Healthy aging does not impair lower extremity motor flexibility while walking across an uneven surface

Human movement strategies in uncertain environments: A synergy-based approach to the stability-agility tradeoff

Humans frequently prepare for agile movements by decreasing stability. This facilitates transitions between movements but increases vulnerability to external disruptions. Therefore, humans might weigh the risk of disruption against the gain in agility and scale their stability to the likelihood of having to perform an agility-demanding action. We used the theory of motor synergies to investigate how humans manage this stability-agility tradeoff under uncertainty. This theory has long quantified stability using the synergy index, and reduction in stability before movement transitions using anticipatory synergy adjustment (ASA). However, the impact of uncertainty - whether a quick action should be executed or inhibited - on ASA is unknown. Furthermore, the impact of ASA on execution and inhibition of the action is unclear. We combined multi-finger, isometric force production with the go/no-go paradigm. Thirty participants performed constant force (no-go task), rapid force pulse (go task), and randomized go and no-go trials (go/no-go task) in response to visual cues. We measured the pre-cue finger forces and computed ASA using the uncontrolled manifold method and quantified the spatio-temporal features of the force after the visual cue. We expected ASA in both go/no-go and go tasks, but larger ASA for the latter. Surprisingly, we observed ASA only for the go task. For the go/no-go task, 53% of participants increased stability before the cue. The high stability hindered performance, leading to increased errors in no-go trials and lower peak forces in go trials. These results align with the stability-agility tradeoff. It is puzzling why some participants increased stability even though 80% of the trials demanded agility. This study indicates that individual differences in the effect of task uncertainty and motor inhibition on ASA is unexplored in motor synergy theory and presents a method for further development.

Evaluating plantar biomechanics while descending a single step with different heights

Objective

This study aims to investigate the plantar biomechanics of healthy young males as they descend a single transition step from varying heights.

Methods

Thirty healthy young males participated the experiment using the F-scan insole plantar pressure system in which participants made single transition steps descent from four step heights (5, 15, 25, and 35 cm), leading with their dominant or non-dominant foot. Plantar pressure data were collected for 5 s during the period between landing touchdown and standing on the ground. Landing at each step height was repeated three times, with a five-minute rest between different height trials.

Results

At 5 cm and 15 cm steps, participants demonstrated a rearfoot landing strategy on both sides. However, forefoot contact was observed at heights of 25 cm and 35 cm. Parameters related to center of plantar pressure (COP) of the leading foot were significantly larger compared to the trailing foot (P < 0.001), increased with higher step heights. Vertical ground reaction forces for the biped, leading and trailing feet decreased with increasing step height (all P < 0.05). The leading foot had a higher proportion of overall and forefoot loads, and a lower proportion of rearfoot load compared to the trailing foot (P < 0.001). The overall load on the dominant side was lower than that on the non-dominant side for both the leading and trailing feet (P < 0.001). For the trailing foot, forefoot load on the dominant side was lower than that on the non-dominant side, however, the opposite result appeared in rearfoot load (P < 0.001). Upon the leading foot landing, forefoot load exceeded the rearfoot load for the dominant (P < 0.001) and non-dominant sides (P < 0.001). Upon the trailing foot landing, forefoot load was lower than the rearfoot load for the dominant (P < 0.001) and non-dominant sides (P = 0.019).

Conclusion

When the characteristics of biomechanical stability are compromised by step height, landing foot, and footedness factors - due to altered foot landing strategies, changing COP, or uneven force distribution - ability to control motion efficiently and respond adaptively to the forces experienced during movement is challenged, increasing the likelihood of loss of dynamic balance, with a consequent increased risk of ankle sprains and falls.

Synergic control of the minimum toe clearance in young and older adults during foot swing on treadmill walking in different speeds

BACKGROUND

The vertical toe position at minimum toe clearance (MTC) in the swing phase is critical for walking safety. Consequently, the joints involved should be strictly controlled and coordinated to stabilize the foot at MTC. The uncontrolled manifold (UCM) hypothesis framework has been used to determine the existence of synergies that stabilize relevant performance variables during walking. However, no study investigated the presence of a multi-joint synergy stabilizing the foot position at MTC and the effects of age and walking speed on this synergy.

