DFA as a window into postural dynamics supporting task performance: does choice of step size matter?

Introduction: Detrended Fluctuation Analysis (DFA) has been used to investigate self-similarity in center of pressure (CoP) time series. For fractional gaussian noise (fGn) signals, the analysis returns a scaling exponent, DFA-α, whose value characterizes the temporal correlations as persistent, random, or anti-persistent. In the study of postural control, DFA has revealed two time scaling regions, one at the short-term and one at the long-term scaling regions in the diffusion plots, suggesting different types of postural dynamics. Much attention has been given to the selection of minimum and maximum scales, but the choice of spacing (step size) between the window sizes at which the fluctuation function is evaluated may also affect the estimates of scaling exponents. The aim of this study is twofold. First, to determine whether DFA can reveal postural adjustments supporting performance of an upper limb task under variable demands. Second, to compare evenly-spaced DFA with two different step sizes, 0.5 and 1.0 in log2 units, applied to CoP time series. Methods: We analyzed time series of anterior-posterior (AP) and medial-lateral (ML) CoP displacement from healthy participants performing a sequential upper limb task under variable demand. Results: DFA diffusion plots revealed two scaling regions in the AP and ML CoP time series. The short-term scaling region generally showed hyper-diffusive dynamics and long-term scaling revealed mildly persistent dynamics in the ML direction and random-like dynamics in the AP direction. There was a systematic tendency for higher estimates of DFA-α and lower estimates for crossover points for the 0.5-unit step size vs. 1.0-unit size. Discussion: Results provide evidence that DFA-α captures task-related differences between postural adjustments in the AP and ML directions. Results also showed that DFA-α estimates and crossover points are sensitive to step size. A step size of 0.5 led to less variable DFA-α for the long-term scaling region, higher estimation for the short-term scaling region, lower estimate for crossover points, and revealed anomalous estimates at the very short range that had implications for choice of minimum window size. We, therefore, recommend the use of 0.5 step size in evenly spaced DFAs for CoP time series similar to ours.

Intellectual Humility: How Recognizing the Fallibility of Our Beliefs and Owning Our Limits May Create a Better Relationship Between the Physical Therapy Profession and Disability.

Despite the many advancements over the history of the profession, physical therapy remains in a somewhat paradoxical relationship with disability. The physical therapist profession values disability as diversity but continues to focus on the normalization of body functions as the primary means to promote functionality in people with disability. This focus, consistent with a medicalized view of disability, may prevent physical therapists from empowering individuals with disability to explore alternative, yet effective, perceptual-motor strategies to achieve their functional goals. Additionally, recent research documents implicit, negative biases of physical therapists and physical therapist assistants toward people with disability, again consistent with the medicalized view that disability is the product of an imperfectly functioning body. Dominant underlying beliefs in any profession are often difficult to counter because they are so pervasive, and those beliefs can be reinforced and made stronger when challenged. The purpose of this Perspective article is to introduce physical therapists to a rising construct in psychology-intellectual humility-that may help to facilitate the profession's relationship with disability. Intellectual humility is predicated on recognizing the fallibility of one's beliefs and related practices. Intellectual humility is a promising construct for physical therapy to address the disability paradox and confront implicit attitudes that have served as the basis for many dominant ideas about disability. This Perspective synthesizes views and evidence from the behavioral and social sciences, philosophy, and critical disability studies to contribute to the ongoing evolution of the profession with respect to disability. IMPACT: The development of enhanced intellectual humility in physical therapy may help to challenge long-held beliefs among physical therapists about disability-many of which are unnoticed, unquestioned, and difficult to counter.

Precision aiming performance with the paretic upper limb is associated with center of pressure patterns in individuals with chronic stroke.

