Chad E. Gooyers

The impact of posture and prolonged cyclic compressive loading on vertebral joint mechanics.

Study Design. An in vitro biomechanics investigation exposing porcine functional spinal units (FSUs) to submaximal cyclic or static compressive forces while in a flexed, neutral, or extended posture. Objective. To investigate the combined effect of cyclically applied compressive force (e.g., vibration) and postural deviation on intervertebral joint mechanics. Summary of Background Data. Independently, prolonged vibration exposure and non-neutral postures are known risk factors for development of low back pain and injury. However, there is limited basic scientific evidence to explain how the risk of low back injury from vibration exposure is modified by other mechanical factors. This work examined the influence of static postural deviation on vertebral joint height loss and compressive stiffness under cyclically applied compressive force. Methods. Forty-eight FSUs, consisting of 2 adjacent vertebrae, ligaments, and the intervening intervertebral disc were included in the study. Each specimen was randomized to 1 of 3 experimental posture conditions (neutral, flexed, or extended) and assigned to 1 of 2 loading protocols, consisting of (1) cyclic (1500 ± 1200 N applied at 5 Hz using a sinusoidal waveform, resulting in 0.2 g rms acceleration) or (2) 1500 N of static compressive force. Results. As expected, FSU height loss followed a typical first-order response in both the static and cyclic loading protocols, with the majority (∼50%) of the loss occurring in the first 20 minutes of testing. A significant interaction between posture and loading protocol (P < 0.001) was noted in the magnitude of FSU height loss. Subsequent analysis of simple effects revealed significant differences between cyclic and static loading protocols in both a neutral (P = 0.016) and a flexed posture (P < 0.0001). No significant differences (P = 0.320) were noted between pre/postmeasurements of FSU compressive stiffness. Conclusion. Posture is an important mechanical factor to consider when assessing the risk of injury from cyclic loading to the lumbar spine.

Is intervertebral disc pressure linked to herniation?: An in-vitro study using a porcine model

Approximately 40% of low back pain cases have been attributed to internal disc disruption. This disruption mechanism may be linked to intradiscal pressure changes, since mechanical loading directly affects the pressure and the stresses that the inner annulus fibrosus experiences. The objective of this study was to characterize cycle-varying changes in four dependent measures (intradiscal pressure, flexion-extension moments, specimen height loss, and specimen rotation angle) using a cyclic flexion-extension (CFE) loading protocol known to induce internal disc disruption. A novel bore-screw pressure sensor system was used to instrument 14 porcine functional spinal units. The CFE loading protocol consisted of 3600 cycles of flexion-extension range of motion (average 18.30 (SD 3.76) degrees) at 1Hz with 1500N of compressive load. On average, intradiscal pressure and specimen height decreased by 47% and 62%, respectively, and peak moments increased by 102%. From 900 to 2100 cycles, all variables exhibited significant changes between successive time points, except for the specimen posture at maximum pressure, which demonstrated a significant shift towards flexion limit after 2700 cycles. There were no further changes in pressure range after 2100 cycles, whereas peak moments and height loss were significantly different from prior time points throughout the CFE protocol. Twelve of the 14 specimens showed partial herniation; however, injury type was not significantly correlated to any of the dependent measures. Although change in pressure was not predictive of damage type, the increase in pressure range seen during this protocol supports the premise that repetitive combined loading (i.e., radial compression, tension and shear) imposes damage to the inner annulus fibrosus, and its failure mechanism may be linked to fatigue.

