Control of POSTURE and BALANCE

Learning objectives Define postural control, distinguish between postural orientation versus stability, and describe a dynamic definition of limits of stability Describe the action components of postural control, being able to define strategies, synergies and how they change according to task and environmental demands. Describe perception systems in postural control.

Define anticipatory postural control and describe the ways that it aids voluntary movement control. Describe the attentional demands of postural control and the implications of this for maintaining stability under multitask situations.

Introduction Postural control emerges from an interaction of the individual with the task and the environment

Defining Postural control Postural control involves controlling the body’s position in space for the dual purposes of stability and orientation Postural orientation is defined as the ability to maintain an appropriate relationship between the body segments, and between the body and the environment for the task.

Postural control Postural control emerge from an interaction of the individual, the task with its inherent postural demands, and the environmental constraints postural control postural actions. Task Individual Environment

More definitions Posture is often used to describe both biomechanical alignment of the body and the orientation of the body to the environment. Postural stability, also referred to as balance, is the ability to control the center of mass in relationship to the base of support. Center of mass (COM), is defined as a point that is at the center of the total body mass.

Continued... Center of mass (COM), is defined as a point that is at the center of the total body mass. The vertical projection of the COM is often defined as the center of gravity (COG). The base of support (BOS) is defined as the area of the body that is in contact with the support surface. The center (COP) of pressure is the center of the distribution of the total force applied to the supporting surface

Postural control requirements vary with task and environment All tasks require postural control. Every task has an orientation component and a stability component. The stability and orientation requirements will vary with the task and the environment. Some tasks place more importance on maintaining an appropriate orientation at the expense of stability. Example?

Sitting on bench and reading – postural orientation requirement Sitting on bench and reading – postural orientation requirement? Stability requirements? Why?

Standing and reading a book. – postural orientation requirement Standing and reading a book. – postural orientation requirement? Stability requirements? Why?

Standing reading a book on a moving train/bus Standing reading a book on a moving train/bus. postural orientation requirement? Stability requirements? Why?

Defining systems for postural control

Defining systems for postural control Postural control for stability and orientation requires a complex interaction of musculoskeletal and neural systems, as shown in the previous slide. Musculoskeletal components: joint range of motion, spinal flexibility, muscle properties, biomechanical relationships among linked body segments.

Defining systems for postural control Neural components: motor processes (organizing muscles throughout the body) , sensory/perceptual processes (organization and integration of visual, vestibular and somatosensory systems) higher level processes (mapping sensation to action and anticipatory and adaptive aspects of postural control)

Stance postural control Control of quiet stance postural stability or balance is defined as the ability to maintain the projected COM within the limits of the BOS, referred to as stability limits. Stability limits are considered the boundaries within which the body can maintain stability without changing the base of support. Previously stability limits during stance were conceptualized rather statically, defined solely by the physical characteristics of the base of support (feet). Recent studies: strength, ROM, characteristics of COM (position and velocity) and various other aspects of the environment

Action systems in Postural control Motor control of quiet stance Static balance? – postural control in quite stance is quite dynamic Quiet stance is characterized by small amounts of spontaneous postural sway. Stability in quiet stance 1. body alignment 2. Muscle tone (intrinsic stiffness of muscles, background muscle tone, postural tone)

1. Alignment The ideal alignment? The ideal alignment in stance allows the body to be maintained in equilibrium with the least expenditure of internal energy Practical trial. Stand with your feet shoulder distance apart for 1 minute. Try leaning forward and backward a little then so far forward or backward as you can without taking a step, so far forward and backward you have to take a step . Come up on your toes and do the same thing. Partner puts three fingers on your sternum and nudges you in a backward direction, first gently, later with more force.

Practical trial continued: during quiet stance – perfectly still/movement? Direction you feel yourself swaying most? Active sway – movement strategies to control body sway? Movement strategies to react to nudges from partner?

Discuss how those strategies change as a function of (a) size of BOS (b) speed of movement (c) centre of mass relative to the BOS List the muscles you think were active to control sway in these conditions (a) What muscles did you feel working to keep you balance when you swayed a little (b) What muscles work when you swayed further (c) What happened when you leaned so far forward that your COM moved out of BOS of feet

2. Muscle tone Muscle tone refers to the force with which a muscle resists being lengthened, that is stiffness Clinically tested by flexing and extending a relaxed patients limbs and feeling the resistance offered by the muscles. A certain level of muscle tone is present in normal conscious, and relaxed person

Postural tone When we stand upright, activity increases in antigravity, postural muscles to counteract the force of gravity; this is referred to as postural tone. Clinical literature: postural tone major mechanism in supporting the body against gravity. Postural control in the trunk segment is the key element for control of normal postural stability in the erect position Research show that many muscles in the body are tonically active during quiet stance – soleus and gastrocnemius – line of gravity slightly in front of knee and ankle, tibialis anterior – body sways backwards; gluteus medius and tensor fascia latae, illiopsoas – prevents hyperextension of hips, thoracic erector spinae in the trunk (along with intermittent activation of the abdominals – because line of gravity falls in front of spinal column

Movement strategies during perturbed stance Displacement of supporting base – movement strategies to recover stability Movement patterns used to recover stability after COM displacement in sagittal plane? Feedback control: Postural control that occurs in response to sensory feedback. Examples? Feedforward control: Postural responses that are made in anticipation of a voluntary movement that is potentially destabilizing in order to maintain stability during movement. Examples?

Movement strategies during perturbed stance Muscle synergies? Anteroposterior stability: Ankle strategy Hip strategy Stepping strategy Practical activity.

Movement strategies during perturbed stance Mediolateral stability Multidirectional stability Practical activity. Adapting motor strategies – Postural control under changing task and environmental conditions requires that we modify how we move when maintaining stability in response to new demands. Adaption is a term that reflects the ability to modify behavior in response to new task demands.

Anticipatory postural control Activity CNS’s anticipation of what the task requires Postural muscle activation patterns

Practical activity - Work with a partner. Tape a ruler vertically to the wall near where you are standing. Stand with your arm outstretched, at about waist height, palm up. Place a heavy book on your outstretched palm and have your partner note the vertical position of your hand on the ruler. Ask your partner to lift the book off that hand. Have your partner note the movement of your hand in this condition.

What did the hand holding the book do when your partner lifted the book? Was it steady? Or did it move upward as the book was lifted off? How much did it move? What happened when you lifted the book yourself? Was it steady? How much did it move?

4. In which of these two conditions is there evidence for anticipatory postural adjustments? 5. What was necessary for the anticipatory postural adjustments to occur?

Clinical application of anticipatory postural control When subject are told to move as fast as possible versus a comfortable speed, anticipatory postural control tend to be earlier and more reliably activated. Weight of a load to be moved also influences anticipatory postural muscle activity – the more heavier the load, the more likely it is to engender instability and thus anticipatory postural response.

The degree of practice has also been shown to influence the timing of anticipatory postural adjustments. Practicing tasks that require anticipatory postural activity may increase the efficacy of this component of postural control over time. Support to a patient who is struggeling with postural control – functional movements of limbs improve.