Transmission of acceleration from a synchronous vibration exercise platform to the head.

Exercise vibration platforms are becoming commonplace in homes and fitness centers. However, excessive mechanical energy transferred to the head and eye can cause injury. The purpose of this study was to evaluate how changes in platform frequency and knee flexion angle affect acceleration transmission to the head. Participants (N=12) stood on a whole-body vibration platform with knee flexion angles of 0°, 20°, and 40° to evaluate how changes in knee flexion affected head acceleration. 7 specific platform frequencies were tested between 20-50 Hz at 2 peak-to-peak displacement settings (1 and 2 mm nominal). Accelerations were measured with triaxial accelerometers at the platform and head to generate transmissibility ratios. Platform-to-head transmissibility was not significantly different between the 2 platform peak-to-peak amplitudes (P>0.05). Transmissibility measures varied depending on platform frequency and knee angle (P < 0.05). Flexing the knees resulted in reduced head transmissibility at all frequencies (P<0.05). Platform-to-head transmissibility values exceeded 1.0 at both 20 and 25 Hz platform vibration frequencies with the knees in full extension. To reduce the risk of injury to structures of the head during vibration exercise, using platforms frequencies below 30 Hz with small knee flexion angles (< 40°) should be avoided.

Application of robotic technology in biomechanics to study joint laxity.

PRIMARY OBJECTIVE: To evaluate whether in vitro joint testing using a robot with six degrees of freedom is useful for evaluating changes in joint laxity as a result of chronic osteoarthritis (OA). RESEARCH DESIGN: Repeated measures. METHODS: Broyden's method of solving nonlinear systems of equations drove a hybrid method of load and position robotic control. Sheep stifles (knee joints) were loaded between 3 Nm of internal load through to 3 Nm of external load in 1 Nm increments. Kinematic and morphologic data from five healthy ovine stifles were compared to the chronic OA effects in four surgically destabilized stifles. RESULTS: Stifles with chronic OA showed increases in stiffness while range of motion decreased. Gross morphologic changes included osteophytes and cartilage fibrillation. DISCUSSION: Robotic testing proved useful for evaluating changes in joint mechanics as a result of chronic OA. We observed morphological changes and associated increases in joint stiffness and decreased laxity.

Six-degree-of-freedom whole-body vibration exposure levels during routine skidder operations.

This research focuses on quantifying six-degree-of-freedom (6-DOF) whole-body vibration (WBV) exposure levels that occur in Northern Ontario skidders during routine field operating tasks. 6-DOF vibration running root-mean-square (RMS) acceleration levels at the operator/seat interface were determined for eight skidders while driving loaded, driving unloaded, picking up a load, dropping off a load and ploughing logs under field operating conditions. The acceleration data were weighted in accordance with ISO 2631-1:1997 and evaluated for both health and comfort outcomes. The mean running RMS weighted translational and rotational accelerations all exceeded 0.36 m/s(2) and 0.14 rad/s(2). The greatest average accelerations occurred while driving unloaded with this condition displaying translational vibration total values (VTV) that exceeded the upper limit of the ISO 2631-1:1997 health caution zone within an average of 2.3 h. Utilizing 6-DOF VTV, virtually all operating conditions would be designated as uncomfortable. STATEMENT OF RELEVANCE: This study provides one of the most comprehensive reports on vibration exposures in seated vehicle operators. The results are geared towards ergonomists with discussions on health effects and measurement concerns, while providing the raw vibration exposure data that will be useful to vehicle, component and vibration sensor designers.

Predicting discomfort scores reported by LHD operators using whole-body vibration exposure values and musculoskeletal pain scores.

