Posture is defined as the position of the body in relation to gravity. Maintaining posture, also known as postural control, requires keeping the body’s center of mass over its support surface. The support surface is the area of the body in contact with the ground, such as the feet while standing.
Challenges to Maintaining Posture:
- Shifting Center of Mass: When the center of mass moves away from the support surface, maintaining posture requires more effort. This can happen by leaning forward, backward, or to the side.
- Smaller Support Surface: A smaller area of contact with the ground, like standing on one leg, makes balancing more challenging.
- Elevated Center of Mass: Holding weight overhead raises the center of mass, making it harder to stay upright.
Muscles and Posture:
- Muscle Contractions: The body uses muscle force generated by contractions to maintain posture. Core muscles play a crucial role in stabilizing the body when the center of mass is displaced.
Applications:
- Exercise Regimens: Programs like Pilates emphasize core muscle activation to improve balance and posture control.
- Advanced Movements: Gymnasts and dancers require exceptional core strength and muscle control to maintain balance with their center of mass far from the support surface.
Automatic Movements - Maintaining Posture
Postural Sway:
- Our bodies are constantly swaying slightly, even when we feel perfectly still. This is called postural sway.
- The sway is slow, with a frequency of about 0.5 Hz (once every 1-2 seconds).
- Babies learn to control postural sway within safe limits to prevent falling.
The Stretch Reflex and Muscle Tone:
- The stretch reflex in muscles like the soleus helps maintain posture by automatically contracting when stretched.
- Maintaining an upright posture is challenging because our center of mass is high, making us top-heavy and prone to tipping.
Anticipatory Adjustments:
- We subconsciously adjust our posture to anticipate movements that might shift our center of mass.
- Examples include:
- Bracing before lifting an object.
- Leaning into a wall to avoid falling when reaching to the side.
- Activating leg muscles before pulling a lever to prevent being pulled off balance.
- Increasing muscle tone before throwing a punch to maintain balance during the movement.
Feedforward vs. Reflexive Mechanisms:
- Postural control mechanisms are feedforward, meaning they anticipate potential problems and make adjustments before they happen.
- This is in contrast to reflexes, which are corrective and respond after a disruption occurs (e.g., the stretch reflex).
The Brainstem’s Role:
- Brainstem motor control centers are responsible for the amazing feat of feedforward postural control.