Gymnastik- och idrottshögskolan, GIH

Muscle disuse has rapid and debilitating effects on muscle mass and overall health, making it an important issue from both scientific and clinical perspectives. However, the myocellular adaptations to muscle disuse are not yet fully understood, particularly those related to the myonuclear permanence hypothesis. Therefore, in this study, we assessed fiber size, number of myonuclei, satellite cells, and capillaries in human gastrocnemius muscle after a period of immobilization following an Achilles tendon rupture. Six physically active patients (5M/1F, 43 {plus minus} 15 years) were recruited to participate after sustaining an acute unilateral Achilles tendon rupture. Muscle biopsies were obtained from the lateral part of the gastrocnemius before and after six weeks of immobilization using a plaster cast and orthosis. Muscle fiber characteristics were analyzed in tissue cross-sections and isolated single fibers using immunofluorescence and high-resolution microscopy. Immobilization did not change muscle fiber type composition nor cross-sectional area of type I or type II fibers, but muscle fiber volume tended to decline by 13% (p=0.077). After immobilization, the volume per myonucleus was significantly reduced by 20% (p=0.008). Myonuclei were not lost in response to immobilization but tended to increase in single fibers and type II fibers. No significant changes were observed for satellite cells or capillaries. Myonuclei were not lost in the gastrocnemius muscle after a prolonged period of immobilization, which may provide support to the myonuclear permanence hypothesis in human muscle. Capillaries remained stable throughout the immobilization period, whereas the response was variable for satellite cells, particularly in type II fibers.

AIMS: Submaximal tests estimating VO₂max have inherent biases; hence, using VO₂max estimations from the same test is essential for reducing this bias. This study aimed to establish sex- and age-specific reference values for estimated VO₂max using the Åstrand-test (Å-test) and the Ekblom-Bak test (EB-test). We also assessed the effects of age, exercise level, and BMI on VO₂max estimations.

METHODS: We included men and women (20-69 years) from the Swedish working population participating in Health Profile Assessments between 2010 and 2020. Excluding those on heart rate-affecting medicines and smokers, n = 263,374 for the Å-test and n = 95,043 for the EB-test were included. VO₂max reference values were based on percentiles 10, 25, 40, 60, 75, and 90 for both sexes across 5-year age groups.

RESULTS: Estimated absolute and relative VO₂max were for men 3.11 L/min and 36.9 mL/min/kg using the Å-test, and 3.58 L/min and 42.4 mL/min/kg using the EB-test. For women, estimated absolute and relative VO₂max were 2.48 L/min and 36.6 mL/min/kg using the Å-test, and 2.41 L/min and 35.5 mL/min/kg using the EB-test. Higher age (negative), higher exercise level (positive), and higher BMI (negative) were associated with estimated VO₂max using both tests. However, explained variance by exercise on estimated VO₂max was low, 10% for the Å-test and 8% for the EB-test, and moderate for BMI, 23% and 29%.

CONCLUSION: We present reference values for estimated VO₂max from two submaximal cycle tests. Age, exercise, and BMI influenced estimated VO₂max. These references can be valuable in clinical evaluations using the same submaximal tests.

AIM: The purpose of this study was to 1. investigate if glucose tolerance is affected after one acute bout of different types of exercise; 2. assess if potential differences between two exercise paradigms are related to changes in mitochondrial function; and 3. determine if endurance athletes differ from nonendurance-trained controls in their metabolic responses to the exercise paradigms.

METHODS: Nine endurance athletes (END) and eight healthy nonendurance-trained controls (CON) were studied. Oral glucose tolerance tests (OGTT) and mitochondrial function were assessed on three occasions: in the morning, 14 h after an overnight fast without prior exercise (RE), as well as after 3 h of prolonged continuous exercise at 65% of VO₂ max (PE) or 5 × 4 min at ~95% of VO₂ max (HIIT) on a cycle ergometer.

RESULTS: Glucose tolerance was markedly reduced in END after PE compared with RE. END also exhibited elevated fasting serum FFA and ketones levels, reduced insulin sensitivity and glucose oxidation, and increased fat oxidation during the OGTT. CON showed insignificant changes in glucose tolerance and the aforementioned measurements compared with RE. HIIT did not alter glucose tolerance in either group. Neither PE nor HIIT affected mitochondrial function in either group. END also exhibited increased activity of 3-hydroxyacyl-CoA dehydrogenase activity in muscle extracts vs. CON.

CONCLUSION: Prolonged exercise reduces glucose tolerance and increases insulin resistance in endurance athletes the following day. These findings are associated with an increased lipid load, a high capacity to oxidize lipids, and increased fat oxidation.

PURPOSE: To study the effect of inhaling a beta-agonist (salbutamol) compared to placebo on skiing and cycling performance in well-trained elite athletes.

METHODS: Three different exercise protocols were used, all with a cross-over double blind placebo-controlled design. Participants inhaled 800 µg salbutamol or a placebo prior to the test, which was repeated on a following day with the participants inhaling the other substance. Fifteen junior elite skiers performed four free-style high intensity sprints (1100 m/work time 3.5-4.5 min). Twelve elite cyclists carried out a short cycling protocol, starting with two 5 min submaximal workloads followed by a maximal intermittent performance test to exhaustion. Another 12 elite cyclists performed the maximal intermittent performance test to exhaustion after a 150 min long submaximal cycling protocol.

RESULTS: Group mean time for the ski sprints increased, with no difference between treatment groups. In the short cycling protocol time to exhaustion was 9.1% (95% CI 52-161) lower after inhaling salbutamol compared to placebo and in the long cycling protocol time to exhaustion was 9.1% (95% CI - 121-267) lower after inhaling salbutamol compared to placebo. Blood lactate, heart rate and ventilation increased during submaximal exercise with salbutamol compared to placebo in the short cycling protocol (p < .05).

CONCLUSION: This study could not confirm any positive performance effects from inhaling 800 µg salbutamol compared to placebo in skiing and high-intensity intermittent cycling performance. Instead, time to exhaustion in the maximal intermittent performance test was lower in both cycling protocols. HighlightsThere was no difference in performance time between salbutamol and placebo treatment in real-life applicable repeated ski sprints.Time to exhaustion in the maximal intermittent performance test was 9.1% lower after inhaling salbutamol compared to placebo, both when performed after 10 and 150 min of submaximal cycling.