RESEARCH QUESTIONS

Is there a multi-joint synergy stabilizing MTC during treadmill walking? Does it depend on the persons' age and walking speed?

METHODS

Kinematic data from 23 young and 15 older adults were analyzed using the UCM approach. The participants walked on a treadmill at three speeds: slow, self-selected, and fast. The sagittal and frontal joint angles from the swing and stance legs and pelvis obliquity were used as motor elements and the vertical toe position at MTC was the performance variable. The variances in the joint space that affected (VORT, 'bad' variance) and did not affect (VUCM, 'good' variance) the toe position at MTC and the synergy index (ΔV) were computed.

RESULTS

The ΔV>0 was revealed for all subjects. Walking speed did not affect ΔV in older adults, whereas ΔV reduced with speed in young adults. ΔV was higher for older than for young adults at self-selected and fast speeds, owing to a lower VORT in the older group.

SIGNIFICANCE

The vertical toe position at MTC was stabilized by a strong multi-joint synergy. In older adults, this synergy was stronger, as they were better at limiting VORT than young adults. Reduced VORT in older adults could be caused by more constrained walking, which may be associated with anxiety due to walking on a treadmill.

Biomechanics of the lower limb in patients with mild knee osteoarthritis during the sit-to-stand task

BACKGROUND

Knee osteoarthritis (KOA) is a prevalent and debilitating condition that markedly affects the sit-to-stand (STS) activity of patients, a prerequisite for daily activities. Biomechanical recognition of movements in patients with mild KOA is currently attracting attention. However, limited studies have been conducted solely on the observed differences in sagittal plane movement and muscle activation.

AIM

This study aimed to identify three-dimensional biomechanical and muscle activation characteristics of the STS activity in patients with mild KOA.

METHODS

A cross-sectional study was conducted to observe the differences between patients with mild KOA and a control group (CG). It was conducted to observe the differences in muscle activation, including root mean square (RMS%) and integrated electromyography (items), kinematic parameters like range of motion (ROM) and maximum angular velocity, as well as dynamic parameters such as joint moment and vertical ground reaction force (vGRF).

RESULTS

Patients with mild KOA had a higher body mass index and longer task duration. In the sagittal plane, patients with KOA showed an increased ROM of the pelvic region, reduced ROM of the hip-knee-ankle joint, and diminished maximum angular velocity of the knee-ankle joint. Furthermore, patients with KOA displayed increased knee-ankle joint ROM in the coronal plane and decreased ankle joint ROM in the horizontal plane. Integrated vGRF was higher in both lower limbs, whereas the vGRF of the affected side was lower. Furthermore, patients showed a decreased peak adduction moment (PADM) and increased peak external rotation moment in the knee joint and smaller PADM and peak internal rotation moment in the ankle joint. The affected side exhibited decreased RMS% and iEMG values of the gluteus medius, vastus medialis, and vastus lateralis muscles, as well as a decreased RMS% of the rectus femoris muscle. Conversely, RMS% and iEMG values of the biceps femoris, lateral gastrocnemius, and medial gastrocnemius muscles were higher.

CONCLUSION

The unbalanced activation characteristics of the anterior and posterior muscle groups, combined with changes in joint moment in the three-dimensional plane of the affected joint, may pose a potential risk of injury to the irritated articular cartilage.

