1 Introduction

In the field of movement science, discussions about development and adaptive processes during aging often reflect a biased or negative perspective [1]. It is difficult not to focus on losses, since the progression of age ultimately marks the end of the cycle of life. The process of aging, as revealed by numerous research findings [e.g., [2]], affects structures and functions of the neuro-muscular-skeletal system in varying degrees. However, according to Sleimen-Malkoun et al. [3], aging also can be viewed as a nonlinear systemic reorganization of a biological system and subsystems (an individual), at complex multi-scale levels or dimensions and in multiple time scales.

Age-associated “frailty” is, in fact, a phenomenon that dynamically integrates the effects of an irreversible biologic process, a sedentary lifestyle, and comorbidities (previous and/or cumulative diseases or injuries). Although aging can be healthy, functional alterations occur in the relatively long time scale of the human life cycle, providing contexts and opportunities for the system to assume new and unique organizations. In general, changes due to aging result from interactions with the lifestyle an individual adopted in past years, as well as the incidence of deterioration associated with diseases. Interactions between aging and lifestyle are particularly noticeable in older individuals who are engaged in active athletic performance. They are found to be at low risk to cardiovascular diseases and obesity, for example [4, 5].

In the motor behavior literature, researchers commonly investigate older individuals’ skills as potential factors that negatively affect adaptation and often lead to loss of independence in daily life. Mobility, dexterity, postural control of the body, and the countless activities of daily living are dynamically integrated as functions that reflect the complex behavioral possibilities of the human body [6]. These possibilities evolve, become more or less sophisticated, and then decline with changes in the adaptive capabilities of the inevitable aging process.

The most prevalent themes of such investigations in motor behavior include posture and gait. Besides maintaining the upright position in humans, posture ensures that other motor skills can functionally exist and evolve. Postural control typically is investigated in quiet or standing positions and then compared to tasks in which perturbation or other demands are applied. Postural control interacts with gait performance continuously, and this is particularly evident in transitional skills (e.g., turning and twisting the body, shifting to backward locomotion, sitting to lay down, sitting to walking, and others). Bipedal locomotion allows humans not only to explore exterior spaces, surfaces, and surrounding objects (e.g., avoiding obstacles, going over objects, passing through apertures) but also to explore alternatives with regard to their own intrinsic dynamics. The architecture of legs, muscles, and neurons guarantees that walking has an invariant pattern among healthy humans, including older adults. Such structures undergo changes in time scales that range from an instant up to periods of years (i.e., multiple time scales). Furthermore, they integrate a multi-scale dimension (spatial) influence that limits behavior under perceived (i.e., informative) task constraints.

Balance and locomotion behaviors are extremely complex and, in addition to the coordination of multiple muscles and joints, require the use of multiple sensory information that help control the movements and that influence adaptations. In the older individual, the functional status of the muscles and joints, which helps detect information for control and adaptation to the environment, undergoes progressive changes. Falls are one of the main concerns for older people [7]. Studies in motor behavior attempt to identify risk factors that predict falls.

About one-third of noninstitutionalized older adults above age 65 experience a fall during a one-year period [8, 9], and most of these falls occur during locomotion [10, 11]. Among institutionalized older persons, this number increases to about two-thirds, or 61 % [12]. Traditionally, falls are related to increased variability in posture and gait [13]. However, variability also can be functional to older adults, and this is one of the topics we address in this chapter. Additionally, we discuss concepts from the dynamic systems approach and how these relate to changes in the posture and gait of older individuals. This chapter starts with a general overview of dynamic systems concepts and their application to the aging process. From a dynamic systems perspective, we discuss the notion of variability and its functional role on posture and locomotion control of older people.

2 The Dynamic Systems Approach and How It Can Explain Changes in Movement Patterns in Older Individuals

One of enduring challenges in aging research is to interpret the changes in performance in physical and mental activities that older people undergo. We often refer to a “decline” in order to explain the way in which our aging brain controls our limbs and movements. Research studies [2, 6, 8, 10] present much evidence about the extent of losses due to aging and of profile changes in bodily functions (e.g., control of the upright body position, mobility, and instrumental motor skills) . Researchers continue to identify differences in older adults as compared to young adults. However, conceptual paradigms have begun to emphasize new ways in which we can discuss adaptation: not just a comparative approach, but those that consider multiple factors that are unique to the process of aging. One such paradigm is the dynamic systems approach [3].