BACKGROUND: Individuals with stroke commonly demonstrate upper-limb sensorimotor impairments. Upper-limb tasks occur against a background level of postural control and thus require a flexible postural control system to facilitate performance. Anterior precision aiming tasks, for example, benefit from lower medial-lateral (ML) center of pressure (COP) fluctuations (where increased fluctuations erode performance) relative to anterior-posterior (AP) fluctuations (where increased fluctuations do not strongly influence performance). After stroke, individuals may compensate for upper-limb impairments by increasing trunk movement which increases overall COP fluctuations and thus may make it more difficult to modulate COP in a task-sensitive manner. RESEARCH QUESTION: Do upper-limb task demands modulate COP movement patterns after stroke? METHODS: In this cross-sectional study, adults with chronic stroke (n = 23) and unilateral upper-limb impairments were immersed in a virtual environment displaying an anterior target. Participants aimed to maintain the position of a virtual laser pointer (via handheld controller) in the target with each hand. COP was concurrently recorded. Mixed effects models and correlations were used to detect differences in COP patterns between limbs and movement planes and evaluate associations between task performance and COP patterns, respectively. RESULTS: Participants showed greater COP standard deviation and regularity in the AP compared to the ML direction. The magnitude of difference between AP and ML COP metrics was greater using the nonparetic limb. Task performance was moderately and positively associated with task-sensitive COP patterns (i.e., higher AP:ML ratios of COP metrics) using the paretic upper limb. Participants consistently demonstrated high levels of task performance and task-sensitive COP movement patterns using the nonparetic limb. SIGNIFICANCE: Impairments in postural control after stroke may be related to the upper limb used. It is important to recognize the role of directional COP variability and regularity in the context of a task goal after stroke.

Narrowing the physiotherapy knowledge-practice gap: faculty training beyond the health sciences.

Physiotherapists seek to improve client movement and promote function within an individual's unique environmental and social realities. Despite this intention, there is a well-noted knowledge-practice gap, that is, therapists generally lack sufficient foundational preparation to effectively navigate societal challenges impacting contemporary healthcare. As one step toward addressing the issue, we propose an educational solution targeting current and future physiotherapy faculty, whose responsibilities for entry-level course development and curriculum design substantially impact student readiness for clinical practice. We propose that physiotherapy faculty trained via postprofessional education in a non-biomedical field (e.g. psychology, education, and philosophy) will be uniquely prepared to provide students with tools for dealing with complex social issues facing their clients; critical analysis skills; statistical and technological training; and a deeper theoretical and philosophical understanding of practice. Taken together, such interdisciplinary tools could help address the knowledge-practice gap for physiotherapists and promote the ongoing evolution of the profession in concert with contemporary healthcare. Physiotherapists who pursue interdisciplinary studies may more deeply understand the challenges faced by clinicians and may be well-positioned to leverage knowledge and methods in another scientific discipline to expand and transform the scope of solutions to these challenges.

Personal factors understood through the Ecological-Enactive Model of Disability and implications for rehabilitation research.

The International Classification of Functioning, Disability and Health (ICF) recognizes that disability arises from the interaction between an individual with a medical condition and the context in which they are embedded. Context in the ICF is comprised of environmental and personal factors. Personal factors, the background life and lifestyle of an individual, are poorly understood in rehabilitation. There is limited knowledge about how personal and environmental factors interact to shape the contextual conditions critical for explaining functioning and disability. In this paper, we explore how a newly proposed model of disability, the Ecological-Enactive Model of Disability, can enhance understanding of personal factors across multiple rehabilitation disciplines. We draw from a review of evidence and phenomenological interviews of individuals with Friedreich's Ataxia. We consider the practical impact of this understanding on disability and rehabilitation research and pathways for the future focusing on representative design.

A Dynamical Approach to the Uncontrolled Manifold: Predicting Performance Error During Steady-State Isometric Force Production.

The uncontrolled manifold (UCM) approach quantifies the presence of compensatory variability between musculoskeletal elements involved in a motor task. This approach has proved useful for identifying synergistic control strategies for a variety of everyday motor tasks and for investigating how control strategies are affected by motor pathology. However, the UCM approach is limited in its ability to relate compensatory motor variance directly to task performance because variability along the UCM is mathematically agnostic to performance. We present a new approach to UCM analysis that quantifies patterns of irregularity in the compensatory variability between motor elements over time. In a bimanual isometric force stabilization task, irregular patterns of compensation between index fingers predicted greater performance error associated with difficult task conditions, in particular for individuals who exploited a larger set of compensatory strategies (i.e., a larger subspace of the UCM). This relationship between the amount and structure of compensatory motor variance might be an expression of underlying processes supporting performance resilience.

The Intelligent Phenotypic Plasticity Platform (IP3) for Precision Medicine-Based Injury Prevention in Sport.

