Repetitive Forces Research Articles

Is There a Relationship between Ritual and Cliché?: Indigenous Fijian Criticism of Kava Drinking

ABSTRACT This article poses the question of what analytical insight can be gained by comparing ritual and cliché, both of which involve evaluations of the significance of repetition. The case study is kava-drinking sessions in indigenous Fiji. Whether purposeful or casual, kava-drinking sessions always follow rules which give them significant form and regularity of the kind some anthropologists consider definitive of ritual practice. Yet many indigenous Fijians criticise present-day kava-drinking sessions for several reasons. One prominent line of criticism is that kava drinking is now a matter of too much repetition – too many people drinking too much too often – and losing the meaningful link to chiefly tradition as a result. Analysing criticism of ritual in terms of cliché clarifies situations in which the iconic force of repetition is treated as an index of excess.

How to break a sperm whale’s teeth: dental damage in a large Miocene physeteroid from the North Sea basin

ABSTRACTIn contrast to the suction-feeding, predominantly teuthophagous extant sperm whale, several Miocene physeteroids display proportionally larger teeth, deeply embedded in both upper and lower jaws. Together with other osteological features, these differences lead to the functional interpretation of these taxa as macroraptorial predators, using their teeth to capture and process large marine vertebrates. However, the assumption that strong forces applied to macroraptorial physeteroid teeth during powerful bites and contacts with bone material should result in major dental damage has not yet been tested. In the present work, we analyzed a large collection of physeteroid teeth with an enameled crown from the Miocene of the North Sea Basin. We especially focused on a set of 45 teeth of Scaldicetus caretti discovered in Antwerp (Belgium, southern North Sea Basin) and tentatively dated to the Tortonian (early late Miocene). Visual inspection and computed tomography (CT) scans revealed dental damage, including wear and breaks. The latter could be interpreted as chipping fractures, occurring along the crown, and vertical root fractures, observed along the apical part of the massive root. Chipping fractures are most likely due to contacts with hard material, whereas vertical root fractures may result from the application of strong and repetitive bite forces and/or contacts with hard material. Such results further support the interpretation of a series of Miocene physeteroids with proportionally large teeth as macroraptorial (rather than suction-feeding) top predators. Considering the size of the teeth of S. caretti, its most likely prey items were other large marine vertebrates.

Laparoscopic Approach to Inguinal Disruption in Athletes: a Retrospective 13-Year Analysis of 198 Patients in a Single-Surgeon Setting

BackgroundInguinal disruption (ID) is a condition of chronic groin pain affecting mainly athletes. ID cannot be defined as a true hernia. Pathogenesis is multifactorial due to repetitive and excessive forces applied to the inguino-pelvic region. Examination reveals tenderness to palpation of the inguinal region. Differential diagnosis is challenging; imaging is helpful for excluding other pathologies. Surgery is the treatment of choice when conservative treatment fails. Primary aim of the study was to evaluate the time to return to full sport activity after transabdominal preperitoneal patch plasty (TAPP) technique in ID. Secondary aim was to evaluate the postoperative complication rate both in the immediate post-operative time and in 1 year follow-up and to verify the relapse rate after surgery. In this study, we consider time to return to full sport activity as the time needed to return to pre-injury sport activity.ResultsA retrospective study is reported by evaluating 198 cases of ID from a single surgeon experience. All patients failed a previous conservative treatment. All cases were treated with the TAPP approach. Time to return to full sport activity was 4 weeks for 94.4% of patients, with a total of 98.5% of active patients at 9 months. Post-operative inguinal pain was the main complication (9.1%). On 13 years follow-up, we report a recurrence rate of 2.5%.ConclusionsCurrent management algorithm for ID, in professional athletes, supports the role of surgery after at least 2 months of conservative treatment. Recently, the role of surgery has been highlighted for a definitive treatment and a faster full recovery to sport activity, especially for elite professional athletes. In our opinion, laparoscopic surgery is the mainstay for non-responsive ID treatment. We present a long-term retrospective evaluation of a wide cohort of professional athletes diagnosed and treated in a systematic way.

