| Body movements during postural stabilization.: Measurements with a motion analysis system. | ||
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Corrective movements are needed to keep the centre of gravity within the base of support. To achieve this goal, co-ordination of the sensory, skeletal muscle and central nervous systems is needed. The parts of the postural control system are presented in Table 2.
Table 2. Postural control systems (Adapted from Era 1997).
| Sensory system | Skeletal muscle system | CNS |
|---|---|---|
| Vestibular system located in the inner ear (semicircular canals, otholiths, maculaes) | Muscles of the upper and lower extremities | Stretch reflex |
| Vision (retina) | Trunk muscles | Long-loop reflexes |
| Proprioceptive system (muscle spindle-type I and II, Golgi tendon organ, joint receptors) | Neck muscles | Preprogrammed reactions (Learned skills) |
| Cutaneous receptors | Synergistic action |
The basic idea of sensory systems is to provide information to the system concerning its own state and that of its surroundings. The information is transferred from the sensory receptors to the CNS via afferent pathways. Sensory receptors convert energy of various forms, such as light, pressure, temperature, and sound (Enoka 1994). The main types of sensory receptors needed in postural control are presented in Table 2.
Visual information is delivered from the retina to at least two different locations in the brain, and these pathways of information have been assumed to be specialised for different purposes; the focal system for object identification and the ambient system for movement control (Trevarthen 1968, Schmidt 1991). The latter has also been shown to affect strongly both stability and balance (Lee & Aronson 1974).
Vision is important for postural control, but it can be compensated for by other information sources (Brandt et al. 1986). Vision seems to influence balance by reacting to motion as a relative image shift on the retina (Brandt et al. 1986), and it also triggers the muscle activation required for postural corrections. The efficiency of vision in postural control depends on visual acuity (Paulus et al. 1984), visual contrast (Leibowitz et al. 1979), object distances (Brandt et al. 1986) and room illumination. The visual system control of balance is best when the visual distance is less than 2 m (Brandt et al. 1986). It has been reported that when the horizon is manipulated so that the vestibular and visual cues are mutually contradictory, elderly persons place more reliance on their visual cues than younger people (Pyykkö et al. 1988)
The semicircular canals respond sensitively to velocity changes of movement at frequencies from 0.2 to 10 Hz, and they have hence been found to be active at the beginning and end of movement, whereas the otholiths operate at low frequencies of less than 5 Hz and provide information of linear acceleration, e.g. gravity (Markham 1987, Toppila & Pyykkö 2000). The information from the otholiths and semicircular canals is conveyed to the vestibular nuclei in the brainstem, which also receives information from other sensory sources. The vestibulo-ocular reflex stabilizes vision by producing the eye movements into opposite direction upon turning of the head (Baloh et al. 1993), and the main goal of the vestibulo-spinal reflex is to stabilize the head and the body. Although it is known that the vestibular system may contribute to the perception of body orientation and thereby be involved in postural control, some studies have shown that the vestibular system does not play an important role in the perception of sway during normal quiet stance (Fitzpatrick & McCloskey 1994).
The somatosensory system provides information related to body position by proprioceptors and exteroceptive receptors. The proprioceptive receptors are located in muscles, tendons and joints (Enbom 1990, Jäntti 1993), and they give information about the position of the limbs and the body and the distension of the respective muscles. Proprioceptors include muscle spindles (type Ia and II), Golgi tendon organs (Ib) and joint receptors (McComas 1996). Exteroceptive information is derived from different type of pressoreceptors on the foot sole. Exteroceptive receptors are located in the cutaneous and subcutaneous tissue (Johansson & Vallbo 1980). The major types of cutaneous receptors are Meissner corpuscles and Merkel disks, located closest to the skin surface, and Ruffini ending and Pacinian corpuscles, located deeper in the skin (Latash 1998)
While the receptors in joint capsules give information about the movements and positions of the body parts relative to each other, their role in postural control has not been fully defined yet. The muscle spindles give information about the changes in muscle length and tension (dynamic stretch), and they can also be activated by passively stretching the entire muscle. In addition to an afferent system, the intrafusal fibers in the muscle spindles also receive an efferent input via γ -motoneuron (Enoka 1994). The pressoreceptors detect the body sway, whereas the mechanoreceptors can determine both the site and velocity of an indentation of the skin, as well as acceleration and pressure changes (Johansson & Vallbo 1980, Magnusson et al. 1990).