Aim: To study how change in cardiorespiratory fitness over time is associated with the development of poor self-rated health in healthy Swedish adults, and whether this association varies with sex, age, body mass index and cardiorespiratory fitness at baseline. A secondary aim was to study the influence of other predictors of self-rated health. Methods: A total of 98,718 participants (45% women, mean age 42.2 years) with two assessments from occupational health service screenings between 1988 and 2019 (mean duration 4.3 years), with good self-rated health at baseline were included. Cardiorespiratory fitness was assessed as estimated maximal oxygen consumption using submaximal cycle testing. Change in cardiorespiratory fitness was expressed as percentage annual change. Poor self-rated health at follow-up was defined as percieving self-rated health as 'poor' or 'very poor'. Results: A large decrease in cardiorespiratory fitness (⩾-3%) was associated with a 34% higher risk of poor self-rated health compared to maintainers (-1 to +1%) after multi-adjustment including change in body mass index, back/neck pain, stress, exercise habits and sleep quality or sleep problems. The associations for decreasers were stronger with longer follow-up time (>10 years). Preserving, or changing to, risk level for body mass index, back/neck pain, stress, exercise and sleep quality/problems were associated with a higher risk of poor self-rated health. Conclusions: Preserving or increasing cardiorespiratory fitness is associated with a lower risk of poor self-rated health, independently of change in other health-related variables, which may act as a protection against future poor self-rated health. This is of high clinical value, and strategies for maintaining or improving cardiorespiratory fitness have the potential to influence both disease and mortality.

Sedan 1988 finns hälsoprofilbedömningar (HPB) genomförda på den arbetsföra befolkningen i Sverige registrerade i en databas.

Till och med år 2021 fanns närmare en miljon HPB i databasen, varav cirka 200 000 var upprepade test. 

I nuläget har 20 artiklar publicerats med utgångspunkt från databasen. Dessa har beskrivit trender i levnadsvanor och hälsa, analyser av samband mellan levnadsvanor och hjärt–kärlsjukdom samt påverkan av covid-19 på levnadsvanor.

Background: The concept of overreaching and super compensation is widely in use by athletes and coaches seeking to maximize performance and adaptations to exercise training. The physiological aspects of acute fatigue, overreaching and non-functional overreaching are, however, not well understood, and well-defined negative physiological outcomes are missing. Instead, the concept relies heavily on performance outcomes for differentiating between the states. Recent advancements in the field of integrated exercise physiology have associated maladaptations in muscular oxidative function to high loads of exercise training.

Method: Eleven female and male subjects that exercised regularly but did not engage in high-intensity interval training (HIIT) were recruited to a 4-week long training intervention where the responses to different training loads were studied. Highly monitored HIIT sessions were performed on a cycle ergometer in a progressive fashion with the intent to accomplish a training overload. Throughout the intervention, physiological and psychological responses to HIIT were assessed, and the results were used to construct a diagnostic model that could indicate maladaptations during excessive training loads.

Results: We here use mitochondrial function as an early marker of excessive training loads and show the dynamic responses of several physiological and psychological measurements during different training loads. During HIIT, a loss of mitochondrial function was associated with reduced glycolytic, glucoregulatory and heart rate responses and increased ratings of perceived exertion in relation to several physiological measurements. The profile of mood states was highly affected after excessive training loads, whereas performance staled rather than decreased. By implementing five of the most affected and relevant measured parameters in a diagnostic model, we could successfully, and in all the subjects, identify the training loads that lead to maladaptations.

Conclusions: As mitochondrial parameters cannot be assessed without donating a muscle biopsy, this test can be used by coaches and exercise physiologists to monitor adaptation to exercise training for improving performance and optimizing the health benefits of exercise. Clinical trial registry number NCT04753021 . Retrospectively registered 2021-02-12.

Testosterone (T) administration has previously been shown to improve muscle size and oxidative capacity. However, the molecular mechanisms underlying these adaptations in human skeletal muscle remain to be determined. Here, we examined the effect of moderate-dose T administration on molecular regulators of muscle protein turnover and mitochondrial remodeling in muscle samples collected from young women. Forty-eight healthy, physically active, young women (28 ± 4 years) were assigned in a random double-blind fashion to receive either T (10 mg/day) or placebo for 10-weeks. Muscle biopsies collected before and after the intervention period were divided into sub-cellular fractions and total protein levels of molecular regulators of muscle protein turnover and mitochondrial remodeling were analyzed using Western blotting. T administration had no effect on androgen receptor or 5α-reductase levels, nor on proteins involved in the mTORC1-signaling pathway (mTOR, S6K1, eEF2 and RPS6). Neither did it affect the abundance of proteins associated with proteasomal protein degradation (MAFbx, MuRF-1 and UBR5) and autophagy-lysosomal degradation (AMPK, ULK1 and p62). T administration also had no effect on proteins in the mitochondria enriched fraction regulating mitophagy (Beclin, BNIP3, LC3B-I, LC3B-II and LC3B-II/I ratio) and morphology (Mitofilin), and it did not alter the expression of mitochondrial fission- (FIS1 and DRP1) or fusion factors (OPA1 and MFN2). In summary, these data indicate that improvements in muscle size and oxidative capacity in young women in response to moderate-dose T administration cannot be explained by alterations in total expression of molecular factors known to regulate muscle protein turnover or mitochondrial remodeling.

BACKGROUND

It is well accepted that exercise training improves glucose uptake and insulin sensitivity, and that endurance trained athletes in general show a high capacity for these parameters and excellent metabolic control. However, some studies fail to observe positive effects on glucose regulation in healthy, trained subjects the day after exercise. These, often unexpected, results have been postulated to be caused by excessive training loads, muscle damage, energy deficit, differences in glucose uptake in the exercised and non-exercised musculature and a metabolic interaction through increased fatty acid metabolism which suppresses glucose oxidation and uptake. The mode or volume of exercise that can lead to glucose intolerance in trained athletes as well as mechanistic insights and its relevance for health and performance are, however, not fully understood.

AIM

We studied the metabolic response to a glucose load the day after a session of high intensity interval training (HIIT) or three hours of continuous exercise (3h) in endurance trained athletes and compared the results with measurements during rest.

METHOD

Nine endurance trained athletes (5 females, 4 males) underwent oral glucose tolerance tests (OGTT) after rest and ~14 hours after exercise on a cycle ergometer (HIIT 5x4 minutes at ~95% of VO2max or 3h at 65% of VO2max). Venous blood was sampled at 15-minute intervals for 120 minutes and concentrations of glucose, insulin, free fatty acids (FFA) and ketones (β-hydroxybutyrate) were measured. Statistical analysis was performed using a RM one-way ANOVA with the Giesser-Greenhouse correction and Dunnett’s test was used to compare the exercise conditions to the resting condition.

