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.

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.

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.

Virtual auditory aperture passability.

Two experiments investigated (1) the ability of individuals to perceive the passability of apertures that are constructed using two virtual sounds sources and (2) the nature of the perceptual information that is used when determining passability in such a way. In the first experiment, participants judged whether they could successfully walk between two sound sources, heard through headphones, without turning their shoulders. We hypothesized that judgements would be accurate and driven by the detection of a proposed informational variable that relates head rotation, forward locomotion and aperture width. To test this hypothesis, we used motion tracking and a gain manipulation to alter apparent head rotation relative to virtual sound source positions and evaluated the effect on performance. Participants were able to accurately judge aperture passability based only on acoustic information. However, the gain manipulation did not show a significant influence on perceptual reports. The unexpected significant influence of lateral head movement on perceptual accuracy, however, does suggest that an alternative informational variable, based on lateral movement, may have been used. In the second experiment, a group of participants with wide shoulders was compared to a group with narrow shoulders on a similar task. Significant differences in minimally acceptable aperture width were found between the wide and narrow groups. When these aperture widths were scaled to the participants' shoulder widths, however, the differences were no longer present. These findings are consistent with previous studies investigating perception of passability and offer promising applications of virtual reality technology in the study of auditory perceptual abilities.

Variable and intermittent grip force control in response to differing load force dynamics.

A recent study (Grover et al. Exp Brain Res 236(10):2531-2544, 2018) found that the grip force applied to maintain grasp of a hand-held object exhibited intermittent coupling to the changing load forces exerted by the object as it was oscillated. In particular, the strength and consistency of grip force response to load force oscillations was tied to overall load force levels and the prominence of load force oscillations. This contrasts with previous reports of grip force-load force coupling as generally continuous and stable and, therefore, has implications for theoretical accounts of grip force control that are predicated on these prior understandings of the coupling. The finding of intermittency additionally raises questions about the consistency of the temporal relation (i.e., lead/lag) between grip force and load force over time. The objective of the current study was, therefore, to investigate how the time-varying pattern (i.e., the regularity vs. complexity) of load force variations contribute to shifts between more intermittent and more continuous grip force control, and to determine the temporal consistency of the coupling. It was found that grip force became more tightly and continuously responsive to load force as load force changes became less predictable. Additionally, we report strong evidence that the temporal (i.e., lead/lag) relation between grip force and load force and the strength of their coupling vary substantially over time.

Biofilm Formation in Different Salmonella Serotypes Isolated from Poultry.

Little is known about Salmonella biofilm assembly, making the prevention of the disease a challenge in the poultry production chain. The objective of the present study was then to evaluate biofilm formation from different serotypes of Salmonella spp. in both polystyrene plates and eggshells. Salmonella Gallinarum and S. Minnesota were both classified as producers of biofilms of moderate intensity. Interestingly, S. Gallinarum produces biofilm even though being a serotype without flagellum and not having the lux gene in its genome, suggesting that there might be other important structures and genes associated with biofilm formation. Regarding Enteritidis, Typhimurium, Typhimurium variant, and Heidelberg serotypes, despite having high counts, BFI (Biofilm Formation Index) showed low biofilm production, probably due to the scarcity of extracellular matrix produced by such strains. A turkey eggshell model was then used for S. Enteritidis and S. Heidelberg biofilm formation. The results from the microbial count and scanning electron microscopy showed that Salmonella serotypes were also able to generate biofilm in eggshells, suggesting the presence of biofilms in poultry producing farms, a main concern for the poultry production industry.

'What's my risk of sustaining an ACL injury while playing football (soccer)?' A systematic review with meta-analysis.

