Introduction

Walking difficulty is a common, costly problem in older adults and it contributes to loss of independence, higher rates of morbidity and increased mortality [1–4, 5••]. Walking is a complex task that places demands on the musculoskeletal, cardiopulmonary, and nervous systems [6]. The changes that occur in walking with age are likely the result of multiple small changes in several different systems more so than the result of one catastrophic event such as a stroke or hip fracture. The walking disability develops gradually, and although many older adults are referred (or self-refer) to a geriatric specialist because of the mobility problem, the reason for the walking difficulties often cannot be identified [6].

The constellation of deficits characteristic of age-related walking problems contributes to inefficient gait [6–10]. Typically the biomechanics are altered (i.e., flexed trunk posture [11], decreased hip extension in mid to late stance [12, 13], and decreased ankle plantarflexion and power at push-off) [11, 13–16] and movement control is disrupted (i.e., reduced rate of forward momentum [17], stride length and time variability [18, 19], and timing issues, including a loss of the rhythm, hesitancy, and difficulty transitioning from stance to swing. swing) [17, 20, 21]. The biomechanical and movement control problems appear to interact [9]. For example, the reduced hip extension blocks the mechanical accumulation of potential energy in the limb tissues during stance to release during swing to fuel the limb forward movement, while also eliminating the hip extension, movement-related feedback stepping signal for the transition from stance to step [17, 22–27].

Interventions to improve walking have historically been multifactorial (i.e., strengthening, endurance and flexibility programs) focusing on improving the underlying impairments of the systems involved. These multifactorial impairment-based programs have resulted in only modest improvements in walking (eg an approximate 5 % increase in gait speed, with a range of 0–16 %) [28–39]. The questions are: “Why are the results suboptimal?” and “Can we [or can’t we] do better?”.

Whether contributors to or consequences of the age-related walking problems, walking is slow [17], less stable [40], inefficient [41], and the timing and coordination of stepping with postures and phases of gait is poor [42–44]. We argue the timing and coordination problems are evidence of the loss of motor skill in walking, more so than a decrease in physiological reserve capacities [muscle strength, motion, and endurance] for walking alone. The loss of motor skill related, high energy cost of walking (e.g., inefficient) is a major factor in the age-related decline in physical function and activity for older adults. Taking a lesson from the sports world and from neurorehabilitation, task-oriented motor skill exercise is the essential component of exercise training to improve motor task performance [45, 46••, 47•]. How do the most skilled movers at their sport prepare for their sport specific skilled tasks? While sport specific skilled movers may exercise to build a foundation of muscle strength, flexibility and endurance capacity for their sport, no athlete steps to the plate with a bat in hand, raises the basketball to the hoop, or tees up the golf ball without substantial task-oriented exercise training. In other words, if walking is the problem, then the intervention should primarily focus on the task of walking through motor skill-based exercise and not on an impairment-based exercise intervention alone.

The purpose of this article as to (1) briefly review the current literature regarding impairment-based intervention approaches for improving mobility in older adults, (2) discuss why the results have been suboptimal (only modest), and (3) suggest an alternative approach to intervention to improve mobility in older adults (i.e., task-oriented motor learning).

Impairment-based Interventions to Improve Mobility

Multifactorial Impairment-Based Walking Exercise

The multifactorial impairment-based therapeutic approach to improve age-related walking problems is to ‘fix’ the impairments of lower extremity weakness, flexibility, and endurance related to walking difficulties. The ‘fix’ involves therapeutic exercise to enhance capacities of muscle strength, range of motion, and aerobic conditioning [48]. Resistance exercises are used to strengthen weak lower extremity muscles for their functions as prime movers of the limbs, as well as for their actions as synergists to stabilize the trunk, or as antagonists, preprogrammed to decelerate limb movement facilitated by the prime mover. Exercises to improve muscle strength and power can enhance the ability of a weak lower limb muscle, such as the ankle plantarflexors to initiate ground reaction forces or the hip abductor muscles to control medial lateral movements during walking. Stretching exercises to increase joint range of motion (ROM) are performed to allow the older adult to: (1) attain specific postures of the limbs or trunk necessary to allow for the muscle forces generated to result in the desired direction of forward propulsion [e.g., plantarflexion ROM], (2) the desired length of muscles for optimal activation (e.g., dorsiflexion ROM at heel strike, lengthening the calf muscles to facilitate their subsequent activation), and (3) appropriate movement-related feedback to the nervous system controlling and adapting stepping patterns [e.g., hip extension]. Aerobic conditioning exercise enhances the delivery and extraction of oxygen to the muscles, necessary to sustain the repeated pattern of muscle activation in walking (Table 1).

