An investigation into the impact of coaching strategies with respect to physical and performance characteristics of male youth of varying biological maturation

This thesis will be presented as two standalone experimental chapters which will culminate in a thesis discussion linking the two papers under the overarching concept of maturation in male youth. Chapter 2 is currently under review in ‘Cogent Medicine’, and the intention of Chapter 2 is to identify physical, injury and performance-based differences between maturational groups within a general school-based population of 8 youth. These observed differences within Chapter 2 will then inform the methodology and coaching strategies utilised within Chapter 3, which will investigate the effectiveness of various coaching methods in order to maximise adaptation, motor ability and injury prevention within each maturational group. The outcome of this thesis hopes to inform practitioners as to how they can best implement their training programmes to maximise learning and adaptation across a range of biological maturation levels. Rather than just knowing when training should occur which has been investigated previously, it is hoped this thesis will provide insight into how coaching should occur to maximise learning within this diverse adolescent population. Due to the layout of this thesis with the individual papers, there is an element of content repetition throughout Chapters 1,2,3 and 4 which needs to be acknowledged, although the various contexts provides uniqueness throughout

Postural control strategy during standing is altered in patients with multiple sclerosis

Disturbances in balance are one of the first reported symptoms of Multiple Sclerosis (MS), yet limited research has been performed to classify the postural control deficits in this population. This study investigated the variability present in the sway patterns during quiet standing in patients with MS (PwMS) and healthy controls. Subjects were assessed (eyes open, closed) standing on a force platform. Variability of the sway patterns was quantified using a measure of amount of variability (root mean square; RMS) and two measures of temporal structure of variability (Lyapunov Exponent – LyE; Approximate Entropy – ApEn). RMS results revealed significantly higher amount of variability in the sway patterns of PwMS. PwMS also exhibit increased regularity (decreased ApEn) and decreased divergence (decreased LyE) during standing compared to healthy controls. Removing vision resulted in significantly decreased divergence (decreased LyE) in the MS subject group. These changes in the temporal structure correspond well with the theoretical model of the optimal movement variability hypothesis and the results support using variability measures to understand the mechanisms that underline postural control in PwMS and possibly other neurodegenerative disease pathologies

Dynamic balance control during treadmill walking in chronic stroke survivors

Maintaining dynamic balance is an important component of walking function that is likely impaired in chronic stroke survivors, evidenced by an increased prevalence of falls. Dynamic balance control requires maintaining the center of mass (COM) within the base of support during movement. During walking, dynamic balance control is achieved largely by modifying foot placement to adjust the base of support. However, chronic stroke survivors have difficulty with both precision control of foot placement, as well as reduced control of COM movement. The objective of this dissertation was to characterize dynamic balance control strategies during walking in chronic stroke survivors. Additionally, we evaluated whether altered sensory feedback could be used to improve balance control in stroke survivors. Dynamic balance control was characterized during challenging walking conditions in stroke survivors and age-matched neurologically intact individuals. Adaptations to perturbations in frontal plane COM, induced using a custom cable-driven device, were used to further probe mechanisms of dynamic balance control. Despite larger amounts of COM movement and step widths, chronic stroke survivors produced a similar ratio of step width to COM sway, indicating that simply increasing step width does not produce a safer walking pattern for the stroke group. Placement of the paretic limb was unchanged in response to the external perturbations of trunk movement, which might underlie deficits in dynamic balance control. Augmented sensory feedback improved paretic foot placement and COM control, when applied during a stepping or treadmill walking task. These results provide insight into differences in dynamic balance control in stroke while also demonstrating that augmented sensory feedback signals might be used to improve balance control, and thus walking function for chronic stroke survivors

Postural control: learning to balance and responses to mechanical and sensory perturbations

The purpose of the current research was to examine how a novel balance task is learnt by individuals with a mature neurological system, and to investigate the responses of experienced hand balancers to mechanical and sensory perturbations. Balance in each posture was assessed by various techniques, including: traditional measures of centre of pressure, nonlinear time series analysis of centre of pressure, estimates of feedback time delay from cross correlations and delayed regression models, and calculation of small, medium, and large movement corrections. Data from this study suggests that the best balance metric for distinguishing between each of the balance conditions was the traditional balance measure of sway velocity. However, sway velocity cannot provide any further information on the underlying process of balance. Nonlinear measures of balance offer insight into the underlying deterministic processes that control balance, offering measures of system determinism, complexity, and predictability. Assessments of feedback time delay and movement corrections provide both an insight into the control of posture and help distinguish one condition from another. Both feedback time delay and movement corrections and magnitudes may be used simultaneously to delve further into the control of posture. Delayed regression models seem to be an appropriate and useful tool for estimating feedback time delays during balance. Findings support the use of the third term in the adapted regression model as a means of estimating the effect of passive stiffness on feedback time delay. Generally, with increased duration in handstand subjects displayed reduced sway as measured by traditional measures of balance. A more marked change in nonlinear measures of balance can be seen, with quicker reductions in variance for some nonlinear measures of balance than in the traditional measures. It may be that more pronounced changes in nonlinear measures represent changes in the subjects underlying process of postural control, whereas less pronounced changes in traditional measures relate more to their general ability or performance in the balance task

