Does ball carrier technique influence tackler head injury assessment risk in elite rugby union?

The aim of this study was to use video evidence of tackles in elite level rugby union to identify ball carrier proficiency characteristics, for both lower and upper body tackles, that have a higher propensity to result in Head Injury Assessments(HIA) for the tackler. HIA (n = 74) and non-HIA tackles (n = 233) were categorised as either front-on or side-on upper or lower body tackles and scored for ball carrying proficiency characteristics. Side-on tackles included tackles from behind. A Chi-Square test (p < 0.05) and Cramer's V were calculated to compare proficiency characteristics in HIA and non-HIA cases. For front-on upper body tackles, the ball carrier "fending into contact" (p < 0.01;ES = Moderate) and "explosiveness on contact" (p = 0.04;ES = Moderate) had a higher propensity to result in a HIA for the tackler. Fending into contact was exhibited in 47% of all upper body Tackle front-on HIA cases. The fending arm contacted the tackler's head in 67% of these cases. Fending into contact can potentially be dangerous and therefore emphasis should be placed on safe fending during tackle-based training drills. Referees should also be alert to arm-to-head contact during the fend. Given the low number of ball carrier characteristics identified, focus should be placed on tackler characteristics for HIA prevention strategies.

Can tackle height influence head injury assessment risk in elite rugby union?

OBJECTIVES: Tackle height laws are an area of controversy in rugby union. It is reported that the tackler is at most risk of a Head Injury Assessment (HIA). Therefore, the aim of this study was to use match video evidence of tackles in elite level rugby union to examine the effect of tackle heights on HIA risk for the tackler. DESIGN: Qualitative observational case-control study. METHODS: Each HIA (n=74) and control tackle (n=965) was categorised based on tackle direction (front- or side-on), tackle type (arm, shoulder or smother) and tackle height (upper trunk, mid-trunk, lower trunk, upper leg or lower leg). The Relative Risk (RR), 95% Confidence Interval (CI) and probability (p) values were calculated for each tackle height. RESULTS: Intended primary contact at the upper trunk of the ball carrier had a greater propensity to result in a HIA for the tackler for front-on upper body shoulder tackles (RR=1.48; 95%CI=1.16-1.90; p<0.01) and side-on upper body smother tackles (RR=2.30; 95%CI=1.82-2.92; p<0.01). Intended primary contact at the upper leg of the ball carrier had a greater propensity to result in a HIA for the tackler for front-on (RR=2.60; 95%CI=1.70-3.97; p<0.01) and side-on (RR=3.34; 95%CI=1.65-6.79; p<0.01) lower body shoulder tackles. CONCLUSIONS: To reduce tackler HIA risk, the results suggest tackling below the upper trunk for upper body tackles. The results also suggest tackling at the lower trunk for lower body tackles and avoiding the upper legs. Prevention strategies should place emphasis on tackling lower risk body regions such as the mid- and lower trunk.

Could lowering the tackle height in rugby union reduce ball carrier inertial head kinematics?

There is mounting evidence of reduced long-term cognitive ability in rugby players, even in those without a reported history of concussion. The tackle height law is an area of controversy. However, little is known about the effects of repetitive inertial head loading in rugby. Furthermore, the magnitude and influencing factors for head kinematics are generally unknown. In this exploratory study, 45 multibody front-on shoulder tackles simulated with the MADYMO pedestrian model and 20 staged rugby tackles executed by professional rugby players in a marker-based 3D motion laboratory were used to assess the effect of tackle height on ball carrier head kinematics. The peak resultant head linear accelerations, angular accelerations and change in angular velocities were measured and examined. The results suggest that tackle height strongly affects the head kinematics experienced by the ball carrier. In particular, higher ball carrier head kinematic values were identified for upper trunk tackles compared to mid/lower trunk tackles in both the multibody simulations and the staged rugby tackles. Average ball carrier peak resultant head linear acceleration, angular acceleration and change in angular velocity values for upper trunk tackles were greater than for mid/lower trunk tackles by a factor of 1.5, 2.5 and 1.7, in the multibody simulations, respectively, and 1.8 (p = 0.102), 2.2 (p = 0.025) and 2.3 (p = 0.004), in the staged tackles, respectively. The results of the study support the proposition of lowering the current tackle height laws to below the chest.

