Michael Kjaer

Michael Kjaer
Bispebjerg Hospital, Copenhagen University | bispebjerghospital · Institute for Sports Medicine

About

672
Publications
136,392
Reads
How we measure 'reads'
A 'read' is counted each time someone views a publication summary (such as the title, abstract, and list of authors), clicks on a figure, or views or downloads the full-text. Learn more
38,923
Citations
Citations since 2016
140 Research Items
16933 Citations
201620172018201920202021202205001,0001,5002,0002,500
201620172018201920202021202205001,0001,5002,0002,500
201620172018201920202021202205001,0001,5002,0002,500
201620172018201920202021202205001,0001,5002,0002,500
Introduction
Skills and Expertise

Publications

Publications (672)
Article
Objectives: This study investigated the effect of lower limb immobilization and retraining on postural control and muscle power in healthy old and young men. Methods: Twenty men, nine old (OM:67.3±4.4 years) and eleven young (YM:24.4±1.6 years) underwent 2 weeks of unilateral whole-leg casting, followed by 4 weeks of retraining. Measures include...
Article
Full-text available
The myotendinous junction (MTJ) is structurally specialized to transmit force. The highly folded muscle membrane at the MTJ increases the contact area between muscle and tendon and potentially the load tolerance of the MTJ. Muscles with a high content of type II fibers are more often subject to strain injury compared to muscles with type I fibers....
Article
Increasing age appears to influence several morphologic changes in major tendons. However, the effects of aging on the cross-sectional area (CSA) of different ankle tendons are much less understood. Furthermore, potential differences in specific tendon regions along the length of the tendons have not been investigated in detail. Sixty healthy adult...
Article
Full-text available
The present pilot study investigated the extent of histological tissue changes in both chronic tendinopathy and in individuals that display early clinical signs of tendinopathy. The study included 8 individuals of whom 3 were healthy without any tendon symptoms, 2 had early symptoms (1–2 months), and 3 had chronic symptoms (>3 months) from their pa...
Article
Background Lateral elbow tendinopathy is a disabling tendon overuse injury. It remains unknown if a corticosteroid injection (CSI) or tendon needling (TN) combined with heavy slow resistance (HSR) training is superior to HSR alone in treating lateral elbow tendinopathy. Purpose/Hypothesis The purpose was to investigate the effects of HSR combined...
Article
Full-text available
Persistent muscle weakness, tendon elongation, and incomplete return to preinjury level are frequent sequelae after acute Achilles tendon rupture, and evidence-based knowledge of how to best rehabilitate the injury is largely absent in the literature. The objective of this review is to illuminate and discuss to what extent an Achilles tendon ruptur...
Article
Full-text available
The myotendinous junction (MTJ), a specialized interface for force transmission between muscle and tendon, has a unique transcriptional activity and is highly susceptible to muscle strain injury. Eccentric exercise training is known to reduce this risk of injury, but knowledge of the influence of exercise on the MTJ at the molecular and cellular le...
Article
Aging is accompanied by morphological and mechanical changes to the intramuscular connective tissue (IMCT) of skeletal muscles, but whether physical exercise can influence these changes is debated. We investigated the effects of aging and exercise with high or low resistance on composition and mechanical properties of the IMCT, including direct mea...
Article
Muscle fiber denervation is a major contributor to the decline in muscle mass and function during aging. Heavy resistance exercise is an effective tool for increasing muscle mass and strength, but whether it can rescue denervated muscle fibers remains unclear. Therefore, the purpose of this study was to investigate the potential of heavy resistance...
Article
Full-text available
Skeletal muscle possesses remarkable adaptability to mechanical loading and regenerative potential following muscle injury primarily due to satellite cell activity. Although the roles of several types of interstitial cells in skeletal muscle have been documented, the signaling interplay between the skeletal muscle and the adjacent tendon tissue has...
Article
The circadian clock controls many aspects of physiology, but it remains undescribed whether extracellular vesicles (EVs), including exosomes, involved in cell-cell communications between tissues are regulated in a circadian pattern. We demonstrate a 24-hour rhythmic abundance of individual proteins in small EVs using liquid chromatography-mass spec...
Article
Full-text available
Tendons are vital collagen-dense specialized connective tissues transducing the force from skeletal muscle to the bone, thus enabling movement of the human body. Tendon cells adjust matrix turnover in response to physiological tissue loading and pathological overloading (tendinopathy). Nevertheless, the regulation of tendon matrix quality control i...
