Neck posture and muscle activity are different when upside down: A human volunteer study

Rollover crashes are dynamic and complex events in which head impacts with the roof can cause catastrophic neck injuries. Ex vivo and computational models are valuable in understanding, and ultimately preventing, these injuries. Although neck posture and muscle activity influence the resulting injury, there is currently no in vivo data describing these parameters immediately prior to a head-first impact. The specific objectives of this study were to determine the in vivo neck vertebral alignment and muscle activation levels when upside down, a condition that occurs during a rollover. Eleven human subjects (6F, 5M) were tested while seated upright and inverted in a custom-built apparatus. Vertebral alignment was measured using fluoroscopy and muscle activity was recorded using surface and indwelling electrodes in eight superficial and deep neck muscles. In vivo vertebral alignment and muscle activation levels differed between the upright and inverted conditions. When inverted and relaxed, the neck was more lordotic, C1 was aligned posterior to C7, the Frankfort plane was extended, and the activity of six muscles increased compared to upright and relaxed. When inverted subjects were asked to look forward to eliminate head extension, flexor muscle activity increased, C7 was more flexed, and C1 was aligned anterior to C7 versus upright and relaxed. Combined with the large inter-subject variability observed, these findings indicate that cadaveric or computational models designed to study injuries and prevention devices while inverted need to consider a variety of postures and muscle conditions to be relevant to the in vivo situation.     

Moving muscle points provide accurate curved muscle paths in a model of the cervical spine

Muscle paths in musculoskeletal models have been modeled using several different methods; however, deformation of soft tissue with changes in posture is rarely accounted for, and often only the neutral posture is used to define a muscle path. The objective of this study was to model curved muscle paths in the cervical spine that take into consideration soft tissue deformation with changes in neck posture. Two subject-specific models were created from magnetic resonance images (MRI) in 5 different sagittal plane neck postures. Curved paths of flexor and extensor muscles were modeled using piecewise linear lines-of-action in two ways; (1) using fixed via points determined from muscle paths in the neutral posture and (2) using moving muscle points that moved relative to the bones determined from muscle paths in all 5 postures. Accuracy of each curved modeled muscle path was evaluated by an error metric, the distance from the anatomic (centroid) muscle path determined from the MRI. Error metric was compared among three modeled muscle path types (straight, fixed via and moving muscle point) using a repeated measures one-way ANOVA (α=0.05). Moving muscle point paths had 21% lower error metric than fixed via point paths over all 15 pairs of neck muscles examined over 5 postures (3.86 mm vs. 4.88 mm). This study highlights the importance of defining muscle paths in multiple postures in order to properly define the changing curvature of a muscle path due to soft tissue deformation with posture.     

A female head–neck model for rear impact simulations

Several mathematical cervical models of the 50th percentile male have been developed and used for impact biomechanics research. However, for the 50th percentile female no similar modelling efforts have been made, despite females being subject to a higher risk of soft tissue neck injuries. This is a limitation for the development of automotive protective systems addressing Whiplash Associated Disorders (WADs), most commonly caused in rear impacts, as the risk for females sustaining WAD symptoms is double that of males.In this study, a finite element head and neck model of a 50th percentile female was validated in rear impacts. A previously validated ligamentous cervical spine model was complemented with a rigid body head, soft tissues and muscles. In both physiological flexion-extension motions and simulated rear impacts, the kinematic response at segment level was comparable to that of human subjects. Evaluation of ligament stress levels in simulations with varied initial cervical curvature revealed that if an individual assumes a more lordotic posture than the neutral, a higher risk of WAD might occur in rear impact.The female head and neck model, together with a kinematical whole body model which is under development, addresses a need for tools for assessment of automotive protection systems for the group which is at the highest risk to sustain WAD.     