Collagen fibril organization in chicken and porcine skeletal muscle perimysium under applied tension and compression.

The tension/compression asymmetry observed in the stress-stretch response of skeletal muscle is not well understood. The collagen network in the extracellular matrix (ECM) almost certainly plays a major role, but the details are unknown. This paper reports qualitatively and quantitatively on skeletal muscle ECM reorganization during applied deformation using confocal imaging of collagen through use of a fluorescently-tagged specific collagen binding protein (CNA35-EGFP) of porcine and chicken muscle samples under tensile and compressive deformation in both the fibre and cross-fibre directions. This reveals the overall three-dimensional structure of collagen in perimysium in planes perpendicular and parallel to the muscle fibres in both species. Furthermore, there is clear evidence of the reorganization of these structures under compression and tension applied in both the muscle fibre and cross-fibre directions. These observations improve our understanding of perimysium structure and response to three-dimensional deformations and are an important basis for constitutive models of passive skeletal muscle. Although overall behaviour was similar, some differences in perimysium structure were observed between chicken and porcine muscle tissue. Further work is required to better understand which structures are responsible for the tension and compression stress-strain asymmetry previously observed in the mechanical response of passive skeletal muscle.

The Effect of Tackler Technique on Head Injury Assessment Risk in Elite Rugby Union.

PURPOSE: This study aimed to use match video evidence of tackles in elite-level rugby union to identify tackler proficiency characteristics, for both lower body and upper body tackles, that result in head injury assessments (HIA) for the tackler. METHODS: A review of international rugby union matches (2013-2017) and Pro 12/European Rugby Champions Cup matches (2014-2017) from a professional rugby union club was conducted. HIA (n = 74) and non-HIA tackles (n = 233) were categorized as either front-on or side-on upper body or lower body tackles and were scored for tackling proficiency characteristics. A chi-square test (P < 0.05) and phi and Cramer's V were calculated to compare HIA and non-HIA tackling proficiency characteristics. RESULTS: In both front- and side-on upper body and lower body tackles, "head up and forward/face up" and "head placement on correct side of ball carrier" were identified as having a lower propensity to result in an HIA for the tackler. For both front-on and side-on upper body tackles, "identify/track ball carrier onto shoulder" and "shortening steps" were identified. In addition, "straight back, centre of gravity forward of support base" and "identify/track ball carrier onto shoulder" were identified for front-on and side-on lower body tackles, respectively. CONCLUSIONS: This study identified tackle characteristics that had a lower propensity to result in an HIA for the tackler in both front-on and side-on upper body and lower body tackles.

Pathological Movements of the Pelvis and Trunk During Gait in Children With Cerebral Palsy: A Cross-Sectional Study With 3-Dimensional Kinematics and Lower Lumbar Spinal Loading.