BACKGROUND: Laboratory studies have typically been used to establish a relationship between whole body vibration (WBV) magnitude, frequency, exposure duration and reported discomfort. However, the relationship between subjective reports of discomfort, and predicted comfort response according to International Standards Organization (ISO) 2631-1, predicted health risks according to ISO 2631-1 and reported musculoskeletal discomfort has not been examined. The purpose here was to compare discomfort values predicted by ISO 2631-1 with the subjective discomfort reported by nine Load-haul-dump (LHD) operators during typical operating conditions. METHODS: Vibration exposure at the operator/seat interface was measured and processed, for one-hour duration, according to criteria established in ISO 2631-1. Vibration total values were determined for 1-minute exposure periods and the LHD operators provided a discomfort score associated with the same vibration exposure period. A linear regression analysis and correlation was carried out to determine the strength of the relationship between the predicted subjective reports of discomfort, ISO 2631-1 discomfort, objectively measured acceleration levels and reported musculoskeletal discomfort. FINDINGS: Reported discomfort was poorly correlated to ISO discomfort scores (r=0.1799). Vibration exposure values and Musculo-Skeletal Disorder (MSD) variables were related to both ISO 2631-1 discomfort and to reported discomfort. The MSD scores produced stronger relationships with reported discomfort scores than did the vibration exposure values.

Contact mechanics of the ovine stifle during simulated early stance in gait. An in vitro study using robotics.

Ovine stifle joint contact pressures and contact areas were measured in vitro using a six degree-of-freedom (DOF) robotic system. The robot generated static joint loads of 1.875 times body weight (BW) compression, 0.15 BW medial shear and 0.625 BW cranial shear at 6.5 degrees of flexion for four specimens, simulating the early stance phase of gait (walking). This condition represents a period of intense loading and was implemented as a worst-case loading scenario for the joint at this gait. We determined that the medial and lateral compartments bore 5.5 +/- 0.9 MPa and 4.4 +/- 1.1 MPa of mean pressure, respectively, on 107.7 +/- 28.7 mm(2) and 60.8 +/- 56.3 mm(2) of area, respectively. The unique contribution of this study is that stifle contact pressures and areas were determined during loading which simulated physiological levels (early stance phase of gait). This information is important to our understanding of the stresses that must be borne by repair tissues/constructs that are implanted into human and animal tibio-femoral joints.

Mapping of donor and recipient site properties for osteochondral graft reconstruction of subchondral cystic lesions in the equine stifle joint.

REASONS FOR PERFORMING STUDY: To improve osteochondral graft reconstruction of subchondral cystic lesions in the medial and lateral femoral condyles by matching the material properties of donor and recipient sites. OBJECTIVES: To measure biomechanical and biochemical parameters that influence the function and healing of osteochondral grafts used to reconstruct subchondral cystic lesions. HYPOTHESIS: Suitable donor sites are available within the stifle joint for reconstructing the femoral condyles, despite considerable regional property variation. METHODS: Fifty-six osteochondral cores were harvested from 6 distal femurs for initial studies that determined subchondral bone modulus of elasticity and ultimate stress. In a second study, 28 osteochondral cores were harvested from 6 distal femurs to measure cartilage aggregate modulus, thickness and sulphated glycosaminoglycan (sGAG) content. Using micro-CT imaging, subchondral bone mineral density and bone volume fraction were also measured. In both studies 2-dimensional contour plots using a bicubic interpolation method and normalised data were generated to allow visual comparison of joint surface characteristics. Statistical comparisons between donor and recipient site raw data were made using an ANOVA for repeated measures with a post hoc Tukey test. RESULTS: Material properties of cartilage and bone vary considerably over the surface of the stifle joint but the central region of the medial condyle, where subchondral cystic lesions freqdently occur, typically demonstrated bone strength and modulus values of the highest observed. Cartilage thickness and aggregate modulus were highest in the medial femoral condyle and axial aspect of the lateral condyle. CONCLUSIONS: Material properties of the grafts from the trochlear groove and axial aspect of the lateral trochlear ridge were the closest match for those found in the medial condyle, whereas properties of the lateral condyle were most similar to those found in the trochlear groove and axial aspect of the medial trochlear ridge.

Effect of specimen length: are the mechanics of individual motion segments comparable in functional spinal units and multisegment specimens?