Relationship between Stance Frontal Plane Kinematics and Initial Impact forces in Runners: Asymmetry Implications

Rearfoot eversion (REV) and contralateral pelvis drop (CPD) play a crucial role in force absorption at impact during running. Controlling and adequately absorbing the repetitive vertical ground reaction force (vGRF) at impact is essential in injury prevention. PURPOSE: This study aimed to examine the relationships between vGRF and REV, and vGRF and CPD bilaterally in collegiate runners. METHODS: Eleven asymptomatic (5 females, 6 males) NCAA Division II cross country runners (age, 19.1 ± 1.1 yrs; height, 174.2 ± 11.2 cm; mass 62.2 ± 6.2 kg; 38.3 ± 15.9 miles/wk, treadmill speed, 3.6 ± 0.5 m/s) underwent 3D motion analysis during a 7 minute steady state run on an instrumented treadmill. vGRF, REV and CPD were evaluated bilaterally for association via Pearson Correlation coefficients, p<0.05. RESULTS: Mean (+SD) peak angles of REV and CPD, and vGRF during left stance were 3.6 ± 6.5°, -5.4 ± 3.8°, and 1.8 ± 0.6 body weights (BW), respectively. Mean peak angles of REV and CPD, and vGRF during right stance were 2.51 ± 2.5°, -4.9 ± 4.9°, and 1.6 ± 0.4 BW, respectively. Significant high negative correlation was found for REV and vGRF during left stance [r (9) = -.967, p=0.03] while CPD and vGRF during right stance were significantly highly positively correlated [r (9) .714, p=0.02]. CONCLUSION: Despite the low number of subjects, we conclude that frontal plane kinematics are associated with impact force magnitude during running, and the associations appear to be asymmetrical necessitating a bilateral examination of running mechanics.

Effect of reverse manual wheelchair propulsion on shoulder kinematics, kinetics and muscular activity in persons with paraplegia

Objective: Shoulder pain after spinal cord injury (SCI) is attributed to increased mobility demands on the arms and negatively impacts independence and quality of life. Repetitive superior and posterior shoulder joint forces produced during traditional wheelchair (WC) locomotion can result in subacromial impingement if unopposed, as with muscular fatigue or weakness. ROWHEELS® (RW), geared rear wheels that produce forward WC movement with backward rim pulling, could alter these forces. Design: Cross sectional. Setting: Research laboratory at a rehabilitation hospital. Participants: Ten manual WC users with paraplegia. Outcome measures: Propulsion characteristics and right upper extremity/trunk kinematics and shoulder muscle activity were collected during ergometer propulsion: (1) self-selected free speed reverse propulsion with RW, (2) matched-speed reverse (rSW), and (3) forward propulsion (fSW) with instrumented Smartwheels (SW). Inverse dynamics using right-side SW rim kinetics and kinematics compared shoulder kinetics during rSW and fSW. Results: Free propulsion velocity, cycle distance and cadence were similar during RW, rSW and fSW. Overall shoulder motion was similar except that peak shoulder extension was significantly reduced in both RW and rSW versus fSW. Anteriorly and inferiorly directed SW rim forces were decreased during rSW versus fSW propulsion, but posteriorly and superiorly directed rim forces were significantly greater. Superior and posterior shoulder joint forces and flexor, adductor, and external rotation moments were significantly less during rSW, without a significant difference in net shoulder forces and moments. Traditional propulsive-phase muscle activity was significantly reduced and recovery-phase muscle activity was increased during reverse propulsion. Conclusion: These results suggest that reverse propulsion may redirect shoulder demands and prevent subacromial impingement, thereby preventing injury and preserving independent mobility for individuals with paraplegia.

Role of Diffusion in Unfolding and Translocation of Multidomain Titin I27 Substrates by a Clp ATPase Nanomachine.

