Postural stability, often called balance control, is the body’s ability to maintain its center of mass (CoM) within the boundaries of its base of support (BoS). The CoM is the point where the body’s mass is concentrated, typically located just in front of the second sacral vertebra when standing upright. The BoS is the area enclosed by the body parts in contact with the supporting surface, such as the space between the feet while standing. Maintaining stability involves continuous, subtle adjustments to keep the CoM projection from moving outside this supporting area. This mechanism is fundamental for all human movements, from standing motionless to navigating complex environments.
The Three Sensory Inputs That Control Postural Stability
The central nervous system orchestrates postural stability by integrating data from three distinct sensory systems. This integration allows the brain to determine the body’s orientation in space and respond appropriately. The visual system provides external information, using environmental cues like the horizon or stationary objects to establish a frame of reference. Changes in the perceived movement of the surroundings help the brain calculate body sway and necessary corrections.
The vestibular system, housed within the inner ear, offers internal information about head position and acceleration. Specialized canals and organs detect rotational movements and the pull of gravity, providing a reliable measure of spatial orientation. This system is important when visual or somatosensory inputs are unreliable or absent.
The somatosensory system provides information from receptors in the skin, joints, and muscles, a process known as proprioception. Pressure receptors on the soles of the feet inform the brain about surface contact and weight distribution. Joint and muscle receptors signal the relative position of body segments. The brain continuously weighs the input from these three systems, prioritizing the most reliable information based on the current environment, such as reducing reliance on somatosensory feedback when standing on a compliant surface.
Mechanical Strategies for Maintaining Equilibrium
When sensory systems detect a deviation, the body employs a hierarchy of motor strategies to reposition the CoM. The simplest and most frequently used adjustment is the ankle strategy, where the body moves as a single, inverted pendulum. This strategy is used during quiet standing or in response to small, slow perturbations, using ankle muscles to generate torque and shift the CoM. For instance, calf muscles correct a slight forward sway by pulling the body back.
If the disturbance is larger or faster, or the surface is narrow, the body transitions to the hip strategy. This involves generating body sway primarily through hip flexion and extension, causing the trunk and hips to move opposite the ankles. The hip strategy is more effective for rapidly accelerating the CoM back toward the center, such as when one is nudged unexpectedly.
When the CoM is pushed so far that ankle and hip movements cannot prevent movement outside the existing BoS, the stepping strategy is deployed. This is the body’s final defense against a fall, involving taking a step to create a new, larger base of support. The stepping action effectively resets the relationship between the CoM and BoS. These mechanical responses work in a rapid, reflexive sequence to maintain an upright posture.
Assessment and Methods for Improving Stability
Postural stability is commonly assessed using clinical tests that challenge the sensory and motor components of balance control. The Romberg test evaluates the somatosensory system by comparing a person’s sway with eyes open versus eyes closed while standing with feet together. A significant increase in sway when vision is removed suggests dependence on visual input or a proprioceptive deficit.
The Single-Leg Stance Test measures the duration a person can maintain balance on one limb, providing insight into lower limb strength and coordination. Variations, such as standing on a foam pad or in a heel-to-toe position, further isolate reliance on different sensory inputs and increase difficulty. These tests help pinpoint which systems contribute most to stability and where deficits may lie.
To improve stability, training programs focus on exercises that systematically challenge the body’s three sensory inputs and mechanical strategies. For the somatosensory system, practicing balance on unstable surfaces, like a wobble board, forces the body to rely less on reliable ground feedback. Challenging the visual system involves performing simple balance tasks, such as standing on one leg, with the eyes closed.
Exercises like reaching movements or controlled leaning target the mechanical strategies, training ankle and hip musculature to react to larger CoM displacements before a step is needed. Integrating these varied tasks, which progress in difficulty by reducing the BoS or increasing perturbation, helps the nervous system refine its sensory integration and motor response capabilities. Regular practice of tasks specific to daily activities, such as standing while turning, provides the most targeted improvement in functional stability.