How older adults regulate lateral stepping on narrowing walking paths

Walking humans often navigate complex, varying walking paths. To reduce falls, we must first determine how older adults purposefully vary their steps in contexts that challenge balance. Here, 20 young (21.7±2.6 yrs) and 18 older (71.6±6.0 yrs) healthy adults walked on virtual paths that slowly narrowed (from 45 cm to as narrow as 5 cm). Participants could switch onto an "easier" path whenever they chose. We applied our Goal Equivalent Manifold framework to quantify how participants adjusted their lateral stepping variability and step-to-step corrections of step width and lateral position as these paths narrowed. We also extracted these characteristics at the locations where participants switched paths. As paths narrowed, all participants reduced their lateral stepping variability, but older adults less so. To stay on the narrowing paths, young adults increasingly corrected step-to-step deviations in lateral position more, by correcting step-to-step deviations in step width less. Conversely, as older adults also increasingly corrected lateral position deviations, they did so without sacrificing correcting step-to-step deviations in step width, presumably to preserve balance. While older adults left the narrowing paths sooner, several of their lateral stepping characteristics remained similar to those of younger adults. Older adults largely maintained overall walking performance per se, but they did so by changing how they balanced the competing stepping regulation requirements intrinsic to the task: maintaining position vs. step width. Thus, balancing how to achieve multiple concurrent stepping goals while walking provides older adults the flexibility they need to appropriately adapt their stepping on continuously narrowing walking paths.

Musculoskeletal adaptation of young and older adults in response to challenging surface conditions

Over 36 million adults over 65 years of age experience accidental falls each year. The underlying neuromechanics (whole-body function) and driving forces behind accidental falls, as well as the effects of aging on the ability of the musculoskeletal system to adapt, are poorly understood. We evaluated differences in kinematics (lower extremity joint angles and range of motion), kinetics (ground reaction force), and electromyography (muscle co-contraction), due to changes in surface conditions during gait in 14 older adults with a history of falling and 14 young adults. We investigated the impact of challenging surfaces on musculoskeletal adaptation and compared the mechanisms of adaptation between age-groups. Older adults displayed greater hip and knee flexion and range of motion during gait, reduced initial vertical loading, and 13 % greater knee muscle co-contraction during early stance compared to young adults. Across age groups, the presence of an uneven challenging surface increased lower-limb flexion compared to an even surface. On a slick surface, older adults displayed 30 % greater ankle muscle co-contraction during early stance as compared to young adults. Older adults respond to challenging surfaces differently than their younger counterparts, employing greater flexion during early stance. This study underscores the need for determining lower-limb musculoskeletal adaptation strategies during gait and assessing how these strategies change with age, risk of accidental falls, and environmental surfaces to reduce the risk of accidental falls.

Legged locomotion over irregular terrains: state of the art of human and robot performance

Legged robotic technologies have moved out of the lab to operate in real environments, characterized by a wide variety of unpredictable irregularities and disturbances, all this in close proximity with humans. Demonstrating the ability of current robots to move robustly and reliably in these conditions is becoming essential to prove their safe operation. Here, we report an in-depth literature review aimed at verifying the existence of common or agreed protocols and metrics to test the performance of legged system in realistic environments. We primarily focused on three types of robotic technologies, i.e., hexapods, quadrupeds and bipeds. We also included a comprehensive overview on human locomotion studies, being it often considered the gold standard for performance, and one of the most important sources of bioinspiration for legged machines. We discovered that very few papers have rigorously studied robotic locomotion under irregular terrain conditions. On the contrary, numerous studies have addressed this problem on human gait, being nonetheless of highly heterogeneous nature in terms of experimental design. This lack of agreed methodology makes it challenging for the community to properly assess, compare and predict the performance of existing legged systems in real environments. On the one hand, this work provides a library of methods, metrics and experimental protocols, with a critical analysis on the limitations of the current approaches and future promising directions. On the other hand, it demonstrates the existence of an important lack of benchmarks in the literature, and the possibility of bridging different disciplines, e.g., the human and robotic, towards the definition of standardized procedures that will boost not only the scientific development of better bioinspired solutions, but also their market uptake.

Effects of Age and Attentional Focus on the Performance and Coordination of the Sit-to-Stand Task

This study investigated whether age and attentional focus affect synergy organization of sit-to-stand (STS). Young and older adults performed STS while holding a cup under internal (IF) and external focus (EF) instructions. Uncontrolled manifold analysis was used to decompose trial-to-trial variability in joint kinematics into variability that preserves (V_UCM) and interferes (V_ORT) with the horizontal and vertical positions of the center of mass (CoM) and cup. V_UCM was significantly higher than V_ORT for all variables in both age groups and focus conditions. Older adults demonstrated higher V_UCM for all variables and higher V_ORT for all variables except the vertical position of the cup. IF instructions benefited older adults, leading to decreased V_ORT of the vertical position of CoM and horizontal and vertical positions of the cup.

