Scott Bonnette

A tutorial introduction to adaptive fractal analysis

The authors present a tutorial description of adaptive fractal analysis (AFA). AFA utilizes an adaptive detrending algorithm to extract globally smooth trend signals from the data and then analyzes the scaling of the residuals to the fit as a function of the time scale at which the fit is computed. The authors present applications to synthetic mathematical signals to verify the accuracy of AFA and demonstrate the basic steps of the analysis. The authors then present results from applying AFA to time series from a cognitive psychology experiment on repeated estimation of durations of time to illustrate some of the complexities of real-world data. AFA shows promise in dealing with many types of signals, but like any fractal analysis method there are special challenges and considerations to take into account, such as determining the presence of linear scaling regions.

Prospectively identified deficits in sagittal plane hip-ankle coordination in female athletes who sustain a second anterior cruciate ligament injury after anterior cruciate ligament reconstruction and return to sport.

BACKGROUND Athletes who return to sport after anterior cruciate ligament reconstruction are at increased risk of future ACL injury. Altered coordination of lower extremity motion may increase this risk. The purpose of this study was to prospectively determine if altered lower extremity coordination patterns exist in athletes who go on to sustain a 2nd anterior cruciate ligament injury. METHODS Sixty-one female athletes who were cleared to return to sport after anterior cruciate ligament reconstruction were included. Hip-ankle coordination was assessed prior to return to sport with a dynamic postural coordination task. Within 12 months, 14 patients sustained a 2nd ACL injury. Fourteen matched subjects were selected for comparative analysis. Cross-recurrence quantification analysis characterized hip-ankle coordination patterns. A group × target speed (slow vs. fast) × leg (involved vs. uninvolved) analysis of variance was used to identify differences. FINDINGS A main effect of group (P = 0.02) indicated that the single injury group exhibited more stable hip-ankle coordination [166.2 (18.9)] compared to the 2nd injury group [108.4 (10.1)]. A leg × group interaction was also observed (P = .04). The affected leg of the single injury group exhibited more stable coordination [M = 187.1 (23.3)] compared to the affected leg of the 2nd injury group [M = 110.13 (9.8)], P = 0.03. INTERPRETATION Hip-ankle coordination was altered in female athletes who sustained a 2nd anterior cruciate ligament injury after return to sport. Failure to coordinate lower extremity movement in the absence of normal knee proprioception may place the knee at risk.

Postconcussion Postural Sway Variability Changes in Youth: The Benefit of Structural Variability Analyses

Purpose: The purpose of this study was to evaluate the utility of postural sway variability as a potential assessment to detect altered postural sway in youth with symptoms related to a concussion. Methods: Forty participants (20 who were healthy and 20 who were injured) aged 10 to 16 years were assessed using the Balance Error Scoring System (BESS) and postural sway variability analyses applied to center-of-pressure data captured using a force plate. Results: Significant differences were observed between the 2 groups for postural sway variability metrics but not for the BESS. Specifically, path length was shorter and Sample and Renyi Entropies were more regular for the participants who were injured compared with the participants who were healthy (P < .05). Conclusion: The results of this study indicate that postural sway variability may be a more valid measure than the BESS to detect postconcussion alterations in postural control in young athletes.

Deficits in Hip-Ankle Coordination in Female Athletes who Suffer a Second Anterior Cruciate Ligament (ACL) Injury after ACL Reconstruction and Return to Sport

Objectives: Athletes who return to sport after ACL reconstruction (ACLR) are at increased risk of future ACL injury. Proprioceptive deficits at the knee, known to be present at thetime of ACL injury, may persist for up to 2 years after ACLR. Coordinated movements of the hip and ankle are critical in the absence of normal knee proprioception during dynamic athletic movements. Failure to optimally position the knee may make the passive structures susceptible to pathologic stresses that increase the risk of subsequent ligament or graft failure. The purpose of this work was to prospectively determine if altered lower extremity coordination patterns exist in female athletes, who go on to suffer a second ACL injury to either limb, after ACLR and return to sport (RTS). The study tested the hypothesis that female athletes who sustained a second ACL injury would demonstrate altered lower extremity coordination patterns indicative of persistent neuromuscular deficits at the time of return to sport compared to female athletes who would not subsequently sustain a second ACL injury. Methods: Sixty-one female athletes, who sustained an ACL injury, underwent ACLR, completed rehabilitation and were medically cleared for RTS were enrolled in this study. Hip-ankle coordination was assessed on all athletes prior to RTS as they performed a dynamic postural coordination task. The task required participants to stand on a single leg and track the anterior-posterior (AP) movement of an oscillating 3-D square target displayed on a computer monitor. Participants tracked the target with the head so as to maintain a constant perceived distance between their head and the target by matching the amplitude and frequency of the target oscillations. Fourteen patients sustained a second ACL injury within 12 months of RTS (ACLR2). Fourteen matched subjects after ACLR, who did not suffer a second ACL injury (ACLR1), were selected for comparative analysis. Cross-recurrence quantification analysis (Figure 1) was used to characterize hip-ankle coordination patterns through the variable cross-maxline (CML). A group (ACLR1 vs. ACLR2) X target speed (slow vs. fast) X leg (affected vs. unaffected) mixed-model analysis of variance was used to identify coordination differences. Results: A significant main effect of group was observed (p = 0.02), and indicated that the ACLR1 group exhibited more stable hip-ankle coordination overall (M = 166.2 ± 18.9) compared to the ACLR2 group (M = 108.4 ± 10.1), irrespective of target speed or tested leg. A leg x group interaction was also observed (p = .04). A Mann-Whitney test was employed due to unequal variances between groups, and indicated that the affected leg of the ACLR1 group exhibited more stable coordination (M = 187.1 ± 23.3)compared to the affected leg of the ACLR2 group (M = 110.13 ± 9.8), p = 0.03. Conclusion: Hip-ankle coordination was altered in female athletes who subsequently sustain a second ACL injury after initial ACLR and RTS. Failure to appropriately coordinate lower extremity movement between the adjoining proximal and distal hip and ankle in the absence of normal knee proprioception, may place the knee in a high-risk position and increase the likelihood of a second ACL injury in this population.