The best predictor of future injury is previous injury and this has not changed in a quarter century despite the introduction of evidence-based medicine and associated revisions to post-injury treatment and care. Nearly nine million sports-related injuries occur annually, and the majority of these require medical intervention prior to clearance for the athlete to return to play (RTP). Regardless of formal care, these athletes remain two to four times more likely to suffer a second injury for several years after RTP. In the case of children and young adults, this sets them up for a lifetime of negative health outcomes. Thus, the initial injury is the tipping point for a post-injury cascade of negative sequelae exposing athletes to more physical and psychological pain, higher medical costs, and greater risk of severe long-term negative health throughout their life. This chapter details the technologies and method that make up the automated Intelligent Phenotypic Plasticity Platform (IP3)-a revolutionary new approach to the current standard of post-injury care that identifies and targets deficits that underly second injury risk in sport. IP3 capitalizes on the biological concept of phenotypic plasticity (PP) to quantify an athlete's functional adaptability across different performance environments, and it is implemented in two distinct steps: (1) phenomic profiling and (2) precision treatment. Phenomic profiling indexes the fitness and subsequent phenotypic plasticity of an individual athlete, which drives the personalization of the precision treatment step. IP3 leverages mixed-reality technologies to present true-to-life environments that test the athlete's ability to adapt to dynamic stressors. The athlete's phenotypic plasticity profile is then used to drive a precision treatment that systematically stresses the athlete, via a combination of behavioral-based and genetic fuzzy system models, to optimally enhance the athlete's functional adaptability. IP3 is computationally light-weight and, through the integration with mixed-reality technologies, promotes real-time prediction, responsiveness, and adaptation. It is also the first ever phenotypic plasticity-based precision medicine platform, and the first precision sports medicine platform of any kind.

Task dynamics define the contextual emergence of human corralling behaviors.

Social animals have the remarkable ability to organize into collectives to achieve goals unobtainable to individual members. Equally striking is the observation that despite differences in perceptual-motor capabilities, different animals often exhibit qualitatively similar collective states of organization and coordination. Such qualitative similarities can be seen in corralling behaviors involving the encirclement of prey that are observed, for example, during collaborative hunting amongst several apex predator species living in disparate environments. Similar encirclement behaviors are also displayed by human participants in a collaborative problem-solving task involving the herding and containment of evasive artificial agents. Inspired by the functional similarities in this behavior across humans and non-human systems, this paper investigated whether the containment strategies displayed by humans emerge as a function of the task's underlying dynamics, which shape patterns of goal-directed corralling more generally. This hypothesis was tested by comparing the strategies naïve human dyads adopt during the containment of a set of evasive artificial agents across two disparate task contexts. Despite the different movement types (manual manipulation or locomotion) required in the different task contexts, the behaviors that humans display can be predicted as emergent properties of the same underlying task-dynamic model.

The Effect of Navigation Demand on Decision Making in a Dynamic, Sport-Inspired Virtual Environment.

Athletes commonly make decisions about the passability of closing gaps when navigating sport environments. This study examined whether increased temporal pressure to arrive at a desired location modifies these decisions. Thirty participants navigated toward a waypoint in a virtual, sport-inspired environment. To do so, they had to decide whether they could pass through closing gaps of virtual humans (and take the shortest route) or steer around them (and take a longer route). The decision boundary of participants who were time pressured to arrive at a waypoint was biased toward end gaps of smaller sizes and was less reliably defined, resulting in a higher number of collisions. Effects of temporal pressure were minimized with experience in the experimental task. Results indicate that temporal pressure affects perceptual-motor processes supporting information pickup and shapes the information-action coupling that drives compliance with navigation demands. Theoretical and practical implications are discussed.

Grasping Embodiment: Haptic Feedback for Artificial Limbs.

Upper-limb prostheses are subject to high rates of abandonment. Prosthesis abandonment is related to a reduced sense of embodiment, the sense of self-location, agency, and ownership that humans feel in relation to their bodies and body parts. If a prosthesis does not evoke a sense of embodiment, users are less likely to view them as useful and integrated with their bodies. Currently, visual feedback is the only option for most prosthesis users to account for their augmented activities. However, for activities of daily living, such as grasping actions, haptic feedback is critically important and may improve sense of embodiment. Therefore, we are investigating how converting natural haptic feedback from the prosthetic fingertips into vibrotactile feedback administered to another location on the body may allow participants to experience haptic feedback and if and how this experience affects embodiment. While we found no differences between our experimental manipulations of feedback type, we found evidence that embodiment was not negatively impacted when switching from natural feedback to proximal vibrotactile feedback. Proximal vibrotactile feedback should be further studied and considered when designing prostheses.

