Age-Related Mobility Limitations
Due to the demographic changes in the western world, healthy aging becomes an important issue on the individual as well as on a population level. Between 2015 and 2050, the proportion of the world’s population over 60 years will increase from 12 to 22%. The WHO defines healthy aging “as the process of developing and maintaining the functional ability that enables wellbeing in older age. One important component of healthy aging is mobility. Self-reported mobility limitations are frequent among older persons but this prevalence varies due to different concepts and models. Nevertheless, mobility limitations are increasingly commonplace in older persons affecting approximately 35% of persons aged 70 and the majority of persons over 85 years. Mobility limitations have been associated with an increased fall, disability, hospitalization, and mortality risk as well as decreased quality of life, and poor psycho-social health next to declining function .
It is commonly understood that a low physical activity level has negative impact on health, and is responsible for many chronic diseases. A low activity level has been linked to sarcopenia and to mobility limitations. With regard to the fact that most people in the western world over 65 years do not meet the recommended physical activity level for healthy aging, this area seems mandatory to include in preventing mobility limitations. Decreasing sedentary behavior or inactivity by maintaining mobility in older persons is, with regard to independence, mortality, and health in older persons, a priority on an individual as well as a population level.
As mobility is such an important factor, two important issues occur: (1) early identification of mobility limitations in older persons and (2) the installation of effective interventions to modify or even reverse mobility limitations.
This narrative review will address on a broader base the mechanism and risk factors for mobility limitations, possible screening methods, and, briefly, on effective interventions.
As addressed above, mobility includes several domains ranging from physical, cognitive, and neuromuscular to psychological domains. On a physical level, gait, balance, and strength play an important role. In the neuromuscular domain, changes in the motor units are important, and in the cognitive domain, age-related changes are relevant as well as psychological factors such as fall-related psychological concern (FrPC)
Age-Related Balance and Gait Changes
Postural control includes two domains: (a) static (balance) and (b) dynamic (gait) components. In the static condition the center of mass remains between the base of support whereas in gait the center of mass as well as the base of support shifts
Balance Changes with Aging
With aging, postural control in the static condition, or balance, is influenced by the visual, sensory, and vestibular systems. The decline of the sensory system occurs with increasing age and results in balance instability and gait limitations. Sensory feedback is necessary for balance control in the light of different environmental circumstances, e.g., different light situations such as sun or shadow, or traffic situations, e.g., sound recognition or localization. Sensory feedback in static balance is necessary to reduce sway movement, e.g., in a situation where the room lights suddenly turn off, the upright position of the body needs feedback from other sensory systems.
A sensory decline occurs with aging especially for vision and hearing. As declining hearing abilities as well as impaired vision have a negative impact on mobility, in addition it also has an impact on quality of life in older persons. The study by Pinto et al. in a US population demonstrated that no single sensory impairment had a negative effect on mobility –measured with the Timed-up and Go test (TUG) – but a global sensory index showed significant effects on mobility.
Gait Changes with Aging
In general, mobility limitations are characterized by temporal or spatial gait changes in numerous studies. In addition, gait has been used as a marker for physical function, as a predictor for falls, and even mortality. Gait is a highly complex process that is influenced by the central/peripheral nervous system, muscular skeletal changes, and by brain changes, e.g., the basal ganglia or the motor cortical regions.
One of the most used variables in aging research is gait speed. Gait speed can be measured in self-selected (often describes as “normal” or “usual”) gait and in fast gait speed to identify resources. Other gait variables are stride length or width and cadence.
Early research by Winter et al. demonstrated a significant reduction in gait speed due to a shortened stride length in a study comparing self-selected gait in younger and healthy older persons. These early findings were later confirmed by Ko et al. adding the age-related changes in step width to the earlier findings. As usual, gait speed had been related to mortality by showing an 89% increased risk in mortality in older persons with the slowest gait speed. It has been suggested that gait is the “6th vital sign” in geriatrics . A recent review by Herssens et al. confirmed the decrease in step length and step time, as well as an increase in stance with aging. In conclusion, evidence now exists that proves that spatio-temporal parameters in gait decline with age. In contrast, the question of any gender specific differences in gait changes with aging is less investigated.