Complex systems theoriesFootnote 1 emphasize the heterarchical importance of multiple subsystems and their contributions to emergent behaviors [19, 20], whether these occur early in life’s development or throughout the lifespan, including the process of aging. These theoretical perspectives provide a unified approach to studying patterned behavior and, according to various scholars [21], can explain behavior patterns in a wide range of organisms and categories of actions with diverse constituent subsystems. When we apply dynamic systems principles to human development, some of the fundamental issues we need to address are: (1) How do changes in patterns emerge? (2) How do behaviors become more stable or less stable? (3) How do task constraints affect the stability and geometry of patterns of movements? (4) And how can diversity in patterns improve our understanding about the meaning of adaptation? For instance, the traditional understanding of motor learning and the development of movement skills is that emergent behavior associates with decreased variability. Conversely, increased variability is often discussed as a cause of impaired or limited outcomes and is associated with aging, disease, or impairment. However, variability is not a parameter outcome that consistently expresses the direction of development [22]. Low variability sometimes has been found among older individuals as a factor that limits a behavior’s flexibility and, therefore, affects adaptation .

Research on aging typically has explained declines in separate systems: neural, cognitive, sensorimotor, and muscular [3]. Sleimen-Malkoun et al. [3] have argued in favor of a conceptual framework that would help us understand “the general principles of age-related reorganization of the neuro-muscle-skeletal system” based on the phenomena of dedifferentiation and loss of complexity. They define dedifferentiation as “a process by which structures, mechanisms of behavior that were specialized for a given function, lose their specialization and become simplified, less distinct or common to different functions” [3] (p. 2). Loss of complexity is a transition marker for aging and is observed when there is a tendency toward more regular fluctuations in physiological parameters [23]. In other words, the system exhibits “less complex patterns of variability” during aging [3].

Vaillancourt and Newell [24] reviewed loss of complexity relative to the notion that complexity has been examined using individual parameters (e.g., less variable heart rate in older individuals). They observed that there are many sources of constraints that influence the biological system—those that sometimes increase, and those that sometimes reduce, variability. In fact, the contexts in which many behavioral as well as physiological events occur are the result of numerous interactions between intrinsic and extrinsic parameters that constrain the functional outcome. In the case of aging, the so-called limitation can be the result of more complex mechanisms and choices of strategies available to the organism. Therefore, the outcome pattern can be less flexible to task contexts because biological constraints could include an (already) established skill that has been interacting with progressive physical changes . For example, Ko and Newell [25] demonstrated recently that center of pressure (COP) complexity , as measured using multi-scale entropy and detrended fluctuation analysis in young and old adults, is dependent on task demand. In a constant task, participants were asked to match COP position over a target line displayed on a monitor by leaning their bodies forward. In this task, COP complexity was lower for older adults as compared to the young adults. On the other hand, for the dynamic task (target moved following a sine-wave pattern), COP complexity was higher for the older than for the young adults.

Thus, even though invariant patterns can be found in motor behavior repertories due to a system’s inherit flexibility, rapid behavior modulations can take place in response to subtle environmental changes (or new task demands). When these modulations go too far beyond a system’s preferred behavioral mode, transitions to new behavioral patterns can occur, whether resilient to context challenges or not. These concepts may help us to understand how motor behavior modifies with the aging process, showing regular and, at the same time, flexible patterns. Yet, the common understanding about adaptive phenomena reinforces our notion that older individuals are less adaptive than younger ones.

3 Variability and the Control of Posture

The study of postural control in older adults started with Sheldon [26], who was the first to investigate the effect of age in the control of body sway. He found that children swayed more than young adults and that body sway started to progressively increase after the age of 60. These findings have been systematically replicated in hundreds of studies about body sway in older adults since this seminal work. However, there is a great deal of controversy about the meaning of this increase in body sway. For some researchers, this increase is indicative of imbalance in older adults, and this could be related to incidence of falls in this population [10]. Others argue that this increase in body sway is not necessarily a problem and, through a dynamic systems perspective, could be seen as an exploratory phenomenon [22].

Posture is understood as the relative position of various body segments in relation to each other and relative to the environment. Thus, humans can take an infinite number of positions during daily and sport activities, such as standing, walking, running, and throwing an object, among others. The posture in which both feet are in contact with the ground indicates the position commonly known as bipedal upright posture. The control of posture is essential for successfully achieving motor actions and implies the control of body position in space, with the double purpose of orientation and stability [27, 28]. Postural orientation involves the proper positioning of the body segments relative to each other and to the environment. Stability during quiet standing involves the maintenance of the center of mass (COM) projection within the boundaries of the base of support.