Effect of vibrotactile stimulation of the hallux nail on segmental coordination: A secondary analysis using uncontrolled manifold analysis

Controlling center of mass (CoM) movement in the mediolateral direction is imperative for stable walking. During walking, CoM movement is adjusted by the coordination of each body segment, which can be evaluated using uncontrolled manifold (UCM) analysis. UCM analysis evaluates segmental coordination by analyzing variablity in motor movement among the different segments of the body. The vibrotactile stimulation of the hallux nail can augment the sensory information of the plantar surface for necessary motor control. This study aims to investigate the effect of the vibrotactile stimulation of the hallux nail on segmental coordination to control CoM movement in the mediolateral direction during walking. Thirteen healthy men participated in the study. A vibrator was attached to each hallux nail, and pressure sensors were placed under the hallux balls. When the hallux ball was in contact with the floor, vibration stimulation was applied. A three-dimensional motion analysis system was used to measure the segment angles during walking, and UCM analysis was used to evaluate kinematic synergy for controlling CoM movement in the mediolateral direction. Subsequently, segment angles were used as an elemental variable. The synergy index and bad variability as motor noise were negatively related to the status without the stimulation. Vibrotactile stimulation in young people was more effective for people with large motor noise and a small synergy index during the single-stance phase. Thus, kinematic synergy can be immediately changed by sensory input using vibrotactile stimulation of the hallux nail, although applying vibration stimulation should be considered in advance.

Changes in kinematic synergy in older adults during walking: A two-year follow-up study

BACKGROUND

A well-controlled center of mass (CoM) in a coordinated segmental manner is required during gait. A synergy index that quantifies the strength of the synergistic control of the body segments that control the CoM can be evaluated using uncontrolled manifold (UCM) analysis. Several studies have compared the synergy index between older and younger adults; however, contradictory results have been found regarding age-related changes in the synergy index. Moreover, no study has investigated these changes longitudinally.

RESEARCH QUESTION

To evaluate age-related changes in the synergy index to control the CoM during gait in a longitudinal study.

METHODS

Twenty-five older adults participated at a baseline visit. The gait task at the two-year follow-up was completed by 16 older adults. Participants walked on a 6-m walkway at baseline and the two-year follow-up, and kinematic data were collected. Using UCM analysis, the synergy indices controlling CoM in the mediolateral and vertical directions were evaluated at baseline and follow-up. We also evaluated the Timed Up and Go (TUG) test and the strength of the knee extensor at both periods.

RESULTS

We found that TUG was significantly slower at follow-up; however, no difference was found in muscle strength. The synergy index in the mediolateral direction increased significantly after two years; such increases were found in individuals with decreased gait speed.

SIGNIFICANCE

This study showed that changes in gait patterns, including decreasing gait speed and increasing segmental coordination, may be important for gait with appropriate postural control relative to the environment and dynamic stability of the body in individuals with low functional mobility.

The effect of peripheral arterial disease and intermittent claudication on gait regularity and symmetry

Peripheral arterial disease (PAD) affects 20-30% of older adults and is associated with intermittent claudication (IC), which is walking-induced pain. This study compared the regularity and symmetry of gait between healthy older adults and adults with PAD, and between IC and non-IC conditions in the PAD group. Eighteen control (70.7 ± 6.3 years) and 11 PAD participants (67.0 ± 10.1 years) walked overground at a continuous, self-selected speed. A waist-mounted accelerometer determined step time, stride time, gait speed and mediolateral (ML), vertical (V) and anteroposterior (AP) gait regularity (step/stride) and symmetry. Correlations between ankle-brachial index (ABI) scores and PAD gait regularity/symmetry were also investigated. PAD step and stride times were greater (p < 0.01), while gait speed, ML and AP step regularity and ML and V stride regularity were significantly less than the controls (p < 0.05). There were no significant differences in gait symmetry. Within the PAD group, post-IC step/stride time and speed increased and decreased, respectively, (p < 0.05), while post-IC step and stride regularity were significantly less in all three directions (p < 0.01). Similarly, ML and V post-IC gait asymmetry increased significantly (p < 0.05). ABI was significantly correlated with pre-and post-IC vertical stride regularity (p < 0.01), and with pre- and post-IC ML gait symmetry (p < 0.05). The results demonstrate that gait regularity decreases as a result of PAD and IC. The association between gait regularity/symmetry and ABI should be investigated further, as it may have clinical application to the assessment of PAD severity.

