Walking on uneven terrain in healthy adults and the implications for people after stroke

AdjuSST: An Adjustable Surface Stiffness Treadmill

Humans have the remarkable ability to manage foot-ground interaction seamlessly across terrain changes despite the high dynamic complexity of the task. Understanding how adaptation in the neuromotor system enables this level of robustness in the face of changing interaction dynamics is critical for developing more effective gait retraining interventions. We developed an adjustable surface stiffness treadmill (AdjuSST) to trigger these adaptation mechanisms and enable studies to better understand human adaptation to changing foot-ground dynamics. The AdjuSST system makes use of fundamental beam-bending principles; it controls surface stiffness by controlling the effective length of a cantilever beam. The beam acts as a spring suspension for the transverse endpoint load applied through the treadmill. The system is capable of enforcing a stiffness range of 15-300kN/m within 340 ms, deflecting linearly downwards up to 10 cm, and comfortably accommodating two full steps of travel along the belt. AdjuSST offers significant enhancements in effective walking surface length compared to similar systems, while also maintaining a useful stiffness range and responsive spring suspension. These improvements enhance our ability to study locomotor control and adaptation to changes in surface stiffness, as well as provide new avenues for gait rehabilitation.

Self-reported factors associated with community ambulation after stroke: The Canadian Longitudinal Study on Aging

Community ambulation is frequently limited for people with stroke. It is, however, considered important to people with stroke. The objectives were to identify factors associated with self-reported community ambulation in Canadians aged 45+ with stroke and to identify factors associated with community ambulation specific to Canadian males and to Canadian females with stroke. Data were utilized from the Canadian Longitudinal Study on Aging Tracking Cohort. Multivariate logistic regression models were developed for community ambulation. Mean age was 68 (SE 0.5) years (45% female). In the final community ambulation model (n = 855), factors associated with being less likely to 'walk outdoors sometimes or often' included difficulty or being unable to walk 2-3 blocks (decreased endurance) vs. no difficulty. Being more likely to walk outdoors was associated with 'better weather' months and being 55-64 years of age vs 75-85. Differences were noted between the models of only males and only females. Decreased walking endurance is associated with a decreased likelihood of walking in the community-a factor that can be addressed by rehabilitation professionals and in community based programs.

Age and beta amyloid deposition impact gait speed, stride length, and gait smoothness while transitioning from an even to an uneven walking surface in older adults

BACKGROUND

Capturing a measure of movement quality during a complex walking task may indicate the earliest signs of detrimental changes to the brain due to beta amyloid (Aβ) deposition and be a potential differentiator of older adults at elevated and low risk of developing Alzheimer's disease. This study aimed to determine: 1) age-related differences in gait speed, stride length, and gait smoothness while transitioning from an even to an uneven walking surface, by comparing young adults (YA) and older adults (OA), and 2) if gait speed, stride length, and gait smoothness in OA while transitioning from an even to an uneven walking surface is influenced by the amount of Aβ deposition present in an OA's brain.

METHODS

Participants included 56 OA (>70 years of age) and 29 YA (25-35 years of age). In OA, Aβ deposition in the brain was quantified by PET imaging. All participants completed a series of cognitive assessments, a functional mobility assessment, and self-report questionnaires. Then participants performed two sets of walking trials on a custom-built walkway containing a mixture of even and uneven surface sections, including three trials with a grass uneven surface and three trials with a rocks uneven surface. Gait data were recorded using a wireless inertial measurement unit system. Stride length, gait speed, and gait smoothness (i.e., log dimensionless lumbar jerk) in the anteroposterior (AP), mediolateral (ML), and vertical (VT) directions were calculated for each stride. Outcomes were retained for five stride locations immediately surrounding the surface transition.

RESULTS

OA exhibited slower gait (Grass: p < 0.001; Rocks: p = 0.006), shorter strides (Grass: p < 0.001; Rocks: p = 0.008), and smoother gait (Grass AP: p < 0.001; Rocks AP: p = 0.002; Rocks ML: p = 0.02) than YA, but they also exhibited greater reductions in gait speed and stride length than YA while transitioning to the uneven grass and rocks surfaces. Within the OA group, those with greater Aβ deposition exhibited decreases in smoothness with age (Grass AP: p = 0.02; Rocks AP: p = 0.03; Grass ML: p = 0.04; Rocks ML: p = 0.03), while those with lower Aβ deposition exhibited increasing smoothness with age (Grass AP: p = 0.01; Rocks AP: p = 0.02; Grass ML: p = 0.08; Rocks ML: p = 0.07). Better functional mobility was associated with less smooth gait (Grass ML: p = 0.02; Rocks ML: p = 0.05) and with less variable gait smoothness (Grass and Rocks AP: both p = 0.04) in the OA group.