Article
Objective Cartilage collagen has very limited repair potential, though some turnover and incorporation has not been fully excluded. We aim to determine the regional turnover of human osteoarthritis cartilage. Design Patients scheduled for knee joint replacement surgery due to osteoarthritis were recruited in this prospective study of four weeks du...
Article
Full-text available
Key points: The detrimental effects of ageing can be partially offset by lifelong self-organized recreational exercise, as evidence by preserved type II myofibre associated satellite cells, a beneficial muscle innervation status and greater fatigue resistance under challenged conditions Satellite cell function (in vitro), muscle fibre size and mus...
Article
Full-text available
Proteomics analysis of skeletal muscle has recently progressed from whole muscle tissue to single myofibers. Here, we further focus on a specific myofiber domain crucial for force transmission from muscle to tendon, the myotendinous junction (MTJ). To overcome the anatomical constraints preventing the isolation of pure MTJs, we performed in-depth a...
Conference Paper
Full-text available
Introduction The myotendinous junction (MTJ) has a unique transcriptional activity and is highly susceptible to strain injury. Eccentric exercise reduces this risk of injury, but the mechanism is unknown. The aim of this study was to investigate the effect of a single bout of eccentric exercise on the expression levels of some of the genes known to...
Article
Background Resistance training is an effective strategy to counteract the age-related loss of muscle mass and -strength in elderly, but whether the benefits of training differ between sexes is unclear. Method 297 elderly men and women were randomized to one year of heavy resistance training (HRT) or control (CON). Changes in muscle function and bo...
Article
Full-text available
Insight into the bidirectional signaling between primary human myogenic cells and neurons is lacking. For this purpose, human myogenic cells were derived from the semitendinosus and gracilis muscles of five healthy individuals and co‐cultured with cerebellar granule neurons from two litters of 7‐day‐old Wistar rat pups, in muscle medium or neural m...
Article
Full-text available
Loading intervention is currently the preferred management of tendinopathy, but to what extent different loading regimes influence the mechanical response in tendons is scarcely investigated. Therefore, the purposes of the investigation were to examine the effect of exercise interventions with either high or low load magnitude applied to the tendin...
Article
Full-text available
Tendon injury is a considerable problem affecting both physically active and sedentary people. The aim of this study was to examine the relationship between markers for metabolic disorders (hyperglycemia, hypercholesterolemia and metabolic syndrome) and the risk of developing tendon injuries requiring referral to a hospital. The Copenhagen City Hea...
Article
Physical exercise results in a duration and intensity dependent vascular response in healthy human tendon. In overused (tendinopathy) and damaged tendon, angiogenic pathways are activated and neovascularization is observed. Whereas no direct relationship exists between the amount of neo‐vessels and degree of tendinopathy symptoms, almost all tendin...
Article
Tendons are connective tissues that transmit mechanical forces from muscle to bone and consist mainly of nano-scale fibrils of type I collagen. Aging has been associated with reduced mechanical function of tendons at the whole-tendon level and also with increased glycation of tendon collagen fibrils. Yet, the mechanical effects of aging at the fibr...
Article
Effects of life-long physical activity on tendon function have been investigated in cross-sectional studies, but these are at risk of "survivorship" bias. Here, we investigate if life-long side-specific loading is associated with greater cross-sectional area (CSA), mechanical properties, cell density (DNA content) and collagen cross-link compositio...
Article
Background Muscle strain injury leads to a high risk of recurrent injury in sports and can cause long-term symptoms such as weakness and pain. Scar tissue formation after strain injuries has been described, yet what ultrastructural changes might occur in the chronic phase of this injury have not. It is also unknown if persistent symptoms and morpho...
Article
The myotendinous junction (MTJ) is a specialised interface for transmitting high forces between muscle and tendon and yet the MTJ is a common site of strain injury with a high recurrence rate. The aim of this study was to identify previously unknown MTJ components in mature animals and humans. Samples were obtained from the superficial digital flex...
Article
It is well known that cells can generate endogenous forces onto the extracellular matrix, but to what extent the mechanical properties of the matrix influences these endogenous cellular forces remains unclear. We therefore sought to quantify the influence of matrix rigidity on cell-matrix interactions by inducing cross-links using increasing concen...
Article
Ageing leads to a decline in white matter microstructure and dexterous function of the hand. In adolescents, it has previously been shown that the degree of right-left asymmetry in the corticospinal tract (CST) is linearly related with right-left asymmetry in dexterity. Here, we tested whether this association is also expressed in older adults. Par...