Uncontrolled manifold analysis of gait kinematic synergy during normal and narrow path walking in individuals with knee osteoarthritis compared to asymptomatic individuals

Knee osteoarthritis (KOA) is a common musculoskeletal disorder resulting in altered gait patterns. Uncontrolled manifold (UCM) analysis has been demonstrated as a useful approach for quantitative analysis of motor variability and synergies. The present study aimed to investigate the changes in the kinematic synergy, controlling the center of mass (COM) position while walking on normal and narrow paths in people with KOA compared to asymptomatic participants. In this cross-sectional study, twenty people with mild to moderate KOA and twenty asymptomatic individuals walked at their comfortable preferred speed across normal and narrow paths on a treadmill. The UCM analysis was performed separately using the lower limb segmental angles as elemental variables and the COM displacement as a performance variable during the stance phase of gait for the frontal and sagittal planes. The results revealed that KOA and asymptomatic individuals could exploit kinematic synergy to control the COM displacement regardless of walking conditions (p < 0.05). Furthermore, the variance within the UCM and synergy index were significantly higher on the narrow path than the normal walking in the mediolateral direction in the KOA group (p < 0.05). The findings of this study suggest that individuals with KOA modify their gait kinematic variability to ensure a stronger kinematic synergy when walking on a challenging narrow path.

The effects of challenging walking conditions on kinematic synergy and stability of gait in people with knee osteoarthritis: A study protocol

Background:

Knee osteoarthritis (KOA) may considerably change the gait parameters, including the gait variability patterns. Uncontrolled manifold (UCM) analysis has been used to evaluate the relationship between motor control and gait variability as a useful index for assessing the multi-segmental movements’ coordination during walking. To our knowledge, no research has evaluated the alterations in the gait kinematic parameters during normal and narrow path walking in individuals with KOA as compared to asymptomatic people.

Materials and Methods:

In this cross-sectional study, individuals diagnosed with mild to moderate medial KOA and asymptomatic people will walk at their comfortable preferred speed on a treadmill. A motion capture system will be used to record at least 50 successful gait cycles. The kinematic variability of joints during gait will be analyzed using UCM, with the center of mass (COM) displacement considered as the performance variable. The primary outcome measure will be the lower limb synergy index. Variability of the COM displacement and changes in angles and angular velocities of lower extremity joints will be assessed as the secondary outcomes.

Results:

The results of this protocol study provide information on the lower limb kinematic synergy during gait on normal and narrow paths for individuals with KOA and asymptomatic controls.

Conclusion:

This information will help the researchers and clinicians understand KOA patients’ gait variability characteristics more deeply. Moreover, it may lead to an enhanced evidence-based approach for clinical decision-making concerning improving gait stability and decreasing the falling risk in these people.

Step length synergy is weaker in older adults during obstacle crossing

Community ambulation requires gait adaptations to navigate environmental obstacles. It is well known that while crossing obstacles, variables quantifying the gait pattern are controlled relative to the obstacle's position. However, the stability of these gait variables is underexplored. We measured foot positions relative to an obstacle as young and older adults stepped over it. We report secondary analysis of this data in which we quantified the stability of the step length when the two feet are placed on either side of the obstacle. We employed the uncontrolled manifold approach to test the hypotheses that (1) synergistic across-trial co-variation in the distances of the front and the back heel from the obstacle edge will stabilize the step length, and (2) older adults will display weaker synergies (i.e., lower step length stability). We observed that the front and back heel distances relative to the obstacle's edge co-varied synergistically to stabilize the step length for both age groups. Therefore, foot placement during obstacle navigation is controlled not only with reference to a feature of the environment (i.e. the obstacle), but also to stabilize the step length, presumably to control COM motion. The synergy index was 38% lower for older adults than young adults. This decline may be associated with aging-related functional deficits and tripping-related falls.

