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Center of Mass
a point that corresponds to the center of the total body mass and is the point where the body is in perfect equilibrium
the product of mass times velocity; linear momentum relates to the velocity of the body along a straight path
Base of Support
the perimeter of the contact area between the body and its support surface; foot placement alters the BOS and changes a person's postural stability
Limits of Stability
the sway boundaries in which an individual can maintain equilibrium without changing his or her base of support
Ground Reaction Force
the contact between our bodies and the ground due to gravity is always accompanied by a reaction from it, the so-called ground reaction force
Center of Pressure
the location of the vertical projection of the ground reaction force; it is equal and opposite to the weighted average of all the downward forces acting on the area in contact with the ground
a complex motor control task involving the detection and integration of sensory information to assess the position and motion of the body in space and the execution of appropriate musculoskeletal responses to control body position within the context of the environments and task
Balance Control Systems
• Nervous system - sensory input, sensorimotor integration to link sensation to response, motor output
• Musculoskeletal - postural alignment, musculoskeletal flexibility, joint integrity, muscle performance, sensation (touch, pressure, vibration, proprioception)
• Contextual Effects - environment, support surface, lighting, gravity and inertial forces, task characteristics
Ankle Strategy (Anteroposterior Plane)
in a quiet stance and during small perturbations movement of the ankle act to restore a person's center of mass to a stable position; muscle activation usually proceeds in a distal to proximal sequence
Weight-Shift Strategy (Lateral Plane)
the movement strategy utilized to control mediolateral perturbations which involves shifting the body weight laterally from one leg to the other
observed during balance tasks when a person quickly lowers his or her body center of mass by flexing the knees, causing associated flexion of the ankles and hips
for rapid and/or large perturbations or for movements executed with the center of gravity near the limits of stability; rapid hip flexion or extension is used to move the center of mass within the base of support
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