Simulation of the effects of different pilot helmets on neck loading during air combat

New generation pilot helmets with mounted devices enhance the capabilities of pilots substantially. However, the additional equipment increases the helmet weight and shifts its center of mass forward. Two helmets with different mass properties were modeled to simulate their effects on the pilot's neck. A musculoskeletal computer model was used, with the methods of inverse dynamics and static optimization, to compute the muscle activations and joint reaction forces for a given range of quasi-static postures at various accelerations experienced during air combat. Head postures which induce much higher loads on the cervical spine than encountered in a neutral position could be identified. The increased weight and the forward shift of the center of mass of a new generation helmet lead to higher muscle activations and higher joint reaction loads over a wide range of head and neck movements. The muscle activations required to balance the head and neck in extreme postures increased the compressive force at the T1-C7 level substantially, while in a neutral posture the muscle activations remained low. The lateral neck muscles can reach activations of 100% and cause compressive joint forces up to 1100N during extensive rotations and extensions at high 'vertical' accelerations (Gz). The calculated values have to be interpreted with care as the model has not been validated. Nevertheless, this systematic analysis could separate the effects of head posture, acceleration and helmet mass on neck loading. More reliable data about mass properties and muscle morphometry with a more detailed motion analysis would help to refine the existing model.     

Analysis of control strategies for VIVA OpenHBM with active reflexive neck muscles

Modeling muscle activity in the neck muscles of a finite element (FE) human body model can be based on two biological reflex systems. One approach is to approximate the Vestibulocollic reflex (VCR) function, which maintains the head orientation relative to a fixed reference in space. The second system tries to maintain the head posture relative to the torso, similar to the Cervicocolic reflex (CCR). Strategies to combine these two neck muscle controller approaches in a single head-neck FE model were tested, optimized, and compared to rear-impact volunteer data. The first approach, Combined-Control, assumed that both controllers simultaneously controlled all neck muscle activations. In the second approach, Distributed-Control, one controller was used to regulate activation of the superficial muscles while a different controller acted on deep neck muscles. The results showed that any muscle controller that combined the two approaches was less effective than only using one of VCR- or CCR-based systems on its own. A passive model had the best objective rating for cervical spine kinematics, but the addition of a single active controller provided the best response for both head and cervical spine kinematics. The present study demonstrates the difficulty in completely capturing representative head and cervical spine responses to rear-impact loading and identified a controller capturing the VCR reflex as the best candidate to investigate whiplash injury mechanisms through FE modeling.

     

Cervical flexion–relaxation response to neck muscle fatigue in males and females

In this study the effect of muscle fatigue on the cervical spine flexion–relaxation response was studied. Twenty healthy participants (10 males and 10 females) were recruited for data collection. The Sorenson protocol was utilized to induce neck muscle fatigue. Surface electromyography and optical motion capture systems were used to measure neck muscle activation and head–neck posture, respectively. A post-fatigue reduction in the Flexion–Relaxation Ratio (FRR) and higher FRR for females compared to males were observed. A post-fatigue decrease was also observed in the onset and offset angles resulting in an expansion of the myoelectric silence period. Gender had no effect on the onset and offset angles of the silence period. Post-fatigue shift in the onset and offset angles and the expansion of the silence period indicate an increased contribution by the passive viscoelastic tissues in stabilizing the cervical spine under fatigued condition.     

The effect of postural correction on muscle activation amplitudes recorded from the cervicobrachial region

In clinical practice, postural correction is a common treatment approach for individuals with neck and shoulder pain. As chronic static muscle use is thought to be associated with the onset of some neck and shoulder pain syndromes, it is important to understand the impact a postural correction program might have on muscle activation amplitudes in the neck and shoulder regions. Normalized surface electromyographic data were recorded from the levator scapulae, upper trapezius, supraspinatus, posterior deltoid, masseter, rhomboid major, cervical erector spinae, and sternocleidomastoid muscles of the dominant side of each of eighteen healthy subjects. Subjects performed five repetitions of each of four seated typing postures (habitual, corrected, head-forward and slouched) and four standing postures (habitual, corrected, and head-forward and slouched). Repeated-measures analysis of variance models (α = 0.05) revealed that in sitting postural correction tended to decreased the level of muscle activation required in all muscles studied during seated computer work, however this finding was not statistically significant. Corrected posture in sitting did, however produce a statistically significant reduction in muscle activity compared to forward head posture. Corrected posture in standing required more muscle activity than habitual or forward head posture in the majority of cervicobrachial and jaw muscles, suggesting that a graduated approach to postural correction exercises might be required in order to train the muscles to appropriately withstand the requirements of the task. A surprising finding was that muscle activity levels and postural changes had the largest impact on the masseter muscle, which demonstrated activation levels in the order of 20% maximum voluntary electrical activation.     

