Sensorimotor function following anterior cruciate ligament injury : movement control, proprioception and neuropsychological perspectives

Abstract: Background: The high incidence of anterior cruciate ligament (ACL) injuries in sports suggests an involvement of both biomechanical and neurocognitive risk factors. Athletes are constantly exposed to challenging sports scenarios, which are often characterised by high-intensity movements combined with a multi-stimuli environment and continuous psychological pressure. Post-injury loss of knee proprioception and long-term injury-associated neuroplasticity arguably place an athlete in a disadvantage when coping with such situations when returning to sports (RTS). This is postulated to contribute to a high rate of re-injuries, seen despite achieving RTS eligibility. Psychological factors such as re-injury fears and anxieties are also suggested to influence central sensorimotor processing and to therefore play a role in the generation and control of functional movements. Their assessment is however based on suboptimal tools, particularly when administered to the athletic population. In general, current clinical assessments focus primarily on coarse outcome measures while disregarding aspects such as multi-joint control and the influence of psychological aspects on motor performance. This thesis focuses on the role of proprioception and re-injury anxiety on functional movement control following ACL injury and reconstruction (ACLR), with implications for risks of re-injury.Methods: This thesis is comprised of four cross-sectional studies (Papers I-IV), that stem from two data collections performed in a motion analysis laboratory. Paper I introduces a novel obstacle clearance test aimed to functionally assess proprioception and sensorimotor control. The goal of the test was to cross an obstacle, downward vision occluded, aiming for minimal foot clearance. Individuals following ACLR and rehabilitation were compared to both mildly active uninjured persons (CTRL) and elite athletes (ATH). A kinematic analysis, using 3D motion capture, included estimates of lower limb movement accuracy, variability and symmetry. Paper II evaluates knee proprioception among the same individuals using a weight-bearing knee joint position sense (JPS) test, and outcomes were compared with associated outcomes from the obstacle clearance test. Paper III explores whether self-reported fear of re-injury is manifested in the biomechanics (kinematics and electromyography) of a standardised rebound side-hop test (SRSH). An ACLR group was stratified into high-fear and low-fear subgroups based on one discriminating question, and compared also to uninjured controls. In Paper IV, a threat-conditioning test paradigm is introduced, aiming to invoke and measure a neurophysiological arousal response to movement-related fear, among uninjured individuals. Conditioned auditory stimuli were occasionally followed by unexpected perturbations of the base of support, and compared with neutral stimuli. Electroencephalography was continuously registered and event-related potentials were explored as potential anxiety biomarkers.Results: Kinematic asymmetry was observed for the ACLR group during obstacle crossing, both for individual joints and for multi-joint movement and velocity curves. In addition, trailing leg trajectory variability during higher obstacle crossings was lower for ACLR compared to both control groups. The less physically-active CTRL group demonstrated less crossing accuracy (larger obstacle distances and JPS errors) compared to both ACLR and ATH. Moderate positive correlations were observed between knee JPS absolute errors and obstacle distances, for the injured leg of the ACLR group only. Individuals with ACLR, classified as having high fear, demonstrated higher biceps femoris amplitudes and anterior-posterior co-contraction index during landing. Side-hop performance was also distinguishable for ACLR (regardless of fear allocation) with greater hip and knee flexion, while high-fear individuals also had more trunk flexion. Perturbation-related fearful response was reflected as a high-amplitude contingent negative variation (CNV) wave in response to conditioned compared to neutral stimuli. The CNV wave was observed over all electrode cites but most significantly over frontal and central cortical areas.Conclusions: Even following rehabilitation, individuals with ACLR showed unique sensorimotor function, characterised by less trajectory variability and greater multi-joint asymmetry when proprioception was challenged (i.e., downward vision occluded). However, knee JPS did not seem to be deficient among these individuals, but instead more related to physical activity, than to the ACLR history. Correlations to JPS errors, seen exclusively for the ACLR leg might suggest a tendency to focus attention more internally when crossing an obstacle (generally an external focus task), though this should be investigated further. Higher levels of self-reported fear of re-injury were manifested in the biomechanics of side hops, with seemingly stiffer landings and protective neuromuscular strategy. This has potential implications for joint degeneration hastening as well as reduced motor adaptability, implying a risk for re-injury. Finally, the balance-perturbation test paradigm seemed to provoke threat-associated arousal in the form of a CNV wave among uninjured individuals. The CNV wave should further be explored as a potential biomarker for re-injury anxiety. Future research should implement this paradigm on individuals with different levels of self-reported movement-related fears and anxieties, striving for a more holistic approach in rehabilitation following ACLR.