Article
Background Heavy slow resistance (HSR) training is currently recommended as part of the treatment of patellar tendon tendinopathy. However, treatment success is not reached in all patients, and combinations of different treatments could be beneficial. Local administration of insulin-like growth factor-1 (IGF-1) in humans has been shown to quickly s...
Article
Background: It is believed that clinical management of osteoarthritis should address muscle weakness to improve physical function and prevent disability and frailty.Objectives: This sub-study investigated the effects of supervised progressive resistance training (RT), supervised Nordic Walking (NW), and unsupervised home-based exercise (HBE) on mus...
Article
Background: Nonsteroidal anti-inflammatory drugs (NSAIDs) are commonly used in the treatment of Achilles tendinopathy, but whether they have any additive clinical effect on physical rehabilitation in the early phase of tendinopathy remains unknown. Purpose/hypothesis: To investigate whether an initial short-term NSAID treatment added to a physic...
Article
Background T2* mapping has proven useful in tendon research and may have the ability to detect subtle changes at an early stage of tendinopathy. Purpose To investigate the difference in T2* between patients with early tendinopathy and healthy controls, and to investigate the relationship between T2* and clinical outcomes, tendon size, and mechanic...
Article
Background Loading interventions have become a predominant treatment strategy for tendinopathy, and positive clinical outcomes and tendon tissue responses may depend on the exercise dose and load magnitude. Purpose/Hypothesis The purpose was to investigate if the load magnitude influenced the effect of a 12-week loading intervention for patellar t...
Article
Objective Age-related loss of muscle mass and function can be attenuated in rodents with life-long voluntary wheel running with moderate resistance. The present study assessed if sarcopenia could be counteracted with ten weeks high intensity training. Method Old (22–23 months) and middle-aged (11 months) mice were divided into three physical activ...
Article
Objectives Primary frozen shoulder (pFS) has three phases that differ in clinical presentation. It is characterized by contracture of the joint capsule. We hypothesized that there is a general upregulation of collagens in pFS, and that this is highest in the first phase of the disease. The aims of this study were to investigate the expression of va...
Article
Full-text available
The presence of adipocytes at the myotendinous junction (MTJ) has not previously been reported in mammals. However, during studies of the MTJ, our group has noticed the presence of adipocytes as a general and surprising phenomenon. The main aim of this study was to describe and quantify the presence of adipocytes in relation to the MTJ in healthy...
Article
Full-text available
Background Fibroblasts are the powerhouses responsible for the production and assembly of extracellular matrix (ECM). Their activity needs to be tightly controlled especially within the musculoskeletal system, where changes to ECM composition affect force transmission and mechanical loading that are required for effective movement of the body. Extr...
Article
Objective During skeletal growth, the articular cartilage expands to maintain its cover of bones in joints, however, it is unclear when and how cartilage grows. We aim to determine the expanding growth pattern and timing across the tibia plateau in human knees. Design Six human tibia plateaus (2 healthy, 2 with osteoarthritis, and 2 with posttraum...
Article
Background The aim of the current study was to examine different features of the rectus abdominis muscle (RA) in patients with and without a midline incisional hernia to characterize the effects of a hernia on abdominal wall skeletal muscle. Material and methods RA tissue from patients undergoing surgical repair of a large midline incisional herni...
Article
Tendon is a highly organized, dense connective tissue that has been demonstrated to have very little turnover. In spite of the low turnover, tendon can grow in response to loading, which may take place primarily at the periphery. Tendon injuries and recurrence of injuries are common in both human and animal in sports. It is unclear why some areas o...
Article
Background Muscle mass, strength and function declines with advancing age. Strength training (ST) improves these parameters in older adults, but the gains often disappear after completion of a short-term intervention. The purpose of the present study was to investigate muscle mass, -strength and -function one year after the completion of a successf...
Article
Full-text available
Integrins are important for mechanosensation in tissue and play, together with nutrition, a role in regulating extracellular matrix (ECM) in skeletal muscle and tendon. Integrin receptors are dimers that consist of an α and β subunit and bridge extracellular and intracellular signals. The present study investigates whether the deletion of the integ...
Article
Marfan Syndrome (MFS) is an autosomal dominant condition caused by a mutation in the fibrillin‐1 producing gene within the connective tissue, which can affect multiple organ systems negatively, including those that play a vital role (e.g. heart and vasculature) in physical activity. The most fatal manifestations of MFS are aortic dilation, dissecti...
Article
Full-text available
The current model for repair of damaged tissue includes immune cells, mediating the progression from a pro-inflammatory to an anti-inflammatory environment. How this process changes with aging in human skeletal muscle under conditions of physiological exercise loading remains unclear. To investigate this, 25 elderly males (mean age 70 ± SD 7 years)...