Background: Increased loading at the lumbar spine, particularly in the coronal plane, has been reported in children with cerebral palsy (CP). As pelvic and trunk movements associated with Trendelenburg and Duchenne type gait are most significant in the coronal plane, the potential exists for lower lumbar spinal loading to be negatively affected in children with CP and these types of movement patterns. Objective: The objective of this study was to assess trunk and pelvic kinematics and lower lumbar spinal loading patterns in children with CP and Trendelenburg and Duchenne type gait. Design: This was a cross-sectional study. Methods: Three-dimensional kinematic (lower limb and thorax) and L5-S1 kinetic data were recorded. Children were divided according to clinical presentation of Trendelenburg or Duchenne type gait. Several discrete kinematic and kinetic parameters were assessed between groups. Results: Three distinct pelvic and trunk movement patterns were identified for children with CP: Trendelenburg, Duchenne, and complex Trendelenburg-Duchenne. Peak L5-S1 lateral bending moments were increased by 62% in children with CP and Duchenne type gait. Children with CP and complex Trendelenburg-Duchenne gait demonstrated the largest deviations from normal, with increased peak ipsilateral and contralateral directed moments of 69% and 54%, respectively, compared with children with typical development. Limitations: A test-retest reliability analysis or measure of minimal detectable change was not conducted as part of this study. Results suggest that measures of minimal detectable change may be high for some of the reported variables. In addition, the inverse dynamic approach determines only the net intersegmental reactive forces that reflect the effect of external loads. Previous studies have shown that spinal loads may be larger than the net intersegmental force. Conclusions: Trendelenburg and Duchenne type movements were not always distinct, and a third type of movement, a combination of the two, was the most common in this study. Clinicians should be aware that children with CP and the Duchenne type or the complex Trendelenburg-Duchenne type of gait pattern experience abnormal loading that may have significant implications for the lower spine in the long term.

A continuum model for tension-compression asymmetry in skeletal muscle.

Experiments on passive skeletal muscle on different species show a strong asymmetry in the observed tension-compression mechanical behavior. This asymmetry shows that the tension modulus is two orders of magnitude higher than the compression modulus. Until now, traditional analytical constitutive models have been unable to capture that strong asymmetry in anisotropic solids using the same material parameters. In this work we present a model which is able to accurately capture five experimental tests in chicken pectoralis muscle, including the observed tension-compression asymmetry. However, aspects of the anisotropy of the tissue are not captured by the model.

Assessment of model-based image-matching for future reconstruction of unhelmeted sport head impact kinematics.

Player-to-player contact inherent in many unhelmeted sports means that head impacts are a frequent occurrence. Model-Based Image-Matching (MBIM) provides a technique for the assessment of three-dimensional linear and rotational motion patterns from multiple camera views of a head impact event, but the accuracy is unknown for this application. The goal of this study is to assess the accuracy of the MBIM method relative to reflective marker-based motion analysis data for estimating six degree of freedom head displacements and velocities in a staged pedestrian impact scenario at 40 km/h. Results showed RMS error was under 20 mm for all linear head displacements and 0.01-0.04 rad for head rotations. For velocities, the MBIM method yielded RMS errors between 0.42 and 1.29 m/s for head linear velocities and 3.53-5.38 rad/s for angular velocities. This method is thus beneficial as a tool to directly measure six degree of freedom head positional data from video of sporting head impacts, but velocity data is less reliable. MBIM data, combined in future with velocity/acceleration data from wearable sensors could be used to provide input conditions and evaluate the outputs of multibody and finite element head models for brain injury assessment of sporting head impacts.

Does player time-in-game affect tackle technique in elite level rugby union?

OBJECTIVES: It has been hypothesised that fatigue may be a major factor in tackle-related injury risk in rugby union and hence more injuries occur in the later stages of a game. The aim of this study is to identify changes in ball carrier or tackler proficiency characteristics, using elite level match video data, as player time-in-game increases. DESIGN: Qualitative observational cohort study. METHODS: Three 2014/15 European Rugby Champions Cup games were selected for ball carrier and tackler proficiency analysis. Analysis was only conducted on players who started and remained on the field for the entire game. A separate analysis was conducted on 10 randomly selected 2014/15 European Rugby Champions Cup/Pro 12 games to assess the time distribution of tackles throughout a game. A Chi-square test and one-way way ANOVA with post-hoc testing was conducted to identify significant differences (p<0.05) for proficiency characteristics and tackle counts between quarters in the game, respectively. RESULTS: Player time-in-game did not affect tackle proficiency for both the ball carrier and tackler. Any results that showed statistical significance did not indicate a trend of deterioration in proficiency with increased player time-in-game. The time distribution of tackles analysis indicated that more tackles occurring in the final quarter of the game than the first (p=0.04) and second (p=<0.01). CONCLUSIONS: It appears that player time-in-game does not affect tackler or ball carrier tackle technique proficiency at the elite level. More tackles occurring in the final quarter of a game provides an alternative explanation to more tackle-related injuries occurring at this stage.

