Niels Ortenblad

University of Southern Denmark | SDU · Institute of Sports Science and Clinical Biomechanics

Decline in mitochondrial function is linked to decreased muscle mass and strength in conditions like sarcopenia and type 2 diabetes. Despite therapeutic opportunities, there is limited and equivocal data regarding molecular cues controlling muscle mitochondrial plasticity. Here we uncovered that the mitochondrial mRNA-stabilizing protein SLIRP, in...

Endurance exercise performance is associated with a well-developed skeletal muscle mitochondrial network, which is composed of subsarcolemmal mitochondria interconnected with intermyofibrillar mitochondria bending around the myofibrils. High-altitude exposure is typically incorporated in elite sport training regimens, but little is known about how...

Purpose This narrative umbrella review evaluates the efficacy of recovery strategies for elite winter sports athletes by comparing their scientific and clinical validity. It aims to provide evidence-based recommendations for coaches and athletes, preparing them for the Milano-Cortina 2026 Olympic Games through a critical evaluation of various post-...

Purpose The aim was to assess the accuracy of a continuous blood glucose monitoring (CGM) device (Abbott FreeStyle Libre 3) against capillary blood glucose measurement (BGM) before, during, and after an intense lower body strength training session in connection with high- versus low-carbohydrate breakfasts. Methods Nine adults (22 ± 2 years) compl...

Piperine has been shown to bind to myosin and shift the distribution of conformational states of myosin molecules from the super‐relaxed state to the disordered relaxed state. However, little is known about the implications for muscle force production and potential underlying mechanisms. Muscle contractility experiments were performed using isolate...

Background Multiples of resting metabolic rate (RMR) are often used to classify physical activity intensity, a concept known as the Metabolic Equivalent of Task (MET). However, the METs metrics may misclassify physical activity intensity in older adults because of age related changes in RMR and maximal aerobic capacity (V̇O2max). This study aimed t...

In skeletal muscle, glycogen particles are distributed both within and between myofibrils, as well as just beneath the sarcolemma. Their precise localisation may influence their degradation rate. Here, we investigated how exercise at different intensities and durations (1‐ and 15‐min maximal exercise) with known variations in glycogenolytic rate an...

Purpose This study investigated the effects of prolonged intermittent cycling exercise on peak power output (PPO) and 6-min time-trial (6 min-TT) performance in elite and professional road cyclists. Moreover, the study aimed to determine whether changes in performance in the fatigued state could be predicted from substrate utilization during exerci...

Decline in mitochondrial function associates with decreased muscle mass and strength in multiple conditions, including sarcopenia and type 2 diabetes. Optimal treatment could include improving mitochondrial function, however, there are limited and equivocal data regarding the molecular cues controlling muscle mitochondrial plasticity. Here we uncov...

Aims: We investigated whether a high-intensity interval training (HIIT)-protocol could restore beta-cell function in type 2 diabetes compared with sedentary obese and lean individuals. Materials and methods: In patients with type 2 diabetes, and age-matched, glucose-tolerant obese and lean controls, we examined the effect of 8-weeks of supervised H...

During submaximal exercise, there is a heterogeneous recruitment of skeletal muscle fibers, with an ensuing heterogeneous depletion of muscle glycogen both within and between fiber types. Here, we show that the mean (95% CI) mitochondrial volume as a percentage of fiber volume of non‐glycogen‐depleted fibers was 2 (−10:6), 5 (−21:11), and 12 (−21:−...

This historical review traces key discoveries regarding K⁺ and Na⁺ ions in skeletal muscle at rest and with exercise, including contents and concentrations, Na⁺,K⁺-ATPase (NKA) and exercise effects on plasma [K⁺] in humans. Following initial measures in 1896 of muscle contents in various species, including humans, electrical stimulation of animal m...

Purpose Short periods of reduced energy availability are commonly undertaken by athletes to decrease body mass, possibly improve the power-to-mass ratio, and enhance physical performance. Our primary aim was to investigate the impact of 10 days of low energy availability (LEA) followed by 2 days of optimal energy availability (OEA) on physical perf...

The impact of training status and sex on intrinsic skeletal muscle mitochondrial respiratory capacity remains unclear. We examined this by analysing human skeletal muscle mitochondrial respiration relative to mitochondrial volume and cristae density across training statuses and sexes. Mitochondrial cristae density was estimated in skeletal muscle b...