Functional spinal units (FSUs) are frequently used for in vitro mechanical testing. This approach assumes that the mechanical behavior of the FSUs is equivalent to the mechanics of these segments within the intact spine. The purpose of this study was to determine whether normal spinal mechanics are compromised in FSU preparations. Flexion-extension pure-moment flexibility testing was performed on 13 L2-L5 porcine specimens. The moment-angle relationship of the L3/L4 segment was recorded, and then the multisegment specimens were cut down to L3/L4 FSUs and retested. Comparisons of stiffness, range of motion, and laxity zone were made between conditions. The neutral zone and range of motion parameters were significantly larger in the L3/L4 motion segment compared to the L3/L4 segment tested within the multisegment specimen; the stiffness was not significantly different. These differences were attributed to cutting the supraspinous ligament as this ligament spans several vertebral levels. Flexion mechanical tests performed on FSUs should be interpreted cautiously as the biomechanics of FSUs is altered from normal. Although the choice of spine length depends on the experimental purpose, spinal flexion studies should be performed on multisegment specimens to appropriately represent the anatomical boundary conditions.

A continuous pure moment loading apparatus for biomechanical testing of multi-segment spine specimens.

An apparatus is described that enables the application of continuous pure moment loads to multi-segment spine specimens. This loading apparatus allows continuous cycling of the spine between specified flexion and extension (or right and left lateral bending) maximum load endpoints. Using a six-degree-of-freedom load cell and three-dimensional optoelectronic stereophotogrammetry, characteristic displacement versus load hysteresis curves can be generated and analyzed for different spinal constructs of interest. Unlike quasi-static loading, the use of continuous loading permits the analysis of the spine's behaviour within the neutral zone. This information is of particular clinical significance given that the instability of a spinal segment is related to its flexibility within the neutral zone. Representative curves for the porcine lumbar spine in flexion-extension and lateral bending are presented to illustrate the capabilities of this system.

Measuring collagen fiber orientation: a two-dimensional quantitative macroscopic technique.

This paper describes the design, evaluation, and application of a new system for quantifying two-dimensional collagen fiber orientation in soft tissue. Series of transmitted polarized light images were collected using a custom-designed macroscope. Combined analysis of pixel brightness, and hue from images collected with a compensator plate, permitted the assignment of each pixel into the appropriate orientation band. Experiments were performed to quantify the linearity and noise of the system. Validation was performed on a specimen composed of strain-birefringent plastic strips at various orientations. Preliminary collagen fiber orientation data is presented from a tendon specimen. This study demonstrates the utility of this approach for studying collagen fiber orientation across large areas.

New insight into the mechanics of the lumbar interspinous ligament.

STUDY DESIGN: Repeated in vitro mechanical tests were performed on porcine and human interspinous ligament specimens with progressive disruption of the collagen fiber network to evaluate the existence of mechanical interactions between collagen fibers. OBJECTIVE: To evaluate the existence of a load pathway in the interspinous ligament whereby loads are transmitted between collagen fibers. SUMMARY OF BACKGROUND DATA: Mechanical tests demonstrate that the interspinous ligament resists spinal flexion, but the collagen fibers are not oriented to oppose separation of the spinous processes. This seeming contradiction could be explained by the existence of mechanical interactions between collagen fibers of the interspinous ligament. METHODS: In vitro mechanical tests were performed on porcine and human bone-interspinous ligament-bone specimens. The collagen network of the ligament substance was disrupted by passing a scalpel blade though the ligament substance. Repeated tests were performed with progressive disruption of the collagen fiber network. RESULTS: The stiffness of the interspinous ligament specimens was reduced as the collagen fiber network was disrupted, but approximately half of the initial stiffness was maintained when the collagen fiber network was severely disrupted. In this case, no intact collagen fibers remained. CONCLUSIONS: Mechanical interactions exist between collagen fibers in the interspinous ligament. The mechanism of the interactions is unknown.

Adaptations in gait resulting from unilateral ischaemic block of the leg.

The purpose of this study was to develop a major temporary perturbation to the peripheral neuromuscular system and to quantify the motor adaptations on the first gait strides taken after the perturbation becomes effective. A unilateral ischaemic block to the leg was induced by a cuff inflated to 200 mmHg; sensory block was evident after 15-20 min and total motor block after about 30 min. Electromyographs from seven muscles on each limb were recorded for the first 16s after walking commenced and were compared with electromyographs from cadence-matched strides recorded 30 min after the cuff was removed. Ensemble-averaged profiles of the ischaemic and control trials from six subjects showed that almost half of the non-ischaemic muscles (4 on the ischaemic limb and 7 on the intact limb) had significantly different profiles. Only two muscles showed consistent changes for all six subjects (gastrocnemius and rectus femoris of the intact limb).