Approximately 80% of the population will experience low-back pain within their lifetime. Significant research efforts have focused on compressive loading as an injury mechanism that could lead to low-back pain and injury. However, the influence of shear loading, and its relationship to vertebral tissue tolerances as well as modulating factors for these tolerances have not been studied as extensively. The primary objective of this thesis was to produce a series of investigations that begin to determine the roles of different modulating factors such as posture, compression, bone density, bone morphology, and repetitive load magnitude on measured vertebral joint shear failure tolerances. The thesis comprises four independent studies using in vitro mechanical testing, imaging modalities, and finite element modeling. Each of the in vitro studies within this thesis used a validated porcine cervical model as a surrogate for the human lumbar spine. The first study employed in vitro mechanical testing to investigate the combined roles of flexion/extension postural deviation and compressive load on the measured ultimate shear failure tolerances. Peripheral quantitative computed tomography scans of the pars interarticularis and measurements of vertebral bone morphology were used in the second investigation along with in vitro mechanical testing to identify the morphological characteristics that can be used to predict ultimate shear failure tolerances. The influence of sub-maximal shear load magnitude on the cumulative shear load and number of loading cycles sustained prior to failure were investigated with in vitro mechanical testing in the third study. Finally, a finite element model of the porcine C3-C4 functional spinal unit was used to investigate the plausibility of hypotheses, developed from previous research and the findings of the first investigation for this thesis, surrounding alterations in measured ultimate shear failure tolerances as a function of changes in facet interaction. Results from the first investigation showed that there was no statistically significant interaction between postural deviation and compressive force on ultimate shear failure tolerance. However, ultimate shear failure tolerance was reduced (compared to neutral) by 13.2% with flexed postures, and increased (compared to neutral) by 12.8% with extended postures. Each 15% increment (up to a maximum of 60% of predicted compressive failure tolerance) in compressive force was met with an average 11.1% increase in ultimate shear failure tolerance. It was hypothesized that alterations in flexion/extension posture and/or compressive force altered the location for the force centroid of facet contact. These changes in the location of facet contact were hypothesized to produce subsequent changes in the bending moment at the pars interarticularis that altered the measured ultimate shear failure tolerance. The three leading factors for calculating of measured ultimate shear failure tolerance were the pars interarticularis length for the cranial vertebra, the average facet angle measured in the transverse plane, and cortical bone area through the pars interarticularis. A bi-variate linear regression model that used the cranial vertebra’s pars interarticularis length and average facet angle as inputs was developed to nondestructively calculate ultimate shear failure tolerances of the porcine cervical spine. Longer pars interarticularis lengths and facets oriented closer to the sagittal plane were associated with higher measured ultimate shear failure tolerances. Fractures observed in this investigation were similar to those reported for studies performed with human specimens and also similar to reported spondylolitic fractures associated with shear loading in humans. This provided additional evidence that the porcine cervical spine is a suitable surrogate in vitro model for studying human lumbar spine mechanics. Altered sub-maximal shear load magnitude create a non-linear decrease in both the number of cycles and the cumulative shear load sustained prior to failure. These findings suggested that estimates of cumulative shear load should assign greater importance to higher instantaneous shear loads. This was due to an increased injury potential at higher instantaneous shear loads. Cumulative load sustained prior to failure was used to develop a tissue-based weighting factor equation that would apply nonlinearly increased weight to higher shear load magnitudes in estimates of cumulative shear load. A finite element model of the porcine C3-C4 functional spinal unit was created, and simulations were performed using similar boundary conditions as the comparable in vitro tests, to assess the plausibility of the moment arm hypothesis offered within the first investigation of this thesis. Moment arm length between the force centroid of facet contact and the location of peak stress within the pars interarticularis was increased for flexed postures and decreased for extended postures. Alterations in moment arm length were larger for postural deviation than compressive force, suggesting a secondary mechanism to explain the observed increase in shear failure tolerance with higher compressive loads from the first investigation. One such possibility was the increase in the number of contacting nodes with higher compressive forces. Alterations in moment arm length were able to explain 50% of the variance in measured ultimate shear failure tolerances from the first study. Thus, the finite element model was successful in demonstrating the plausibility of moment arm length between the force centroid of facet contact and the pars interarticularis as a modulator of measured ultimate shear failure tolerance. This thesis has developed the basis for understanding how failure of the vertebral joint exposed to shear loading can be modulated. In particular, this thesis has developed novel equations to predict the ultimate shear failure tolerance measured during in vitro testing, and to determine appropriate weighting factors for sub-maximal shear forces in calculations of cumulative shear load. Evidence presented within this thesis also provides support for the long-standing moment arm hypothesis for modulation of shear injury potential.