White adipose tissue (WAT) is important for metabolic homeostasis. We established the differential proteomic signatures of WAT in glucose-tolerant lean and obese individuals and patients with type 2 diabetes (T2D) and the response to 8 weeks of high-intensity interval training (HIIT). Using a high-throughput and reproducible mass spectrometry–based...

Purpose O 2 -transport and endurance exercise performance are greatly influenced by hemoglobin mass (Hb mass ), which largely depends on lean body mass (LBM). This study investigated the effects of 8 weeks with 3 weekly sessions of conventional (3-SET: 3x10 reps) or high-volume strength training (10-SET: 5-10x10 reps) on LBM, Hb mass , muscle stren...

and graphical abstract The decline in mitochondrial function is associated with decreased muscle mass and strength in multiple conditions, including sarcopenia, type 2 diabetes, and cancer. Treatment of these diseases could include improving mitochondrial function, however, there are limited and equivocal data regarding the molecular cues that cont...

Intramuscular lipids are stored as subsarcolemmal or intramyofibrillar droplets with potential diverse roles in energy metabolism. We examined intramuscular lipid utilization through transmission electron microscopy during repeated high-intensity intermittent exercise, an aspect that is hitherto unexplored. Seventeen moderately-to-well-trained male...

Distinct subcellular pools of glycogen particles exist within skeletal muscle fibres, distributed both within and between myofibrils and can be found in proximity to, or at a distance from mitochondria. Their precise localisation may influence their degradation rate and role in muscle function. Here, we investigated how exercise at different intens...

Perturbations in K⁺ have long been considered a key factor in skeletal muscle fatigue. However, the exercise-induced changes in K⁺ intra-to-extracellular gradient is by itself insufficiently large to be a major cause for the force decrease during fatigue unless combined to other ion gradient changes such as for Na⁺. Whilst several studies described...

Mitochondria are the cellular organelles responsible for resynthesising the majority of ATP. In skeletal muscle, there is an increased ATP turnover during resistance exercise to sustain the energetic demands of muscle contraction. Despite this, little is known regarding the mitochondrial characteristics of chronically strength‐trained individuals a...

Intramuscular lipid droplets (LDs) and mitochondria are essential organelles in cellular communication and metabolism, supporting local energy demands during muscle contractions. While insulin resistance impacts cellular functions and systems within the skeletal muscle, it remains unclear whether the interaction of LDs and mitochondria is affected...

Performance in short-duration sports is highly dependent on muscle glycogen, but the total degradation is only moderate and considering the water-binding property of glycogen, unnecessary storing of glycogen may cause an unfavorable increase in body mass. To investigate this, we determined the effect of manipulating dietary carbohydrates (CHO) on m...

Mitochondria are the cellular organelles responsible for resynthesising the majority of ATP. In skeletal muscle, there is an increased ATP turnover during resistance exercise to sustain the energetic demands of muscle contraction. Despite this, little is known regarding the mitochondrial characteristics of chronically strength-trained individuals a...

Excessive storage of lipid droplets (LDs) in skeletal muscles is a hallmark of type 2 diabetes. However, LD morphology displays a high degree of subcellular heterogeneity and varies between single muscle fibers, which impedes the current understanding of lipid-induced insulin resistance. Using quantitative transmission electron microscopy (TEM), we...

Low-frequency fatigue (LFF) is defined by a relatively larger deficit in isometric force elicited by low-frequency electrical stimulation compared to high-frequency stimulation. However, the effects of LFF on power during dynamic contractions has not been thoroughly characterized. In the current study, rat soleus muscles underwent fatiguing concent...

Aims Non-weight-bearing high-intensity interval training (HIIT) involving several muscle groups may efficiently improve metabolic health without compromising adherence in obesity and type 2 diabetes. In a non-randomized intervention study, we examined the effect of a novel HIIT-protocol, recruiting both lower and upper body muscles, on insulin sens...

Glycogen particles are situated in key areas of the muscle cell in the vicinity of the main energy‐consumption sites and may be utilised heterogeneously dependent on the nature of the metabolic demands. The present study aimed to investigate the time course of fibre type‐specific utilisation of muscle glycogen in three distinct subcellular fraction...

The 70-kDa heat shock protein (HSP70) is a ubiquitous molecular chaperone which is highly inducible by cellular stress such as exercise. To investigate the role of muscle glycogen content on the HSP70 expression, muscle glycogen was manipulated by consumption of either water (H2O) or a carbohydrate-enriched diet (CHO) during recovery from 4 h of gl...