CONCLUSION

These results suggest that, relative to YA, OA may be adopting more cautious, compensatory gait strategies to maintain smoothness when approaching surface transitions. However, OA with greater Aβ deposition may have limited ability to adopt compensatory gait strategies to increase the smoothness of their walking as they get older because of neuropathological changes altering the sensory integration process and causing worse dynamic balance (i.e., jerkier gait). Functional mobility, in addition to age and Aβ deposition, may be an important factor of whether or not an OA chooses to employ compensatory strategies to prioritize smoothness while walking and what type of compensatory strategy an OA chooses.

Electrical Brain Activity during Human Walking with Parametric Variations in Terrain Unevenness and Walking Speed

Mobile brain imaging with high-density electroencephalography (EEG) can provide insight into the cortical processes involved in complex human walking tasks. While uneven terrain is common in the natural environment and poses challenges to human balance control, there is limited understanding of the supraspinal processes involved with traversing uneven terrain. The primary objective of this study was to quantify electrocortical activity related to parametric variations in terrain unevenness for neurotypical young adults. We used high-density EEG to measure brain activity when thirty-two young adults walked on a novel custom-made uneven terrain treadmill surface with four levels of difficulty at a walking speed tailored to each participant. We identified multiple brain regions associated with uneven terrain walking. Alpha (8 - 13 Hz) and beta (13 - 30 Hz) spectral power decreased in the sensorimotor and posterior parietal areas with increasing terrain unevenness while theta (4 - 8 Hz) power increased in the mid/posterior cingulate area with terrain unevenness. We also found that within stride spectral power fluctuations increased with terrain unevenness. Our secondary goal was to investigate the effect of parametric changes in walking speed (0.25 m/s, 0.5m/s, 0.75 m/s, 1.0 m/s) to differentiate the effects of walking speed from uneven terrain. Our results revealed that electrocortical activities only changed substantially with speed within the sensorimotor area but not in other brain areas. Together, these results indicate there are distinct cortical processes contributing to the control of walking over uneven terrain versus modulation of walking speed on smooth, flat terrain. Our findings increase our understanding of cortical involvement in an ecologically valid walking task and could serve as a benchmark for identifying deficits in cortical dynamics that occur in people with mobility deficits.

The bumpy road ahead: the role of substrate roughness on animal walking and a proposed comparative metric

Outside laboratory conditions and human-made structures, animals rarely encounter flat surfaces. Instead, natural substrates are uneven surfaces with height variation that ranges from the microscopic scale to the macroscopic scale. For walking animals (which we define as encompassing any form of legged movement across the ground, such as walking, running, galloping, etc.), such substrate 'roughness' influences locomotion in a multitude of ways across scales, from roughness that influences how each toe or foot contacts the ground, to larger obstacles that animals must move over or navigate around. Historically, the unpredictability and variability of natural environments has limited the ability to collect data on animal walking biomechanics. However, recent technical advances, such as more sensitive and portable cameras, biologgers, laboratory tools to fabricate rough terrain, as well as the ability to efficiently store and analyze large variable datasets, have expanded the opportunity to study how animals move under naturalistic conditions. As more researchers endeavor to assess walking over rough terrain, we lack a consistent approach to quantifying roughness and contextualizing these findings. This Review summarizes existing literature that examines non-human animals walking on rough terrain and presents a metric for characterizing the relative substrate roughness compared with animal size. This framework can be applied across terrain and body scales, facilitating direct comparisons of walking over rough surfaces in animals ranging in size from ants to elephants.

Comprehensive rehabilitation outcome measurement scale (CROMS): development and preliminary validation of an interdisciplinary measure for rehabilitation outcomes

INTRODUCTION

Comprehensive and interdisciplinary measurement of rehabilitation outcome is an essential part of the assessment and prognosis of a patient. Thus, this requires substantial contributions from the patient, their family and the rehabilitation professional working with them. Moreover, the measurement tool should be comprehensive and must consider the cultural compatibility, cost efficiency and contextual factors of the region.