Body sway is traditionally assessed based on the displacement of the COP obtained by force platforms. The resulting force applied to the ground due to the body’s weight and internal forces generates the ground reaction force (GRF, vector equal and opposite to the force applied to the ground). The resulting GRF originates at a point called COP. The COP and GRF reflect the effect of the postural muscles, activated by the control system to actively control the position of COM and, therefore, stability [28]. During upright standing, the control of COP displacement in the anterior-posterior (AP) direction is due to the activity of the plantar flexor and dorsiflexor muscles of the ankle joint. In the medial-lateral (ML) direction, the control of COP displacement is achieved by the loading and unloading mechanism related to the action of hip adductors/abductors [29].

The ground reaction force opposes the force of gravity, which, in turn, affects the acceleration of the COM to maintain support against gravity. At the same time, the maintenance of balance during quiet standing requires, also, the maintenance of the COM projection within the limits established by the base of support. In quiet standing, the body continually oscillates both in the AP and ML directions, which is called spontaneous sway [30]. Interestingly, the COP is in phase with the COM, indicating a stable temporal relationship between these two variables. Furthermore, the amplitude of displacement of the COP is greater than the COM. This relationship between COP and COM has led to the suggestion that the COM is the variable to be controlled by the postural control system, while the COP is the variable that controls the position of the COM [29]. According to this interpretation and based on an inverted pendulum model, the increase in the COP displacement is deliberate, as this allows an individual to change the direction of the COM movement, moving it away from the limits of the base of support. Winter et al. [31] showed that the difference between COP and COM position correlates negatively with COM acceleration, indicating that the position of the COP ahead of the COM, for example, accelerates the COM backward. In this way, COP keeps the COM within the limits of the base of support .

The prevalent idea is that an increase in body sway represents postural instability [32]. In this traditional approach, feedback provided by the different sensory systems is essential to control body sway and, consequently, reduce it [30]. Additionally, increase in body sway is seen as a consequence of increase in noise within the system. In a study that employed this traditional background, Maurer and Peterka [30] developed a feedback model of postural control in humans. Their model was able to reproduce sway behaviors that resembled those of young and older adults. The model is based on the assumption that any angular deviation from the upright reference position, detected by sensory systems, should be corrected by the body’s generation of appropriate corrective torque. Particularly important to this model is the addition of disturbance torque in the form of Gaussian noise to simulate spontaneous body sway. In this sense, noise would be responsible for the increase in nonfunctional aspects of the variability in COP excursions. Interestingly, in order to resemble spontaneous body sway in older adults, the noise level would have to increase by 50 % as compared to young adults. As suggested by the authors, this increase in noise level could be due to a degradation of the sensory system, as observed during the aging process. This would result in impaired information about body sway in this population. Thus, the increase in COP sway observed with aging is seen as a result of increased noise within the system and, consequently, as an unstable system . This traditional perspective treats increase in sway (or variability) as detrimental to the individual [22].

Although the results of the model developed by Maurer and Peterka [30] are consistent with a continuous feedback regulation control mechanism, they admit that their results do not prove the model is necessarily used by the control system. In fact, a dynamic systems approach based on nonlinearity and complex systems argues in favor of the possibility that variability is functional to the postural control system [22, 33].

Carpenter et al. [34] designed a study to test the relationship between COP and COM during quiet standing. They developed an apparatus to artificially “lock” movements of the COM without the participants’ awareness. They postulated that locking the COM would decrease COP displacement, which supports the traditional approach. However, their results showed a consistent and surprising increase in COP displacement after locking the COM. This finding contradicts the traditional approach, and the authors interpreted this increase in COP displacement as an exploratory role of postural sway. Even in the presence of visual feedback of COP or COM excursions in the locked condition, COP displacement increased [35]. Similarly, the provision of an explicit verbal cue that the participants would be “locked” did not reduce COP displacement [36]. In fact, COP displacement was again greater in the locked as compared to the unlocked condition, independent of the cue about being “locked” or not.

These authors argued that the central nervous system actively sways the body—using this as an exploratory mechanism to acquire sensory information. Thus, the information obtained through body sway would be essential to detect the position of the body relative to the limits of stability. This strategy would avoid adaptation of the sensory receptors while at the same time stimulate a wider range of receptors. Similarly, the use of stochastic resonance can boost the activation of muscle spindles [37, 38]. In a recent study, the use of stochastic resonance with participants who lightly touched their fingertip on a rigid surface resulted in additional reduction in body sway as compared to the traditional light touch paradigm without stochastic resonance [39]. According to Carpenter et al. [34], movement variability through postural sway can stimulate more receptors and an increased variety of sensory receptors (slow and fast adapting receptors). Yet, postural sway also can ensure that different sensory systems will be stimulated and provide converging and reliable information .