Degradation of multidomain substrate proteins (SPs) by AAA+ nanomachines takes place processively from the tagged terminal. Ring-shaped ATPase components, such as ClpY, apply repetitive mechanical forces to effect domain unfolding and translocation of polypeptide segments through a narrow central channel. We study these mechanisms through atomistic Langevin dynamics simulations of C-terminal-tagged SPs in allosteric ClpY cycles. We find that monomeric SPs are processed through single unfolding pathways and fast timescales, whereas multimeric SPs involve branched pathways and slower timescales. These distinct mechanisms are attributed to the slower rotational diffusion of the C-terminal domain in multidomain SPs that hinders access to the soft mechanical direction. In the geometry specific to laser optical tweezers experiments, involving a restrained SP N-terminal, a single unfolding pathway is found for both monomeric and tetrameric SPs as pulling is applied along the N-C direction. Non-native interactions modulate unfolding of unrestrained monomers by weakening the C-terminal interface but do not contribute significantly to unfolding of restrained SPs. On the basis of these results, we propose that the interplay of restricted SP dynamics and ATPase kinetics underlies partitioning of multidomain SPs into completely degraded products and undegraded fragments comprising folded domains.

The Impact of Fatigue Testing and Surface Processing on Nickel Release in Nitinol Stents

Immersion testing of medical implants is typically performed under non-loaded conditions to assess susceptibility of metal ion release from a device. However, many implants are subjected to repetitive forces in vivo, which may increase ion release into surrounding tissues. Therefore, the objective of this study was to determine whether fatigue loading of stents with different surface finishes accelerates nickel release. Nitinol stents with an electropolished (EP) oxide or blue oxide (BO) were assigned to the following groups: (1) static immersion without loading, (2) static fluid pressure and bend, (3) dynamic pressure on a static bend, and (4) dynamic bending with static pressure. Immersion fluid was collected periodically and analyzed for nickel concentration using ICP-MS. Results indicated nickel release under dynamic bending was greater by up to 280× for EP and 190× for BO finishes compared to all other groups (p 0.27), nickel release under dynamic pressure conditions was up to 50× higher compared to static groups. In addition, BO stents released up to 6× greater levels of nickel compared to EP stents for static immersion, dynamic pressure, and dynamic bending groups (p < 0.05). These findings highlight the impact of fatigue loading on uniform corrosion for different nitinol stent surfaces.

Two-dimensional biomechanical thumb model for pipetting

Manual pipetting imposes repetitive movements, high force, and awkward postures on the thumb, fingers, and wrist, increasing the risk of musculoskeletal injuries. The purpose of this study is to provide a linear regression model to estimate the optimal pipetting grip height based on the two-dimensional biomechanical static thumb model developed in this study. This biomechanical static thumb model uses hand anatomy and static equilibrium conditions to estimate internal tendon forces against a given external force. Based on the model, we conclude that the optimal grip height is 3 cm in males and 2 cm in females in terms of grip strength and force efficiency. In addition, the model is validated by RMS EMG data from correlation analysis between the predicted internal tendon forces, with RMS EMG values of 0.65 for Flexor Pollicis Longus, 0.57 for Flexor Pollicis Brevis, and 0.61 for Adductor Pollicis.

Subject-specific Musculoskeletal Model for Studying Bone Strain During Dynamic Motion

Bone stress injuries are common in sports and military trainings. Repetitive large ground impact forces during training could be the cause. It is essential to determine the effect of high ground impact forces on lower-body bone deformation to better understand the mechanisms of bone stress injuries. Conventional strain gauge measurement has been used to study in vivo tibia deformation. This method is associated with limitations including invasiveness of the procedure, involvement of few human subjects, and limited strain data from small bone surface areas. The current study intends to introduce a novel approach to study tibia bone strain under high impact loading conditions. A subject-specific musculoskeletal model was created to represent a healthy male (19 years, 80 kg, 1,800 mm). A flexible finite element tibia model was created based on a computed tomography (CT) scan of the subject's right tibia. Laboratory motion capture was performed to obtain kinematics and ground reaction forces of drop-landings from different heights (26, 39, 52 cm). Multibody dynamic computer simulations combined with a modal analysis of the flexible tibia were performed to quantify tibia strain during drop-landings. Calculated tibia strain data were in good agreement with previous in vivo studies. It is evident that this non-invasive approach can be applied to study tibia bone strain during high impact activities for a large cohort, which will lead to a better understanding of injury mechanism of tibia stress fractures.

