The state of balance is maintained through complex postural control mechanisms which are reliant on adequate sensory input. The principal input systems to balance are the vestibular system (S2.10), the visual system (S2.10) and the somatosensory systems (S3.23) (Massion et al. 2004). Visual input provides a reference for upright vertical but is also essential in predicting forthcoming threats to balance from the environment. The somatosensory system, primarily proprioception, provides a reference for the body’s position in relation to the supporting surface and to other body parts and finally the vestibular system provides a reference for head position and movement of the head in relation to gravity.

In general this sensory input has two main functions:

Sensory feed forward which allows a preparation for movement. These anticipatory adjustments require input from both the internal and external environment and are an integral part of every movement. They are strongly linked to previous experiences.

Sensory feedback which allows ongoing regulation and appropriate muscle adjustments to be made in response to planned and unplanned displacement of the COM during movement.

Integration

Integration of this sensory information for an appropriate motor response is facilitated by various higher centres, including: the brain stem and cranial nerves (S2.10); the cerebellum (S2.12); reticular formation (S2.10) and the cerebral cortex (S2.7). The amount of cognitive processing by the cortex for postural control is usually minimal, with its contribution depending on the complexity of the task and the capability of the individual’s postural control system. For example, postural control relies on accurate sensory input, but also on the ability of the central nervous system (CNS) to attend to the relevant sensory cues and to prioritize or weight the input according to its relevance to the context and task (Horak 2006). The CNS also intervenes if a sensory conflict exists when it must weight the sources and reject the potential source of error (Karnath et al. 2000b).

Motor output and balance

During movement there is inevitably a displacement of the COM in relation to the BOS. This occurs whether the movement involves the trunk, the upper limb, the lower limb, turning the head or simply breathing. What determines whether the displacement leads to a fall is the motor response by which balance is recovered. These motor adjustments are flexible and varied and dependent on the task, the environmental context and the individual. An appropriate motor response requires a certain level of muscle strength, endurance and an available range of movement (Cholewicki et al. 1997; Ebenbichler and Oddsson 2001; Hodges and Richardson 1997) but also fine grading of agonists, antagonists and synergistic muscles, appropriate co-contraction and a high level of reciprocal innervation.

Although postural control mechanisms are varied, certain patterns of activation are described, however in an adult these are integrated into movement and may not always be evident:

Movement strategies

An individual can slow down the displacement of the COM by rapidly generating muscle torque at the ankles, hips or other joints around a fixed BOS. For anteroposterior displacements of the COM the predominant torque is generated via an ankle strategy (Fig. 32.1) although a hip strategy (Fig. 32.2) may also be present. For a mediolateral displacement, the hip strategy is dominant (Maki and McIlroy 2006). These strategies occur in a feed forward manner to maintain balance and are continuously interchangeable during movement. However, if the perturbation is large and these strategies are unsuccessful, the individual may actually change the BOS by stepping (Fig. 32.3) or using an outstretched arm. Although the latter response is often referred to as protective it can also occur during small perturbations as a normal strategy for balance.