The effects of tackle height on inertial loading of the head and neck in Rugby Union: A multibody model analysis.

OBJECTIVE: There is evidence of chronic injury to the head-and-neck region of Rugby Union players. The aim of this study was to use multibody simulations to examine the effects of tackle height on both Tackler and Ball Carrier head kinematics and neck dynamics. RESEARCH DESIGN: Quantitative Exploratory Study Methods and procedures: 45 front-on shoulder tackles with no direct contact to the head/neck were simulated with the MADYMO pedestrian model and used to assess differences between upper body tackles and lower body tackles. The average resultant head linear and angular accelerations as well as neck forces and moments were assessed. MAIN OUTCOMES AND RESULTS: Much higher Ball Carrier head kinematic values and neck loading were predicted for upper body tackles compared to lower body tackles, and principal findings were unaffected by a sensitivity analysis. Tackler results were less straightforward and trends were influenced by the sensitivity analysis for muscle activation. CONCLUSION: Although further model validation is required, the results of this study indicate the need for further research on tackle heights and inertial head-and-neck loading in the tackle phase of play in Rugby Union.

The suture pullout characteristics of human and porcine linea alba.

There is a substantial prevalence of post-operative incisional hernia for both laparoscopic and laparotomy procedures, but there have been few attempts at quantifying abdominal wound closure methodology in the literature. One method to ascertain a more robust method of wound closure is the identification of the influence of suture placement parameters on suture pullout force. Current surgical practice involves a recommended bite depth and bite separation of 10mm, but the evidence base for this is not clear. In this paper, the suture pullout characteristics of both porcine and human linea alba were investigated to ascertain a suture placement protocol for surgical wound closure. Uniaxial suture pullout force testing on fresh frozen porcine and human linea alba samples was performed using standard materials testing machines. The influence of the number of suture loops, the bite depth and the bite separation of the sutures and the orientation of the sutures with respect to the principal fibre direction in the linea alba were assessed. Results showed a clearly identifiable relationship between pullout force of the suture, bite separation and bite depth, with low suture separation and high suture depth as optimal parameters for increasing pullout force. Resistance to pullout could be improved by as much as 290% when optimizing test conditions. Both human and porcine tissue were observed to exhibit very similar pullout force characteristics, corroborating the use of a porcine model for investigations into wound closure methodology. Orientation of suture application was also found to significantly affect the magnitude of suture pullout, with suturing applied longitudinally across a transverse defect resulting in higher pullout forces for small suture bite separations. Although further assessment in an environment more representative of in vivo conditions is required, these findings indicate that increasing the bite depth and reducing the bite separation with respect to the current surgical recommendations may reduce the risk of post-operative incisional hernia.

Visualisation of Collagen in fixed skeletal muscle tissue using fluorescently tagged Collagen binding protein CNA35.

Detection and visualisation of Collagen structure are important to understand the relationship between mechanical behaviour and microstructure in skeletal muscle since Collagen is the main structural protein in animal connective tissues, and is primarily responsible for their passive load-bearing properties. In the current study, the direct detection and visualization of Collagen using fluorescently tagged CNA35 binding protein (fused to EGFP or tdTomato) is reported for the first time on fixed skeletal muscle tissue. This Technical Note also establishes a working protocol by examining tissue preparation, dilution factor, exposure time etc. for sensitivity and specificity. Penetration of the binding protein into intact mature skeletal muscle was found to be very limited, but detection works well on tissue sections with higher sensitivity on wax embedded sections compared to frozen sections. CNA35 fused to tdTomato has a higher sensitivity than CNA35 fused to EGFP but both show specific detection. Best results were obtained with 15µm wax embedded sections, with blocking of non-specific binding in 1% BSA and antigen retrieval in Sodium Citrate. There was a play-off between dilution of the binding protein and time of incubation but both CNA35-tdTomato and CNA35-EGFP worked well with approximately 100µg/ml of purified protein with overnight incubation, while CNA35-tdTomato could be utilized at 5 fold less concentration. This approach can be applied to study the relationship between skeletal muscle micro-structure and to observe mechanical response to applied deformation. It can be used more broadly to detect Collagen in a variety of fixed tissues, useful for structure-functions studies, constitutive modelling, tissue engineering and assessment of muscle tissue pathologies.