Subject-specific Musculoskeletal Model for Studying Bone Strain During Dynamic Motion.

Bone stress injuries are common in sports and military trainings. Repetitive large ground impact forces during training could be the cause. It is essential to determine the effect of high ground impact forces on lower-body bone deformation to better understand the mechanisms of bone stress injuries. Conventional strain gauge measurement has been used to study in vivo tibia deformation. This method is associated with limitations including invasiveness of the procedure, involvement of few human subjects, and limited strain data from small bone surface areas. The current study intends to introduce a novel approach to study tibia bone strain under high impact loading conditions. A subject-specific musculoskeletal model was created to represent a healthy male (19 years, 80 kg, 1,800 mm). A flexible finite element tibia model was created based on a computed tomography (CT) scan of the subject's right tibia. Laboratory motion capture was performed to obtain kinematics and ground reaction forces of drop-landings from different heights (26, 39, 52 cm). Multibody dynamic computer simulations combined with a modal analysis of the flexible tibia were performed to quantify tibia strain during drop-landings. Calculated tibia strain data were in good agreement with previous in vivo studies. It is evident that this non-invasive approach can be applied to study tibia bone strain during high impact activities for a large cohort, which will lead to a better understanding of injury mechanism of tibia stress fractures.

Diversity in Myosin Heavy Chain Composition of the Papionin Masseter Muscle Indicates the Importance of Hybrid Phenotypes for Feeding

The contractile performance of a muscle fiber is directly related to fiber type. Variation in fiber type within and among muscles leads to a diversity of phenotypes, providing the flexibility needed to perform an assortment of behaviors with varying functional demands. Previous analyses have demonstrated variation in fiber phenotype of the chewing muscles in a number of primates and nonprimate mammals. Some of this variation has been linked to species‐specific feeding behaviors. To further evaluate the relationship between fiber phenotype and feeding behavior, we compare the distribution of masseter muscle fiber types in four papionin species: the hard‐object feeding sooty mangabey (Cercocebus atys) and three papionin species that do not specialize on hard objects.Immunohistological staining was used to determine the presence of slow (MyHC‐1 and MyHC α –cardiac) and fast (MyHC‐2 and MyHC‐M) isoforms. Samples were taken from the anterior superficial masseter of C. atys (n=8), M. mulatta (n=2), M. fascicularis (n=2), and P. anubis (n=2). Histological sections were subsequently digitized to determine the relative proportions of MyHC types and fiber phenotype in each specimen.The presence of MyHC α‐cardiac was ubiquitous throughout the sample (C. atys=99%; M. mulatta=76%; M. fascicularis=77%; P. anubis=79%). MyHC α‐cardiac maintains fatigue resistance while producing twitch tension more rapidly than MyHC‐1, thus providing an advantage for animals that require a fiber phenotype combining fatigue resistance and intermediate speed of contraction. Previously, we reported high proportions of MyHC α‐cardiac in C. atys, which we functionally linked to the need for sustained and repetitive high bite forces required for crushing hard seeds. Our finding that C. atys consistently express a higher percentage of MyHC α‐cardiac in comparison to the other papionins further supports this interpretation. The presence of hybrid fibers in all four papionin species was notable. Slow types MyHC‐1 and MyHC α‐cardiac were co‐expressed in nearly 100% of fibers sampled in each of the species. Hybrid fibers co‐expressing slow and fast‐type phenotypes were also prevalent. The abundance of hybrid fibers across papionins suggests a shared fiber phenotype that provides for versatility in the contractile properties of the anterior superficial masseter, a muscular region that must meet diverse performance requirements. This initial work is part of a larger ongoing study to assess jaw‐muscle fiber phenotype in non‐human primates. Future analyses will include larger samples and a greater diversity of species to enable us to test key functional hypotheses about fiber type diversity in primates.Support or Funding InformationFunded in part by National Science Foundation Grants (BCS 1719743, BCS 0962677, BCS 0095522) and Yerkes National Primate Research Center Base Grant ORIP/OD P51OD011132 and the Comparative AIDS CoreThis abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