In their study cohort, the normal gait speed was stable in persons up to 65–70 years whereas the fast gait speed performance started to decline even after the age of 40–50 years.
Another important marker is the walking speed reserve, which is calculated by the difference between fast to normal gait speed . In a daily routine, catching the bus or keeping up with peers can require reserve capacity in gait speed. The walking speed reserve marker can become more important in the future. In the study by Callisaya et al. about 12.8% of the participants could not increase their gait speed to the level for a safe road crossing speed.
Age-Related Neuromuscular Changes Associated with Mobility
On a muscular level, a change in muscle fiber sizes occurs with aging. It is commonly understood that the type II fibers (fast twitch) are especially affected – they are responsible for generating the power in chair-rise performance. Next to the reduction in muscle fiber size, other age-related changes are coming into the focus, e.g., the role of motor units (MU).
Motor units are responsible for the organization of neural control in any muscle. The MUs are composed of a single alpha motor neuron and the connected muscle fibers. The number of MUs can be estimated with the motor unit number index. At present, in the neuro-centric approach the loss of MUs is responsible for one pathophysiologic pathway of sarcopenia. In addition, there is an increase in the size of the surviving MUs (meaning an increased number of innervated muscle fibers per alpha motor neuron) reported to compensate for the MUs loss. The compensation and remodeling process can lead to the re-innervation by axonal sprouting from other motor neurons. Next to this process, a greater variability in MU discharge is reported. A variety of firing rates, in muscles with reduced MU (up to 30–40%) are reported during maximal isometric contractions .
Clark demonstrated that neural activation of skeletal muscle is a key component for muscle weakness. Other processes of interest are impaired voluntary muscle activation and/or increased antagonist activation.
In conclusion, morphological and physiological changes to MU due to aging is followed by alterations in the discharge properties of the MU.
Age-Related Changes in Muscle Mass, Strength, and Power
Muscle performance declines with age. Muscle performance is linked to muscle mass, muscle strength, and muscle power. Muscle strength is the ability to generate maximal muscle force whereas muscle power refers to the product of force and velocity of the muscle contraction.
Muscle mass and strength have their peak on average in the third decade of life and slowly decline afterwards. Changes in muscle mass occur due to fat infiltration and loss of muscle fibers. Interestingly, evidence is accumulating that the loss of muscle mass and muscle strength deviate with advanced age. Muscle strength declines faster compared to the loss of muscle mass with a reduction of about 3 vs. 1% in older age. The loss of muscle mass can reach about 40% in persons older than 80 years but this decline can be modified by gender and lifestyle behavior. Furthermore, the national US sample by Chen et al. revealed that women have lower muscle mass and lower strength compared to their male counterparts.
Sarcopenia has formerly been recognized solely as the loss of muscle mass. Due to the evidence that the loss of muscle mass is not congruent to the loss of muscle strength, the new definition of sarcopenia includes muscle mass and muscle quality with strength and gait parameters . Based on the understanding that both muscle mass as well as strength have an impact on physical function, in the present sarcopenia definition, gait speed is included as a marker of physical performance . Research has shown that sarcopenia is related to impairments in physical function, e.g., mobility limitations, and negative health outcomes as well as hospitalizations or falls.
The impact of muscle power on mobility in older age has been investigated with chair-rise or stair-climbing performance. With regard to strength decline, it seems that muscle power deteriorates on a faster slope. Reid et al. demonstrated in a longitudinal study that muscle power in their study population declined by about 3% per year. The rate of decline in muscle power seems to be 10% greater than the loss of muscle strength. Through research regarding muscle power and its impact on mobility limitation, a picture emerged that suggested muscle power has a higher impact on mobility than muscle strength. Research demonstrated that low leg power leads to a 2 to 3-fold higher risk of mobility limitation. Bean et al. revealed in their study that leg power was more related to reduced gait speed and chair-rise times then leg strength.