RESULTS

The area under the curve (AUC) during the OGTT increased greatly after 3h (668±124 mM · min) (p<0.01) compared to rest (532±89) but was found to be unchanged after HIIT (541±96). Resting values of FFA and ketones were increased after 3h (p<0.01 and p<0.05, respectively) but not after HIIT. Insulin was found to be unaltered during all conditions.

CONCLUSIONS AND RELEVANCE

Here, we show manifestation of glucose intolerance in endurance trained athletes together with concomitant increases in plasma concentrations of FFA and ketones the day after a session of prolonged exercise training but not after HIIT. This could be a protective response for securing glucose delivery to the brain and therefore have a positive effect on endurance. It also has the potential to reduce the recovery of glycogen depots, glucose uptake during exercise and performance at higher work rates.

Administration of branched-chain amino acids (BCAA) has been suggested to enhance mitochondrial biogenesis, including levels of PGC-1α, which may, in turn, alter kynurenine metabolism. Ten healthy subjects performed 60 min of dynamic one-leg exercise at ~70% of Wmax on two occasions. They were in random order supplied either a mixture of BCAA or flavored water (placebo) during the experiment. Blood samples were collected during exercise and recovery, and muscle biopsies were taken from both legs before, after and 90 and 180 min following exercise. Ingestion of BCAA doubled their concentration in both plasma and muscle while causing a 30-40% reduction (P<0.05 vs. placebo) in levels of aromatic amino acids in both resting and exercising muscle during 3-h recovery. The muscle concentration of kynurenine decreased by 25% (P<0.05) during recovery, similar in both resting and exercising leg and with both supplements, although plasma concentration of kynurenine during recovery was 10% lower (P<0.05) when BCAA were ingested. Ingestion of BCAA reduced the plasma concentration of kynurenic acid by 60% (P<0.01) during exercise and recovery, while the level remained unchanged with placebo. Exercise induced a 3-4-fold increase (P<0.05) in muscle content of PGC-1a1 mRNA after 90 min of recovery under both conditions, whereas levels of KAT4 mRNA and protein were unaffected by exercise or supplement. In conclusion, the reduction of plasma levels of kynurenine and kynurenic acid caused by BCAA were not associated with any changes in the level of muscle kynurenine, suggesting that kynurenine metabolism was altered in tissues other than muscle.

BACKGROUND: The impact of cardiorespiratory fitness (CRF) and other lifestyle-related factors on severe COVID-19 risk is understudied. The present study aims to investigate lifestyle-related and socioeconomic factors as possible predictors of COVID-19, with special focus on CRF, and to further study whether these factors may attenuate obesity- and hypertension-related risks, as well as mediate associations between socioeconomic factors and severe COVID-19 risk.

METHODS: Out of initially 407,131 participants who participated in nationwide occupational health service screening between 1992 and 2020, n = 857 cases (70% men, mean age 49.9 years) of severe COVID-19 were identified. CRF was estimated using a sub-maximum cycle test, and other lifestyle variables were self-reported. Analyses were performed including both unmatched, n = 278,598, and sex-and age-matched, n = 3426, controls. Severe COVID-19 included hospitalization, intensive care or death due to COVID-19.

RESULTS: Patients with more severe COVID-19 had significantly lower CRF, higher BMI, a greater presence of comorbidities and were more often daily smokers. In matched analyses, there was a graded decrease in odds for severe COVID-19 with each ml in CRF (OR = 0.98, 95% CI 0.970 to 0.998), and a two-fold increase in odds between the lowest and highest (< 32 vs. ≥ 46 ml·min^-1·kg^-1) CRF group. Higher BMI (per unit increase, OR = 1.09, 1.06 to 1.12), larger waist circumference (per cm, OR = 1.04, 1.02 to 1.06), daily smoking (OR = 0.60, 0.41 to 0.89) and high overall stress (OR = 1.36, 1.001 to 1.84) also remained significantly associated with severe COVID-19 risk. Obesity- and blood pressure-related risks were attenuated by adjustment for CRF and lifestyle variables. Mediation through CRF, BMI and smoking accounted for 9% to 54% of the associations between low education, low income and blue collar/low skilled occupations and severe COVID-19 risk. The results were consistent using either matched or unmatched controls.

CONCLUSIONS: Both lifestyle-related and socioeconomic factors were associated with risk of severe COVID-19. However, higher CRF attenuated the risk associated with obesity and high blood pressure, and mediated the risk associated with various socioeconomic factors. This emphasises the importance of interventions to maintain or increase CRF in the general population to strengthen the resilience to severe COVID-19, especially in high-risk individuals.

Blood flow restriction (BFR) with low-load resistance exercise (RE) is often used as a surrogate to traditional high-load RE to stimulate muscular adaptations, such as hypertrophy and strength. However, it is not clear whether such adaptations are achieved through similar cellular and molecular processes. We compared changes in muscle function, morphology and signaling pathways between these differing training protocols. Twenty-one males and females (mean ± SD: 24.3 ± 3.1 years) experienced with resistance training (4.9 ± 2.6 years) performed nine weeks of resistance training (three times per week) with either high-loads (75-80% 1RM; HL-RT), or low-loads with BFR (30-40% 1RM; LL-BFR). Before and after the training intervention, resting muscle biopsies were collected, and quadricep cross-sectional area (CSA), muscular strength and power were measured. Approximately 5 days following the intervention, the same individuals performed an additional 'acute' exercise session under the same conditions, and serial muscle biopsies were collected to assess hypertrophic- and ribosomal-based signaling stimuli. Quadricep CSA increased with both LL-BFR (7.4±4.3%) and HL-RT (4.6±2.9%), with no significant differences between training groups (p=0.37). Muscular strength also increased in both training groups, but with superior gains in squat 1RM occurring with HL-RT (p<0.01). Acute phosphorylation of several key proteins involved in hypertrophy signaling pathways, and expression of ribosomal RNA transcription factors occurred to a similar degree with LL-BFR and HL-RT (all p>0.05 for between-group comparisons). Together, these findings validate low-load resistance training with continuous BFR as an effective alternative to traditional high-load resistance training for increasing muscle hypertrophy in trained individuals.