ВЛИЯНИЕ ТЕРМИЧЕСКОЙ ОБРАБОТКИ НА СТРУКТУРУ И ИЗНОСОСТОЙКОСТЬ СТАЛЕЙ ПЕРЛИТНОГО КЛАССА

From the ecological point of view, high-manganese austenitic steels of 110G13L type are undesirable in the production because of the high emission of manganese oxides to the atmosphere while melting in the electric arc furnaces, gas-oxygen cutting, and welding operations. Another disadvantage of steels of this class is their low initial hardness, which causes the revet closing capacity of the working parts of castings under the conditions of dynamic loads. Moreover, these steels are poorly machinable. The paper presents the results of the comparative study of the relationship between the structure formed in the process of heat treatment and the abrasive wear resistance of steels of two structural classes - high-manganese austenitic 110G13L steel and the 70H2GSML perlite-class steel. The wide use of 110G13L steel for the production of parts operating under the impact-abrasive wear and the repetitive contact-impact force is caused by the ability for high friction hardening of stable manganese austenite with low energy of packaging defects combined with the good impact strength. The disadvantages of 110G13L steel are the high quenching temperature, poor environmental performance and economic reasons that make it necessary in some cases to replace it with the perlite steels. The study of wear resistance of 70H2GSML perlite-class steel depending on the tempering temperature after normalization showed that this steel could be used for the casting of replaceable cast parts for crushing-milling equipment operating under the abrasive action without significant impact loads after normalization and high tempering. The wear resistance of samples made of 70H2GSML steel with the secondary sorbite structure, when testing steady abrasive, makes 55-60 % of level provided by samples of 110G13L steel, however, the lower cost, manufacturability and the environmental problems in some cases can settle the issue in favor of the application of perlite steel. The additional reserves to increase the wear resistance of 70H2GSML steel involve the quenching at high temperatures due to the formation of the structure of retained austenite that, in the process of abrasive wear, transforms into deformation martensite on the working surface increasing the steel’s friction hardening ability.

Anatomical head model to measure impact force transfer through the head layers and their displacement

When the human head is subjected to blunt force impact, there are several mechanical responses that may result from the forces involved, including absorption of impact forces through the various layers of the head. The purpose of this study was to develop an anatomical head model to measure force transfer through the various head layers and their displacement when subject to short-duration high-velocity impacts. An anatomical head model was constructed using previously validated simulant materials: epoxy resin (skull), polyvinyl siloxane (scalp), agar/glycerol/water (brain) and modified intravenous fluid for the cerebrospinal fluid. An array of accelerometers (4 mm × 4 mm × 1.45 mm) was incorporated into the various layers of the head to measure forces in x- (anterior/posterior), y- (left/right) and z- (up/down) axis. All sensors were connected to a signal conditioning board and USB powered data loggers. The head model was placed into a rigid metal stand with an optical sensor to trigger data capturing. A weight (750 g) was dropped from a height of 0.5 m (n= 20). Impact forces (z-axis) of 1107.05 N were recorded on top of the skin, with decreasing values through the different layers (bottom of skin 78.48 N, top of skull 319.82 N, bottom of skull 87.30 N, top and centre of brain 47.09 N and base of brain 78.41 N. Forces in the x- and y-axes were similar to those of the z-axis. With the base of the brain still receiving 78.41 N, this highlights the potential danger of repetitive impact forces to the head. Upon impact the layers of the head are displaced in the x-, y- and z-direction, with the highest values shown in the z-axis. In conclusion, this study identified the importance of considering short-duration high-intensity impacts to the head and their effect on underlying tissues.