Brain-Behavior Mechanisms for the Transfer of Neuromuscular Training Adaptions to Simulated Sport: Initial Findings From the Train the Brain Project

CONTEXT A limiting factor for reducing anterior cruciate ligament injury risk is ensuring that the movement adaptions made during the prevention program transfer to sport-specific activity. Virtual reality provides a mechanism to assess transferability, and neuroimaging provides a means to assay the neural processes allowing for such skill transfer. OBJECTIVE To determine the neural mechanisms for injury risk-reducing biomechanics transfer to sport after anterior cruciate ligament injury prevention training. DESIGN Cohort study. SETTING Research laboratory. PARTICIPANTS Four healthy high school soccer athletes. INTERVENTIONS Participants completed augmented neuromuscular training utilizing real-time visual feedback. An unloaded knee extension task and a loaded leg press task were completed with neuroimaging before and after training. A virtual reality soccer-specific landing task was also competed following training to assess transfer of movement mechanics. MAIN OUTCOME MEASURES Landing mechanics during the virtual reality soccer task and blood oxygen level-dependent signal change during neuroimaging. RESULTS Increased motor planning, sensory and visual region activity during unloaded knee extension and decreased motor cortex activity during loaded leg press were highly correlated with improvements in landing mechanics (decreased hip adduction and knee rotation). CONCLUSION Changes in brain activity may underlie adaptation and transfer of injury risk-reducing movement mechanics to sport activity. Clinicians may be able to target these specific brain processes with adjunctive therapy to facilitate intervention improvements transferring to sport.

An alternative perspective on postural stability and variability.

In this issue of Exercise and Sport Sciences Reviews, the article ‘‘Task-Dependent Postural Control Throughout the Lifespan’’ by Haddad et al. (2) promotes two major ideas: 1) Postural control is highly task dependent and 2) Postural variability is functionalV it reflects the availability of multiple solutions (resulting from kinematic and neuromuscular redundancy) and may play an exploratory role by generating sensory information. These ideas are central to an ecologicaldynamical perspective on postural control. Haddad et al. (2) make important contributions to that perspective by summarizing recent findings on postural control across the lifespan and by emphasizing the relevance of this approach for clinical and rehabilitation applications. Postural stability typically is described in terms of minimizing spontaneous postural sway and counteracting perturbations that displace the center of mass away from a single reference point centered within the base of support. The alternative proposed by Haddad et al. (2) is that successful postural control amounts to achieving body configurations that fall somewhere within a stable subspace of the postural configuration space (depicted graphically in Configuration Space Diagrams). Variability within the stable region of the configuration space is acceptable and will not lead to falls. The boundary of the stable region is the reference for upright stance; there is not just one body configuration that is most desirable or stable. The boundaries are dynamic; they change on a moment-to-moment basis as the actor moves, the environment changes, or task demands arise and dissolve. What is ‘‘stable’’ reflects suprapostural task goals and the behavioral (including social) and environmental contexts. Configuration space diagrams potentially are useful, but there is a need to develop the idea beyond its current qualitative descriptive form. The uncontrolled manifold (4), goalequivalent manifold (1), and the Tolerance-Noise-Covariation (3) methods may be well suited for that purpose. These methods differ in important ways yet share in common the idea that motor performance can be understood by describing how relatively microscopic motor system degrees of freedom (DF) map onto relatively macroscopic task variables. In redundant systems, many combinations of the microscale DF preserve a value of the task variable. This is equivalent to saying that there is a stable subregion (or manifold) of the configuration space, whose dimensions are the microscale DF, within which the task variable remains constant. Variations within that manifold can remain unchecked without leading to task failure. For quiet stance, joint angles can serve as the microscale DF and center of mass position can serve as the task variable, but expanding this idea to incorporate the approach of Haddad et al. (2) requires defining the task variable in terms of the functional goals of the suprapostural task. Postural variability is informative for postural control; Postural sway generates information (visual, somatosensory, vestibular, etc.) that is specific to the changing position of the body and thus can be useful for controlling posture. A challenge that remains for researchers is to determine whether (and when) postural sway is explicitly exploratory (i.e., it is an intentional information-seeking act) rather than performatory (in the service of a behavioral goal) or unintended noise. At some level, this may not matter; regardless of intent, postural sway may provide information useful for postural control. But the term ‘‘exploratory’’ connotes an intention, and, if this is indeed the case, then efforts to model postural control will benefit from fleshing out this idea more definitely. Lastly, the authors raise important implications for rehabilitation. Models that account for properties of the actor, task, and environment may be more useful in clinical settings than models based on quiet stance. Quiet stance may lack the sensitivity and specificity desirable in a diagnostic tool for clinical applications because the absence of an explicit suprapostural task leaves postural control underconstrained. Potentially fruitful directions for clinical research may be to identify the limits to which an actor’s postural control system is appropriately adaptive so as to support a variety of suprapostural task goals and to track the evolution of the stable region during the course of rehabilitation. Authors for this section are recruited by Commentary Editor: Russell R. Pate, Ph.D., FACSM, Department of Exercise Science, University of South Carolina, Columbia, SC 29208 (E-mail: rpate@mailbox.sc.edu).