Apraxia of speech (AOS) is a neurogenic speech disorder that is characterised by deficits in the articulatory and prosodic domains of speech production. A range of physiologic assessment techniques have been employed in an attempt to elucidate the physiological underpinnings of articulatory and prosodic defects in AOS. However, despite the advancement of electromagnetic articulography (EMA), a technique that facilitates safe, non-invasive assessment of intra-oral structures, little research has investigated lingual kinematics during speech production in participants with AOS. Tongue-to-palate contact patterns, on the contrary, have been investigated in AOS. However, most of this research relied upon descriptive analysis, rather than instrumental techniques including electropalatography (EPG). Therefore, the present thesis aimed to utilise EMA and EPG to provide a comprehensive assessment of lingual movement and tongue-to-palate contact patterns during word-initial consonant singletons and consonant clusters, during mono- and multisyllabic words, in AOS. The strength of coupling between the tongue and jaw and tongue-tip and tongue-back was also examined, as was consonant cluster coarticulation. Five participants (three females and two males) with AOS and a concomitant non-fluent aphasia participated in the project. The mean age of the group at the time of the EMA assessment was 53.6 years (SD = 12.60; range 35 - 67 years). At the time of initial assessment, all participants were a minimum of 12 months post onset of stroke (M = 1.67 years; SD = 0.72). Perceptual analysis indicated that each of the five participants with AOS presented with the following mandatory characteristics: sound distortions, sound prolongations, syllabic speech output, and dysprosody. A control group of 12 neurologically unimpaired participants (8 male, 4 female; M = 52.08 years; SD = 12.52; age range = 29 - 70 years) also participated in the study. The apraxic speakers’ tongue-tip and tongue-back movements were initially profiled during monosyllabic word production using EMA. Movement duration, distance, maximum velocity, maximum acceleration and deceleration, and velocity profile index values were recorded during word-initial consonant singletons (i.e., /t, s, l, k/) and consonant clusters (i.e., /kl, sk/). Results indicated that the participants with AOS evidenced significantly prolonged movement durations and, in some instances, significantly greater articulatory distances, relative to the control group. All measures pertaining to speed appeared to be relatively unimpaired. Phonetic complexity had a variable impact on the articulation of word-initial consonants. The results were able to account for the overall slow rate of speech exhibited by the participants with AOS. In a subsequent study, EMA was employed to investigate the impact of increasing word length on lingual kinematics for five participants with AOS. Target consonant singletons and consonant clusters were embedded in the word-initial position of one, two, and three syllable words (e.g., tar, target, targeting). Movement duration appeared to be most sensitive to the effect of word length during consonant singleton production. However, word length effects were absent during consonant cluster production. The data were discussed in the context of motor theories of speech production. The final EMA investigation examined the strength of coupling between the tongue and jaw and tongue-tip and tongue-back during /ta, sa, la, ka/ syllable repetitions, in a group of five participants with AOS. In comparison to the control group, four participants with AOS exhibited significantly stronger articulatory coupling for alveolar and/or velar targets, indicative of decreased functional movement independence. The reduction in functional movement independence was thought to reflect an attempt to simplify articulatory control, or alternatively, a decrease in the ability to differentially control distinct articulatory regions. To complement the EMA data, EPG was employed to investigate the spatial characteristics of linguopalatal contact during word-initial consonant singletons (i.e., /t, s, l, k/) and consonant clusters (i.e., /kl, sk/) in three participants with AOS. Through the use of quantitative and qualitative analysis techniques, misdirected articulatory gestures (e.g., double articulation patterns), distorted linguopalatal contact patterns (alveolar fricatives), lingual overshoot, and for one participant, significantly greater spatial variability were identified in the linguopalatal contact data. Pattern of closure appeared to be relatively unimpaired during alveolar plosive and approximant productions, and lingual undershoot and true omission errors were absent. The results were discussed in relation to their impact on phonetic distortion. A subsequent EPG study examined the temporal and spatial aspects of consonant cluster coarticulation in three participants with AOS. Target stimuli included ‘scar’ and ‘class’. In contrast to what was expected, each of the participants with AOS appeared able to coproduce elements within a consonant cluster. Appropriately, pattern of linguopalatal contact did not appear to be influenced by coproduction. Amount of linguopalatal contact did differ significantly on occasion. Coarticulatory effects were appropriately absent for each of the participants with AOS during alveolar fricative production in ‘scar’; however, the control group and each of the apraxic speakers exhibited place of articulation assimilation during velar stop production. The control group and two participants with AOS produced discrete velar and alveolar articulations during ‘class’; one participant with AOS evidenced coarticulatory effects during the /kl/ cluster. The research findings indicated that consonant cluster coarticulation was generally maintained in word-onset position, and it was postulated that future research should endeavour to investigate consonant cluster coarticulation in consonant sequences that span a syllable boundary. The EMA and EPG research findings presented in this thesis inform about the underlying physiological nature of articulatory disturbances in AOS. These findings will be discussed in the context of contemporary theories of speech motor control.