4 Older Adults and Their Strategies for Postural Control

As for the advocates of reduction in physiological complexity [23], the explanation about the effects of biological aging and/or disease on the postural system is that a particular parameter reflects the system’s reduced physiological complexity, and, therefore, its limited adaptive capacity [40]. In their study, older participants with poor visual acuity and impaired foot sole sensation exhibited the lowest complexity in postural sway.

An alternative explanation—that is, in dynamic systems terms—of why older people undergo so many changes as the years advance is that the complexity of a system is determined by both deterministic and stochastic influences . These influences interact from moment to moment (multiple time scales) with the ever-changing state (aging) of the system [24].

The context of a particular behavior can illustrate how older individuals respond to perturbations to their balance. When performing balance tasks, healthy older individuals can use exploratory strategies that compensate for disruptions as long as meaningful contexts create the need for adaptation and as long as the system has resources to resolve them (the perturbations). For example, haptic cues during the use of tools can be integrated into a balance task—as in haptic tasks Footnote 2—in order for these individuals to exploit the context of perturbation. The extent of postural perturbation is crucial for individuals to exploit the haptic task. In Fig. 1.1, a blindfolded 60-year-old adult reduces sway during a haptic anchoring task only when her foot position is sufficiently disruptive to her balance (i.e., the tandem position). When her feet are in parallel position, the COP path length reduces ~21 % during the anchoring task. However, when her body is anchored in tandem position, a reduction of 56 % is observed .

Fig. 1.1
figure 1

COP displacement of an older blindfolded individual during a balance task while standing on a soft surface. Left side depicts baseline condition and right side the anchoring task in two-foot positions: parallel (a) and tandem position (b)

Full size image

In a preliminary study about effects of haptic information embedded in a postural task (called “haptic anchoring”), Mauerberg-deCastro and MagreFootnote 3 reported that an older group of blindfolded adults (mean age 69) showed a larger amount of sway when the balance task included a tandem foot position in comparison to a parallel foot position. As shown in Fig. 1.2, anchoring provided haptic cues to reduce body sway in older participants in all task conditions, though to a much greater extent when the feet were in a tandem position (50 % decrease when compared to the baseline) than when parallel (23 % decrease when compared to the baseline).

Fig. 1.2
figure 2

Mean and standard error of COP ’s path length during a balance task on a soft surface, under baseline and anchoring task conditions, with vision available and blindfolded, in parallel foot position (a) and tandem position (b)

Full size image

The task demands of controlling the anchors while keeping the body position stable revealed the extent of adaptability of the older group. The reduced ability to significantly alter the dynamics from one task to another appeared to depend on the degree of the task demand. The older group exploited the anchor context when the point of support on the surface was narrower (i.e., tandem foot position), more prominently reducing the magnitude of sway. This ability to adaptively weigh and select sensory references integrates task constraints. For older people , the exploitation of such a solution in order to maintain a relatively stable posture is a matter of safety [43]. While young adults generally have less traumatic consequences with falls, they can expand instability of the COM to a greater boundary. Cabe and Pittenger [44] considered the range of excursions of an object’s COM to be limited by a toppling point , conceptualized as a haptic angular tau . As the postural control strategies of older adults include biomechanical parameters of the body (i.e., different anatomical structures and their lengths, mass, degrees of freedom) and perceptual assemblies (i.e., information embedded in a particular task), the toppling point becomes the unstable equilibrium point that marks an impending moment for falling, or a phase transition to another behavioral pattern.

Concerning the notion of variability in postural control, as discussed above, the increase in postural sway can be seen as a functional aspect of motor control in older adults. Carpenter et al. [34] argued that the central nervous system intentionally increases COP sway “to ensure a certain quality and quantity of sensory information.” Because of increased thresholds for sensory detection, older adults increase their body sway as a way to gain enhanced information about the limits of stability. On the other hand, the reduction in body sway with the addition of haptic cues reduces the need to sway the body to actively determine the limits of the base of support. Interestingly, Freitas et al. [45] showed, in a training study, that the intermittent use of an anchor system was the only condition that resulted in long-term effects on COP sway (i.e., reduced sway). These authors argued that the task variability embedded in the intermittent use of the anchor system was more beneficial to the postural control system of older adults than the repetitive use of the anchors.