Risks associated with significant head impact events in elite rugby union.

PRIMARY OBJECTIVES: To conduct video and statistical analysis on Rugby Union play, focusing mainly on the tackle, to establish the player to player configurations for significant direct head impacts and non-direct head impacts. RESEARCH DESIGN: Quantitative, observational cohort study. METHODS AND PROCEDURES: Video analysis of 52 significant direct head impacts (31 Tackle, 10 Ruck, 7 Dive and 4 Ground) and 40 non-direct head impact tackles from 2014/15 International Rugby Union matches. Relative risk, 95% CI and p-values were calculated for a range of tackle variables. MAIN OUTCOME AND RESULTS: Upper body tackles and lower body tackles accounted for 37% (19) and 23% (12) of cases, respectively, with the tackler as the head impacted player for 97% (30) of cases. The majority (81%) of tackle-related significant direct head impacts occurred in the second half of the game, with 63% of upper body tackle significant direct head impacts occurring in the final quarter. Tackler head placement and high speed tackles had statistical significance for causing tackle related significant direct head impacts as well as foot planting and difference in tackler and ball carrier mass for upper body tackles and ball carrier change in direction for lower body tackles. CONCLUSION: Tackle variables that statistically increased the risk of significant direct head impact were identified, which can aid player protection strategies.

Uniaxial and biaxial tensile stress-stretch response of human linea alba.

There are few studies on the stress-stretch behaviour of human linea alba, yet understanding the mechanics of this tissue is important for developing better methods of abdominal wound closure. Published data focuses mainly on porcine linea alba and for human tissue there are conflicting results and no bi-axial data available. This variability is likely due to challenges with the physical dimensions of the tissue and differences in experimental methodology. This study focussed on the tensile mechanical characterisation of the human linea alba using uniaxial and equi-load biaxial testing performed using image-based strain measurement methods. Thirteen freshly frozen human cadaveric abdominal walls were obtained and used to prepare 7 samples in both the transverse and longitudinal directions for uniaxial testing, and 13 square samples for bi-axial testing. The results showed significant anisotropy and for the equi-load biaxial tests the deformation was heavily biased in the longitudinal direction. In comparison with similar tests on porcine tissue from a previous study, it was found that the response of porcine linea alba to uniaxial loading is similar to that of human linea alba, with no statistically significant differences observed. Under biaxial loading human and porcine linea showed no statistical significance in the difference between their means in the transverse direction. However, a significant difference was observed in the longitudinal direction, and further study of the respective tissue structures is needed to better understand this result. These results provide the first data on the biaxial tensile properties of human linea alba and can aid in an improved assessment of wound closure mechanics.

The in vitro passive elastic response of chicken pectoralis muscle to applied tensile and compressive deformation.