Article
There is growing evidence for a link between loss of skeletal muscle, impaired muscle performance, and systemic markers of acute inflammation in hospitalized geriatric patients. The present literature suggests a negative effect of acute inflammation at the time of hospital admission upon muscle performance and the change of this during the hospital...
Article
Full-text available
Objectives Bone and other human tissues remodel through life, for example, as a response to increasing load, and this prevents permanent destruction of the tissue. Non-traumatic meniscal rupture is a common musculoskeletal disease, but it is unknown if it is caused by inability of the menisci to remodel. The aim of this study was to determine wheth...
Article
Background There is currently a lack of imaging modalities that can be used as a sensitive measure in tendinopathy. Recent findings suggest the applicability of ultra-short echo time (UTE) magnetic resonance imaging (MRI) T2* mapping in tendons, but the reproducibility remains unknown. Purpose To evaluate test–retest reproducibility of UTE MRI T2*...
Article
Full-text available
The decline in muscle mass and function with age is partly caused by a loss of muscle fibres through denervation. The purpose of this study was to investigate the potential of exercise to influence molecular targets involved in neuromuscular junction (NMJ) stability in healthy elderly individuals. Participants from two studies (one group of 12 youn...
Article
Aim: To examine changes in lean mass during hospitalization in geriatric patients and the effect of muscle activation by neuromuscular electrical stimulation. Methods: Thirteen patients (69-94 years) at a geriatric ward completed tests at hospital admission (day 2-3) and discharge (day 8-10). One leg received daily stimulation of the knee extensor...
Article
Background Physical muscle function and brain hippocampus size declines with age, accelerating after the age of 60. Strength training over a few months improves physical function, but less is known about how long-term strength training affects physical function and hippocampus volume. Therefore, we aimed to investigate the effect of 1-year strength...
Article
Blunted muscle hypertrophy and impaired regeneration with aging have been partly attributed to satellite cell (SC) dysfunction. However, true muscle regeneration has not yet been studied in elderly individuals. To investigate this, muscle injury was induced by 200 electrically stimulated (ES) eccentric contractions of the vastus lateralis (VL) of o...
Article
Tendon pathology (tendinopathy) typically occurs in specific regions of a tendon and growth in response to exercise also appears to be more pronounced in specific regions. In a previous study in animals we found evidence of regional differences in tendon turnover, but whether the turnover of human patellar tendon differs in different regions still...
Article
Heavy controlled loads of tendon with prolonged time under tension remains the treatment‐of‐choice in tendinopathy rehabilitation. The use of low‐load resistance exercise (LL, 20‐40% 1RM) in combination with blood flow restriction (BFR) has been advocated as a clinically important rehabilitation tool for persons not tolerating high muscle‐tendon lo...
Article
Background: Tendon loading might play a role in the development of heterotopic ossification after Achilles tendon ruptures. Early heavy loading on a healing tendon in animals has been shown to prolong the proinflammatory response, and inflammatory cells are thought to drive heterotopic ossification formation. Taken together, this suggests that ear...
Article
Overloading of tendon tissue with resulting chronic pain (tendinopathy) is a common disorder in occupational-, leisure- and sports-activity, but its pathogenesis remains poorly understood. To investigate the very early phase of tendinopathy, Achilles and patellar tendons were investigated in 200 physically active patients and 50 healthy control per...
Article
Full-text available
Fibrillin‐1 mutations cause pathological changes in connective tissue that constitute the complex phenotype of Marfan syndrome. In this study, we used fibrillin‐1 hypomorphic and haploinsufficient mice (Fbn1mgr/mgR and Fbn1+/− mice, respectively) to investigate the impact of fibrillin‐1 deficiency alone or in combination with regular physical activ...
Article
Tendon cells exist in a dense extracellular matrix (ECM) and mechanical loading is important for the strength development of this matrix. We therefore use a three‐dimensional (3D) culture system for tendon formation in vitro. The objectives of this study were to elucidate the temporal expression of tendon related genes during the formation of artif...
Article
Aims To examine satellite cell and myonuclear content in very old (>83 yrs) individuals, and the response to heavy resistance training. Methods A group of very old men and women (Old, 83‐94 yrs, n=29) was randomized to 12 weeks of heavy resistance training or untrained controls. A group of young men who did not resistance train (Young, 19‐27 yrs,...