METHODS

The Comprehensive Rehabilitation Outcome Measurement Scale (CROMS) was developed through consensus and followed the Delphi process incorporating inputs from various rehabilitation professionals. The domains and items were finalized using Principal Component Analysis (PCA). The tool was validated in two native languages and back-translated considering the semantic equivalence of the scale. Intra-class correlation coefficient was performed to determine the agreement between the therapist and patient-reported scales.

RESULTS

The final CROMS carries 32 comprehensive items that can be completed by the person with disability and the professional team. CROMS compares well to similar items on FIM (l ICC of 0.93) and has good internal consistency with a Cronbach's Alpha of 0.92 for both patient and therapist reported measures.

CONCLUSIONS

The 32 item CROMS is a tool that can potentially be used to evaluate the functional independence of various patient populations, predominantly patients with neurological disabilities.

Mobile electroencephalography captures differences of walking over even and uneven terrain but not of single and dual-task gait

Walking on natural terrain while performing a dual-task, such as typing on a smartphone is a common behavior. Since dual-tasking and terrain change gait characteristics, it is of interest to understand how altered gait is reflected by changes in gait-associated neural signatures. A study was performed with 64-channel electroencephalography (EEG) of healthy volunteers, which was recorded while they walked over uneven and even terrain outdoors with and without performing a concurrent task (self-paced button pressing with both thumbs). Data from n = 19 participants (M = 24 years, 13 females) were analyzed regarding gait-phase related power modulations (GPM) and gait performance (stride time and stride time-variability). GPMs changed significantly with terrain, but not with the task. Descriptively, a greater beta power decrease following right-heel strikes was observed on uneven compared to even terrain. No evidence of an interaction was observed. Beta band power reduction following the initial contact of the right foot was more pronounced on uneven than on even terrain. Stride times were longer on uneven compared to even terrain and during dual- compared to single-task gait, but no significant interaction was observed. Stride time variability increased on uneven terrain compared to even terrain but not during single- compared to dual-tasking. The results reflect that as the terrain difficulty increases, the strides become slower and more irregular, whereas a secondary task slows stride duration only. Mobile EEG captures GPM differences linked to terrain changes, suggesting that the altered gait control demands and associated cortical processes can be identified. This and further studies may help to lay the foundation for protocols assessing the cognitive demand of natural gait on the motor system.

Women carry for less: body size, pelvis width, loading position and energetics

The energetic cost of walking varies with mass and speed; however, the metabolic cost of carrying loads has not consistently increased proportionally to the mass carried. The cost of carrying mass, and the speed at which human walkers carry this mass, has been shown to vary with load position and load description (e.g. child vs. groceries). Additionally, the preponderance of women carriers around the world, and the tendency for certain kinds of population-level sexual dimorphism has led to the hypothesis that women might be more effective carriers than men. Here, I investigate the energetic cost and speed changes of women (N = 9) and men (N = 6) walking through the woods carrying their own babies (mean baby mass = 10.6 kg) in three different positions - on their front, side and back using the same Ergo fabric baby sling. People carrying their babies on their backs are able to maintain their unloaded walking speed (1.4 m/s) and show the lowest increase in metabolic cost per distance (J/m, 17.4%). Women carry the babies for a lower energetic cost than men at all conditions (p < 0.01). Further energetic and kinematic evidence elucidates the preponderance of back-carrying cross-culturally, and illustrates the importance of relatively wider bi-trochanteric breadths for reducing the energetic costs of carrying.

The Foot Fault Scoring System to Assess Skilled Walking in Rodents: A Reliability Study

The foot fault scoring system of the ladder rung walking test (LRWT) is used to assess skilled walking in rodents. However, the reliability of the LRWT foot fault score has not been properly addressed. This study was designed to address this issue. Two independent and blinded raters analyzed 20 rats and 20 mice videos. Each video was analyzed twice by the same rater (80 analyses per rater). The intraclass correlation coefficient (ICC) and the Kappa coefficient were employed to check the accuracy of agreement and reliability in the intra- and inter-rater analyses of the LRWT outcomes. Excellent intra- and inter-rater agreements were found for the forelimb, hindlimb, and both limbs combined in rats and mice. The agreement level was also excellent for total crossing time, total time stopped, and the number of stops during the walking path. Rating individual scores in the foot fault score system (0–6) ranged from satisfactory to excellent, in terms of the intraclass correlation indexes. Moreover, we showed that experienced and inexperienced raters can obtain reliable results if supervised training is provided. We concluded that the LRWT is a reliable and useful tool to study skilled walking in rodents and can help researchers address walking-related neurobiological questions.