Concurrent Validation and Reference Values of Gluteus Medius Clinical Test.

CONTEXT: The hip abductor muscles, mainly the gluteus medius, are responsible for controlling hip adduction in a closed kinetic chain. Frontal plane knee alignment, assessed during functional activities such squatting, jumping and running, may overload joint structures, like the anterior cruciate ligament and patellofemoral joint. The hand-held dynamometer is reliable and effective for testing the muscular strength of the hip abductors. OBJECTIVES: 1. To assess the concurrent validity between the gluteus medius clinical test and a maximum isometric force test of the hip abductors using the hand-held dynamometer; (2) to determine the intra and inter-examiner reliability for the application of the gluteus medius clinical test; and (3) to describe reference values of gluteus medius clinical test on a population of youth athletes. DESIGN: Cross-sectional. METHODS: Thirty healthy individuals were recruited for validity and reliability testing. On the first day, participants performed the maximal isometric test of the hip abductors, measured via hand-held dynamometry. On the following week, the gluteus medius clinical test was performed. Intraclass correlation coefficients (ICC2,2) were computed for the reliability analysis, with a 95% confidence interval. To generate reference values, the gluteus medius clinical test was performed on 273 athletes. RESULTS: The results of this study indicated a weak positive correlation (r = 0.436, p = 0.001) between tests, which indicates that they examine different domains of gluteus medius muscle function, likely endurance and muscle strength. The magnitude of computed ICCs (>0.95) indicates excellent intra- and inter-examiner reliability. CONCLUSION: The findings of the current study indicate that the gluteus medius clinical test is reliable and examines a domain of muscular function not fully captured by HHD. The clinical test developed in this study is low-cost and can be included for gluteus medius assessment. LEVEL OF EVIDENCE: Level 3.

Grip force anticipation of nonlinear, underactuated load force.

Feedforward internal model-based control enabled by efference copies of motor commands is the prevailing theoretical account of motor anticipation. Grip force control during object manipulation-a paradigmatic example of motor anticipation-is a key line of evidence for that account. However, the internal model approach has not addressed the computational challenges faced by the act of manipulating mechanically complex objects with nonlinear, underactuated degrees of freedom. These objects exhibit complex and unpredictable load force dynamics which cannot be encoded by efference copies of underlying motor commands, leading to the prediction from the perspective of an efference copy-enabled feedforward control scheme that grip force should either lag or fail to coordinate with changes in load force. In contrast to that prediction, we found evidence for strong, precise, anticipatory grip force control during manipulations of a complex object. The results are therefore inconsistent with the internal forward model approach and suggest that efference copies of motor commands are not necessary to enable anticipatory control during active object manipulation.NEW & NOTEWORTHY From the perspective of feedforward internal model-based control, precise, anticipatory grip force (GF) control when manipulating a complex object should not be possible as the object's changing load forces (LFs) cannot be encoded by efference copies of the underlying movements. However, we observed that GF exhibited strong, precise, anticipatory coupling with LF during extended manipulations of a complex object. These findings suggest that an alternative theoretical framework is needed to account for anticipatory GF control.

A novel single-leg squat test with speed and accuracy requirements: Reliability and validity in anterior cruciate ligament reconstructed individuals.

BACKGROUND: Some traditional single-leg squat tests focused on number of repetitions may not demand precise control of lower limb dynamic alignment, especially in the frontal and transverse planes. The primary objective of this study was to evaluate test-retest reliability and construct validity of a novel single-leg squat test - the 'precision-squat test' (PST) - designed to assess performance under varying task demands that can impact the execution of lower limb movements. A secondary objective was to investigate whether musculoskeletal factors predict performance in the PST in healthy individuals. METHODS: Thirty healthy participants were assessed to verify test-retest reliability. To verify the test's construct validity, we compared the performance of 21 anterior cruciate ligament reconstructed (ACLR) individuals and 21 matched controls. Finally, 36 healthy individuals were assessed to verify the musculoskeletal factors related to PST performance. All participants performed the PST: they executed single-leg squats while moving a laser pointer (attached to the thigh) between two targets. We varied target size and distance between targets to manipulate the task difficulty. RESULTS: Reliability of the PST was excellent at all demand levels (intraclass correlation coefficient (ICC)(3,2) > 0.93). Squat time increased under test conditions involving higher task difficulty (P < 0.001) and in ACLR individuals compared with age-matched controls (P < 0.05). Regression analyses revealed that reduced knee extensors and hip external rotators torques are related to increased squat time (P < 0.05). CONCLUSIONS: PST is a valid and reliable tool to assess performance of healthy and ACLR individuals. In addition, hip and knee strength are associated with performance during the test.