Human skeletal muscle characteristics such as fiber type composition, fiber size and myonuclear content are widely studied in clinical and sports related contexts. Being aware of the methodological and biological variability of the characteristics is a critical aspect in study design and outcome interpretation, but comprehensive data on the variability of morphological features in human skeletal muscle is currently limited. Accordingly, in the present study, m. vastus lateralis biopsies (10 per subject) from young and healthy individuals, collected in a systematic manner, were analyzed for various characteristics using immunohistochemistry (n=7) and SDS-PAGE (n=25). None of the analyzed parameters; fiber type % (FT%), type I and II CSA (fCSA), percentage fiber type area (fCSA%), myosin heavy chain composition (MyHC%), type IIX content, myonuclear content or myonuclear domain varied in a systematic manner longitudinally along the muscle or between the two legs. The average within subject coefficient of variation for FT%, fCSA, fCSA%, and MyHC% ranged between 13-18%, but was only 5% for fiber specific myonuclear content, which reduced the variability for myonuclear domain size to 11-12%. Pure type IIX fibers and type IIX MyHC were randomly distributed and present in <24% of the analyzed samples, with the average content being 0.1 and 1.1%, respectively. In conclusion, leg or longitudinal orientation does not seem to be an important aspect to consider when investigating human vastus lateralis characteristics. However, single muscle biopsies should preferably not be used when studying fiber type and fiber size related aspects given the notable sample to sample variability.

BACKGROUND: Total sitting time is associated with a higher risk for cardio metabolic disease and mortality, while breaks in prolonged sitting attenuate these effects. However, less is known about associations of different specific domains and breaks of sitting on general health, back/neck pain and if physical activity could influence these associations. The aim was to investigate how workplace sitting and frequency of breaking up workplace sitting is associated with self-reported general health and self-reported back/neck pain.

METHODS: 44,978 participants (42% women) from the Swedish working population, who participated in a nationwide occupational health service screening 2014-2019, were included in this cross-sectional study. Self-reported sitting duration and frequency of breaks from sitting at work, general health, back/neck pain, exercise, leisure time sitting, diet, smoking, stress and body mass index were assessed. Occupation was classified as requiring higher education qualifications or not. Logistic regression modelling was used to assess the association between workplace sitting/frequency of breaks in workplace sitting and poor general health and back/neck pain, respectively.

RESULTS: Compared to sitting all the time at work, sitting ≤75% of the time showed significantly lower risks for poor general health (OR range 0.50-0.65), and sitting between 25 and 75% of the time showed significantly lower risks (OR 0.82-0.87) for often reported back/neck pain. For participants reporting sitting half of their working time or more, breaking up workplace sitting occasionally or more often showed significantly lower OR than seldom breaking up workplace sitting; OR ranged 0.40-0.50 for poor health and 0.74-0.81 for back/neck pain.

CONCLUSIONS: Sitting almost all the time at work and not taking breaks is associated with an increased risk for self-reported poor general health and back/neck pain. People sitting almost all their time at work are recommended to take breaks from prolonged sitting, exercise regularly and decrease their leisure time sitting to reduce the risk for poor health.

This study examined acute molecular responses to concurrent exercise involving different muscles. Eight men participated in a randomized crossover-trial with two sessions, one where they performed interval cycling followed by upper body resistance exercise (ER-Arm), and one with upper body resistance exercise only (R-Arm). Biopsies were taken from the triceps prior to and immediately, 90- and 180-min following exercise. Immediately after resistance exercise, the elevation in S6K1 activity was smaller and the 4E-BP1:eIF4E interaction greater in ER-Arm, but this acute attenuation disappeared during recovery. The protein synthetic rate in triceps was greater following exercise than at rest, with no difference between trials. The level of PGC-1α1 mRNA increased to greater extent in ER-Arm than R-Arm after 90 min of recovery, as was PGC-1α4 mRNA after both 90 and 180 min. Levels of MuRF-1 mRNA was unchanged in R-Arm, but elevated during recovery in ER-Arm, whereas MAFbx mRNA levels increased slightly in both trials. RNA sequencing in a subgroup of subjects revealed 862 differently expressed genes with ER-Arm versus R-Arm during recovery. These findings suggest that leg cycling prior to arm resistance exercise causes systemic changes that potentiate induction of specific genes in the triceps, without compromising the anabolic response.

Exercise training positively affects metabolic health through increased mitochondrial oxidative capacity and improved glucose regulation and is the first line of treatment in several metabolic diseases. However, the upper limit of the amount of exercise associated with beneficial therapeutic effects has not been clearly identified. Here, we used a training model with a progressively increasing exercise load during an intervention over 4 weeks. We closely followed changes in glucose tolerance, mitochondrial function and dynamics, physical exercise capacity, and whole-body metabolism. Following the week with the highest exercise load, we found a striking reduction in intrinsic mitochondrial function that coincided with a disturbance in glucose tolerance and insulin secretion. We also assessed continuous blood glucose profiles in world-class endurance athletes and found that they had impaired glucose control compared with a matched control group.

The COVID-19 pandemic has become a public health emergency of international concern, which may have affected lifestyle habits and mental health. Based on national health profile assessments, this study investigated perceived changes of lifestyle habits in response to the COVID-19 pandemic and associations between perceived lifestyle changes and mental health in Swedish working adults. Among 5599 individuals (50% women, 46.3 years), the majority reported no change (sitting 77%, daily physical activity 71%, exercise 69%, diet 87%, alcohol 90%, and smoking 97%) due to the pandemic. Changes were more pronounced during the first wave (April–June) compared to the second (October–December). Women, individuals &lt;60 years, those with a university degree, white-collar workers, and those with unhealthy lifestyle habits at baseline had higher odds of changing lifestyle habits compared to their counterparts. Negative changes in lifestyle habits and more time in a mentally passive state sitting at home were associated with higher odds of mental ill-health (including health anxiety regarding one’s own and relatives’ health, generalized anxiety and depression symptoms, and concerns regarding employment and economy). The results emphasize the need to support healthy lifestyle habits to strengthen the resilience in vulnerable groups of individuals to future viral pandemics and prevent health inequalities in society.

Background: Protein availability around aerobic exercise might benefit aerobic capacity and body composition in normal weight adults. However, it is unknown if individuals with overweight/obesity elicit similar adaptations or improve other cardiometabolic/health-related markers in response to different types of protein. Thus, our aim was to study the effect of supplementation of two different protein drinks in conjunction with exercise on aerobic capacity, body composition and blood health markers in untrained subjects with overweight or obesity. Methods: The present study measured training adaptation and health parameters over a 6 week period in untrained men with overweight/obesity (n = 28; BMI 30.4 ± 2.2 kg/m2) ingesting either plant- (Oat/Potato; n = 8) or animal-based (Milk; n = 10) protein-carbohydrate drinks (10 g of protein/serving), or a control carbohydrate drink (n = 10) acutely before and after each training session (average three sessions/week @ 70% HRmax). Pre-post intervention ˙VO2peakV˙O2peak , muscle biopsies and blood samples were collected, body composition measured (DXA) and two different exercise tests performed. Body weight was controlled with participants remaining weight stable throughout the intervention. Results: For the groups combined, the training intervention significantly increased ˙VO2peakV˙O2peak (8%; P < 0.001), performance in a time-to-exhaustion trial (~ 100%; P < 0.001), mitochondrial protein content and enzyme activity (~20-200%). Lean body mass increased (1%; P < 0.01) and fat mass decreased (3%; P < 0.01). No significant effects on fasting blood glucose, insulin, lipids or markers of immune function were observed. There were no significant interactions between drink conditions for training adaptation or blood measurements. For body composition, the Oat/Potato and carbohydrate group decreased leg fat mass significantly more than the Milk group (interaction P < 0.05). Conclusions: Aerobic capacity and body composition were improved and a number of mitochondrial, glycolytic and oxidative skeletal muscle proteins and enzyme activities were upregulated by a 6 week training intervention. However, none of the parameters for endurance training adaptation were influenced by protein supplementation before and after each training session.

