visual, vestibular and somatosensory systems to generate motor commands, resulting in muscular responses. Several balance control mechanisms can be used during standing and walking. One of these balance control mechanisms is to activate muscles around the ankle to generate moments around the ankle, called the ankle mechanism. These ankle moments are reflected in a shift of the center of pressure of the ground reaction force (CoP). Another mechanism that can be used is foot placement. Foot placement determines the BoS and therefore constrains the location of the CoP. Combined, foot placement and ankle moments determine the position of the CoP relative to the CoM, which in turn determines the acceleration of the CoM and thus affects the location of the CoM relative to the BoS. Shifts of the CoP through modulation of foot placement and ankle moments are coined the CoP-mechanism. A different mechanism that can be used to accelerate the CoM, is the counter-rotation mechanism, i.e., changing angular momentum of segments around the CoM to change the direction of the ground reaction force. Rotation of the trunk and pelvis around the hip, which has been called the hip mechanism in literature, is an example of the counter-rotation mechanism. Research on the role of the counter-rotation mechanism is limited during walking and standing. If, and how, the counter-rotation mechanism is used and how it interacts with the CoP-mechanism can be investigated by trying to provoke the use of the counter-rotation mechanism by applying unexpected perturbations or by constraining the CoP-mechanism (e.g., standing on a balance board or constraining foot placement). Since children and older adults fall more often than younger adults, it would be worthwhile to assess the influence of age on the use of the balance control mechanisms. In children, the sensory systems and the integration of the sensory input are immature and in older adults the complete sensorimotor system deteriorates, limiting balance control. The first aim of this doctoral thesis was to obtain insight in the use of the balance control mechanisms in younger adults during walking and standing. The second aim of this doctoral thesis was to obtain insight in the use of the balance control mechanisms in children, younger adults, and older adults during standing. The CoP-mechanism was the main contributor to CoM accelerations during walking and standing in younger adults, but also in children and older adults. The use of the counter-rotation mechanism may be limited, because angular accelerations ultimately need to be reversed and because of interference with other task constraints, such as stabilizing the orientation of the head in space and preventing interference with the gait pattern. It would be worthwhile to assess in the future if different use of a balance control mechanisms could be related to falling. When understanding the link between the use of the balance control mechanisms and falling, evaluating the use of the balance control mechanisms could be used to determine training targets to potentially decrease fall incidence. In addition, determining the use of the balance control mechanisms could be useful to detect abnormal motor development in children early