Sarcomere length organization as a design for cooperative function amongst all lumbar spine muscles

The functional design of spine muscles in part dictates their role in moving, loading, and stabilizing the lumbar spine. There have been numerous studies that have examined the isolated properties of these individual muscles. Understanding how these muscles interact and work together, necessary for the prediction of muscle function, spine loading, and stability, is lacking. The objective of this study was to measure sarcomere lengths of lumbar muscles in a neutral cadaveric position and predict the sarcomere operating ranges of these muscles throughout full ranges of spine movements. Sarcomere lengths of seven lumbar muscles in each of seven cadaveric donors were measured using laser diffraction. Using published anatomical coordinate data, superior muscle attachment sites were rotated about each intervertebral joint and the total change in muscle length was used to predict sarcomere length operating ranges. The extensor muscles had short sarcomere lengths in a neutral spine posture and there were no statistically significant differences between extensor muscles. The quadratus lumborum was the only muscle with sarcomere lengths that were optimal for force production in a neutral spine position, and the psoas muscles had the longest lengths in this position. During modeled flexion the extensor, quadratus lumborum, and intertransversarii muscles lengthened so that all muscles operated in the approximate same location on the descending limb of the force-length relationship. The intrinsic properties of lumbar muscles are designed to complement each other. The extensor muscles are all designed to produce maximum force in a mid-flexed posture, and all muscles are designed to operate at similar locations of the force-length relationship at full spine flexion.     

An examination of the flexion-relaxation phenomenon in the cervical spine in lumbo-pelvic sitting

The flexion-relaxation phenomenon (FRP) is well documented at end-range lumbar spine flexion in both standing and sitting however, the FRP has been insufficiently investigated in cervico-thoracic musculature. The aim of this study was to determine whether the FRP occurs during forward flexion of the neck, in lumbo-pelvic sitting, amongst a pain-free population. Surface electromyography (EMG) was used to measure muscle activation in 20 (10 men, 10 women) asymptomatic subjects in selected cervico-thoracic muscles during four, 5-s phases (upright posture, forward flexion, full flexion and return to upright) while subjects were positioned in lumbo-pelvic sitting. Spinal kinematics were simultaneously measured using an electromagnetic motion tracking device. No FRP was observed in upper trapezius or thoracic erector spinae (T4). When using visual methods to determine the presence/absence of the FRP, five subjects were believed to show evidence of the FRP in the cervical erector spinae. However, when using various non-visual criteria to determine the existence of the FRP, substantial variations (0–13 subjects) were evident. We recommend that criteria based upon relatively large differences in muscle activation should be considered when defining the FRP. These findings are of significance for future investigations examining specific cervical pain disorders.     

Validation of an EMG-driven, graphically based isometric musculoskeletal model of the cervical spine

EMG-driven musculoskeletal modeling is a method in which loading on the active and passive structures of the cervical spine may be investigated. A model of the cervical spine exists; however, it has yet to be criterion validated. Furthermore, neck muscle morphometry in this model was derived from elderly cadavers, threatening model validity. Therefore, the overall aim of this study was to modify and criterion validate this preexisting graphically based musculoskeletal model of the cervical spine. Five male subjects with no neck pain participated in this study. The study consisted of three parts. First, subject-specific neck muscle morphometry data were derived by using magnetic resonance imaging. Second, EMG drive for the model was generated from both surface (Drive 1: N=5) and surface and deep muscles (Drive 2: N=3). Finally, to criterion validate the modified model, net moments predicted by the model were compared against net moments measured by an isokinetic dynamometer in both maximal and submaximal isometric contractions with the head in the neutral posture, 20 deg of flexion, and 35 deg of extension. Neck muscle physiological cross sectional area values were greater in this study when compared to previously reported data. Predictions of neck torque by the model were better in flexion (18.2% coefficient of variation (CV)) when compared to extension (28.5% CV) and using indwelling EMG did not enhance model predictions. There were, however, large variations in predictions when all the contractions were compared. It is our belief that further work needs to be done to improve the validity of the modified EMG-driven neck model examined in this study. A number of factors could potentially improve the model with the most promising probably being optimizing various modeling parameters by using methods established by previous researchers investigating other joints of the body.     