Purpose: We investigated the coupling between muscle glycogen content and localization and high-intensity exercise performance using a randomized, placebo-controlled, parallel-group design with emphasis on single-fiber subcellular glycogen concentrations and sarcoplasmic reticulum Ca2+ kinetics. Methods: Eighteen well-trained participants perfor...

Glycogen is a key energy substrate in excitable tissue, including in skeletal muscle fibers where it also contributes to local energy production. Transmission electron microscopy imaging has revealed the existence of a heterogenic subcellular distribution of three distinct glycogen pools in skeletal muscle, which are thought to reflect the requirem...

Muscle glycogen is an important fuel source for contracting skeletal muscle, and it is well documented that exercise performance is impaired when the muscle’s stores of glycogen are exhausted. The role of carbohydrate (CHO) availability on exercise performance has been known for more than a century, while the specific role of muscle glycogen for mu...

With the use of transmission electron microscopy, high-resolution images of fixed samples containing individual muscle fibers can be obtained. This enables quantifications of ultrastructural aspects such as volume fractions, surface area to volume ratios, morphometry, and physical contact sites of different subcellular structures. In the 1970s, a p...

Glycogen is a key energy substrate in excitable tissue and especially in skeletal muscle fibers it contributes with a substantial, but also local energy production. A heterogenic subcellular distribution of three distinct glycogen pools in skeletal muscle is proved by transmission electron microscopy (TEM), which is thought to represent the require...

Muscle glycogen is the main substrate during high-intensity exercise and large reductions can occur after relatively short durations. Moreover, muscle glycogen is stored heterogeneously and similarly displays a heterogeneous and fiber-type specific depletion pattern with utilization in both fast- and slow-twitch fibers during high-intensity exercis...

Cross-country (XC) ski races involve a variety of formats, two different techniques and tracks with highly variable topography and environmental conditions. In addition, XC skiing is a major component of both Nordic combined and biathlon competitions. Research in this area, both in the laboratory and field, encounters certain difficulties that may...

The present study examined skeletal muscle metabolism and changes in repeated sprint performance during match play for n = 20 competitive elite women outfield players. We obtained musculus vastus lateralis biopsies and blood samples before, after, and following intense periods in each half of a friendly match, along with 5 × 30‐meter sprint tests a...

Introduction Little is known about potential differences in contractile properties of muscle fibers of the same type in arms and legs. Accordingly, the present study was designed to compare the force-generating capacity and Ca²⁺ sensitivity of fibers from arm and leg muscles of highly trained cross-country skiers. Method Single muscle fibers of m....

Aim: The maintenance of healthy and functional mitochondria is the result of a complex mitochondrial turnover and herein quality-control program which includes both mitochondrial biogenesis and autophagy of mitochondria. The aim of this study was to examine the effect of an intensified training load on skeletal muscle mitochondrial quality control...

Objective NAD⁺ is a co-factor and substrate for enzymes maintaining energy homeostasis. Nicotinamide phosphoribosyltransferase (NAMPT) controls NAD⁺ synthesis, and in skeletal muscle, NAD⁺ is important for muscle integrity. However, the underlying molecular mechanisms by which NAD⁺ synthesis affects muscle health remain poorly understood. Thus, the...

Objective: To explore the relationship between the contents of subcellular localization specific mitochondria (mitochondria between myofibrils, mitochondria under the muscle membrane, total mitochondria) and different muscle fiber types, and the effect of short-term diet intervention on the content of subcellular localization specific mitochondria...

Ischaemic preconditioning (IPC) protects against myocardial ischaemia–reperfusion injury. The metabolic and ionic effects of IPC remain to be clarified in detail. We aimed to investigate the effect of IPC (2 times 5 min ischaemia) on the subcellular distribution of glycogen and Ca2+-uptake and leakiness by the sarcoplasmic reticulum (SR) in respons...

New findings: What is the central question of this study? Glycogen supercompensation following glycogen-depleting exercise can be achieved by consuming a carbohydrate-enriched diet, but the associated effects on the size, number and localization of intramuscular glycogen particles is unknown. What is the main finding and its importance? Using tran...