Article
Full-text available
Visceral adipose tissue is an immunogenic tissue, which turns detrimental during obesity by activation of proinflammatory macrophages. During aging, chronic inflammation increases proportional to visceral adipose tissue (VAT) mass and associates with escalating morbidity and mortality. Here, we utilize a mouse model to investigate the inflammatory...
Article
Exercise is recommended as first‐line treatment to reduce pain and improve physical function in patients with hip osteoarthritis, but the mechanism behind the effect remains unknown. Blood and urine were collected at baseline and after four months of exercise from 60+‐year‐old persons (n=39) with hip osteoarthritis included in a randomized controll...
Article
Full-text available
Introduction: The majority of young women use oral contraceptives (OCs). Use of OCs has been associated with lower myofibrillar protein and tendon collagen synthesis rates, but it is unknown whether OCs will limit the adaptive response of myotendinous tissue to resistance training. Design and Methods: Fourteen healthy untrained young regular OC use...
Article
Introduction: Muscle fiber denervation increases with age and yet studies at the tissue level are sparse, due to the challenging nature of establishing the relative role of regeneration and denervation. Methods: Muscle biopsies were obtained from the vastus lateralis of 70 healthy male participants (72±6 yrs, range: 65-94 yrs). mRNA levels of ac...
Article
Full-text available
Advanced glycation end-products (AGEs) accumulate with aging and have been associated with tissue modifications and metabolic disease. Regular exercise has several health benefits, and the purpose of this study was to investigate the effect of regular long-term exercise and diet on skin autofluorescence (SAF) as a measure of glycation and on Achill...
Article
Traumatic strain injury in skeletal muscle is often associated with fluid accumulation at the site of rupture, but the role of this injury exudate (EX) in cellular responses and healing is unknown. We aimed to characterize the EX sampled from human hamstring or calf muscles following a strain injury (n = 12). The cytokine and growth-factor profile,...
Article
Full-text available
Immobilization of the lower limbs promotes a catabolic state that reduces muscle mass, whereas physical training promotes an anabolic state that increases muscle mass. Understanding the molecular mechanisms underlying this is of clinical interest, as loss of muscle mass is a major complication to critical illness in humans. To determine the molecul...
Article
Full-text available
Purpose The discovery of musculoskeletal tissues, including muscle, tendons, and cartilage, as peripheral circadian clocks strongly implicates their role in tissue-specific homeostasis. Age-related dampening and misalignment of the tendon circadian rhythm and its outputs may be responsible for the decline in tendon homeostasis. It is unknown which...
Article
Full-text available
Purpose: Recent data suggest that there is a lack of turnover in the core of human tendon, but it remains unknown whether there are regional differences between core and periphery of the cross section. The purpose of this project was to investigate regional differences in turnover as estimated by the accumulation of fluorescent Advanced Glycation E...
Article
Background and Purpose Hospital admission for older patients has been linked to loss of muscle mass and function, and the former can be measured with ultrasonography during hospitalization. Methods Two studies were carried out. In Study 1 69 patients’ (85±8 yrs.) activity level was measured with ActivPal and quadriceps thickness measured twice wit...
Article
Underpinning skeletal muscle plasticity is the interplay between many cell types, of which fibroblasts are emerging as potent players, both negatively in the development of fibrosis but also positively in stimulating muscle repair through enhancing myogenesis. The mechanisms behind this interaction however remain unknown. To investigate this, waste...
Article
The decline in skeletal muscle regenerative capacity with age is partly attributed to muscle stem cell (satellite cell) dysfunction. Recent evidence has pointed to a strong interaction between myoblasts and fibroblasts, but the influence of age on this interaction is unknown. Additionally, while the native tissue environment is known to determine t...
Article
Aim Chronic inflammation increases with age and is correlated positively to visceral fat mass, but inversely to muscle mass. We investigated the hypothesis that resistance training would increase muscle mass and strength together with a concomitant drop in local and systemic inflammation level independent of any changes in visceral fat tissue in el...
Article
Full-text available
From 19th to 22nd November 2018, 26 researchers representing nine countries and a variety of academic disciplines met in Snekkersten, Denmark, to reach evidence-based consensus about physical activity and older adults. It was recognised that the term ‘older adults’ represents a highly heterogeneous population. It encompasses those that remain highl...
Article
This review summarises the treatment of acute muscle injuries. Muscle injuries are frequent traumatic injuries caused by either excessive strain on the muscle tendon unit (strain injury) or a forceful blow to the muscle (contusion). An early start of rehabilitation after acute strain injuries is a key to shortening the time to return to sport. The...