Table 1 Components of impairment-based and task oriented motor learning interventions
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While focus of the impairment-based exercise approach to age-related walking problems is to remedy the reduced capacity of the body systems that contribute to the movements in gait, in the context of walking the rationale for the approach can be inadequate. In our clinical experience, many older adults with age-related walking difficulties demonstrate adequate muscle force production for mobility. The relation of muscle strength to gait performance is modest at best [49],and strengthening exercises have only minimal effect on walking ability [50].Lower extremity range of motion deficits noted among older adults have often been small, and walking typically involves limited range of motion of the lower limb joints [13, 15]. Interventions targeted at improving ROM have resulted in little change in gait characteristics of older adults, particularly with the joint motion during walking [15, 51]. Enhancing aerobic capacity may provide the energy needed to sustain the muscle activity the older adult uses for walking and reduce restriction of walking activity in prolonged walking conditions [48, 52]. However, there is little reason to believe aerobic conditioning would impact the age-related changes in brief bouts of walking characteristic of activities of daily living (ADLs) and during performance of a short [e.g., 4 m] timed walk (the latter the observational method by which age-related walking changes have most often been described) [2–4].

In addition to addressing impairments, multifactorial approaches typically include progressive ambulation training. In progressive ambulation exercise component tasks of walking (e.g. propulsion, weight-shifting of the center of mass, toe clearance during swing, weight acceptance at heel strike) are broken down and practiced individually, using visual, verbal and manual cueing [53]. The older adult is provided feedback on the walking components to facilitate their ability to recognize the incorrect actions and make conscious choices to correct the walking pattern. Increased demands for cognitive processing are difficult for older adults, particularly related to tasks that require recognition of peripheral sensory input, specifically when a shift is required to relate sensory input to a self-organized, walking movement plan [20, 54–57]. Among older adults, key factors in slowed movement performance are signal recognition and central plan processing [58, 59]. Slowed movement performance while participating in gait training, may accentuate the timing and coordination problems in walking [9, 17, 20]. Thus, multifactorial, impairment-based exercise aims to increase physiologic capacity in body systems that contribute to walking, but does not include task specific exercise necessary to make use of the physiological capacity in body systems (i.e., musculoskeletal and cardiopulmonary systems) for the walking. The result is a bigger engine for walking and maximum performance; the ability to use the excess capacity to tolerate the age-related gait abnormalities and high energy cost of poor walking (Fig. 1).

Fig. 1
figure 1

Building a bigger engine versus a better engine: A comparison between impairment-based and task-specific motor learning exercise programs. We relate differences in the exercise programs to differences in building an engine for maximum versus optimal performance. The impairment-based program results in greater physiological capacity in body systems that can be used for walking (i.e., maximum performance). The motor learning program results in a more efficient use of physiological capacity in body systems for walking (i.e., optimal performance)

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Alternative Approach – Task-oriented Motor Learning

Task-Oriented Motor Skill-Based Walking Exercise

Motor skill exercise for walking is task-oriented, motor sequence learning, intended to improve the older adult’s appropriate motor plan selection from an enhanced repertoire of motor plans for walking. Changes in the capacities of body systems [48] necessitates a change in the choice of motor plans to meet the performance demands [60, 61]. To make the appropriate motor plan selection, the brain needs continual and relevant movement experiences. To make appropriate changes in the selection of motor plans, the brain needs to be updated about the body capacities through continued recent and relevant movement experiences in walking [61].

Welford [60] defined motor skill as the selection of the minimum capacities to meet task demands, or the most appropriate motor plan for the task. The highly skilled or expert mover uses the minimum neural, muscle and joint motion capacities for successful task performance and because only the minimum capacities are selected, skilled movement is efficient [62]. For the highly skilled, at the motor task, movement acceleration and deceleration are planned together, the movement is continuous, without guidance, and as a result the movement is usually fast and efficient. In comparison, the less skilled, rely on non-programmed (discontinuous) movements that require feedback and the result is slow and inefficient movement [61]. Motor skill-based walking exercise has increased efficiency and speed of walking [63, 64•]. Motor skill in well-learned movement tasks is maintained through information gained by the person’s sensing and moving environmental experiences. The regular and relevant experiences serve to adapt and refresh motor programs. Adjustments are made for changes in the limbs, muscles and posture, which enables the skilled mover to focus on the movement goal, while spinal and supraspinal motor centers organize and implement successful movement strategies [61, 65, 66].