A three-dimensional model of the human cervical spine for impact simulation

A three-dimensional analytical model of the cervical spine is described. The cervical vertebrae and the head are modeled as rigid bodies which are interconnected by deformable elements representing the intervertebral disks, facet joints, ligaments and muscles. A special pentahedral continuum element for representing the articular facets is described which effectively maintains stability of the cervical spine in both lateral and frontal plane accelerations, which is very difficult with multi-spring models of the facets. A simplified representation is used for the spine and body below the level of T1. The neck musculature is modeled by over 100 muscle elements representing 22 major muscle groups in the neck. The model has been validated for frontal and sideways impact accelerations by simulating published experimental data. Results are also presented to show the effects of the stretch reflex response on the dynamics of the head and neck under moderate acceleration.     

Torsion and Bending in the Neck and Tail of Sauropod Dinosaurs and the Function of Cervical Ribs: Insights from Functional Morphology and Biomechanics

The long necks of sauropods have been subject to many studies regarding their posture and flexibility. Length of the neck varies among groups. Here, we investigate neck posture and morphology in several clades from a mechanical viewpoint. Emphasis is put on comparing sauropod necks and tails with structures in living archosaurs and mammals. Differences in the use made of necks and tails lead to clear-cut differences in the mechanical loads occurring in the same models. Ways of sustaining loads are identified by theoretical considerations. If the observed skeletal structures are suited to resist the estimated loading in a particular posture, this concordance is taken as an argument that this posture or movement was of importance during the life of the individual. Apart from the often-discussed bending in side view, we analyze the often overlooked torsion. Because torsional stresses in a homogenous element concentrate near the periphery, a cylindrical cross section gives greatest strength, and the direction of forces is oblique. In a vertebrate neck, during e.g. shaking the head and twisting the neck, oblique muscles, like the mm. scaleni, if activated unilaterally initiate movement, counterbalance the torsional moments and keep the joints between neck vertebrae in equilibrium. If activated bilaterally, these muscles keep the neck balanced in an energy-saving upright posture. The tendons of the mm. scaleni may have ossified as cervical ribs The long cervical ribs in brachiosaurids and mamenchisaurids seem to have limited flexibility, whereas the shorter cervical ribs in Diplodocidae allowed free movement. The tails of sauropods do not show pronounced adaptation to torsion, and seem to have been carried more or less in a horizontal, extended posture. In this respect, sauropod tails resemble the necks of herbivorous cursorial mammals. These analyses provide an improved understanding of neck use that will be extended to other sauropods in subsequent studies.     

Load-sharing in the lumbosacral spine in neutral standing & flexed postures – A combined finite element and inverse static study

Understanding load-sharing in the spine during in-vivo conditions is critical for better spinal implant design and testing. Previous studies of load-sharing that considered actual spinal geometry applied compressive follower load, with or without moment, to simulate muscle forces. Other studies used musculoskeletal models, which include muscle forces, but model the discs by simple beams or spherical joints and ignore the articular facet joints.This study investigated load-sharing in neutral standing and flexed postures using a detailed Finite Element (FE) model of the ligamentous lumbosacral spine, where muscle forces, gravity loads and intra-abdominal pressure, as predicted by a musculoskeletal model of the upper body, are input into the FE model. Flexion was simulated by applying vertebral rotations following spine rhythm measured in a previous in-vivo study, to the musculoskeletal model. The FE model predicted intradiscal pressure (IDP), strains in the annular fibers, contact forces in the facet joints, and forces in the ligaments. The disc forces and moments were determined using equilibrium equations, which considered the applied loads, including muscle forces and IDP, as well as forces in the ligaments and facet joints predicted by the FE model. Load-sharing was calculated as the portion of the total spinal load carried along the spine by each individual spinal structure. The results revealed that spinal loads which increased substantially from the upright to the flexed posture were mainly supported by the discs in the upright posture, whereas the ligaments’ contribution in resisting shear, compression, and moment was more significant in the flexed posture.     