The influence of resistance bands on frontal plane knee mechanics during body-weight squat and vertical jump movements

The purpose of this study was to examine the impact of wearing a resistance band around the distal thigh on frontal plane knee mechanics during bodyweight squat and jumping exercises. Three closed-kinetic-chain exercises were examined, including: (1) bodyweight squat, (2) countermovement jump, and (3) squat jump. For each exercise, three experimental conditions were tested: (1) control condition with no band; (2) light-tension band applied around the distal thighs; and (3) medium-tension band applied around the distal thighs. Two dependent measures were used for analyses: (1) knee width normalized to ankle width and (2) peak external knee moment. In the absence of any feedback, application of the resistance bands failed to promote ‘neutral’ knee alignment when squatting and jumping. The stiffest resistance band resulted in significantly lower (p = 0.002) peak-width index values during the ascent phase of the countermovement jump. Additionally, the use of the medium-tension band resulted in significantly larger (p = 0.002) peak knee abduction moments compared to the no bands condition during the descent portion of the bodyweight squat and countermovement jump exercises. These findings conflict with previous clinical case reports on the proprioceptive response induced by resistance bands.

Exploring interactions between force, repetition and posture on intervertebral disc height loss and bulging in isolated porcine cervical functional spinal units from sub-acute-failure magnitudes of cyclic compressive loading

Most in vitro studies are limited in the ability to partition intervertebral disc (IVD) height loss from total specimen height loss since the net changes in the actuator position of the materials testing system simply reflect net changes to functional spinal units (FSUs) used for testing. Three levels of peak compressive force, three cycle rates and two dynamic postural conditions were examined using a full-factorial design. Cyclic compressive force was applied using a time-varying waveform with synchronous flexion/extension for 5000 cycles. Surface scans from the anterior aspect of the IVD were recorded in a neutral and flexed posture before and after the cyclic loading protocol using a 3D laser scanner to characterise changes in IVD height loss and bulging. A significant three-way interaction (p=0.0092) between the magnitude of peak compressive force, cycle rate and degree of postural deviation was observed in cycle-varying specimen height loss data. A significant main effect of peak compressive force (p=0.0003) was also observed in IVD height loss calculated from the surface profiles of the IVD. The relative contribution of IVD height loss (measured on the anterior surface) to total specimen height loss across experimental conditions varied considerably, ranging from 19% to 58%. Postural deviation was the only factor that significantly affected the magnitude of peak AF bulge (p=0.0016). This investigation provides evidence that total specimen height loss is not an accurate depiction of cycle-varying changes in the IVD across a range of in vivo scenarios that were replicated with in vitro testing.

Identifying interactive effects of task demands in lifting on estimates of in vivo low back joint loads

This investigation examined interactions between the magnitude of external load, movement speed and (a)symmetry of load placement on estimates of in vivo joint loading in the lumbar spine during simulated occupational lifting. Thirty-two participants with manual materials handling experience were included in the study. Three-dimensional motion data, ground reaction forces, and activation of six bilateral trunk muscle groups were captured while participants performed lifts with two loads at two movement speeds and using two load locations. L4-L5 joint compression and shear force-time histories were estimated using an EMG-assisted musculoskeletal model of the lumbar spine. Results from this investigation provide strong evidence that known mechanical low back injury risk factors should not be viewed in isolation. Rather, injury prevention efforts need to consider the complex interactions that exist between external task demands and their combined influence on internal joint loading.

The Impact of Posture on the Mechanical Properties of a Functional Spinal Unit During Cyclic Compressive Loading

To assess how posture affects the transmission of mechanical energy up the spinal column during vibration, 18 porcine functional spinal units (FSUs) were exposed to a sinusoidal force (1500 ± 1200 N) at 5 Hz for 120 min in either a flexed, extended, or neutral posture. Force and FSU height were measured continuously throughout the collection. From these data, specimen height loss, dynamic stiffness, hysteresis, and parameters from a standard linear solid (SLS) model were determined and analyzed for differences between postures. Posture had an influence on all of these parameters. In extension, the FSU had higher dynamic stiffness values than when neutral or flexed (p < 0.0001). In flexion, the FSU had higher hysteresis than both an extended or neutral posture (p < 0.0001). Height loss was greatest in a flexed posture and smallest in an extended posture (p < 0.0001). In extension, the series spring element in the SLS model had a stiffness value higher than both flexed and neutral posture conditions, whereas the stiffness in the parallel spring was the same between extension and neutral (p < 0.01), both higher than in flexion. Viscosity coefficients were highest in extension compared to both flexed and neutral (p < 0.01). Based on these results, it was determined that posture had a significant influence in determining the mechanical properties of the spine when exposed to cyclic compressive loading.