Important components to task-oriented, motor skill exercise are combining different conditions of performance, body position and task demands to yield variations in exercise complexity and the skill required [67]. When the participant accomplishes the goal of the motor task, the successful performance rewards and reinforces the motor plan selection [66]. Motor skill-based exercise includes: (1) a defined movement goal; (2) movement to gain knowledge of muscles and postures (i.e., implicit motor learning or movement awareness); (3) practice to correct errors in movement, develop and adjust motor plans, and 4) challenge to select the optimal motor plan (Table 1) [61]. The following are specific examples of motor skill-based exercise principles applied to walking.

Defined Movement Goal

Defining the goal of the task limits the degrees of freedom and enhances the probability that the person will generate the appropriate sequence of muscles and movements. The defined goal links the motor sequence generated to the neural circuit for the intended task [68••, 69, 70]. Motor sequence learning of the pattern and timing of muscle activation for the intended task occurs primarily without conscious attention to the recruitment of specific muscles and timing of the movement series. For example, a defined goal for step initiation in gait, such as ‘step across’ and toward the forward line of progression of the stance limb, enhances forward momentum in the stepping pattern of gait. By accomplishing the step across task goal, the center of mass of the body accelerates toward the stance limb and loads the limb through midstance. Stance limb loading not only facilitates plantarflexor power while the limb accelerates behind the body, but the loading also accelerates the thigh into hip extension—the neural signal for stepping. The step across task goal facilitates activation of the abductors on the limb in transition to swing with the adductors on the limb in transition to stance and accelerates the center of mass of the body forward and toward the stance limb [17]. During walking, defined goals for walking that promote forward momentum by the interaction of the limbs with the ground sustains the pattern of stepping and reduces the need to generate individual steps that tax balance control mechanisms [11, 17, 40]. Forward momentum in walking is also facilitated by a defined goal of a gait speed [71•] or a cadence to maintain throughout walking and can be accomplished by the use of a metronome or the rhythm of music [72–74]. The combined activation of the hip abductors of one limb with the adductor muscles of the opposite limb is facilitated by a curved path goal for walking [63, 75–77].

Move to Gain Knowledge

Walking activities that involve alternating the direction of trunk and limb movements simulate the smooth switching between agonist and antagonist muscle groups in gait. In sitting, a forward reach on a diagonal with the upper body leaning on to an exercise ball, followed by reversal of the movement, reproduces the pattern of trunk movements and generates the movement-related feedback characteristic of the alternating locomotor pattern in walking. A step backward and across prior to a step forward and across exercise induces the muscle pattern and limb loading necessary to generate momentum forward toward the limb preparing for stance. The stepping exercise provides the experience of moving the body center of mass without asking the person to ‘think’ about weight shifting and progression. Walking circular paths, alternating clockwise and counterclockwise directions provide similar patterned muscle activation and limb loading experiences as in the stepping exercises but in the context of walking.

Practice to Refine and Develop a Repertoire of Motor Plans

Accurate repetition [eg practice] of the motor plan for the walking drives experience dependent changes in neurons and their connections, or the neuroplastic changes characteristic of motor skill acquisition) [69, 78–80]. Repetition of stepping and walking pattern activities in one direction or side only (e.g., 10 repetitions), then to the opposite side (e.g., 10 repetitions), prior to switching the exercise tasks more frequently progressing to switching on every repetition is a method for promoting accuracy before advancing to the more difficult skill of continuous switching movement plans. Components of the motor skill for walking practiced in one exercise task are incorporated into subsequent exercises—for example “step across” muscle pattern activation could be practiced again in curved path walking patterns. Treadmill assisted-walking provides accurate practice at a designated speed [goal]. The moving belt of the treadmill facilitates consistent step timing and the hip extension movement that elicits the neural signal for stepping. Demands on cognitive processing are minimal as treadmill-assisted walking is motor skill walking practice ‘by doing’ [42, 81–84].