Differential effects of mental concentration and acute psychosocial stress on cervical muscle activity and posture

Physical and psychosocial stressors in the workplace have been independently associated with the development of neck pain, yet interactions among these risk factors remain unclear. The purpose of this study was to compare the effects of mentally challenging computer work performed with and without exposure to a psychosocial stressor on cervical muscle activity and posture. Changes in cervical posture and electromyography of upper trapezius, cervical extensor, and sternocleidomastoid muscles were compared between a resting seated posture at baseline, a low stress condition with mental concentration, and a high stress condition with mental concentration and psychosocial stress in sixty healthy office workers. Forward head posture significantly increased with mental concentration compared to baseline, but did not change with further introduction of the stressor. Muscle activity significantly increased from the low stress to high stress condition for both the dominant and non-dominant upper trapezius, with no corresponding change in activity of the cervical extensors or flexors between stress conditions. These findings suggest that upper trapezius muscles are selectively activated by psychosocial stress independent of changes in concentration or posture, which may have implications for the prevention of stress-related trapezius myalgia in the workplace.     

Reconstruction of the neck of Azhdarcho lancicollis and lifestyle of azhdarchids (Pterosauria, Azhdarchidae)

A computer reconstruction of isolated cervical vertebrae of Azhdarcho lancicollis from the Turonian of Uzbekistan allows three-dimensional model of the cervical region of the vertebral column of this animal. The relative length of cervical vertebrae (I + II < III < IV < V > VI > VII > VIII > IX) is the same as in pterodactyloids with short cervical vertebrae. An increase in neck length is provided mostly by the middle cervical vertebrae (IV–VI). In a neutral posture, the neck of azhdarchids was not straight, as often reconstructed, but S-shaped, with the maximum angles between the V–VI (20°), VI–VII (20°), and VIII–IX (17°) vertebrae. The feeding strategy of azhdarchids was probably similar to that of pelicans. In a search for prey, azhdarchids were soaring above the water surface of large inland or nearshore marine water bodies. Their prey (predominantly fish) was captured by the widely open mouth and fell into the throat sac, the presence of which is suggested by the spiral jaw joint. Prey was swallowed during the abrupt neck flexion in the posterior segment, which brought the head in an almost horizontal position. A storklike wading ecology for azhdarchids is less probable, because these clumsy on land animals were vulnerable to terrestrial predators.     

Quantitative Method for Biomechanical Evaluation of Bedding Comfortableness

Comfortable bedding is usually designed subjectively because of the difficulty in performing a quantitative evaluation. This paper proposes a quantitative evaluation method of comfortableness of beddings. The bedding shape determining how comfortable an individual may feel in using it depends on the body shape and normal posture of individuals. The internal physical load is expected to relate to the comfortableness of bedding. However, only a few quantitative discussions exist on the relation between the comfortableness of bedding and physical load. This study proposes a new evaluation method of physical load in a relaxed posture. The strain energy of muscles and joints was used as an indicator of physical load. To estimate physical load, a neutral body position was simulated from a natural standing posture and was used as a reference posture with the neutral condition of muscle lengths. By considering individual differences, multiple models of neutral body position were provided. We simulated individual differences of a comfortable pillow height using the proposed models. Physical load in a relaxed posture was varied according to the models. Calculated results show that physical load becomes small when a pillow is comfortable. For both subjective pillow comfortableness and smallness of physical load, there is a similar tendency that the low pillow with the small height difference between head and neck is preferable if the concave depth of back shape of head and neck is small. Moreover, the results show that muscles and joints equally affect the comfortableness of designed pillow. This implies that less total energy required for maintaining the posture contributes to pillow comfortableness.     

Functional Synergies Underlying Control of Upright Posture during Changes in Head Orientation

Background

Studies of human upright posture typically have stressed the need to control ankle and hip joints to achieve postural stability. Recent studies, however, suggest that postural stability involves multi degree-of-freedom (DOF) coordination, especially when performing supra-postural tasks. This study investigated kinematic synergies related to control of the body’s position in space (two, four and six DOF models) and changes in the head’s orientation (six DOF model).