Forest terrains influence walking kinematics among indigenous Tsimane of the Bolivian Amazon

Laboratory-based studies indicate that a major evolutionary advantage of bipedalism is enabling humans to walk with relatively low energy expenditure. However, such studies typically record subjects walking on even surfaces or treadmills that do not represent the irregular terrains our species encounters in natural environments. To date, few studies have quantified walking kinematics on natural terrains. Here we used high-speed video to record marker-based kinematics of 21 individuals from a Tsimane forager-horticulturalist community in the Bolivian Amazon walking on three different terrains: a dirt field, a forest trail and an unbroken forest transect. Compared with the field, in the unbroken forest participants contacted the ground with more protracted legs and flatter foot postures, had more inclined trunks, more flexed hips and knees, and raised their feet higher during leg swing. In contrast, kinematics were generally similar between trail and field walking. These results provide preliminary support for the idea that irregular natural surfaces like those in forests cause humans to alter their walking kinematics, such that travel in these environments could be more energetically expensive than would be assumed from laboratory-based data. These findings have important implications for the evolutionary energetics of human foraging in environments with challenging terrains.

The impact of outdoor walking surfaces on lower-limb coordination and variability during gait in healthy adults

BACKGROUND

Inter-joint coordination and variability during gait provide insight into control and adaptability of the neuromuscular system. To date, coordination research has been restricted to laboratory settings, and it is unclear how these findings translate to real-world, outdoor walking environments.

RESEARCH QUESTION

Compared to flat walking, to what extent do outdoor surfaces impact lower-limb inter-joint coordination and variability during gait, in healthy adults?

METHODS

Data from inertial measurement units placed on the lower-back, thigh, and shank were extracted from thirty healthy young adults (15 females, 23.5 ± 4.2 years) during outdoor walking on flat (paved sidewalk); irregular (cobblestone, grass); sloped (slope-up, slope-down); and banked (banked-right, banked-left) surfaces. Sagittal joint angles for the right knee and hip were computed and partitioned by gait phase (stance and swing). Continuous Relative Phase analysis determined inter-joint coordination and variability for the knee-hip joint pair using Mean Absolute Relative Phase (MARP) and Deviation Phase (DP), respectively. One-way repeated measures ANOVAs tested surface effects. Post-hoc Bonferroni adjusted surface comparisons were assessed.

RESULTS

Significant knee-hip surface effects were seen during all gait phases for MARP (p < 0.001) and DP (p ≤ 0.001). Compared to flat walking, grass prompted more in-phase coordination (smaller MARP) during stance and swing phase (p ≤ 0.003). Slope-up caused more in-phase coordination during stance (p < 0.001), while slope-down caused more out-of-phase coordination during stance and swing (p ≤ 0.003), compared to the flat surface. Sloped surfaces prompted more variable (larger DP) knee-hip coordination (p ≤ 0.001), compared to flat walking during stance and swing phase.

SIGNIFICANCE

Compared to flat walking, changes in knee-hip coordination and variability were greatest on slope-up/slope-down surfaces. This could reflect greater changes in lower-limb kinematics on sloped surfaces and/or a neuromuscular response to the demands of a more challenging task.

Increased Arm Swing and Rocky Surfaces Reduces Postural Control in Healthy Young Adults

