From as far back as ancient Greece two key observations were recognised; Posture is maintained by tonic muscle contractions acting against gravity; Lesions of the central nervous system result in often profound changes in posture.

Posture is managed by some of the most ancient parts of our brain and is often poorly understood, thankfully when looking at the balance system we can break it down into three different components; sensory, central processing systems and the motor/muscular systems.

Information from out sensory system comes in, integrates in our central nervous system and then motor commands are sent out, allowing us to stand with as little sway as our graceful 4-legged friends (see Standing Balance) despite being handicapped with only 2 legs.

Our marvellous balance requires no conscious effort is based on an overlap of multiple systems, reflexes and strategies.

Part 1) The sensory system;

1. The proprioceptive system consisting muscle, joint and cutaneous receptors. This provides a huge amount of information (think big data, just bigger) with joint positions, muscle lengths, muscle activity, temperature, contact surface condition, pressure and any nociceptive stimulus. This data from all the receptors of joints, muscles, ligaments, tendons, and skin are firing to the central nervous system all the time and integrated at multiple levels.
2. The ocular system provides information about both the position of the body through the amount of torsion of the eyes to maintain a level horizon, and about the environment itself including potential future perturbations (That car is definitely going to perturbate me if I keep practicing my static balance on the road)
3. The vestibular system gives information about the position of the head with respect to both gravity and feedback about movement

By using so many different receptor systems, balance can be maintained even when one system fails or deteriorates. It is also helpful letting you calibrate and verify truth when the sensory input is conflicting.

Part 2) Integration:

The integration of balance ranges in complexity throughout the entire neuraxis, there are cord based monosynaptic reflexes, multi-level/multi-neuronal spinal cord reflexes, cerebellar circuits, brainstem circuits and cortical pathways all interacting with each other.

The vestibular nucleus is a primary integration centre for balance, combining inputs from the body, eyes and vestibular system and sending efferent commands to muscles to maintain posture. The vestibular nucleus also is in constant communication with the cerebellum, which corrects, refines and adjusts the vestibular motor outputs and all the other motor outputs of the pyramidal system.

Part 3) The Motor System;

1. Posture – from tonic contractions of postural muscles;
   • Cortical output; supresses the red nucleus and drives extension in the upper limb and flexion in the lower limb
   • Rubrospinal tonic activity increases flexion in the upper limb
   • Olivospinal and vestibulospinal tonic activity increases extension in the upper and lower limbs
2. Extrapyramidal reflexes; inputs that reach the various brainstem nuclei have an immediate reflex response based on their central integrative state. Using these reflexes, we are able to adjust to our environment very quickly without having to consciously think about the appropriate action.
3. Cortical output regarding motor planning and activity specific goals, this is done via the pyramidal tracts for volitional movement, extrapyramidal tracts for modulation of sensory inputs and reflexes and with modulation from the cerebellum.

The complex arrangement of inputs and outputs forms a branching tree of reflexive behaviours and strategies for managing balance. During quiet standing where the centre of pressure oscillates only to a small degree, sway is managed primarily through positioning of the ankles, called an ankle strategy. In this situation as you sway forward, the movement is noticed, and reflexively you tighten posterior muscles in the leg (calf muscles) pushing backwards and stopping your forward travel replacing it with a posterior sway. The sway backward is then noticed, and reflexively you tighten anterior muscles (tib and fib anterior muscles) pushing you forwards and stopping your backward travel……. This cyclic pattern of sway forward and backwards and left and right can be then modelled like an inverted pendulum.

Because the muscles around the ankle are relatively small given their distance to your centre of mass they are only able to accommodate small perturbations to maintain balance. If the perturbation is bigger, we need to involve bigger muscles.

The bigger muscles are the hip muscles and “cough” originally named a hip strategy, activation of the hip flexors vs extensors giving the anterior and posterior control and left vs right via the adductors/adductors. Because the hip muscles are larger and closer to the centre of mass they have a bigger impact to the centre of mass and are activated when an ankle strategy alone is not enough.

The next “step” in maintaining standing posture becomes dynamic with a stepping strategy. The stepping strategy occurs when a large perturbation moves the COP moved beyond the base of support, so you move the base of support to catch yourself. If the perturbation is too large or unnoticed then you fall over, there are a host of new reflexes driven by the same balance systems to minimise injury as you fall or to try and catch yourself, this may involve raising the arms to protect your vital structures, or to catch something to break the fall.

If we compare our bipedal posture to our closest great ape relatives (gorillas and chimps) you can easily see that ours is the only comfortably upright posture. This has many advantages, and the advantages must have outweighed the costs for us to be here; it frees the arms for other tasks; lets us see further into the distance; and lets us wade into deeper waters. The trade-off is related to balance, our centre of mass is higher, our base of support is smaller, and if we fall are more likely to injure ourselves. This position is inherently less stable. However, with the past 4-5 million years of evolution we have developed a wonderfully complex balance system that just works, no concentration required. This system is made from: millions of sensory receptors coming from your joints, muscles and skin; multiple integration levels within the spinal cord, brainstem and then also cerebellum and cortex. All these systems work to maintain balance, to make predictions on how you are going to move and to calibrate one system verses the others. All this work happens without any conscious oversight and when its working well the amount of sway is barely noticeable. It protects against falls and injury throughout life while allowing easy and graceful movement in an uncertain world.