Compensation of stochastic time-continuous perturbations during walking in healthy young adults: An analysis of the structure of gait variability

BACKGROUND

During everyday locomotion, we cope with various internal or external perturbations (e.g. uneven surface). Uncertainty exists on how unpredictable external perturbations increase noise within the motor system and if they are compensated by employing covariation of the limb joints or rather due to decreased sensitivity of an altered posture.

RESEARCH QUESTION

Do continuous stochastic perturbations affect the structure of gait variability in young and healthy adults?

METHODS

In a cross-over study, gait kinematics of 21 healthy young sports students were registered during treadmill walking with and without continuous stochastic perturbations. Using the TNC method, the following aspects were analyzed: (a) the sensitivity of body posture to perturbations ('tolerance') decreasing gait variability, (b) the unstructured motor 'noise' increasing gait variability and (c) the amount of 'covariation' of the limb joints.

RESULTS

Compared to normal walking, gait variability was significantly increased (p < .001) during walking with perturbations. The negative effect of noise was partly compensated by improved 'covariation' of leg joints (p < .001). The aspect 'tolerance' had a small effect on increasing gait variability during stance phase (p < .001) and decreasing gait variability during swing phase (p < .001).

SIGNIFICANCE

Increased motor noise due to external perturbations is partly compensated by improved covariation of the limb joints. However, the effect of an altered posture slightly affects gait variability. Further studies should focus on different populations (e.g. older participants) to see if they use the same mechanism (improved covariation) to compensate for stochastic perturbations.

The patterning of local variability during the acquisition of a novel whole-body continuous motor skill in young adults

There is increasing evidence that movement variability during motor skill acquisition plays a functional role. Specifically, initial variability might represent exploration of the possible motor space for solutions and error identification. Following practice, individuals might exploit a reduced amount of motor solutions to execute the task. While this variability pattern has been supported during discrete upper limb and multi-finger force tasks, there is a paucity of evidence for continuous whole-body motor tasks. Therefore, the purpose of this study was to characterize the role of variability during the acquisition of a whole-body continuous motor task across practice sessions in young adults. Twelve young adults aged 18-35-years participated in this study. Subjects practiced a novel, sagittal plane task, the kettlebell swing, using an online training video. We conducted an uncontrolled manifold analysis to partition local variability of the configuration of the kettlebell and body segments based on their impact on the position of the center-of-mass (COM) in the sagittal plane. Our results demonstrated that following initial practice, variability that did not affect the COM position remained elevated, suggesting sustained exploration of motor solutions. Following multiple practice sessions, variability related to motor solutions decreased, potentially indicating exploitation. The results from this study support the proposal that young adults initially utilize a range of motor solutions when acquiring a whole-body motor skill, followed by exploitation of stereotypic movement.

Older but not younger adults rely on multijoint coordination to stabilize the swinging limb when performing a novel cued walking task