Identifying Plant and Feedback in Human Posture Control

Human upright bipedal stance is a classic example of a control system consisting of a plant (i.e., the physical body and its actuators) and feedback (i.e., neural control) operating continuously in a closed loop. Determining the mechanistic basis of behavior in a closed loop control system is problematic because experimental manipulations or deficits due to trauma/injury influence all parts of the loop. Moreover, experimental techniques to open the loop (e.g., isolate the plant) are not viable because bipedal upright stance is not possible without feedback. The goal of the proposed study is to use a technique called closed loop system identification (CLSI) to investigate properties of the plant and feedback separately. Human upright stance has typically been approximated as a single-joint inverted pendulum, simplifying not only the control of a multi-linked body but also how sensory information is processed relative to body dynamics. However, a recent study showed that a single-joint approximation is inadequate. Trunk and leg segments are in-phase at frequencies below 1 Hz of body sway and simultaneously anti-phase at frequencies above 1 Hz during quiet stance. My dissertation studies have investigated the coordination between the leg and trunk segments and how sensory information is processed relative to that coordination. For example, additional sensory information provided through visual or light touch information led to a change of the in-phase pattern but not the anti-phase pattern, indicating that the anti-phase pattern may not be neurally controlled, but more a function of biomechanical properties of a two-segment body. In a subsequent study, I probed whether an internal model of the body processes visual information relative to a single or double-linked body. The results suggested a simple control strategy that processes sensory information relative to a single-joint internal model providing further evidence that the anti-phase pattern is biomechanically driven. These studies suggest potential mechanisms but cannot rule out alternative hypotheses because the source of behavioral changes can be attributed to properties of the plant and/or feedback. Here I adopt the CLSI approach using perturbations to probe separate processes within the postural control loop. Mechanical perturbations introduce sway as an input to the feedback, which in turn generates muscle activity as an output. Visual perturbations elicit muscle activity (a motor command) as an input to the plant, which then triggers body sway as an output. Mappings of muscle activity to body sway and body sway to muscle activity are used to identify properties of the plant and feedback, respectively. The results suggest that feedback compensates for the low-pass properties of the plant, except at higher frequencies. An optimal control model minimizing the amount of muscle activation suggests that the mechanism underlying this lack of compensation may be due to an uncompensated time delay. These techniques have the potential for more precise identification of the source of deficits in the postural control loop, leading to improved rehabilitation techniques and treatment of balance deficits, which currently contributes to 40% of nursing home admissions and costs the US health care system over $20B per year

Interpersonal interactions for haptic guidance during maximum forward reaching

Caregiver-patient interactions rely on interpersonal coordination (IPC) involving the haptic and visual modalities. We investigated in healthy individuals spontaneous IPC during joint maximum forward reaching. A 'contact-provider' (CP; n=2) kept light interpersonal touch (IPT) laterally with the wrist of the extended arm of a forward reaching, blind-folded 'contact-receiver' (CR; n=22). Due to the stance configuration, CP was intrinsically more stable. CR received haptic feedback during forward reaching in two ways: (1) presence of a light object (OBT) at the fingertips, (2) provision of IPT. CP delivered IPT with or without vision or tracked manually with vision but without IPT. CR's variabilities of Centre-of-Pressure velocity (CoP) and wrist velocity, interpersonal cross-correlations and time lags served as outcome variables. OBT presence increased CR's reaching amplitude and reduced postural variability in the reach end-state. CR's variability was lowest when CP applied IPT without vision. OBT decreased the strength of IPC. Correlation time lags indicated that CP retained a predominantly reactive mode with CR taking the lead. When CP had no vision, presumably preventing an effect of visual dominance, OBT presence made a qualitative difference: with OBT absent, CP was leading CR. This observation might indicate a switch in CR's coordinative strategy by attending mainly to CP's haptic 'anchor'. Our paradigm implies that in clinical settings the sensorimotor states of both interacting partners need to be considered. We speculate that haptic guidance by a caregiver is more effective when IPT resembles the only link between both partners