Early Diagnosis and Prevention of Repetitive Strain Injury Induced Carpal Tunnel Syndrome among Computer Users

Carpal tunnel syndrome (CTS) is one of the most frequent compression neuropathies involving the median nerve during its course through the carpal tunnel. The prevalence in the general population is about 3.0–5.8 percent among women and 0.6–2.1% among men, based on both clinical symptoms and nerve conduction study (NCS). High stress and pressure in the carpal tunnel may lead to the development of CTS. This is now mostly accepted that exposure to hand-arm vibrations and combination of repetitive hand force use may be the causative factor. In the recent years, the widespread use of computers and laptops has lead to a matter of concern whether the use of these devices could be a risk factor in the causation of CTS. Hence this article reviews the relation between computer users and CTS, the methods of early diagnosis and ways to prevent it.

A Proposed Mechanism for Development of CTE Following Concussive Events: Head Impact, Water Hammer Injury, Neurofilament Release, and Autoimmune Processes.

During the past decade, there has been an increasing interest in early diagnosis and treatment of traumatic brain injuries (TBI) that lead to chronic traumatic encephalopathy (CTE). The subjects involved range from soldiers exposed to concussive injuries from improvised explosive devices (IEDs) to a significant number of athletes involved in repetitive high force impacts. Although the forces from IEDs are much greater by a magnitude than those from contact sports, the higher frequency associated with contact sports allows for more controlled assessment of the mechanism of action. In our study, we report findings in university-level women soccer athletes followed over a period of four and a half years from accession to graduation. Parameters investigated included T1-, T2-, and susceptibility-weighted magnetic resonance images (SWI), IMPACT (Immediate Post-Concussion Assessment and Cognitive Testing), and C3 Logix behavioral and physiological assessment measures. The MRI Studies show several significant findings: first, a marked increase in the width of sulci in the frontal to occipital cortices; second, an appearance of subtle hemorrhagic changes at the base of the sulci; third was a sustained reduction in total brain volume in several soccer players at a developmental time when brain growth is generally seen. Although all of the athletes successfully completed their college degree and none exhibited long term clinical deficits at the time of graduation, the changes documented by MRI represent a clue to the pathological mechanism following an injury paradigm. The authors propose that our findings and those of prior publications support a mechanism of injury in CTE caused by an autoimmune process associated with the release of neural proteins from nerve cells at the base of the sulcus from a water hammer injury effect. As evidence accumulates to support this hypothesis, there are pharmacological treatment strategies that may be able to mitigate the development of long-term disability from TBI.

Increasing motor neuron excitability to treat weakness in sepsis.

Weakness induced by critical illness (intensive care unit acquired weakness) is a major cause of disability in patients and is currently untreatable. We recently identified a defect in repetitive firing of lower motor neurons as a novel contributor to intensive care unit acquired weakness. To develop therapy for intensive care unit acquired weakness, it was necessary to determine the mechanism underlying the defect in repetitive firing. Both computer simulation and in vivo dynamic voltage clamp of spinal motor neurons in septic rats were employed to explore potential mechanisms underlying defective repetitive firing. Our results suggest alteration in subthreshold voltage-activated currents might be the mechanism underlying defective repetitive firing. It has been shown previously that pharmacologic activation of serotonin receptors on motor neurons increases motor neuron excitability, in part by enhancing subthreshold voltage-activated inward currents. Administration of a U.S. Food and Drug Administration-approved serotonin agonist (lorcaserin) to septic rats greatly improved repetitive firing and motor unit force generation. Our findings suggest activation of serotonin receptors with lorcaserin may provide the first ever therapy for intensive care unit acquired weakness in patients. Ann Neurol 2017;82:961-971.

Trabecular bone in the calcaneus of runners.