Growing evidence supports that pharmacological application of growth differentiation factor 15 (GDF15) suppresses appetite but also promotes sickness-like behaviors in rodents via GDNF family receptor α-like (GFRAL)-dependent mechanisms. Conversely, the endogenous regulation of GDF15 and its physiological effects on energy homeostasis and behavior...

Context Women show an accelerated loss of muscle mass around menopause, possibly related to the decline in estrogen. Furthermore, the anabolic response to resistance exercise seems to be hampered in postmenopausal women. Objective We aimed to test the hypothesis that transdermal estrogen therapy (ET) amplifies the skeletal muscle response to resis...

In this study, we examined the effect of beta 2 -agonist salbutamol at oral doses during a period of resistance training on sprint performance, quadriceps contractile function, skeletal muscle hypertrophy, fiber-type composition, maximal activity of enzymes of importance for anaerobic energy turnover, and sarcoplasmic reticulum Ca ²⁺ -handling in y...

Growing evidence supports that pharmacological application of growth differentiation factor 15 (GDF15) suppresses appetite but also promotes sickness-like behaviors in rodents via GDNF family receptor α-like (GFRAL)-dependent mechanisms 1,2 . Conversely, the endogenous regulation and secretion of GDF15 and its physiological effects on energy homeos...

Objective: to determine whether skeletal muscle molecular markers and SC number were influenced differently in users and non-users of oral contraceptives (OCs) following 10 weeks of resistance training Methods: Thirty-eight young healthy untrained users (n=20) and non-users of OC (n=18) completed a 10-week supervised progressive resistance training...

Aim: Glycogen particles are found in different subcellular localizations, which are utilized heterogeneously in different fibre types during endurance exercise. Although resistance exercise typically involves only a moderate use of mixed muscle glycogen, the hypothesis of the present study was that high-volume heavy-load resistance exercise would...

Background: Protein supplementation has been suggested to augment endurance training adaptations by increasing mixed muscle and myofibrillar protein synthesis and lean body mass. However, a potential beneficial effect on mitochondrial adaptations is yet to be clarified. The aim of the present study was to investigate the effect of consuming whey p...

Key points When muscle biopsies first began to be used routinely in research on exercise physiology five decades ago, it soon become clear that the muscle content of glycogen is an important determinant of exercise performance. Glycogen particles are stored in distinct pools within the muscles, but the role of each pool during exercise and how this...

Background Long-term testosterone replacement therapy (TRT) increases muscle mass in elderly men with subnormal testosterone levels. However, the molecular mechanisms underlying this effect of TRT on protein balance in human skeletal muscle in vivo remain to be established. Methods Here, we examined skeletal muscle biopsies obtained before and 24-...

San-Millán and colleagues [...]

Little is presently known about the effects of acute high-intensity exercise or training on release and uptake of Ca²⁺ by the sarcoplasmic reticulum (SR). The aims here were to characterize this regulation in highly trained athletes following (1) repeated bouts of high-intensity exercise and (2) a period of endurance training including high-intensi...

Little is presently known about the effects of acute high-intensity exercise or training on release and uptake of Ca 2+ by the sarcoplasmic reticulum (SR). The aims here were to characterize this regulation in highly trained athletes following (1) repeated bouts of high-intensity exercise and (2) a period of endurance training including high-intens...

Background: Protein supplementation has been suggested to augment endurance training adaptations by increasing mixed muscle and myofibrillar protein synthesis and lean body mass. However, a potential beneficial effect on mitochondrial adaptations is yet to be clarified. The aim of the present study was to investigate the effect of consuming whey pr...

Background: Protein supplementation has been suggested to augment endurance training adaptations by increasing mixed muscle and myofibrillar protein synthesis and lean body mass. However, a potential beneficial effect on mitochondrial adaptations is yet to be clarified. The aim of the present study was to investigate the effect of consuming whey pr...

Key points Muscle glycogen content is associated with muscle function, but the physiological link between the two is poorly understood. This study investigated the effects of inhibiting glycogenolysis, while maintaining high overall energy status, on different aspects of muscle function. We demonstrate here that Na⁺,K⁺‐ATPase activity depends on gl...

Intramuscular triglycerides (IMTG) are a key substrate during prolonged exercise, but little is known about the rate of IMTG resynthesis in the post-exercise period. We investigated the hypothesis that the distribution of the lipid droplet (LD)-associated perilipin (PLIN) proteins is linked to IMTG storage following exercise. 14 elite male triathle...