The Effect of Local Hydration Environment on the Mechanical Properties and Unloaded Temporal Changes of Isolated Porcine Annular Samples

Preventing dehydration during in vitro testing of isolated layers of annulus fibrosus tissue may require different test conditions than functional spine units. The purpose of the study was twofold: (A) to quantify changes in mass and thickness of multilayer annulus samples in four hydration environments over 120 min; and (B) to quantify cycle-varying biaxial tensile properties of annulus samples in the four environments. The environments included a saline bath, air, relative humidity control, and misting combined with controlled humidity. The loading protocol implemented 24 cycles of biaxial tensile loading to 20% strain at a rate of 2%/s with 3-, 8-, and 13-min of intermittent rest. Specimen mass increased an average (standard deviation) 72% (11) when immersed for 120 min (p < 0.0001). The air condition and the combined mist and relative humidity conditions reduced mass by 45% (15) and 25% (23), respectively, after 120 min (p < 0.0014). Stress at 16% stretch in the air condition was higher at cycle 18 (18 min of exposure) and cycle 24 (33 min of exposure) compared to all other environments in both the axial and circumferential directions (p < 0.0460). There was no significant change in mass or thickness over time in the relative humidity condition and the change in circumferential stress at 16% stretch between cycles 6 and 24 was a maximum of 0.099 MPa and not statistically significant. Implementation of a controlled relative humidity environment is recommended to maintain hydration of isolated annulus layers during cyclic tensile testing.

Peak Stress in the Annulus Fibrosus Under Cyclic Biaxial Tensile Loading.

Numerous in vitro studies have examined the initiation and propagation of fatigue injury pathways in the annulus fibrosus (AF) using isolated motion segments; however, the cycle-varying changes to the AF under cyclic biaxial tensile loading conditions have yet to be examined. Therefore, the primary objective of this study was to characterize the cycle-varying changes in peak tensile stress in multilayer AF tissue samples within a range of physiologically relevant loading conditions at subacute magnitudes of tissue stretch up to 100 loading cycles. A secondary aim was to examine whether the stress-relaxation response would be different across loading axes (axial and circumferential) and whether this response would vary across regions of the intervertebral disk (IVD) (anterior and posterior-lateral). The results from the study demonstrate that several significant interactions emerged between independent factors that were examined in the study. Specifically, a three-way interaction between the radial location, magnitude of peak tissue stretch, and cycle rate (p = 0.0053) emerged. Significant two-way interactions between the magnitude of tissue stretch and cycle number (p < 0.0001) and the magnitude of tissue stretch and loading axis (p < 0.0001) were also observed. These findings are discussed in the context of known mechanisms for structural damage, which have been linked to fatigue loading in the IVD (e.g., cleft formation, radial tearing, increased neutral zone, disk bulging, and loss of intradiscal pressure).

The impact of compressive force magnitude on the in vitro neutral zone range and passive stiffness during a flexion–extension range of motion test

Abstract The objective of this work was to examine the influence of compressive force magnitude on a functional spinal unit’s (FSU) flexion–extension neutral zone measured during pure moment (PM) tests. Each porcine cervical FSU received four repeats of a PM test with 10, 300, 900 and 1,800 N of compressive force, in a randomized order. Increasing the magnitude of compression significantly decreased the neutral zone range (p < 0.001), while increasing passive stiffness (p < 0.001). The flexion limit at 10 N was significantly lower (p < 0.05) than the other loading conditions. Reporting the compressive force magnitude is important when posture is a standardized experimental factor considered in the design of in vitro spine biomechanics studies.