There are some essential inputs for postural control during stance produced by proprioception. First, the information from ankle joints should be recognized, as it is effected by the movement of the centre of gravity, resulting in torque around the ankle joint. Second, the information from the neck muscles gives important references concerning head movement in relation to the trunk. And third, it has been suggested that the eye muscles reflect the eye position in relation to the head (Spirduso 1995).
Although the calf musculature is activated first to provide postural control during body movements (Nashner 1983), the co-activation of certain “prime postural muscles”, such as the neck muscles, the hamstring musculature, the soleus and supraspinalis muscles, occurs in this order (Nashner 1983, Johansson & Magnusson 1991). Apart from these, however, several muscles participate in producing both reflective movements with different latency times (Nashner 1983) and voluntary movements to balance the body position. Whenever the muscles are stretched, the proprioceptive receptors within the muscle and tendon signal the change in muscle length to the central mechanism of the postural control system (Prochazka & Wand 1980, Spirduso 1995).
Postural control requires coordinated muscle action (Johansson & Magnusson 1991), for example, to produce adequate muscular contractions (Era et al. 1996). As the muscles act about the joints in balancing the body, especially the role of the ankle, knee and hip joints is essential (e.g. movement strategies page 14). According to the passive stiffness control model, ankle stiffness, as a result of the CNS being limited to the selection of appropriate muscle tonus, stabilizes the unstable mechanical system in quiet stance (Winter et al. 1998). However, other researchers have pointed out the active mechanism of postural stabilization in balanced stance (Johansson et al. 1988, Morasso & Schieppati 1999), where the muscle and foot skin receptors play an essential role (Morasso & Schieppati 1999).
Several parts of the central nervous system (CNS), which consists of the spinal cord and the brain, take part in controlling posture. Input signals to the cortical neurons come mainly from the thalamic nuclei, which transmit information from the spinal cord, basal ganglia and cerebellum and from the parietal and frontal areas of the cortex. The first and fastest response to a change in stance is triggered by spinal reflexes (e.g. Allum & Keshner 1986). The excitation within the CNS is mediated through the synapses consisting of afferent fibers on neurons, whereas the inhibitory synapses use special mediators, interneurons. For example, reciprocal Ia inhibition is evoked by the low-threshold afferents in antagonist muscles (e.g. Tanaka 1983), and recurrent inhibition of motoneurons is meditated by interneurons called Renshaw cells (e.g. Pierrot-Deseilligny et al. 1983).
The voluntary movements needed for balancing posture are planned within the brain. The output commands are sent to the muscles via the pyramidal and extrapyramidal systems. The pyramidal cells, with their connections to the premotor and parietal cortex, transmit information to the spinal motoneurons and interneurons, which control voluntary movements and the segmental reflexes needed for balancing posture (Jäntti 1993) The output of the cortical motor areas also includes projections to the basal ganglia, cerebellum and red nucleus. The basal ganglia, which constitute the major component of the extrapyramidal system, consist of the substantia nigra and subthalamic nuclei. They are connected to three nuclear groups (caudate nucleus, putamen and globus pallidus). The basal ganglia and the nuclear groups take part in the facilitation and planning of both voluntary and reflex movement during postural control. The cerebellum and its connections are responsible for the co-ordination and smoothing of the reflex movements and the regulation of voluntary movement.
To ensure proper postural control, the sensory influx must be integrated in the CNS to provide adequate motor output. The sensory information from the visual, vestibular and proprioceptive and exteroceptive systems is used as input. However, it is suggested that only one of the three main sources of afferent input is necessary to ensure balance in non-complicated circumstances (Rothwell 1994). Forget and Lamarre (1990) demonstrated that even in the absence of peripheral feedback, postural adjustments may be adequate.
At the spinal cord, afferent impulses trigger stretch reflexes, whereas at the higher levels in the CNS, neural connections mediate more complicated motor responses. On the effector side, one important precondition for balancing is the ability to choose appropriate responses, to modify these responses on the basis of sensory input and, finally, to produce the needed muscular contractions to maintain posture (Era et al. 1996). The context-dependent responses that utilise all available sensory input and lead to preprogrammed motor responses are based on past experience.
Although it is impossible to differentiate reflective postural adjustments from voluntary movements in any reliable way, the reflexes can be differentiated by the time series (Allum & Keshner 1986). The role of reflex movements is to regulate muscle force, whereas the stretch reflex-based movements (automatic reactions) resist possible disturbances. Purposeful voluntary movements affect balance directly or indirectly, for example, when we move our COG with the intent of rising from a chair or opening a door (Nashner 2001).