Muscle short-range stiffness behaves like a maxwell element, not a spring: Implications for joint stability.

Introduction: Muscles play a critical role in supporting joints during activities of daily living, owing, in part, to the phenomenon of short-range stiffness. Briefly, when an active muscle is lengthened, bound cross-bridges are stretched, yielding forces greater than what is predicted from the force length relationship. For this reason, short-range stiffness has been proposed as an attractive mechanism for providing joint stability.

Exploring the Influence of Facet Orientation and Tropism on Neutral Zone Properties.

Lumbar spine pathologies have been linked independently to both neutral zone (NZ) properties and facet joint anatomical characteristics; however, the effect of facet joint orientation (FO) and tropism (FT) on NZ properties remains unclear. The aim of the present study was to investigate how axial plane FO and FT relate to NZ range and stiffness in the human lumbar spine and porcine cervical spine. Seven human lumbar functional spine units (FSUs) and 94 porcine cervical FSUs were examined.

Strain inhibition of bacterial collagenase is consistent with a collagen fibril uncrimping mechanism in rat tail tendons.

Mechanical strain inhibits bacterial collagenase from cleaving collagen. Additionally, the toe region of a soft tissue's force-elongation curve arises from sequentially engaging collagen fibrils as the tissue lengthens. Together, these phenomena suggest that mechanical strain may gradually inhibit collagenase activity through a soft tissue's toe region.

Lower limb postures resembling sitting and standing alter lumbar angles along the passive stiffness curve.

In vivo lumbar passive stiffness is often used to assess time-dependent changes in lumbar tissues and to define the neutral zone. We tested the hypothesis that flexing the hips would alter tension in hip and spine musculature, leading to a more extended passive stiffness curve (i.e.

Cyclic loading history alters the joint compression tolerance and regional indentation responses in the cartilaginous endplate.

This study quantified the effect of subthreshold loading histories that differed by joint posture (neutral, flexed), peak loading variation (10%, 20%, 40%), and loading duration (1000, 3000, 5000 cycles) on the post-loading Ultimate Compressive Tolerance (UCT), yield force, and regional Cartilaginous End Plate (CEP) indentation responses (loading stiffness and creep displacement). One hundred and fourteen porcine spinal units were included. Following conditioning and cyclic compression exposures, spinal units were transected and one endplate from each vertebra underwent subsequent UCT or microindentation testing.

Defining the lumbar and trunk-thigh neutral zone from the passive stiffness curve: application to hybrid sit-stand postures and chair design.

Minimal data exist on the neutral position for the lumbar spine, trunk, and thighs when adopting a hybrid posture. This study examined sex differences in the neutral zone lumbar stiffness and the lumbar and trunk-thigh angle boundaries of the neutral zone, and determined if the standing lumbar angle fell within the neutral zone. Passive lumbar flexion and extension moment-angle curves were generated for 31 participants (13 M, 18 F), pooled from two datasets, with trunk-thigh angles available for 10 participants.

Posture and Helmet Configuration Effects on Joint Reaction Loads in the Middle Cervical Spine.

Between 43 and 97% of helicopter pilots in the Canadian Armed Forces report neck pain. Potential contributing factors include the weight of their helmet, night vision goggles (NVG), and counterweight (CW) combined with deviated neck postures. Therefore, the purpose of this investigation was to quantify changes in neck loads associated with posture, helmet, NVG, and CW.

Cervical Spine Motion Requirements From Night Vision Goggles May Play a Greater Role in Chronic Neck Pain than Helmet Mass Properties.

Background: Chronic Neck Pain (CNP) among rotary-wing aircrew is thought to stem from night vision goggles (NVG) and counterweight (CW) systems which displace the centre of mass of the head. This investigation aimed to quantify the loads acting on the neck as a function of movement magnitude (MM), helmet conditions, and movement axes in rapid movements.

Methods: Cervical spine kinematics during rapid head repositioning tasks for flexion-extension (FE) and axial rotation (AR) movements were measured from 15 males and 15 females.

Are rotational passive stiffness and translational passive stiffness correlated? A porcine in vitro study.

Background: Qualitative clinical assessments of spinal stiffness have been demonstrated to show moderate correlations with one-another. We hypothesized that these correlations would improve in an in vitro model of the functional spinal unit. If the stiffness of spinal units are different across loading regimes (e.

Characterizing the Mechanical and Viscoelastic Response of the Porcine Facet Joint Capsule Ligament in Response to a Simulated Impact.

The facet capsule ligament (FCL) is a structure in the lumbar spine that constrains motions of the vertebrae. Subfailure loads can produce microdamage resulting in increased laxity, decreased stiffness, and altered viscoelastic responses. Therefore, the purpose of this investigation was to determine the mechanical and viscoelastic properties of the FCL under various magnitudes of strain from control samples and samples that had been through an impact protocol.

Strain Response in the Facet Joint Capsule During Physiological Joint Rotation and Translation Following a Simulated Impact Exposure: An In Vitro Porcine Model.

Low back pain (LBP) is frequently reported following rear impact collisions. Knowledge of how the facet joint capsule (FJC) mechanically behaves before and after rear impact collisions may help explain LBP development despite negative radiographic evidence of gross tissue failure. This study quantified the Green strain tensor in the facet joint capsule during rotation and translation range-of-motion tests completed before and following an in vitro simulation of a rear impact collision.

Passive stiffness changes in the lumbar spine following simulated automotive low speed rear-end collisions.