In conclusion, the role of muscle mass alone in mobility is less important than the role of muscle strength. Muscle strength only partly contributes to mobility but it especially contributes if strength is reduced in the lower extremity and when falling below a “threshold” it contributes to mobility limitations .
Through research regarding muscle power and its impact on mobility limitation, a picture emerged that suggested muscle power has a higher impact on mobility than muscle strength. Research demonstrated that low leg power leads to a 2 to 3-fold higher risk of mobility limitation. Bean et al. revealed in their study that leg power was more related to reduced gait speed and chair-rise times then leg strength.
In conclusion, the role of muscle mass alone in mobility is less important than the role of muscle strength. Muscle strength only partly contributes to mobility but it especially contributes if strength is reduced in the lower extremity and when falling below a “threshold” it contributes to mobility limitations.
In the last two decades, research has demonstrated that gait is no longer an “automatically controlled” but a cognitive influenced process.
Emerging evidence shows that a decline in gait speed can predict cognitive decline by more than a decade
Several studies have demonstrated that chronic diseases are a risk factor for mobility limitations. In the Twin study by Kujala et al. about 23.2% of their participants reported mobility restriction by a disease. Most commonly were musculoskeletal (60.2%), followed by cardiovascular (18.8%), and neurological disease (7.7%). neurological diseases such as dementia or mild cognitive impairments are related to mobility limitations
Factors having an impact on muscle performance (muscle mass, strength, and power) are complex and multifactorial. Numerous research papers have described risk factors for sarcopenia (the loss of strength, muscle mass, and muscle performance) including a decreased anabolism pathway with sedentary lifestyle, bed rest, malnutrition, anorexia, age-related hormonal changes, and aging. An increased catabolism pathway fostered by disease, injury, inflammation, oxidative stress, mitochondrial dysfunction, and an increase in myostatin also adds to the risk of sarcopenia
Sedentary Behavior as a Risk Factor for Mobility Limitation
Research has demonstrated that older persons are more prone to sedentary behavior
possible assessments for mobility have including performance measures such as the well-known TUG, the Short Physical performance Test (SPPB), or the different walk tests
Self-reported mobility ranges from simple questionnaires to life-space mobility
As strength, balance, and gait are important components for mobility the SPPB is an excellent tool with good psychometric properties and is often used in research as well as in the clinic to identify mobility limitations or functional decline
Gait can be measured by a stopwatch and different lengths (ranging from 4 to 10 m to the 400 m walk) or with extensive technology. To show subclinical gait decline, the dual task paradigm is recommended.
Exercise Intervention on Mobility
One of the most effective interventions in counteracting mobility limitation is exercise. Taking into account the physiological risk factors, it seems evident that an exercise intervention is based on strength, gait, and balance. The most effective interventions have addressed the muscle pathway by strength training exercises or in combination with balance and gait exercises
exercise intervention was individualized and tailored with regard to intensity including supervised and unsupervised sessions. The intervention was provided for between 24 and 36 months
One of the most disseminated and effective exercise programs is the OTAGO program developed by Campbell & Robertson in New Zealand. The first intervention was a home-based strength and balance exercise accompanied by a walking activity in women aged 80 years and older. Several other interventions with the same components (balance and strength exercises) replicated the positive effects on physical function, reduced fall rate, and other health outcomes
A common component of the above-mentioned exercise programs is the structured format with increasing intensity and standardized repetitive exercises and the supervision and social feedback by experienced trainers. However, long-term adherence without this tight monitoring is questionable. Therefore, new concepts are developed to integrate exercise into daily routine. One approach would use a daily walking routine, e.g., walking to the store whereas another approach would integrate functional exercises to help improve balance and strength in the daily routine. Integrating training exercises into a daily routine seems to have several advantages: requires no additional time to perform the exercise, includes a relationship to the daily routine (balance exercises, e.g., semi-tandem during cooking or washing), and improvements are linked to the daily routine thus enhancing motivation and compliance