Objective: To evaluate and compare ratio and allometric scaling models of maximal oxygen consumption (VO₂max) for different body size measurements in relation to cardiovascular disease (CVD) incidence and all-cause mortality.

Methods: 316 116 individuals participating in occupational health screenings, initially free from CVD, were included. VO₂max was estimated using submaximal cycle test. Height, body mass and waist circumference (WC) were assessed, and eight different scaling models (two evaluated in a restricted sample with WC data) were derived. Participants were followed in national registers for first-time CVD event or all-cause mortality from their health screening to first CVD event, death or 31 December 2015.

Results: Increasing deciles of VO₂max showed lower CVD risk and all-cause mortality for all six models in the full sample (p<0.001) as well as with increasing quintiles in the restricted sample (eight models) (p<0.001). For CVD risk and all-cause mortality, significantly weaker associations with increasing deciles for models 1 (L·min^-1) and 5 (mL·min^-1·height^-2) were seen compared with model 2 (mL·min^-1·kg^-1), (CVD, p<0.00001; p<0.00001: all-cause mortality, p=0.008; p=0.001) and in some subgroups. For CVD, model 6 (mL·min^-1·(kg¹·height^-1)^-1) had a stronger association compared with model 2 (p<0.00001) and in some subgroups.In the restricted sample, trends for significantly stronger associations for models including WC compared with model 2 were seen in women for both CVD and all-cause mortality, and those under 50 for CVD.

Conclusion: In association to CVD and all-cause mortality, only small differences were found between ratio scaling and allometric scaling models where body dimensions were added, with some stronger associations when adding WC in the models.

The repair, remodeling and regeneration of myofibers is dependent on satellite cells (SCs), although, the distribution of SCs in different fiber types of human muscle remains inconclusive. There is also a paucity of research comparing muscle fiber characteristics in a sex-specific manner. Therefore, the aim of this study was to investigate fiber type-specific SC content in men and women. Muscle biopsies from vastus lateralis were collected from 64 young (mean age 27 ± 5), moderately trained men (n=34) and women (n=30). SCs were identified by Pax7-staining together with immunofluorescent analyses of fiber type composition, fiber size and myonuclei content. In a mixed population, comparable number of SCs were associated to type I and type II fibers (0.07 ± 0.02 vs 0.07 ± 0.02 SCs per fiber, respectively). However, unlike men, women displayed a fiber type-specific distribution, with SC content being lower in type II than type I fibers (P=0.041). Sex-based differences were found specifically for type II fibers, where women displayed lower SC content compared to men (P<0.001). In addition, positive correlations (r-values between 0.36-0.56) were found between SC content and type I and type II fiber size in men (P=0.03 and P<0.01, respectively), whereas similar relationships could not be detected in women. Sex-based differences were also noted for fiber type composition and fiber size, but not for myonuclei content. We hereby provide evidence for sex-based differences present at the myocellular level, which may have important implications when studying exercise- and training induced myogenic responses in skeletal muscle.

AIMS: To explore how change in cardiorespiratory fitness is associated with incident hypertension in adults, and whether the association varies between sex, age, body mass index, cardiorespiratory fitness at baseline and follow-up time. A second aim is to study how change in other lifestyle-related variables affects the results.

METHODS: A total of 91,728 participants (48% women), normotensive at baseline, with two examinations from occupational health service screenings between 1982 and 2019 (mean duration 4.3 years) were included. Cardiorespiratory fitness was assessed as estimated maximal oxygen consumption using submaximal cycle testing. Change in cardiorespiratory fitness was expressed as the percentage change per year. Incident hypertension was defined as systolic blood pressure of 140 mmHg or greater or diastolic blood pressure of 90 mmHg or greater, or self-reported physician-diagnosed hypertension, at second examination.

RESULTS: A large increase (≥3% annual change) in cardiorespiratory fitness was associated with a 11% lower risk of incident hypertension compared with maintainers (-1 to +1%), after multi-adjustment including change in smoking, body mass index, diet, stress and exercise habits. On the contrary, a small (-1 to -<3%) and large (≥-3%) decrease in cardiorespiratory fitness associated with a 21% and 25% higher risk compared with maintainers. Longer duration between the examinations was associated with stronger risk associations. Preserving, or changing to, risk level for the other lifestyle variables was associated with a higher risk of incident hypertension. However, a simultaneous maintenance of or increase in cardiorespiratory fitness attenuated the risk associated with smoking, and stress.

CONCLUSION: Preserving or increasing cardiorespiratory fitness should be part of any long-term strategy to decrease the risk of incident hypertension.

Aims: The purpose of this research was to describe the current prevalence and historic trends in overweight, obesity and severe obesity in Sweden. Methods: Data on BMI, age, gender, education and geographic region were obtained in n=447,925 Swedish adults through a nationwide screening test (1995?2017). To account for sampling variations, we quantified prevalence estimates and time trends using standardized values (direct method) to all 18?74-year-old Swedes, using nationwide databases. Rates of overweight (BMI ?25 kg/m2), obesity (BMI ?30 kg/m2) and severe obesity (BMI ?35 kg/m2) were calculated across gender, age, education and geographic categories. Years were grouped into two-year sampling periods (except the first period where we used three years) for increased power. We used multivariable logistic regression to quantify independent associations between age, gender, education and region with obesity development and current prevalence rates. Results: In 2016/17 the unstandardized prevalence of overweight, obesity and severe obesity were 55.1%, 16.6% and 4.2%, respectively. Factors associated with a higher obesity prevalence were male gender, older age, lower education and residing in a rural region (all P<0.001). Between 1995 and 2017 the prevalence of severe obesity increased by 153%, compared to obesity (+86%) and overweight (+23%). While there were similar increases in obesity across gender and age groups, people with low education (vs high) and rural areas (vs urban) had a higher prevalence increase (both P<0.001). Conclusions: Rates of overweight, obesity and severe obesity have increased markedly in Swedish adults since 1995. Priority groups for prevention efforts include individuals with low education and those living in rural areas.