OBJECTIVE: To estimate the incidence proportion (IP) and incidence rate (IR) of ACL injury in football players. DESIGN: Systematic review with meta-analysis. DATA SOURCES: PubMed, CINAHL and SPORTDiscus electronic databases were searched from inception to 20 January 2017. ELIGIBILITY CRITERIA FOR SELECTING STUDY: Studies that reported the total number of participants/population by sex, total number of ACL injuries by sex and total person-time by sex were included. RESULTS: Twenty-eight studies were included. The IP and IR of ACL injury in female football players were 2.0% (95% CI 1.2% to 3.1%) and 2.0/10 000 athlete exposures (AEs) (95% CI 1.6 to 2.6; I2=91%) over a period of one season to 4 years. The IP and IR of ACL injury in male players were 3.5% (95% CI 0.7% to 8.2%) and 0.9/10 000 AEs (95% CI 0.7 to 1.1; I2=94%). Studies that evaluated matched cohorts of female and male players showed no difference in IP (relative risk=1.2; 95% CI 0.9 to 1.6; P=0.47) over a period of one season to 4 years. Women were at greater risk than men (incidence rate ratio (IRR)=2.2; 95% CI 1.6 to 3.1; I2=83%; P<0.001). When accounting for participation level, the difference in IR between women and men was greatest for intermediate players (IRR=2.9; 95% CI 2.4 to 3.6) compared with amateur (IRR=2.6; 95% CI 1.4 to 4.8) and elite (IRR=2.0; 95% CI 1.1 to 3.4) players. SUMMARY/CONCLUSION: Overall, more men sustained ACL injury in football. There was no difference in the relative risk of ACL injury between female and male football players in a window that spanned one season to 4 years. The IR of ACL injury among women was 2.2 times higher than the IR of ACL injury among men. The reported sex disparity in ACL injury was independent of participation level.

Intermittent coupling between grip force and load force during oscillations of a hand-held object.

Tightly coordinated grip force adaptations in response to changing load forces have been reported as continuous, stable, and proportional to the load force changes. Considering the existence of inherent sensorimotor feedback delays, current accounts of grip force-load force coupling invoke explicit predictive mechanisms in the form of internal models for feedforward control to account for anticipatory grip force modulations. However, recent findings suggest that the stability and regularity of grip force-load force coupling is less persistent than previously thought. Thus, the objective of the current study was to comprehensively quantify the time-varying characteristics of grip force-load force coupling. Investigations into the coupling's dynamics during continuous 30 s bouts of load force oscillation revealed intermittent phases of coordination, as well as phases that varied in stability, rather than a persistent and continuously stable pattern of coordination. These findings have important implications for accounts of grip force-load force coupling and of anticipation in motor control, more broadly.

The Amount and Pattern of Reciprocal Compensations Predict Performance Stability in a Visually Guided Finger Force Production Task.

Previous work suggests that synergistic activity among motor elements implicated in force production tasks underlies enhanced performance stability associated with visual feedback. A hallmark of synergistic activity is reciprocal compensation, that is, covariation in the states of motor elements that stabilizes critical performance variables. The present study examined if characteristics of reciprocal compensation are indicators of individuals' capacity to respond adaptively to variations in the resolution of visual feedback about criterion performance. Twenty healthy adults (19.25 ± 1.25 years; 15 females and five males) pressed two sensors with their index fingers to produce a total target force equivalent to 20% of their maximal voluntary contraction under nine conditions that differed in the spatial resolution of real-time feedback about their performance. By combining within-trial uncontrolled manifold and sample entropy analyses, we quantified the amount and degree of irregularity (i.e., non-repetitiveness) of reciprocal compensations over time. We found a U-shaped relationship between performance stability and gain. Importantly, this relationship was moderated by the degree of irregularity of reciprocal compensation. Lower irregularity in reciprocal compensation patterns was related to individuals' capacity to maintain (or minimize losses in) performance under changes in feedback resolution. Results invite future investigation into how interindividual variations in reciprocal compensation patterns relate to differences in control strategies supporting adaptive responses in complex, visually guided motor tasks.

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.

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.

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.

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.

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.

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.

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.

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.

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.