The dynamics of plant nutation.

In this article we advance a cutting-edge methodology for the study of the dynamics of plant movements of nutation. Our approach, unlike customary kinematic analyses of shape, period, or amplitude, is based on three typical signatures of adaptively controlled processes and motions, as reported in the biological and behavioral dynamics literature: harmonicity, predictability, and complexity. We illustrate the application of a dynamical methodology to the bending movements of shoots of common beans (Phaseolus vulgaris L.) in two conditions: with and without a support to climb onto. The results herewith reported support the hypothesis that patterns of nutation are influenced by the presence of a support to climb in their vicinity. The methodology is in principle applicable to a whole range of plant movements.

Early learning differences between intra- and interpersonal interlimb coordination.

Intrinsic coordination patterns exist between limbs such that 1) coordination at these states is inherently stable, 2) any other pattern requires learning to produce, and 3) this learning is subject to interference from a systemic bias towards intrinsic patterns. The dynamics that govern intrapersonal interlimb coordination also govern interpersonal coordination. However, intrapersonal coordination exhibits greater coupling strength and thus more stable intrinsic dynamics than interpersonal coordination. Because the strength of intrinsic coordination tendencies has consequences for learning coordination patterns, the differences in coupling strength between intra- and interpersonal coordination should impact the ability to perform new coordination patterns via greater or less interference from intrinsic dynamics. This was investigated by measuring participants' performance as they learned a new coordination pattern alone (intrapersonal) or in pairs (interpersonal). Participants were implicitly tasked with learning the pattern as they separately controlled the vertical and horizontal position of an on-screen cursor to trace a circling target. We observed better performance of dyads on first trial and steeper learning trajectories for individuals. Overall, these results indicate that individuals experienced greater interference from stronger intrinsic coordination dynamics during early learning but could overcome this interference and achieve similar performance to that of dyads with very little practice.

Children and adolescents with cerebral palsy flexibly adapt grip control in response to variable task demands.

BACKGROUND: Children and adolescents with cerebral palsy demonstrate impairments in grip control with associated limitations in functional grasp. Previous work in cerebral palsy has focused on grip control using relatively predictable task demands, a feature which may limit generalizability of those study results in light of recent evidence in typically developing adults suggesting that grip control strategies are task-dependent. The purpose of this study was to determine whether and how varying upper extremity task demands affect grip control in children and adolescents with cerebral palsy. METHODS: Children and adolescents with mild spastic cerebral palsy (n = 10) and age- and gender-matched typically developing controls (n = 10) participated. Participants grasped an object while immersed in a virtual environment displaying a moving target and a virtual representation of the held object. Participants aimed to track the target by maintaining the position of the virtual object within the target as it moved in predictable and unpredictable trajectories. FINDINGS: Grip control in children with cerebral palsy was less efficient and less responsive to object load force than in typically developing children, but only in the predictable trajectory condition. Both groups of participants demonstrated more responsive grip control in the unpredictable compared to the predictable trajectory condition. INTERPRETATION: Grip control impairments in children with cerebral palsy are task-dependent. Children and adolescents with cerebral palsy demonstrated commonly observed grip impairments in the predictable trajectory condition. Unpredictable task demands, however, appeared to attenuate impairments and, thus, could be exploited in the design of therapeutic interventions.

Antifragility in Climbing: Determining Optimal Stress Loads for Athletic Performance Training.