The influence of visual information on multi-muscle control during quiet stance: a spectral analysis approach

Standing upright requires the coordination of neural drives to a large set of muscles involved in controlling human bipedal stance (i.e., postural muscles). The coordination may deteriorate in situations where standing is performed under more challenging circumstances, such as standing on a smaller base of support or not having adequate visual information. The present study investigates the role of common neural inputs in the organization of multi-muscle synergies and the effects of visual input disruption to this mechanism of control. We analyzed the strength and distribution of correlated neural inputs (measured by intermuscular coherence) to six postural muscles previously recognized as components of synergistic groups involved in the maintenance of the body's vertical positioning. Two experimental conditions were studied: quiet bipedal stance performed with opened eyes (OEs) and closed eyes (CEs). Nine participants stood quietly for 30 s while the activity of the soleus, biceps femoris, lumbar erector spinae, tibialis anterior, rectus femoris, and rectus abdominis muscles were recorded using surface electrodes. Intermuscular (EMG-EMG) coherence was estimated for 12 muscle pairs formed by these muscles, including pairs formed solely by either posterior, anterior, or mixed (one posterior and one anterior) muscles. Intermuscular coherence was only found to be significant for muscle pairs formed solely by either posterior or anterior muscles, and no significant coherence was found for mixed muscle pairs. Significant intermuscular coherence was only found within a distinct frequency interval bounded between 1 and 10 Hz when visual input was available (OEs trials). The strength of correlated neural inputs was similar across muscle pairs located in different joints but executing a similar function (pushing body either backward or forward) suggesting that synergistic postural groups are likely formed based on their functional role instead of their anatomical location. Absence of visual information caused a significant decrease in intermuscular coherence. These findings are consistent with the hypothesis that correlated neural inputs are a mechanism used by the CNS to assemble synergistic muscle groups. Further, this mechanism is affected by interruption of visual input

Age-related impairments and influence of visual feedback when learning to stand with unexpected sensorimotor delays

Background:While standing upright, the brain must accurately accommodate for delays between sensory feedback and self-generated motor commands. Natural aging may limit adaptation to sensorimotor delays due to age-related decline in sensory acuity, neuromuscular capacity and cognitive function. This study examined balance learning in young and older adults as they stood with robot-induced sensorimotor delays.Methods:A cohort of community dwelling young (mean = 23.6 years, N = 20) and older adults (mean = 70.1 years, N = 20) participated in this balance learning study. Participants stood on a robotic balance simulator which was used to artificially impose a 250 ms delay into their control of standing. Young and older adults practiced to balance with the imposed delay either with or without visual feedback (i.e., eyes open or closed), resulting in four training groups. We assessed their balance behavior and performance (i.e., variability in postural sway and ability to maintain upright posture) before, during and after training. We further evaluated whether training benefits gained in one visual condition transferred to the untrained condition.Results:All participants, regardless of age or visual training condition, improved their balance performance through training to stand with the imposed delay. Compared to young adults, however, older adults had larger postural oscillations at all stages of the experiments, exhibited less relative learning to balance with the delay and had slower rates of balance improvement. Visual feedback was not required to learn to stand with the imposed delay, but it had a modest effect on the amount of time participants could remain upright. For all groups, balance improvements gained from training in one visual condition transferred to the untrained visual condition.Conclusion:Our study reveals that while advanced age partially impairs balance learning, the older nervous system maintains the ability to recalibrate motor control to stand with initially destabilizing sensorimotor delays under differing visual feedback conditions

Consolidation of the postural set during voluntary intermittent light finger contact as a function of hand dominance

Light fingertip contact with an earth-fixed referent decreases body sway. In a previous study Johannsen et al. (2014) demonstrated longer return-to-baseline of body sway for intermittent contacts of more than 2 seconds duration. This indicates that sway reduction with light tactile contact involves postural control strategies independent of the availability of tactile feedback and may depend on the intention to control body sway with light touch feedback. In the present study, we investigated the effect of hand dominance on post-contact return-to-baseline to probe for potential inter-hemispheric differences in the utilization of light finger contact for sway control. Twelve healthy, right-handed young adults stood in normal bipedal stance with eyes closed on a force plate with an earth-fixed referent directly in front. Acoustic signals instructed onset and removal of intermittent light touch. We found that return-to-baseline of sway following longer contact durations is affected by hand dominance with the dominant hand resulting in a slower return to No-contact levels of sway. Our results indicate that the light touch postural set is more persistent and might need longer to disengage when established with the dominant hand or takes longer to consolidate when established with the non-dominant hand