The mechanics of passive skeletal muscle are important in impact biomechanics, surgical simulation, and rehabilitation engineering. Existing data from porcine tissue has shown a significant tension/compression asymmetry, which is not captured by current constitutive modelling approaches using a single set of material parameters, and an adequate explanation for this effect remains elusive. In this paper, the passive elastic deformation properties of chicken pectoralis muscle are assessed for the first time, to provide deformation data on a skeletal muscle which is very different to porcine tissue. Uniaxial, quasi-static compression and tensile tests were performed on fresh chicken pectoralis muscle in the fibre and cross-fibre directions, and at 45° to the fibre direction. Results show that chicken muscle elastic behaviour is nonlinear and anisotropic. The tensile stress-stretch response is two orders of magnitude larger than in compression for all directions tested, which reflects the tension/compression asymmetry previously observed in porcine tissue. In compression the tissue is stiffest in the cross-fibre direction. However, tensile deformation applied at 45° gives the stiffest response, and this is different to previous findings relating to porcine tissue. Chicken muscle tissue is most compliant in the fibre direction for both tensile and compressive applied deformation. Generally, a small percentage of fluid exudation was observed in the compressive samples. In the future these data will be combined with microstructural analysis to assess the architectural basis for the tension/compression asymmetry now observed in two different species of skeletal muscle.

Control of tension-compression asymmetry in Ogden hyperelasticity with application to soft tissue modelling.

This paper discusses tension-compression asymmetry properties of Ogden hyperelastic formulations. It is shown that if all negative or all positive Ogden coefficients are used, tension-compression asymmetry occurs the degree of which cannot be separately controlled from the degree of non-linearity. A simple hybrid form is therefore proposed providing separate control over the tension-compression asymmetry. It is demonstrated how this form relates to a newly introduced generalised strain tensor class which encompasses both the tension-compression asymmetric Seth-Hill strain class and the tension-compression symmetric Bazant strain class. If the control parameter is set to q=0.5 a tension-compression symmetric form involving Bazant strains is obtained with the property Ψ(λ1,λ2,λ3)=Ψ(1λ1,1λ2,1λ3). The symmetric form may be desirable for the definition of ground matrix contributions in soft tissue modelling allowing all deviation from the symmetry to stem solely from fibrous reinforcement. Such an application is also presented demonstrating the use of the proposed formulation in the modelling of the non-linear elastic and transversely isotropic behaviour of skeletal muscle tissue in compression (the model implementation and fitting procedure have been made freely available). The presented hyperelastic formulations may aid researchers in independently controlling the degree of tension-compression asymmetry from the degree of non-linearity, and in the case of anisotropic materials may assist in determining the role played by, either the ground matrix, or the fibrous reinforcing structures, in generating asymmetry.

Impact characteristics of a vehicle population in low speed front to rear collisions.

Rear impact collisions are mostly low severity, but carry a very high societal cost due to reported symptoms of whiplash and related soft tissue injuries. Given the difficulty in physiological measurement of damage in whiplash patients, there is a significant need to assess rear impact severity on the basis of vehicle damage. This paper presents fundamental impact equations on the basis of an equivalent single vehicle to rigid barrier collision in order to predict relationships between impact speed, maximum dynamic crush, mean and peak acceleration, time to common velocity and vehicle stiffness. These are then applied in regression analysis of published staged low speed rear impact tests. The equivalent mean and peak accelerations are linear functions of the collision closing speed, while the time to common velocity is independent of the collision closing speed. Furthermore, the time to common velocity can be used as a surrogate measure of the normalized vehicle stiffness, which provides opportunity for future accident reconstruction.

Uniaxial and biaxial mechanical properties of porcine linea alba.

Incisional hernia is a severe complication post-laparoscopic/laparotomy surgery that is commonly associated with the linea alba. However, the few studies on the mechanical properties of the linea alba in the literature appear contradictory, possible due to challenges with the physical dimensions of samples and variations in protocol. This study focuses on the tensile mechanical characterisation of the porcine linea alba, as determined by uniaxial and equi-load biaxial testing using image-based strain measurement methods. Results show that the linea alba demonstrated a non-linear elastic, anisotropic behaviour which is often observed in biological soft tissues. The transverse direction (parallel to fibres) was found to be approximately eight times stiffer than the longitudinal (cross-fibre) direction under both uniaxial and equi-load biaxial loading. The equi-load biaxial tensile tests revealed that contraction could occur in the transverse direction despite increasing load, probably due to the anisotropy of the tissue. Optical surface marker tracking and digital image correlation methods were found to greatly improve the accuracy of stretch measurement, resulting in a 75% change in the apparent stiffness compared to using strain derived from machine cross-head displacement. Additionally, a finite element model of the experiments using a combination of an Ogden and fibre exponential power law model for the linea alba was implemented to quantify the effect of clamping and tissue dimensions (which are suboptimal for tensile testing) on the results. The preliminary model results were used to apply a correction factor to the uniaxial experimental data prior to inverse optimisation to derive best fit material parameters for the fibre reinforced Ogden model. Application of the model to the equi-load biaxial case showed some differences compared to the experimental data, suggesting a more complex anisotropic model may be necessary to capture biaxial behaviour. These results provide an improved assessment of the mechanical properties of the porcine linea alba for wound closure and other studies.