En stor grupp av arbetsföra svenskar har haft en negativ utveckling av såväl kondition, förekomst av övervikt och fetma samt mående och upplevd hälsa under de senaste 30 åren. Det visar flera studier som baserats på en unik databas med hälsoprofiler.

Background: Recently, it was shown that exogenously administered testosterone enhances endurance capacity in women. In this study, our understanding on the effects of exogenous testosterone on key determinants of oxygen transport and utilization in skeletal muscle is expanded.Methods: In a double-blinded, randomized, placebo-controlled trial, 48 healthy active women were randomized to 10 weeks of daily application of 10 mg of testosterone cream or placebo. Before and after the intervention, VO₂ max, body composition, total hemoglobin (Hb) mass and blood volumes were assessed. Biopsies from the vastus lateralis muscle were obtained before and after the intervention to assess mitochondrial protein abundance, capillary density, capillary-to-fiber (C/F) ratio, and skeletal muscle oxidative capacity.Results: Maximal oxygen consumption per muscle mass, Hb mass, blood, plasma and red blood cell volumes, capillary density, and the abundance of mitochondrial protein levels (i.e., citrate synthase, complexes I, II, III, IV-subunit 2, IV-subunit 4, and V) were unchanged by the intervention. However, the C/F ratio, specific mitochondrial respiratory flux activating complex I and linked complex I and II, uncoupled respiration and electron transport system capacity, but not leak respiration or fat respiration, were significantly increased following testosterone administration compared to placebo.Conclusion: This study provides novel insights into physiological actions of increased testosterone exposure on key determinants of oxygen diffusion and utilization in skeletal muscle of women. Our findings show that higher skeletal muscle oxidative capacity coupled to higher C/F ratio could be major contributing factors that improve endurance performance following moderately increased testosterone exposure.

Lactate has been implicated as a potential signaling molecule. In myotubes, lactate incubation increases mTORC1- and ERK-signaling and induces hypertrophy, indicating that lactate could be a mediator of muscle adaptations to resistance exercise. However, the potential signaling properties of lactate, at rest or with exercise, have not been explored in human tissue. In a cross-over design study, 8 men and 8 women performed one-legged resistance exercise while receiving venous infusion of saline or sodium lactate. Blood was sampled repeatedly, and muscle biopsies were collected at rest and at 0, 90,180 min and 24 h after exercise. The primary outcomes examined were intracellular signaling, fractional protein synthesis rate (FSR), and blood/muscle levels of lactate and pH. Post-exercise blood lactate concentrations were 130% higher in the Lactate trial (3.0 vs 7.0 mmol×l^-1_, p<0.001) whereas muscle levels were only marginally higher (27 vs 32 mmol×kg^-1 d.w., p=0.003) compared to the Saline-trial. Post-exercise blood pH was higher in the Lactate-trial (7.34 vs 7.44, p<0.001), with no differences in intramuscular pH. Exercise increased the phosphorylation of mTOR^S2448 (~40%), S6K1^T389 (~3-fold), and p44^T202/T204 (~80%) during recovery, without any differences between trials. FSR over the 24-h recovery period did not differ between the Saline (0.067 %/h) and Lactate (0.062 %/h) trials. This study does not support the hypothesis that blood lactate levels can modulate anabolic signaling in contracted human muscle. Further in vivo research investigating the impact of exercised versus rested muscle and the role of intramuscular lactate is needed to elucidate its potential signaling properties.

The purpose of the present study was to compare the myosin heavy chain (MHC) isoform composition of the deltoid and vastus lateralis muscles of the dominant and non-dominant limbs in handball players. Eleven male Greek elite handball players (age 22.6 ± 1.9 yrs, training experience 10.6 ± 2.1 yrs, height 184.1 ± 4.1 cm, and weight 81.0 ± 12.5 kg) participated in the study. Four muscle biopsies were obtained from the dominant and non-dominant deltoid and vastus lateralis muscles during the in-season period. The MHC composition was determined using SDS-PAGE. No significant difference was found between the dominant and non-dominant muscles; Deltoid muscle: MHC I [(95%CI = -1.22, 0.33), P = 0.228], MHC ΙΙa [(95%CI = -0.32, 1.59), P = 0.168] and MHC IIx [(95%CI = -1.49, 1.10), P = 0.749]; Vastus lateralis muscle: MHC I [(95%CI = -0.38, 0.63), P = 0.586], MHC ΙΙa [(95%CI = -0.50, 0.65), P = 0.783] and MHC IIx [(95%CI = -1.08, 0.42), P = 0.355]. The findings of the present study indicate that the greater use of the dominant limbs for throwing actions and body movements in handball do not lead to altered MHC isoform composition compared to the non-dominant limbs.

AIM: To identify and describe the characteristics of naturally occurring patterns of exercise, sitting in leisure time and at work and cardiorespiratory fitness, and the association of such profiles with metabolic risk factors, perceived health, and perceived symptoms.

METHODS: 64,970 participants (42% women, 18-75 years) participating in an occupational health service screening in 2014-2018 were included. Exercise and sitting were self-reported. Cardiorespiratory fitness was estimated using a submaximal cycle test. Latent profile analysis was used to identify profiles. BMI and blood pressure were assessed through physical examination. Perceived back/neck pain, overall stress, global health, and sleeping problems were self-reported.

RESULTS: Six profiles based on exercise, sitting in leisure time and at work and cardiorespiratory fitness were identified and labelled; Profile 1 "Inactive, low fit and average sitting in leisure, with less sitting at work"; Profile 2 "Inactive, low fit and sedentary"; Profile 3 "Active and average fit, with less sitting at work"; Profile 4 "Active, average fit and sedentary in leisure, with a sedentary work" (the most common profile, 35% of the population); Profile 5 "Active and fit, with a sedentary work"; Profile 6 "Active and fit, with less sitting at work". Some pairwise similarities were found between profiles (1 and 2, 3 and 4, 5 and 6), mainly based on similar levels of exercise, leisure time sitting and fitness, which translated into similar dose-response associations with the outcomes. In general, profile 1 and 2 demonstrated most adverse metabolic and perceived health, profile 4 had a more beneficial health than profile 3, as did profile 6 compared to profile 5.

CONCLUSIONS: The present results implies a large variation in exercise, sitting, and fitness when studying naturally occurring patterns, and emphasize the possibility to target exercise, sitting time, and/or fitness in health enhancing promotion intervention and strategies.