5 Gait Patterns Are Flexible During Locomotion, Yet Regularity Is Task Context-Dependent

At any age, and for any open biological system, constraints set the boundaries of the system’s behavior, leading to the emergence of semi-predictable, stable patterns. Even though the aging process is associated with losses, this adaptive phenomenon should not necessarily be interpreted as something bad or inefficient. Humans demonstrate multiple levels of flexibility in their behaviors [46]. At early ages, novice, immature infants demonstrate highly disorganized movement patterns, yet these patterns seem to provide opportunities for the infants to seek stable coordination solutions. This also is the case for older adults. Adaptation arises as a common, yet flexible, solution, which then results in movement patterns that appear similar to, and as stable as, those used by individuals in a wide range of ages [46].

Patterns of movements observed during our younger years versus our older years represent only an apparent difference. Spatial-temporal parameters of walking comparisons between older and young adults reflect an increase or decrease in parameters rather than in qualitative changes of movement patterns, as observed in comparisons between very young walkers and adults [47]. For example, novice walkers change the degrees of freedom of the lower limbs by freezing them as they acquire experience (e.g., limiting range of motion in the medial-lateral plane of walking step cycles) or releasing them (e.g., increasing range of motion in the knee and ankle joints to improve dampening of forces).

As for older adults who have experienced falls, changes in walking often include a significant decrease in speed and step length [48]. Such adjustments often are related to challenges in the environment (e.g., inclination of terrain, slippery surfaces, and presence of obstacles) and in the task requirements (e.g., performing a simultaneous cognitive and motor task) and to the fact that humans use a variety of shoes to perform daily locomotion. For instance, shoes can be a risk factor for falls in older people [13]. While these authors found no differences between young and older adults’ walking patterns with a variety of shoes, both adopted a conservative walking pattern when wearing elevated heel shoes. Rinaldi and Moraes [49] found that older adults with a history of falls (i.e., fallers) were unable to perform walking and prehension movements concomitantly. Fallers reduced their gait speed, especially when the prehension task was more difficult, which helped them to increase their margin of stability when grasping an object. Therefore, changes in behavioral outcomes often are a measure of compounded parameters, those that represent interplay of constraints rather than simple, individual variables (e.g., getting old).

Because comparative studies commonly use older individuals who are sedentary or potentially unhealthy, variable or unstable patterns of walking reflect particular strategies of the aging system, as well as physical limitations. In older adults with various health and physical conditions, Cozzani and Mauerberg-deCastro [50] found several common strategies in walking and stepping over obstacles. Sedentary and active community-dwelling older adults and institutionalized older adults slowed down while walking just prior to crossing over a 15-cm high obstacle, although the active group walked faster than the two other groups. These groups all exaggerated the vertical displacement of the clearing foot when crossing over a 15-cm obstacle, with the active group reaching a height of 20 cm and the sedentary group a height of 18 cm. The institutionalized group cleared the obstacle by only 2 cm, which, in this case, could have been the result of muscle weakness of the legs, which likely limited the amount of vertical foot elevation. The typical vulnerability (e.g., weak muscle strength of lower limbs) of older individuals who are institutionalized likely reflects their limited ability to exaggerate the movement. When crossing over obstacles or irregular surfaces, this weakness exposes them to risks for tripping and falling. All groups crossed the 2-cm obstacle height with a similar vertical foot elevation (~13-cm vertical displacement). To compensate for physical limitations, the sedentary and institutionalized individuals varied the horizontal distance of the last step according to the obstacle’s height. The active individuals maintained a constant step length across obstacle heights. The exaggeration of vertical elevation imposes a high demand in the maintenance of balance—standing on one foot while the other swings to cross over the obstacle—but it likely serves as a compensation strategy to prevent collisions with obstacles. Is this exaggeration of movement a sign of awareness and fear of the potential danger of collisions leading to falls, even by physically active older individuals? The literature shows that, indeed, older individuals slow their locomotion patterns when a challenge is perceived to be a risk for falls [51]. Healthy and non-healthy older people equally perceive that they are at risk for falls.