Assessing the microstructural response to applied deformation in porcine passive skeletal muscle.

The passive micro-structural mechanical response of muscle tissue is important for numerous medical applications. However, the recently observed tension/compression asymmetry in porcine muscle remains poorly explained. In particular there remains a lack of understanding of how external tension or compression applied in the fibre or cross-fibre direction translates internally to deformation of muscle fibres and the extra-cellular matrix. Accordingly, fresh porcine skeletal muscle tissue was harvested, deformed by 30% in uniaxial tension or compression in both the fibre and cross-fibre directions and prepared for optical microscope, polarised light microscope and SEM analysis. The average deformed specimen results were compared to the average control results in each case. For compressive or tensile stretch applied in the muscle fibre direction the average measured muscle fibre cross-sectional area changes are in close correspondence with predictions based on global Poisson's ratio measurements and these deformation modes did not cause shape changes in the muscle fibre cross-sections. However, muscle tissue reacted to the applied cross-fibre direction deformations as follows: compression flattened muscle fibre cross-sections, aligning them perpendicular to the direction of the applied deformation while tensile deformations stretched the cross-sections of muscle fibres, aligning them parallel to the direction of applied deformation. No evidence of structural reorganisation of endomysium collagen fibres in response to applied stretch was observed. The observed micro-structural responses do not appear to be influenced by the surrounding endomysium, but appear to be significantly influenced by proximity to the perimysium network. It is hypothesised that the perimysium and its interaction with the surrounding muscle fibres is therefore likely to be predominantly responsible for the tension/compression asymmetry observed in macroscopic tests of passive skeletal muscle stress strain behaviour.

Mechanical characterisation of porcine rectus sheath under uniaxial and biaxial tension.

Incisional hernia development is a significant complication after laparoscopic abdominal surgery. Intra-abdominal pressure (IAP) is known to initiate the extrusion of intestines through the abdominal wall, but there is limited data on the mechanics of IAP generation and the structural properties of rectus sheath. This paper presents an explanation of the mechanics of IAP development, a study of the uniaxial and biaxial tensile properties of porcine rectus sheath, and a simple computational investigation of the tissue. Analysis using Laplace׳s law showed a circumferential stress in the abdominal wall of approx. 1.1MPa due to an IAP of 11kPa, commonly seen during coughing. Uniaxial and biaxial tensile tests were conducted on samples of porcine rectus sheath to characterise the stress-stretch responses of the tissue. Under uniaxial tension, fibre direction samples failed on average at a stress of 4.5MPa at a stretch of 1.07 while cross-fibre samples failed at a stress of 1.6MPa under a stretch of 1.29. Under equi-biaxial tension, failure occurred at 1.6MPa with the fibre direction stretching to only 1.02 while the cross-fibre direction stretched to 1.13. Uniaxial and biaxial stress-stretch plots are presented allowing detailed modelling of the tissue either in silico or in a surrogate material. An FeBio computational model of the tissue is presented using a combination of an Ogden and an exponential power law model to represent the matrix and fibres respectively. The structural properties of porcine rectus sheath have been characterised and add to the small set of human data in the literature with which it may be possible to develop methods to reduce the incidence of incisional hernia development.