INTRODUCTION: The ability to carry heavy loads is an important and necessary task during numerous outdoor activities and especially in military operations. The aim of this study was to investigate factors associated with load-carrying ability in men and women with and without extensive load-carrying experience.

MATERIALS AND METHODS: The energy expenditure during carrying no load, 20, 35, and 50 kg at 2 walking speeds, 3 and 5 km h-1, was studied in 36 healthy participants, 19 men (30 ± 6 years, 82.5 ± 7.0 kg) and 17 women (29 ± 6 years, 66.1 ± 8.9 kg), experienced (>5 years) in carrying heavy loads (n = 16, 8 women) or with minor or no such experience (n = 20, 9 women). A standard backpack filled with weights to according carry load was used during the walks. Anthropometric data, leg muscle strength, as well as trunk muscle endurance and muscle fiber distribution of the thigh, were also obtained. Extra Load Index (ELI)-the oxygen uptake (VO2) during total load over unloaded walking-was used as a proxy for load-carrying ability at 20, 35, and 50 kg (ELI20, ELI35, and ELI50, respectively). In addition to analyzing factors of importance for the ELI values, we also conducted mediator analyses using sex and long-term carrying experience as causal variables for ELI as the outcome value. The study was approved by the Regional Ethics Committee in Stockholm, Sweden.

RESULTS: For the lowest load (20 kg), ELI20, was correlated with body mass but no other factors. Walking with 35 and 50 kg load at 5 km h-1 body mass, body height, leg muscle strength, and absolute VO2max were correlated, while relative VO2max, trunk muscle endurance, and leg muscle fiber distribution were not correlated to ELI35 and ELI50.ELI50 at 5 km h-1 differed between the sexes. This difference was only mediated by the difference in body mass. Neither muscle fiber distribution, leg muscle strength, trunk muscle endurance, and body height nor did absolute or relative VO2max explain the difference.Participants with long-term experience of heavy load carrying had significant lower ELI20 and ELI50 values than those with minor or no experience, but none of the above studied factors could explain this difference.

CONCLUSION: The study showed that body mass, without sex differences, and experience of carrying heavy loads are the dominant factors for the ability to carry heavy loads. Even though the effect of experience alludes to the need for extensive carrying training, no causality can be proven. Load carry training intervention studies is suggested for future investigations.

It is well established that testosterone administration induces muscle fiber hypertrophy and myonuclear addition in men, however, it remains to be determined whether similar morphological adaptations can be achieved in women. The aim of the present study was therefore to investigate whether exogenously administered testosterone alters muscle fiber morphology in skeletal muscle of young healthy, physically active women. Thirty-five young (20-35 years), recreationally trained women were randomly assigned to either 10-week testosterone administration (10 mg daily) or placebo. Before and after the intervention, hormone concentrations and body composition were assessed, and muscle biopsies were obtained from the vastus lateralis. Fiber type composition, fiber size, satellite cell- and myonuclei content, as well as muscle capillarization were assessed in a fiber type-specific manner using immunohistochemistry. Following the intervention, testosterone administration elevated serum testosterone concentration (5.1-fold increase, P=0.001), and induced significant accretion of total lean mass (+1.9%, P=0.002) and leg lean mass (+2.4%, P=0.001). On the muscle fiber level, testosterone increased mixed fiber cross-sectional area (+8.2%, P=0.001), an effect primarily driven by increases in type II fiber size (9.2%, P=0.006). Whereas myonuclei content remained unchanged, a numerical increase (+30.8%) was found for satellite cells associated with type II fibers in the Testosterone group. In parallel with fiber hypertrophy, testosterone significantly increased capillary contacts (+7.5%, P=0.015) and capillary-to-fiber ratio (+9.2%, P=0.001) in type II muscle fibers. The current study provides novel insight into fiber type-specific adaptations present already after 10 weeks of only moderately elevated testosterone levels in women.

INTRODUCTION: Human skeletal muscle is thought to have heightened sensitivity to exercise stimulus when it has been previously trained (i.e., it possesses "muscle memory"). We investigated whether basal and acute resistance exercise-induced gene expression and cell signaling events are influenced by previous strength training history.

METHODS: Accordingly, 19 training naïve women and men completed 10 weeks of unilateral leg strength training, followed by 20 weeks of detraining. Subsequently, an acute resistance exercise session was performed for both legs, with vastus lateralis biopsies taken at rest and 1 h after exercise in both legs (memory and control).

RESULTS: The phosphorylation of AMPK and eEF2 was higher in the memory leg than in the control leg at both time points. Post-exercise phosphorylation of 4E-BP1 was higher in the memory leg than in the control leg. The memory leg had lower basal mRNA levels of total PGC1α, and, unlike the control leg, exhibited increases in PGC1α-ex1a transcripts after exercise. In the genes related to myogenesis (SETD3, MYOD1, and MYOG), mRNA levels differed between the memory and the untrained leg; these effects were evident primarily in the male subjects. Expression of the novel gene SPRYD7 was lower in the memory leg at rest and decreased after exercise only in the control leg, but SPRYD7 protein levels were higher in the memory leg.

CONCLUSION: In conclusion, several key regulatory genes and proteins involved in muscular adaptations to resistance exercise are influenced by previous training history. Although the relevance and mechanistic explanation for these findings need further investigation, they support the view of a molecular muscle memory in response to training.

The prevalence of poor health, in particular stress-related mental ill-health, is increasing over time and birth cohorts. As rapid societal changes have occurred in the last decade and still are occurring, there is an interest in investigating the trends in health-related factors. The aim of the present study was to investigate trends in self-reported general health, overall stress, work-related stress, feelings of loneliness, and sleeping problems in 335,625 Swedish adults across categories of gender, geographic regions, length of education, and age from 2000 to 2016. On population level, sleeping problems and poor general health have increased markedly and significantly, while experiences of work stress decreased between 2000 and 2016 (p < 0.05). Overall stress and level of loneliness were unchanged (p > 0.05). The risk of having ≥3 symptoms (any of poor or very poor general health, often or very often perceived overall stress, loneliness, or sleeping problems) has increased significantly from 2000 to 2016 (ß = 1034 (1027-1040)). This increase was significantly higher in young (ß = 1052 (1038-1065)) and individuals with lower education (ß = 1056 (1037-1076)) compared to older and high length of education.

BACKGROUND: Trend analyses of active commuting and potential variations in trends and association with cardiovascular disease (CVD) risk within sub-groups are unknown.

OBJECTIVES: To a) describe trends in active commuting between 1998 to 2015 and b) to study the association between different amounts of active commuting and the incidence risk of CVD in a large sample of Swedish workers, and analyses of potential variations across sub-groups of socio-demographics, physical activity and BMI.

