Ageing and Postural Stability

Approximately 30 percent of the elderly population sustain a fall each year, constituting an important health problem [16]. Falls have different immediate consequences such as hip fractures, and long-term consequences, ranging from restriction of activity to institutionalization [16, 21]. Postural instability is strongly associated with falls and is the best single predictor of falls. Postural reactions in response to external perturbations are essential for maintaining equilibrium [13]. From the theoretical point of view (Lapunov’s stability theorem) stability is a feature of any system in which sufficiently small changes in the input parameters result in limited changes of the output. Consistent with this definition is the assumption that a stable postural control system is characterized by its insensitivity to changes in the input (in this case, sensory) values that mediate its reactions. The process of aging is associated with a well-documented decline in the integrity of many physiological systems that participate in the control of postural stability [12, 17, 25]. Age-related neurodegenerative changes in the neuromuscular control and decreased resolution of the sensory inputs result in the sensory signals which are contaminated with greater noise and physiological delays as compared with young healthy subjects [4, 6, 23]. Additional time and attention resources need to be allocated to the postural task when there is a reduction in the available sensory information [12, 14, 19]. In spite of its very different courses the ageing process takes place in each individual. Generally it can be stated that the stability of posture declines with age. This deficiency is due to increasing deficits in the control system including lower sensitivity of the sensory inputs, slowness of central processing and the decreased power of the motor output. These age-related deficits result in compensatory modification of the control. For example, the standard recovery strategies used in normal subjects are modified and adapted to a new physiological context. Such a functional adaptation to somatosensory and vestibular loss has been well documented [12]. These authors showed that sensory deficits altered the type of postural response selected under a given set of conditions, e.g. vestibular loss resulted in a lack of hip strategy, whereas this strategy was increased in the case of somatosensory loss. The selection of a balance recovery program depends on many factors such as the availability of sensory information as well as physiological and environmental context. In the case of greater balance perturbations, the only potential strategy to recover equilibrium is to take a step or grab onto a stable object, since the ankle or hip strategies might not be successful any more. The relative strength of the perturbation and the selection of an appropriate corrective strategy depend on the quality of postural control, which declines dramatically with age [12], whereas the effectiveness of ankle and hip strategies depends on stance symmetry. The tendency for the step strategy is evidenced by limb load asymmetry, which helps to initiate a step [10]. Complex impairment of the postural system in the elderly increases the relative strength of a postural perturbation. Thus, one could expect a preference for the step strategy to increase with age as indicated by an asymmetrical limb load distribution.