Sources of constraints during the aging process are diverse enough to maintain a system’s sensitivity to its own changes as well as to changes in the task context [52]. Whether older individuals misjudge or exaggerate their perceptions, a single explanation is not sufficient for us to understand everything about the aging process. For example, research shows a contrast between age groups with regard to sensitivity to movement and actual physiological condition [53]. Moraes and Mauerberg-deCastro [54] found that older individuals perceived levels of difficulty of sitting and standing up tasks, from different chair heights, in a similar way as do young adults. However, the older individuals slowed down their motion, particularly during the descent phase of sitting down. Their movement patterns reflect instability and irregularity as the height of the chairs was lowered less than the typical 90° sitting position. One potential concern is that older adults may be unaware of their motor instabilities, and, by erroneously perceiving all chair heights as equal, they expose themselves to falls.

As sitting down and standing up can be integrated tasks in daily living activities, locomotion is a contingency of mobility and, therefore, a criterion for independence. Although locomotion is a relatively simple skill, older individuals’ locomotion can reflect risks for falls, especially in the presence of obstacles. Thus, the immediate adaptations due to the demands of the environment and the task have direct implications with the functional conditions of the perceptual-motor systems [55]. For example, changes in locomotion patterns that are originally caused by limitations in postural control include reduction of speed, changes in the durations of the phases of stepping, and the addition of new control strategies (e.g., dragging the feet), among others.

Task constraints such as speed, direction, and movement magnitude are important variables that impact movement performance. This can be illustrated, for example, when healthy and older individuals with Down syndrome walk on a padded surface, a 15-cm thick mattress [46]. The older individuals attempted to dampen their steps, which they perceived were too fast and unsafe. If a person has balance problems, changes in their locomotion behavior may increase risk for falls as soon as the parameters escalate (i.e., speed, cadence). Older people become quite cautious and do not go beyond their perceived safe limits with regard to a particular behavior or skill. Figure 1.3 illustrates the gait pattern (i.e., phase portrait) of a young adult and an older adult with Down syndrome. The young adult shows a smooth, regular kinematic pattern of the shank displacements during walking. To the contrary, the walking topology of the older adult with Down syndrome is marked by irregular trajectories and limited excursions of the attractor in the state space [46].

Fig. 1.3
figure 3

Phase portraits of the shank segment of a young adult (top) and an older adult (bottom) with Down syndrome walking on a padded surface (adapted from data published in Mauerberg-deCastro and Angulo-Kinzler [46])

Full size image

During locomotion, older individuals maximize their biomechanical possibilities, whether in the presence of a disability condition or physical deteriorations (e.g., loss of vision acuity), and they respond to task and environmental perturbations by adjusting their skill parameters (e.g., slowing down the movement). In order to explain the adaptability of the locomotion behavior, we need to consider that the irregular, controlled, and slow motions are solutions to the perturbation, caused by the unstable surface. When locomotion slows down drastically, there is a higher demand on the postural control system, and ballistic portions of the motion have to be suppressed by an increased dampening of the body.

In a similar way as above, the addition of haptic cues (i.e., dragging an anchor system on the floor while performing various walking tasks) shows the adaptability of the walking behavior in older adults. During tandem walking, older adults who dragged the anchors showed reduced trunk sway acceleration in the frontal plane [41]. The haptic cues provided by the anchors allow an individual to expand his or her possibilities for exploring the adjacent environment and, consequently, to use this information about his or her body position relative to the ground to adaptively reduce body sway while walking .

6 Final Remarks

In this chapter, we employed general concepts of a dynamic systems perspective to discuss the aging process. We used the concepts of variability, adaptive potential, complex multi-scale and multi-level, and different time-varying sources of constraints that challenge the traditional notion of frailty in aging. Specifically, we approached these discussions with regard to the functional role of aging on posture and locomotion control.

While individual differences exist and provide potential for adaptive success, the variability of behavior outcome does not necessarily rely on the notion of “good” or “bad” control solutions. Stability, or regularity, in movement patterns is found to be complex, and it reflects diverse strategies in individuals of various ages. Often, older people experience the combined effects of aging and health deterioration, and, therefore, their systems and subsystems organize solutions with different levels of complexity.

Sometimes a solution is the result of an organism’s lack of flexibility, sometimes not. Older individuals may resort to a few motor behavior solutions that can be considered effective. Exploitation of a task context must take into account a number of factors, including those that are not directly relevant to a function (e.g., unrealistic fear of falling when there is no explicit postural perturbation). The dynamic systems perspective provides a useful lens from which to reexamine the discontinuous process of aging and reinterpret change not as the body’s inevitable journey to “frailty,” but as a transition to possibilities for adaptation.