Methodology/Principal Findings

Subjects either tracked a vertically moving target with a head-mounted laser pointer or fixated a stationary point during 4-min trials. Uncontrolled manifold (UCM) analysis was performed across tracking cycles at each point in time to determine the structure of joint configuration variance related to postural stability or tracking consistency. The effect of simulated removal of covariance among joints on that structure was investigated to further determine the role of multijoint coordination. Results indicated that cervical joint motion was poorly coordinated with other joints to stabilize the position of the body center of mass (CM). However, cervical joints were coordinated in a flexible manner with more caudal joints to achieve consistent changes in head orientation.

Conclusions/Significance

An understanding of multijoint coordination requires reference to the stability/control of important performance variables. The nature of that coordination differs depending on the reference variable. Stability of upright posture primarily involved multijoint coordination of lower extremity and lower trunk joints. Consistent changes in the orientation of the head, however, required flexible coordination of those joints with motion of the cervical spine. A two-segment model of postural control was unable to account for the observed stability of the CM position during the tracking task, further supporting the need to consider multijoint coordination to understand postural stability.     

Activity of respiratory muscles in upright and recumbent humans

It is established that during tidal breathing the rib cage expands more than the abdomen in the upright posture, whereas the reverse is usually true in the supine posture. To explore the reasons for this, we studied nine normal subjects in the supine, standing, and sitting postures, measuring thoracoabdominal movement with magnetometers and respiratory muscle activity via integrated electromyograms. In eight of the subjects, gastric and esophageal pressures and diaphragmatic electromyograms via esophageal electrodes were also measured. In the upright postures, there was generally more phasic and tonic activity in the scalene, sternocleidomastoid, and parasternal intercostal muscles. The diaphragm showed more phasic (but not more tonic) activity in the upright postures, and the abdominal oblique muscle showed more tonic (but not phasic) activity in the standing posture. Relative to the esophageal pressure change with inspiration, the inspiratory gastric pressure change was greater in the upright than in the supine posture. We conclude that the increased rib cage motion characteristic of the upright posture owes to a combination of increased activation of rib cage inspiratory muscles plus greater activation of the diaphragm that, together with a stiffened abdomen, acts to move the rib cage more effectively.     

The effect of static neck flexion on mechanical and neuromuscular behaviors of the cervical spine

Occupations that involve sustained or repetitive neck flexion are associated with a higher incidence of neck pain. Little in vivo information is available on the impact of static neck flexion on cervical spinal tissue. The aim of this study was to assess changes in mechanical and neuromuscular behaviors to sustained neck flexion in healthy adults. Sixty healthy subjects aged 20–35 years participated in this study. The participants were exposed to static neck flexion at a fixed angle of full flexion for 10 min. Mechanical and neuromuscular responses of the cervical spine to sudden perturbations were measured pre- and post-exposure. Magnitude of load-relaxation during flexion exposure, stiffness, peak head angular velocity, and reflexive activities of cervical muscles were recorded. Effective neck stiffness decreased significantly, especially in female participants (P = 0.0001). The reflexive response of the cervical erector spinae muscles to head perturbation delayed significantly (P = 0.0001). Peak head angular velocity was significantly increased after exposure to neck flexion for 10 min, especially in female participants (P = 0.001). In the present study, static flexion resulted in changes in mechanical and neuromuscular behavior of the cervical spine, potentially leading to decreased stiffness of the cervical spine. The results confirm the importance of maintaining a correct head and neck position during work and improving the work environment to reduce the cervical spinal load and work-related neck pain.     

The potential interplay of posture, digestive anatomy, density of ingesta and gravity in mammalian herbivores: why sloths do not rest upside down

1. The interaction between the density of ingesta and gravity observed in the digestive systems of ruminant herbivores should receive attention in other non‐ruminant herbivorous mammals. The resting postures adopted by non‐ruminants are of particular interest. 2. A new interpretation of established findings regarding the digestive tract of sloths illustrates that the interplay of posture, anatomy, the density of ingesta and gravity can provide a novel explanation of behavioural and morphological adaptations in herbivores, as the average particle size and dry‐matter content increases within their forestomach from its caudal towards its cranial portion. In sloths, this could be indicative of a stratification of ingesta occurring in the upright sitting posture adopted while resting, as opposed to their characteristic upside down posture when moving. 3. The sitting resting posture of sloths could therefore be an adaptation to exploit the tendency of the forestomach contents to stratify in order to pass larger, more difficult‐to‐digest particles faster from the fermentation chamber.