Challenges to Select the Optimal Motor Plan

In addition to the switching between movement directions and alternating movement patterns described above (move to gain knowledge), motor skill acquisition can be enhanced by challenges of accuracy and amplitude and varying the demand for accuracy, amplitude and direction in walking motor tasks. Oval [curved] walking paths can be made narrower, or progressively smaller ovals as in a spiral walking pattern. The goal for consistent speed or pace is maintained for the curve and the straighter parts of the oval and spiral paths. Varying the selection of motor plans while walking can be implemented by: (1) changing directions on the oval or spiral paths; (2) walking a figure-of-eight path [curve direction changes at each end of the ‘8’]; and (3) walking a serpentine path.

The challenge to select the optimal motor plan for the walking conditions also sets a criterion for performance. The performer’s ability to recognize success in walking may be a key factor in the maintenance of intervention-related improvements [85, 86] in walking [71•]. The defined movement goal for the repetitive task-oriented exercise provides a performance criterion that allows the performer to clearly recognize success throughout the practice as well as at the completion of an entire intervention episode. Visual feedback [87••], verbal positive reinforcement of the task [71•, 88], and reinforcement in a virtual reality environment [86] are all examples of success/reward recognition task-oriented exercise strategies that have had a positive impact on the motor skill of walking. Interventions that focus on timing and coordination of gait reduced the energy cost of walking and increased gait speed [63, 64•, 89]. Intervention strategies that facilitates the performer’s ability to recognize successful walking performance may positively influence the sustainability of the skilled walking ability. Among mobility-limited older adults, the increased walking confidence outcome of task-oriented but not the impairment-oriented walking exercise may be attributed to the characteristics of the motor skill-based walking exercise approach [63, 90•].

In summary, the task-oriented motor learning intervention aims to improve the motor skill of walking by re-aligning biomechanical and neuromotor control, strengthening motor programs, and improving feedback for adjusting movements. In a sense it is comparable to a “tune-up” of an engine for optimal performance. The result is a more efficient system for walking which requires less energy and can last longer (Fig. 1). Older adults who are skilled, efficient walkers likely tire less easily and thus have the ability to participate in many life activities [63, 64•, 90•].

Evidence Supporting Task-Oriented Motor Learning Exercise

We recently conducted two randomized clinical trials to examine the impact of task-oriented motor learning exercise on mobility in two distinct groups of older adults, those with moderate mobility difficulty (the RESTORE study) [63, 90•] and those with mild subclinical mobility limitations (the PRIME study) [64•]. In the RESTORE study, 47 older adults with slow and variable gait were randomly assigned to either an impairment-based or a task-oriented motor learning exercise program. These older adults walked slowly, mean gait speed 0.85 m/s, with ineffecient gait (i.e. mean baseline energy cost of walking was 0.30 mL/kg/m, almost twice the energy cost of normal walking). The exercise was delivered by a physical therapist and lasted one hour, twice a week for 12 weeks. At the end of 12 weeks, older adults in the task-specific group had greater improvements in the energy cost of walking, gait quality, walking confidence, and physical function compared to older adults in the impairment-based group [63, 90•].

The PRIME study included 40 older adults with subclinical gait dysfunction defined as near normal gait speed and impaired skill in walking. Though their mean baseline gait speed of 1.18 m/s was near normal (i.e., 1.2–1.3 m/s) their baseline energy cost of walking of 0.22 mL/kg/m was nearly 50 % greater than the 0.15-mL/kg/m energy cost of walking in young adults indicating an inefficient walking pattern. Subjects were randomly assigned to either an impairment-based or a task-oriented motor learning exercise program that met twice weekly for twelve weeks. After the intervention, the subjects in the task-oriented group improved more than the subjects in the impairment-based group in gait speed and measures of motor skill in walking. In fact, the task-oriented group improved more than twice as much in gait speed (i.e., 0.13 m/s versus 0.05 m/s) as the impairment-based group. Although not statistically significant, the difference in change in energy cost of walking between the groups was clinically meaningful with the task-oriented group demonstrating greater improvements than the impairment-based group [64•]. In older adults with either mild (PRIME) or moderate (RESTORE) gait dysfunction, a task-oriented motor learning exercise program resulted in greater improvements in several mobility indicators than an impairment-based program. The task-oriented training resulted in older adults who walked faster, more efficiently with improved motor skill and confidence (i.e., better walking ability).

Conclusion

Age-related walking problems may be responsible for greater mortality and morbidity of older adults than acute illness and disease [7]. Multicomponent impairment-based walking exercise can enhance strength, flexibility and endurance capacities important for walking (a ‘bigger walking machine’), but not necessarily better walking ability. The outcome of task-oriented motor learning walking exercise is the timing and coordination for better walking ability—a ‘well-tuned walking machine’.