Sensorimotor coordination relies on the fine calibration and integration of visual, vestibular, and somatosensory input. Using virtual environments (VE) allows for the dissociation of visual and inertial inputs to manipulate human behavioral outputs. Our goal was to employ VE technology in a novel manner to investigate how head stabilization is affected by spatiotemporal properties of dynamic visual input when combined with passive motion on a linear sled. Healthy adults (n = 12) wore a head-mounted display during naso-occipital sinusoidal horizontal whole body translations while seated. Subjects were secured in a seat with a five-point harness, with the head free to move. Frequency and amplitude of sinusoidal input (i.e., inertial conditions) were set to create overlapping conditions of maximum acceleration (amax) or velocity (vmax). Four inertial conditions were combined with four visual conditions (VIS). VIS were created so that direction of optic flow either matched direction of passive motion or did not. The effect of near and far fixation distance within the VE was also tested. Head kinematics were collected with a three-axis gyro. Head stability showed a complex interaction dependent on changes in weighting of visual and inertial inputs that changed with the sled driving frequency. Inertial condition affected amplitude (p < 0.0000) and phase (p < 0.0000) of head pitch angular velocity. In the absence of visual input, head pitch velocity amplitude increased (p < 0.01). An interaction effect between inertial and VIS conditions on head yaw occurred in SW (p < 0.05). There was also a significant interaction of depth of field and inertial condition on amplitude (p < 0.001) and phase (p < 0.05) of head yaw velocity in SW, especially during high vmax conditions. We conclude visual flow can organize lateral cervical responses despite being discordant with inertial input. When using VE for rehabilitation, possible unintended, involuntary or reflexive motor responses that may not be present in traditional training environments should be taken into consideration.

Sensorimotor coordination relies on fine calibration of the interaction among visual, vestibular, and somatosensory input. Our goal was to investigate how the spatiotemporal properties of passive inertial motion and visual input affect head stabilization. Healthy young adults (n=12) wore a head-mounted display during A/P sinusoidal horizontal translations of the whole body. Visual conditions (VIS) displayed forward (EO), sideways (SW), or backward (BW) visual motion relative to the head, plus an eyes-closed conditions (EC) which were combined with 4 inertial conditions to comprise 16 conditions in total. In SW either near or far DEPTH of field with 180° phase shift was displayed. Subjects were secured in a seat with head free to move. Frequency and amplitude of sinusoidal input included overlapping max acceleration (a max ) or max velocity (v max ). Amplitude and phase of angular velocity was collected with a 3-axis gyro. A main effect of inertial condition on amplitude for all axes of head motion (p<.0000) and a shift (p<.0000) from phase lead to lag of head pitch with increasing freq (121°, 127°, 83°, -32°) were found. A main effect of VIS on head pitch (p<0.01) was due to the absence of vision (EC). An interaction effect between inertial and VIS conditions on head yaw occurred with SW (p<0.05). In SW, a significant interaction of depth of field and inertia on amplitude (p<0.001) and phase (p<0.05) of head yaw occurred, especially during high v max conditions. Thus, visual flow can organize lateral cervical responses despite being discordant with inertial input.

Purpose The purposes of this study were to suppress estradiol levels in adolescent (postpubertal rats) using gonadotropin-releasing hormone antagonist (GnRH-a) injections and to determine the changes in bone structure and mechanical strength. Methods In an Institutional Animal Care and Use Committee–approved study, female rats at 23 d of age were assigned to a baseline group (BL65; n = 10) sacrificed on day 65, a control group (Control; n = 15) sacrificed on day 90, or an experimental group (AMEN; n = 9) sacrificed on day 90 that received daily injections of GnRH-a for a 25-d period from 65 to 90 d of age (2.5 mg·kg−1 per dose). Results Body weights were similar on day 65; however, the AMEN group was significantly heavier than the Control group (17%, P = 0.001) on day 90. In the AMEN rats relative to the Control group, plasma estradiol levels were reduced by 36% (P = 0.0001) and plasma insulin-like growth factor 1 levels were 24% higher (P = 0.003). In the femur, there was no change in periosteal bone apposition or total cross-sectional area. The marrow area increased by 13.7% (P = 0.05) resulting in a 7.8% decrease in relative cortical area (P = 0.012), and endocortical bone formation rate increased by 39.4% (P = 0.04). Trabecular volume and number decreased by 51.5% (P = 0.0003) and 49.5% (P = 0.0003), respectively. The absolute peak moments of the tibiae and femurs were unchanged in the AMEN group relative to the Control group, but these were reduced by 8.8% (P = 0.03) and 7.5% (P = 0.09), respectively, when normalized by body weight. Conclusions Suppression of estradiol by 25 d of GnRH-a administration to 65-d-old (postpubertal) rats reduced trabecular volume and number by about 50%, increased endocortical bone turnover, and reduced relative cortical thickness without changing tibial and femoral total area. These changes in bone structure were associated with no change in absolute mechanical strength possibly because of increases in body weight or in insulin-like growth factor 1 concentrations.

Tortuosity and buckling of blood vessels are defined as the deviation from original configuration and has been observed throughout the vascular system. The blood flow in the regions down-stream of tortuous section decreases, which may cause a deficiency in blood supply to the organs and ischemia. Although tortuosity of blood vessels has been associated with aging, atherosclerosis, hypertension, genetic and other cardiovascular disease, the mechanism behind its initiation and development is not yet understood. In a series of theoretical and experimental studies, biomechanical aspects of buckling of arteries has been investigated under quasi-static loading (Han, 2007; Liu and Han, 2011); however, it has been shown theoretically that the buckling behavior of arteries under dynamic loading are different and arteries may become mechanically unstable at pressures other than the static critical loading (Rachev, 2009). This work addresses buckling of porcine aorta and experimental verification of dynamic buckling in this case. We hypothesize that dynamic buckling can partly contribute to the traumatic rupture of aorta that is a leading cause of fatality in motor vehicle crashes.

Understanding the mechanical behavior of aorta under supra-physiological loadings is an important aspect of modeling tissue behavior in various applications that involve large deformations. Utilizing inflation-extension experiments, the mechanical behavior of porcine descending thoracic aortic segments under physiological and supra-physiological intraluminal pressures was investigated. The pressure was changed in the range of 0-70 kPa and the deformation of the segment was determined in three dimensions using a custom-made motion capture system. An orthotropic Fung-type constitutive model was characterized by implementing a novel computationally efficient framework that ensured material stability for numerical simulations. The nonlinear rising trend of circumferential stretch ratio [Formula: see text] from outer toward inner wall was significantly increased at higher pressures. The increase in [Formula: see text] from physiological pressure (13 kPa) to 70 kPa was 13% at the outer wall and 22% at the inner wall; in this pressure range, the longitudinal stretch ratio [Formula: see text] increased 20%. A significant nonlinearity in the material behavior was observed as in the same pressure range, and the circumferential and longitudinal Cauchy stresses at the inner wall were increased 16 and 18 times, respectively. The overall constitutive model was verified in several loading paths in the [Formula: see text] space to confirm its applicability in multi-axial loading conditions.