Motor flexibility, the ability to employ multiple motor strategies to meet task demands, may facilitate ambulation in complex environments that constrain movements; loss of motor flexibility may impair mobility. The purpose of this study was to determine the effects of obesity (a specific model of mobility impairment) and advanced age on motor flexibility during a task that constrained foot placement while walking. Twenty-one community-dwelling obese (OB) and 25 normal weight (NW) older adults (46 total older adults-OA) and 10 younger adults (YA) walked normally on a treadmill (baseline) then walked while stepping on lighted cues projected onto the treadmill at locations corresponding to average foot placement during normal walking (cued). The uncontrolled manifold (UCM) analysis was used to partition total variance in a set of seven lower-limb segment angles into components that did ("bad" variance) and did not ("good" variance) affect step-to-step variance in the trajectory of the swing foot. Motor flexibility was operationalized as an increase (baseline to cued) in total variance with an increase in good variance that exceeded the change in bad variance. There was no significant group × walking task interaction for total and good variance for OB vs NW, but there was a strong and significant interaction effect for OA vs YA (p < 0.01; Cohen's d > 1.0). Whereas YA reduced both good and bad variance, OA increased good variance beyond the change in bad variance. In OA, these changes were associated with several functional measures of mobility. Cued walking may place greater demands on OA requiring greater reliance on motor flexibility, although otherwise healthy older obese adults may be able to compensate for functional and cognitive declines associated with obesity by increasing motor flexibility under such tasks. The extent to which motor flexibility is employed during novel or constrained tasks may be a biomarker of healthy aging and a target for (re)habilitation.

How healthy older adults regulate lateral foot placement while walking in laterally destabilizing environments

Gait variability is generally associated with falls, but specific connections remain disputed. To reduce falls, we must first understand how older adults maintain lateral balance while walking, particularly when their stability is challenged. We recently developed computational models of lateral stepping, based on Goal Equivalent Manifolds, that separate effects of step-to-step regulation from variability. These show walking humans seek to strongly maintain step width, but also lateral position on their path. Here, 17 healthy older (ages 60+) and 17 healthy young (ages 18-31) adults walked in a virtual environment with no perturbations and with laterally destabilizing perturbations of either the visual field or treadmill platform. For step-to-step time series of step widths and lateral positions, we computed variability, statistical persistence and how much participants directly corrected deviations at each step. All participants exhibited significantly increased variability, decreased persistence and tighter direct control when perturbed. Simulations from our stepping regulation models indicate people responded to the increased variability imposed by these perturbations by either maintaining or tightening control of both step width and lateral position. Thus, while people strive to maintain lateral balance, they also actively strive to stay on their path. Healthy older participants exhibited slightly increased variability, but no differences from young in stepping regulation and no evidence of greater reliance on visual feedback, even when subjected to substantially destabilizing perturbations. Thus, age alone need not degrade lateral stepping control. This may help explain why directly connecting gait variability to fall risk has proven difficult.

The relation between limb segment coordination during walking and fall history in community-dwelling older adults

Control of the swing foot during walking is important to prevent falls. The trajectories of the swing foot are adjusted by coordination of the lower limbs, which is evaluated with uncontrolled manifold (UCM) analysis. A previous study that applied this analysis to walking revealed that older adults with fall history had compensatorily great segment coordination to stabilize the swing foot during normal walking. However, it is unknown whether the increase in segment coordination helps for preventing incident falls in the future. At baseline measurement, 30 older adults walked for 20 times at a comfortable speed. UCM analysis was performed to evaluate how the segment configuration in the lower limbs contributes to the swing foot stability. One year after the baseline visit, we asked the subjects if there were incident falls through a questionnaire. The univariate and multivariable logistic regression analyses were performed to assess the association between the index of segment coordination and incident falls with and without adjustment for gait velocity. Twenty-eight older adults who responded to the questionnaire were classified into older adults (n = 12) who had the incident fall and those (n = 16) who did not have falls. It was revealed that older adults who increased the segment coordination associated with swing foot stability tended to experience at least one fall within one year of measurement. The index of the UCM analysis can be a sensitive predictor of incident falls.