Fall-induced injuries can stem from a disruption in the postural control system and place a financial burden on the healthcare system. Most gait research focused on lower extremities and neglected the contribution of arm swing, which have been shown to affect the movement of the center of mass when walking. This study evaluated the effect of arm swing on postural control and stability during regular and rocky surface walking. Fifteen healthy young adults (age = 23.4 ± 2.8) walked on these two surfaces with three arm motions (normal, held, and active) using the CAREN Extended-System (Motek Medical, Amsterdam, NL). Mean, standard deviation and maximal values of trunk linear and angular velocity were calculated in all three axes. Moreover, step length, time and width mean and coefficient of variation as well as margin of stability mean and standard deviation were calculated. Active arm swing increased trunk linear and angular velocity variability and peak values compared to normal and held arm conditions. Active arm swing also increased participants’ step length and step time, as well as the variability of margin of stability. Similarly, rocky surface walking increased trunk kinematics variability and peak values compared to regular surface walking. Furthermore, rocky surface increased the average step width while reducing the average step time. Though this surface type increased the coefficient of variation of all spatiotemporal parameters, rocky surface also led to increased margin of stability mean and variation. The spatiotemporal adaptations showed the use of “cautious” gait to mitigate the destabilizing effects of both the active arm swing and rocky surface walking and, ultimately, maintain dynamic stability.

Canan Outdoor Multisurface Terrain Enhance the Effects of Fall Prevention Exercise in Older Adults? A Randomized Controlled Trial

Walking on complex surface conditions in outdoor environments is important for active aging. This study aimed at examining whether fall prevention exercise integrated with an outdoor multisurface terrain compared with indoor solid ground was more beneficial for older adults. Twenty-two older nursing home residents were randomly assigned to outdoor multisurface terrain (n = 11, 79.5 ± 2.1 years) or indoor solid ground (n = 11, 78.8 ± 5.2 years) groups. Training occurred five times per week (30 min) for 3 weeks. The following performance test outcomes were measured: 10 m walk test (10 mWT), multisurface terrain walk test (MTWT), 2 min walk test (2 MWT), timed up and go test (TUGT), single-leg standing test with eyes open (SLSTEO), single-leg standing test with eyes closed (SLSTEC), and closed cycles test (CCT). Compared with baseline, the outdoor multisurface terrain training significantly improved performance in all tests (p < 0.01). The improvements of the outdoor multisurface terrain group after intervention were significantly higher than those of the indoor solid group in the 10 mWT (p = 0.049), MTWT (p = 0.02), and 2 MWT (p = 0.000). Exercise combined with outdoor multisurface terrain training may be an efficacious approach and a feasible environmental intervention for fall prevention in older adults.

Does the Environment Cause Changes in Hemiparetic Lower Limb Muscle Activity and Gait Velocity During Walking in Stroke Survivors?

Stroke survivors often face difficulty in community ambulation though they attain steady-state walking in clinical setups. Compliance and unpredictability of the environment may alter the muscle activity and challenge the individual's gait. Successful reintegration into the community requires gait assessment and training in a real-life challenging environment. Little is known about the assessment and training of gait in the community environment under challenging mobility dimensions. Hence, we aimed to study the changes that real-life environmental dimensions have on the activity of selected muscles in hemiparetic lower limb and gait velocity in stroke survivors.

METHODS

An observational cross-sectional study was conducted on 16 ambulatory stroke survivors to assess the hemiparetic lower limb muscle activity during walking in real-life environmental dimensions. Participants were made to walk in the community on a walkway consisting of even surface, ramp, stairs, uneven terrain and obstacles. They were also made to manoeuvre through traffic and pick a load while walking for a distance in the walkway. Muscle activity of Rectus Femoris, Biceps Femoris, Gastrocnemius Medialis and Tibialis Anterior of the paretic lower limb were continuously recorded while walking using wireless surface electromyography. Gait velocity for the entire walkway and level of perceived difficulty while walking in different dimensions were also measured. Paired t-test was used to compare the percentage Maximum Voluntary Contraction (%MVC) of lower limb muscles between even surface and real-life environment dimensions while walking. One sample t-test was used to compare the gait velocity in real-life dimensions versus gait velocity in even surface measured in an earlier study.

RESULTS

There was a significant reduction (p < 0.01) in the activity of all four hemiparetic lower limb muscles while walking under the influence of real-life environmental dimensions compared to even surface. Gait velocity (0.33 ± 0.17 m/s) was significantly lower than that is essential to be a community ambulator. The level of perceived difficulty across all dimensions was reported qualitatively with the highest difficulty reported during stair and obstacle clearance.

CONCLUSION

Real-life environmental dimensions lead to the reduction of paretic lower limb muscle activities and gait velocity during walking in community-dwelling stroke survivors. Stroke survivors perceived more difficulty while walking in real-life environment dimensions particularly while negotiating stairs and obstacles.