In the past decades, much research has examined the negative effects of stressors on the performance of athletes. However, according to evolutionary biology, organisms may exhibit growth under stress, a phenomenon called antifragility. For both coaches and their athletes, a key question is how to design training conditions to help athletes develop the kinds of physical, physiological, and behavioral adaptations underlying antifragility. An answer to this important question requires a better understanding of how individual athletes respond to stress or loads in the context of relevant sports tasks. In order to contribute to such understanding, the present study leverages a theoretical and methodological approach to generate individualized load-response profiles in the context of a climbing task. Climbers (n = 37) were asked to complete different bouldering (climbing) routes with increasing loading (i.e. difficulty). We quantified the behavioral responses of each individual athlete by mathematically combining two measures obtained for each route: (a) maximal performance (i.e. the percentage of the route that was completed) and (b) number of attempts required to achieve maximal performance. We mapped this composite response variable as a function of route difficulty. This procedure resulted in load-response curves that captured each athlete's adaptability to stress, termed phenotypic plasticity (PP), specifically operationalized as the area under the generated curves. The results indicate individual load-response profiles (and by extension PP) for athletes who perform at similar maximum levels. We discuss how these profiles might be used by coaches to systematically select stress loads that may be ideally featured in performance training.

Effects of baby walker use on the development of gait by typically developing toddlers.

BACKGROUND: Decisions about the use of baby walker are in part predicated on caregivers´ beliefs about its effect on gait development. The actual effects of baby walkers, however, have not been established. RESEARCH QUESTION: What are the effects of the use of baby walker prior to gait onset on age of acquisition of this milestone and on early walking kinematics? METHODS: Thirty-two toddlers, 16 in the baby walker group (BWG) and 16 in the non-users group (BWNG), were evaluated in the week of gait acquisition and monthly up to six months after this event. Spatial and temporal gait parameters and lower limb kinematics during walking were assessed using a tridimensional motion analysis system. An independent t-test compared age of gait acquisition between groups. A mixed ANOVA examined the effects of group, moment of assessment and the group x moment of assessment interaction effect on the amplitude of joint motions during walking and on spatial and temporal gait parameters. RESULTS: The age of gait acquisition was not different between groups. BWG had lower gait speed (specifically in the first, third, fourth, and fifth months after gait acquisition) and longer duration of stance and swing phases than BWNG. Additionally, BWG had smaller knee amplitude and greater hip amplitude in sagittal plane than BWNG in the week of gait acquisition. SIGNIFICANCE: The results demonstrated that there is no delay in the age of gait acquisition, but there are differences in kinematics. These results can contribute to evidence-based recommendations by health care professionals about the use of baby walker by toddlers during emergence and early development of gait.

Flexible organization of grip force control during movement frequency scaling.

The grip force applied to maintain grasp of a handheld object has been typically reported as tightly coupled to the load force exerted by the object as it is actively manipulated, occurring proportionally and consistently in phase with changes in load force. However, continuous grip force-load force coupling breaks down when overall load force levels and oscillation amplitudes are lower (Grover F, Lamb M, Bonnette S, Silva PL, Lorenz T, Riley MA. Exp Brain Res 236: 2531-2544, 2018) or more predictable (Grover FM, Nalepka P, Silva PL, Lorenz T, Riley MA. Exp Brain Res 237: 687-703, 2019). Under these circumstances, grip force is instead only intermittently coupled to load force; continuous coupling is prompted only when load force levels or variations become sufficiently high or unpredictable. The current study investigated the nature of the transition between continuous and intermittent modes of grip force control by scaling the load force level and the oscillation amplitude continuously in time by means of scaling the required frequency of movement oscillations. Participants grasped a cylindrical object between the thumb and forefinger and oscillated their arm about the shoulder in the sagittal plane. Oscillation frequencies were paced with a metronome that scaled through an ascending or descending frequency progression. Due to greater accelerations, faster frequencies produced greater overall load force levels and more pronounced load oscillations. We observed smooth but nonlinear transitions between clear regimes of intermittent and continuous grip force-load force coordination, for both scaling directions, indicating that grip force control can flexibly reorganize as parameters affecting grasp (e.g., variations in load force) change over time.NEW & NOTEWORTHY Grip force (GF) is synchronously coupled to changing load forces (LF) during object manipulation when LF levels are high or unpredictable, but only intermittently coupled to LF during less challenging grasp conditions. This study characterized the nature of transitions between synchronous and intermittent GF-LF coupling, revealing a smooth but nonlinear change in intermittent GF modulation in response to continuous scaling of LF amplitude. Intermittent, "drift-and-act" control may provide an alternative framework for understanding GF-LF coupling.