Trabecular bone of the human calcaneus is subjected to extreme repetitive forces during endurance running and should adapt in response to this strain. To assess possible bone functional adaptation in the posterior region of the calcaneus, we recruited forefoot-striking runners (n = 6), rearfoot-striking runners (n = 6), and non-runners (n = 6), all males aged 20–41 for this institutionally approved study. Foot strike pattern was confirmed for each runner using a motion capture system. We obtained high resolution peripheral computed tomography scans of the posterior calcaneus for both runners and non-runners. No statistically significant differences were found between runners and nonrunners or forefoot strikers and rearfoot strikers. Mean trabecular thickness and mineral density were greatest in forefoot runners with strong effect sizes (<0.80). Trabecular thickness was positively correlated with weekly running distance (r2 = 0.417, p<0.05) and years running (r2 = 0.339, p<0.05) and negatively correlated with age at onset of running (r2 = 0.515, p<0.01) Trabecular thickness, mineral density and bone volume ratio of nonrunners were highly correlated with body mass (r2 = 0.824, p<0.05) and nonrunners were significantly heavier than runners (p<0.05). Adjusting for body mass revealed significantly thicker trabeculae in the posterior calcaneus of forefoot strikers, likely an artifact of greater running volume and earlier onset of running in this subgroup; thus, individuals with the greatest summative loading stimulus had, after body mass adjustment, the thickest trabeculae. Further study with larger sample sizes is necessary to elucidate the role of footstrike on calcaneal trabecular structure. To our knowledge, intraspecific body mass correlations with measures of trabecular robusticity have not been reported elsewhere. We hypothesize that early adoption of running and years of sustained moderate volume running stimulate bone modeling in trabeculae of the posterior calcaneus.

Influence of footwear comfort on the variability of running kinematics

Footwear comfort is an important factor in design, purchase and use of running shoes but current measures require multiple subjective assessments. Therefore, an objective and more reliable surrogate measure of footwear comfort would be of high relevance. In other research fields, perceived comfort was found to influence the variability of movement execution. Consequently, the purpose of this study was to investigate the influence of perceived footwear comfort on the variability of running kinematics as a potential surrogate measure of comfort. Thirty-six recreational athletes ran in five different running shoes on an indoor track while their running kinematics were recorded using a foot-mounted tri-axial inertial measurement unit (IMU). Footwear comfort was measured through multiple subjective assessments. The relative variability of IMU data was determined across the swing phase of 45 gait cycles and compared between the most and least comfortable shoes. Lower footwear comfort was associated with lower kinematic variability especially in the second half of the swing phase but only for variables that are not directly linked to the forward propulsion during running and mainly describe frontal and transverse joint rotations. The constraints of an uncomfortable shoe may lead to the adaptation of a more monotonous running style with the goal to stay in the least uncomfortable movement path. This finding may partially explain a previously described higher injury risk when exercising in footwear of lower comfort, as more repetitive forces could increase the risk of overuse injury. The results of this study implicate the possible use of IMU-based kinematic variability as a surrogate measure of footwear comfort, which could complement subjective measures.

Posterior Shoulder Instability.

Posterior shoulder instability is a relatively uncommon condition, occurring in ∼10% of those with shoulder instability. Because of the rarity of the condition and the lack of knowledge in treatment, it is often misdiagnosed or patients experience a delay in diagnosis. Posterior instability typically affects athletes participating in contact or overhead sports and is usually the result of repetitive microtrauma or blunt force with the shoulder in the provocative position of flexion, adduction, and internal rotation, leading to recurrent subluxation events. Acute traumatic posterior dislocations are rare injuries with an incidence rate of 1.1 per 100,000 person years. This rate is ∼20 times lower than that of anterior shoulder dislocations. Risk factors for recurrent instability are: (1) age below 40 at time of first instability; (2) dislocation during a seizure; (3) a large reverse Hill-Sachs lesion; and (4) glenoid retroversion. A firm understanding of the pathoanatomy, along with pertinent clinical and diagnostic modalities is required to accurately diagnosis and manage this condition.

Treatment of Valgus Extension Overload

Valgus extension overload (VEO) can cause significant morbidity in the throwing athlete. VEO results from repetitive extreme valgus forces across the elbow and comprises a constellation of injuries located at the posteromedial compartment (osteophyte formation, olecranon stress fractures, and impingement), the ulnar collateral ligament (UCL), and the radiocapitellar joint (lateral compartment chondrosis). The following chapter provides an overview of the relevant anatomy, biomechanics, and diagnosis of VEO. It further details the arthroscopic surgical technique to successfully treat patients with VEO.