Instability Resistance Training Decreases Motor Noise During Challenging Walking Tasks in Older Adults: A 10-Week Double-Blinded RCT

Locomotor stability is challenged by internal perturbations, e.g., motor noise, and external perturbations, e.g., changes in surface compliance. One means to compensate for such perturbations is to employ motor synergies, defined here as co-variation among a set of elements that acts to stabilize, or provide similar trial-to-trial (or step-to-step) output, even in the presence of small variations in initial conditions. Whereas evidence exists that synergies related to the upper extremities can be trained, the extent to which lower limb synergies, such as those which may be needed to successfully locomote in complex environments, remains unknown. The purpose of this study was to evaluate if resistance training (RT) in unstable environments could promote coordination patterns associated with stronger synergies during gait. Sixty-eight participants between the age of 65 and 80 were randomly assigned to one of three different RT modalities: stable whole-limb machine-based RT (S-MRT), instability free-weight RT (I-FRT), and stable machine-based adductor/abductor RT (S-MRT_HIP). Before and after RT, participants walked across an even lab floor and a more challenging uneven surface with and without holding a weighted bag. The uncontrolled manifold control analysis (UCM) was used to calculate the synergy index (i.e., strength of the kinematic synergy) related to stabilization of our performance variable, the mediolateral trajectory of the swing foot, under each condition. Regardless of RT group, there was no effect of RT on the synergy index when walking across the even lab floor. However, the synergy index during the two uneven surface conditions was stronger after I-FRT but was not affected by the other RT modalities. The stronger synergy index for the I-FRT group was due to improved coordination as quantified by an overall increase in variability in elemental variable space but a decrease in the variability that negatively affects performance. The unstable environment offered by I-FRT allows for exploration of motor solutions in a manner that appears to transfer to challenging locomotor tasks. Introducing tasks that promote, rather than limit, exploration of motor solutions seems to be a valuable exercise modality to strengthen kinematic synergies that cannot be achieved with traditional strengthening paradigms (e.g., S-MRT).

Clinical Trial Registration:www.ClinicalTrials.gov, identifier NCT03017365.

Flexibility in joint coordination remains unaffected by force and balance demands in young and old adults during simple sit-to-stand tasks

PURPOSE

We examined the possibility that old adults use flexibility in joint coordination as a compensatory mechanism for the age-related decline in muscle strength when performing the sit-to-stand (STS) task repeatedly under high force and balance demands.

METHOD

Young (n = 14, 22.4 ± 2.1) and old (n = 12, 70 ± 3.2) healthy adults performed repeated STSs under high and low force and balance demands. The balance demand was manipulated by reducing the base of support and the force demand by increasing body weight with a weight vest. Uncontrolled manifold analysis was used to quantify age differences in motor flexibility.

RESULTS

While there were age-typical differences in kinematic STS strategies, flexibility in joint coordination was independent of age and task difficulty during repeated STSs.

DISCUSSION

That simple manipulations of force and balance demands did not affect flexibility in joint coordination in old and young adults suggests that motor flexibility acts as a compensatory mechanism only at the limits of available muscle strength and balance abilities during STS movements. Intervention studies should identify how changes in specific neuromuscular functions affect flexibility in joint coordination during activities of daily living such as STS.

Recommendation for the minimum number of steps to analyze when performing the uncontrolled manifold analysis on walking data

The uncontrolled manifold (UCM) analysis quantifies the extent to which co-variation among a set of variables facilitates consistent performance by partitioning variance in those variables into two components then calculating their normalized difference (i.e., the synergy index). Although UCM-derived measures are thought to depend on the number of data points analyzed, the minimum number needed to reasonably approximate true values of these measures is unknown. For each of two performance variables related to mechanical stability of gait, we evaluated changes in UCM-derived measures when increasing the number of analyzed points, here steps. Fourteen older adults walked on a treadmill while motion capture tracked movement. For each subject, n steps (where n = 2-99) were randomly sampled from the first 100, then used to calculate UCM-derived variables. For each subject, variables were expressed as a percent of the subject-specific value with n = 100 and averaged across 50 simulations. For each n, 95% confidence intervals (CIs) were calculated from group data. The minimum number of steps to "reasonably approximate" a variables was defined as the value of n for which the lower CI was >90% of the value with n = 100. Regardless of performance variable, reasonable approximations of the synergy index were attained with n = 16 steps, whereas n = 50 steps were needed for each of the variance components However, the differences between using 16 steps and 50 steps were small. Collecting 15-20 steps is recommended for a reasonable approximation of the synergy indices considered herein, particularly when data collection is constrained to a limited number of steps.