METHODS: A total of 318,309 participants (47% women, 18-74 years) who participated in a nationwide occupational health service screening between 1998 and 2015 were included. Commuting habits were self-reported, and data on first-time CVD events were derived from national registers.

RESULTS: Self-reported passive commuters decreased between 1998 and 2015 (64% to 56%), transferring to an increase in mainly moderate/high-dose active commuters (12% to 19%). Changes were seen in all subgroups. The characteristics and life-style habits of the typical passive and active commuter changed little over the study period. Low- and moderate/high-dose active commuters had significantly decreased risks for a first time CVD during follow-up. This was accentuated in men, middle-aged and in participants with light physical work situations, irregular exercise habits, being overweight/obese and with low fitness.

CONCLUSION: Increases in active commuting were observed between 1998 and 2015, however still leaving a majority who do not actively commute. As active commuting, regardless dose, is associated with a lower CVD risk, encouraging more people to actively commute may provide an easily accessible and time-efficient possibility to increase physical activity and health in the general population.

Objective To investigate the effects of a moderate increase in serum testosterone on physical performance in young, physically active, healthy women.Methods A double blind, randomised, placebo controlled trial was conducted between May 2017 and June 2018 (ClinicalTrials.gov ID: NCT03210558). 48 healthy, physically active women aged 18–35 years were randomised to 10 weeks of treatment with 10 mg of testosterone cream daily or placebo (1:1). All participants completed the study. The primary outcome measure was aerobic performance measured by running time to exhaustion (TTE). Secondary outcomes were anaerobic performance (Wingate test) and muscle strength (squat jump (SJ), counter movement jump (CMJ) and knee extension peak torque). Hormone levels were analysed and body composition assessed by dual energy X-ray absorptiometry.Results Serum levels of testosterone increased from 0.9 (0.4) nmol/L to 4.3 (2.8) nmol/L in the testosterone supplemented group. TTE increased significantly by 21.17 s (8.5%) in the testosterone group compared with the placebo group (mean difference 15.5 s; P=0.045). Wingate average power, which increased by 15.2 W in the testosterone group compared with 3.2 W in the placebo group, was not significantly different between the groups (P=0.084). There were no significant changes in CMJ, SJ and knee extension. Mean change from baseline in total lean mass was 923 g for the testosterone group and 135 g for the placebo group (P=0.040). Mean change in lean mass in the lower limbs was 398 g and 91 g, respectively (P=0.041).Conclusion The study supports a causal effect of testosterone in the increase in aerobic running time as well as lean mass in young, physically active women.

Purpose: To measure isokinetic peak torque during elbow, knee and trunk extension and flexion and muscle endurance during elbow and knee extension.

Method: Muscle endurance and peak torque were measured using Cybex methodology in totally 115 boys and 48 girls from 8 to 15 years of age most of them for five years.

Results: Knee muscle endurance was mainly unchanged while elbow fatigue index was lightly reduced from 11 to 15 years in both sexes with no difference between sexes. From the youngest ages to adolescence peak absolute (N.m) and normalized (N.m/kg body mass) torque increases in all measures with highest increase in the trunk and lowest in elbow activities. During elbow activity boys are stronger than girls from age 11. For trunk and knee activity the sex differences start at age 14 years. Knee, elbow and trunk extension/flexion ratios as well as knee/elbow both extension and flexion ratios are mainly unchanged with increasing age with no differences between sexes.

Conclusion: Data indicate that elbow isokinetic strength and endurance profile is partly different from corresponding data during knee and trunk activity.

Dose-response relationship between exercise load and mitochondrial function

Flockhart M, Nilsson L, Bergman K, Apro W, Ekblom B, Larsen FJ

A dose-dependent relationship exists between exercise load and muscular adaptation. Mitochondria adapt to the increased ATP-demand by alterations in mass and/or quality. How mitochondrial mass and quality changes as a function of exercise load is not well investigated and we have previously found mitochondrial dysfunction after short-term intensive exercise. We therefore aimed to study mitochondrial function by altering exercise load during a three week interval training regimen to understand the dose-response relationship between exercise load and mitochondrial function. We took four muscle biopsies throughout the study, and as expected, mitochondrial function was positively affected during the first two weeks. After the third week, a dramatic mitochondrial dysfunction was evident as mitochondrial intrinsic respiration was reduced by 26% despite a 32% increase in mitochondrial yield. We hereby present evidence of a striking exercise-induced reduction in mitochondrial function after a period of very intense interval training.

Biphasic relationship between training load and glucose tolerance

Nilsson, L, Flockhart M, Bergman K, Apro W, Ekblom B, Larsen FJ

There is a well-established construct regarding the positive effects of exercise on glucose tolerance and insulin sensitivity, as well as muscle glycogen storage. In insulin resistance, physical activity is an essential part of the treatment. However, the optimal dose is unknown. Reduced muscular glycogen stores, resulting from exercise, should stimulate an increased uptake of blood glucose. In this study we investigated the relation between training load, glucose tolerance and insulin sensitivity during three weeks of increasing interval training. Three times during the intervention, oral glucose tests were conducted to investigate the rate of glucose uptake. We found a biphasic dose-response relationship between training load and glucose tolerance, where an excessive training load led to a paradoxical reduction in glucose tolerance and impaired insulin release despite an unchanged amount of muscle glycogen. In light of these results, an upper limit of physical exercise exist where the negative effects overpowers the positive.

The aim of this study was to examine the correlation between muscle fiber type and sprint performance in elite young soccer players of different age groups of the same team. Twenty-eight young players participated in this study (group U15, n = 8; group U13, n = 9; and group U11, n = 11). Anthropometric assessments, acceleration (10 m), and Bangsbo modified sprint test (30 m) were performed. Muscle biopsies were obtained from the vastus lateralis, and after that, fiber-type composition was determined by immunohistochemistry. No significant correlations were found between the sprint test and muscle fiber distribution for the groups U13 and U11 (p > 0.05). Also, no correlations were found between cross-sectional areas in the types of fibers with the sprint test in all groups (p > 0.05). A positive correlation was found between type I fibers and the performance in the acceleration test (10 m) (r = 0.77, p < 0.05) was found only in group U15 and a negative correlation between type IIA fibers and the performance in the acceleration test (10 m) (r = -0.89, p < 0.05). The correlations were observed only in group U15, which may indicate that the duration and the intensity of the soccer systematic training can affect the plasticity of the muscle fibers. Specific soccer training in youth is one of the factors that can affect fiber-type plasticity. The specific training programs and status of U15 are more intensive, and the exercises are oriented more to improve physical fitness.