SIGNIFICANCE

Knowledge about the influence of real-life environmental dimensions will help the clinicians to target rehabilitation methods to improve walking adaptability.

Uneven substrates constrain walking speed in ants through modulation of stride frequency more than stride length

Natural terrain is rarely flat. Substrate irregularities challenge walking animals to maintain stability, yet we lack quantitative assessments of walking performance and limb kinematics on naturally uneven ground. We measured how continually uneven 3D-printed substrates influence walking performance of Argentine ants by measuring walking speeds of workers from laboratory colonies and by testing colony-wide substrate preference in field experiments. Tracking limb motion in over 8000 videos, we used statistical models that associate walking speed with limb kinematic parameters to compare movement over flat versus uneven ground of controlled dimensions. We found that uneven substrates reduced preferred and peak walking speeds by up to 42% and that ants actively avoided uneven terrain in the field. Observed speed reductions were modulated primarily by shifts in stride frequency instead of stride length (flat R 2: 0.91 versus 0.50), a pattern consistent across flat and uneven substrates. Mixed effect modelling revealed that walking speeds on uneven substrates were accurately predicted based on flat walking data for over 89% of strides. Those strides that were not well modelled primarily involved limb perturbations, including missteps, active foot repositioning and slipping. Together these findings relate kinematic mechanisms underlying walking performance on uneven terrain to ecologically relevant measures under field conditions.

Assessment of backward walking unmasks mobility impairments in post-stroke community ambulators

Background: While over half of stroke survivors recover the ability to walk without assistance, deficits persist in the performance of walking adaptations necessary for safe home and community mobility. One such adaptation is the ability to walk or step backward. Post-stroke rehabilitation rarely includes backward walking (BW) assessment and BW deficits have not been quantified in post-stroke community ambulators. Objective: To quantify spatiotemporal and kinematic BW characteristics in post-stroke community ambulators and compare their performance to controls. Methods: Individuals post-stroke (n = 15, 60.1 ± 12.9 years, forward speed: 1.13 ± 0.23 m/s) and healthy adults (n = 12, 61.2 ± 16.2 years, forward speed: 1.40 ± 0.13 m/s) performed forward walking (FW) and BW during a single session. Step characteristics and peak lower extremity joint angles were extracted using 3D motion analysis and analyzed with mixed-method ANOVAs (group, walking condition). Results: The stroke group demonstrated greater reductions in speed, step length and cadence and a greater increase in double-support time during BW compared to FW (p < .01). Compared to FW, the post-stroke group demonstrated greater reductions in hip extension and knee flexion during BW (p < .05). The control group demonstrated decreased plantarflexion and increased dorsiflexion during BW, but these increases were attenuated in the post-stroke group (p < .05). Conclusions: Assessment of BW can unmask post-stroke walking impairments not detected during typical FW. BW impairments may contribute to the mobility difficulties reported by adults post-stroke. Therefore, BW should be assessed when determining readiness for home and community ambulation.

Models for temporal-spatial parameters in walking with cadence ratio as the independent variable

Accurate models that describe temporal-spatial parameters are desirable in gait estimation and rehabilitation. This study aimed to explore simple but relatively accurate models to describe stride length (SL), speed (SP) and walk ratio (WR) at various cadences. Twenty-four able-bodied participants (16 in a test group and 8 in a validation group) walked at seven cadence ratios (CRs). The individual and group mean SL, SP and WR were studied. Suitable temporal-spatial model structures were proposed and used to approximate the individual SL, SP and WR at various CRs. After the temporal-spatial model structures were found to be feasible, the general temporal-spatial models were analysed using the test group mean SL, SP and WR. Accuracy was assessed using the validation group mean values. Individual approximation accuracies showed that the proposed model structure deduced from the linear SL model was suitable for WR approximation. The linear, deduced quadratic and power functions approximated the individual SL, SP and WR, respectively, with high accuracy. Based on the test group mean SL, SP and WR, the general temporal-spatial models were obtained and produced comparable approximation accuracies in the validation group. The general temporal-spatial models predicted well the individual gait parameters with similar individual errors for both groups. These temporal-spatial models clearly describe SL, SP and especially WR at various cadences. They provide accurate reference data for gait estimation and have potential to guide speed modulation in robot-assisted gait rehabilitation.

Graphical abstract