Background: Historically, there has been a lack of focus on the lumbar spine during rear impacts because of the perception that the automotive seat back should protect the lumbar spine from injury. As a result, there have been no studies involving human volunteers to address the risk of low back injury in low velocity rear impact collisions.

Methods: A custom-built crash sled was used to simulate rear impact collisions.

An Electromyographically Driven Cervical Spine Model in OpenSim.

Relatively few biomechanical models exist aimed at quantifying the mechanical risk factors associated with neck pain. In addition, there is a need to validate spinal-rhythm techniques for inverse dynamics spine models. Therefore, the present investigation was 3-fold: (1) the development of a cervical spine model in OpenSim, (2) a test of a novel spinal-rhythm technique based on minimizing the potential energy in the passive tissues, and (3) comparison of an electromyographically driven approach to estimating compression and shear to other cervical spine models.

The Influence of Simulated Low Speed Vehicle Impacts and Posture on Passive Intervertebral Mechanics.

Study Design: An in vitro biomechanics investigation exposing porcine functional spinal units (FSUs) to sudden impact loading although in a flexed, neutral, or extended posture.

Objective: To investigate the combined effect of impact severity and postural deviation on intervertebral joint mechanics.

Summary Of Background Data: To date, no in vitro studies have been conducted to explore lumbar tissue injury potential and altered mechanical properties from exposure to impact forces.

Exploring the influence of impact severity and posture on vertebral joint mechanics in an in-vitro porcine model.

To date, no in vitro studies have been conducted to explore lumbar soft tissue injury potential and altered mechanical properties from exposure to impact forces. After a motor vehicle collision (MVC), the cause of reported acute onset low back pain is difficult to associate with potential soft tissue strain injury sites including the facet joint and innervated facet joint capsule ligament (FJC). Thus, the purpose of this investigation was to quantify intervertebral anterior-posterior (AP) translation and facet joint capsule strain under varying postures and impact severities.

Model-Aided Design of a Rear-Impact Collision Testing System for In Vivo Investigations.

A collision testing device used to simulate rear-end impacts on human volunteers was developed and validated. The testing device was designed using impact parameters obtained from real crash-test-derived vehicle-to-vehicle rear-end collisions. Experimental results show the proposed testing device achieves repeatable impact parameters well within the reported ranges of real vehicle-to-vehicle rear-end impact simulations reported in the literature.

A novel least-squares method to characterize in-vivo joint functional passive regional stiffness zones.

Objective: To present and evaluate a method to objectively quantify the functional regions of joint lumped passive stiffness.

Background: Joint passive stiffness has an important clinical role in constraining the degrees of freedom at a given joint. Links between passive stiffness and injury, pathology and function may be better understood if joint passive stiffness can be accurately quantified.

The rate of tendon failure in a collagen fibre recruitment-based model.

Accurate characterization of the mechanical response of collagenous tissues is critical for investigations into mechanisms of soft tissue injury. These tissues are inherently viscoelastic, exhibiting strain-rate dependent stiffnesses, creep, and stress-relaxation. The strain-rate features of the failure portion of the stress-strain curve are less well developed.

A Comparison of Clinical Spinal Mobility Measures to Experimentally Derived Lumbar Spine Passive Stiffness.

Spinal stiffness and mobility assessments vary between clinical and research settings, potentially hindering the understanding and treatment of low back pain. A total of 71 healthy participants were evaluated using 2 clinical assessments (posteroanterior spring and passive intervertebral motion) and 2 quantitative measures: lumped mechanical stiffness of the lumbar spine and local tissue stiffness (lumbar erector spinae and supraspinous ligament) measured via myotonometry. The authors hypothesized that clinical, mechanical, and local tissue measures would be correlated, that clinical tests would not alter mechanical stiffness, and that males would demonstrate greater lumbar stiffness than females.

A one-dimensional collagen-based biomechanical model of passive soft tissue with viscoelasticity and failure.

Introduction: Strains and sprains of soft tissues, including tendons and ligaments, are frequently occurring injuries. Musculoskeletal models show great promise in prediction and prevention of these injuries. However, these models rarely account for the viscoelastic properties of ligaments and tendons, much less their failure properties.

A Mobile Application to Measure Trunk Flexion Angles in Lifting Tasks.

The aim of this project was to develop and test an application capable of quickly and repeatedly measuring trunk flexion angles during sagittal plane lifting tasks. The developed application uses the built-in accelerometer in mobile devices to approximate trunk flexion angle, as the user follows an operator as they perform a lift. A black line is superimposed over the camera feed, allowing the user to approximate the angle of inclination of a line connecting the operator's seventh cervical and first sacral vertebrae-thereby estimating the trunk flexion angle.

Expressing angles relative to reference postures: A mathematical comparison of four approaches.

Three-dimensional joint angles are most often quantified using Euler Angles. These measures are often easier to interpret if they are reported relative to a reference posture. However, since Euler Angles are not vectors, directly subtracting their values is difficult to justify mathematically.

Cervical spine joint loading with neck flexion.

Cervical spine flexion is a common posture for those using computers, tablets, and smartphones; the latter being dubbed 'text-neck' in recent years. Coincidentally, flexion has been flagged as a significant mechanical risk factor for the development of chronic neck pain. Unfortunately, few modelling endeavours have attempted to quantify the intervertebral joint loads throughout the cervical spine in flexion, while still accounting for muscular activation.

A versatile approach to determine instantaneous co-activation: Development, implementation and comparison to existing measures.

An adaptable method of computing muscle co-activity is presented. Instantaneous muscle activities from multiple muscles are reduced into commonality and activity level dimensions. Both dimensions are weighted independently and combined into a co-activation measure, Φ.