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Biomedical subjects

Michael Kjaer

Publications and source records attributed to Michael Kjaer.

At least 37 records · Page 2Linked to original sources
PubMed2006

Muscle and blood metabolites during a soccer game: implications for sprint performance.

PURPOSE: To examine muscle and blood metabolites during soccer match play and relate it to possible changes in sprint performance. METHODS: Thirty-one Danish fourth division players took part in three friendly games. Blood samples were collected frequently during the game, and muscle biopsies were taken before and after the game as well as immediately after an intense period in each half. The players performed five 30-m sprints interspersed by 25-s recovery periods before the game and immediately after each half (N=11) or after an intense exercise period in each half (N=20). RESULTS: Muscle lactate was 15.9+/-1.9 and 16.9+/-2.3 mmol.kg d.w. during the first and second halves, respectively, with blood lactate being 6.0+/-0.4 and 5.0+/-0.4 mM, respectively. Muscle lactate was not correlated with blood lactate (r=0.06-0.25, P>0.05). Muscle glycogen decreased (P<0.05) from 449+/-23 to 255+/-22 mmol.kg d.w. during the game, with 47+/-7% of the muscle fibers being completely or almost empty of glycogen after the game. Blood glucose remained elevated during the game, whereas plasma FFA increased (P<0.05) from 0.45+/-0.05 to 1.37+/-0.23 mM. Mean sprint time was unaltered after the first half, but longer (P<0.05) after the game (2.8+/-0.7%) as well as after intense periods in the first (1.6+/-0.6%) and second halves (3.6+/-0.5%). The decline in sprint performance during the game was not correlated with muscle lactate, muscle pH, or total glycogen content. CONCLUSION: Sprint performance is reduced both temporarily during a game and at the end of a soccer game. The latter finding may be explained by low glycogen levels in individual muscle fibers. Blood lactate is a poor indicator of muscle lactate during soccer match play.

Adenine Nucleotides↗
PubMed2005

No effect of menstrual cycle on myofibrillar and connective tissue protein synthesis in contracting skeletal muscle.

We tested the hypothesis that acute exercise would stimulate synthesis of myofibrillar protein and intramuscular collagen in women and that the phase of the menstrual cycle at which the exercise took place would influence the extent of the change. Fifteen young, healthy female subjects were studied in the follicular (FP, n=8) or the luteal phase (LP, n=7, n=1 out of phase) 24 h after an acute bout of one-legged exercise (60 min of kicking at 67% W(max)), samples being taken from the vastus lateralis in both the exercised and resting legs. Rates of synthesis of myofibrillar and muscle collagen proteins were measured by incorporation of [(13)C]leucine. Myofibrillar protein synthesis (means+/-SD; rest FP: 0.053+/-0.009%/h, LP: 0.055+/-0.013%/h) was increased at 24-h postexercise (FP: 0.131+/-0.018%/h, P<0.05, LP: 0.134+/-0.018%/h, P< 0.05) with no differences between phases. Similarly, muscle collagen synthesis (rest FP: 0.024+/- 0.017%/h, LP: 0.021+/- 0.006%/h) was elevated at 24-h postexercise (FP: 0.073+/- 0.016%/h, P<0.05, LP: 0.072+/- 0.015%/h, P<0.05), but the responses did not differ between menstrual phases. Therefore, there is no effect of menstrual cycle phase, at rest or in response to an acute bout of exercise, on myofibrillar protein synthesis and muscle collagen synthesis in women.

Adult↗
PubMed2005

The behaviour of satellite cells in response to exercise: what have we learned from human studies?

Understanding the complex role played by satellite cells in the adaptive response to exercise in human skeletal muscle has just begun. The development of reliable markers for the identification of satellite cell status (quiescence/activation/proliferation) is an important step towards the understanding of satellite cell behaviour in exercised human muscles. It is hypothesised currently that exercise in humans can induce (1) the activation of satellite cells without proliferation, (2) proliferation and withdrawal from differentiation, (3) proliferation and differentiation to provide myonuclei and (4) proliferation and differentiation to generate new muscle fibres or to repair segmental fibre injuries. In humans, the satellite cell pool can increase as early as 4 days following a single bout of exercise and is maintained at higher level following several weeks of training. Cessation of training is associated with a gradual reduction of the previously enhanced satellite cell pool. In the elderly, training counteracts the normal decline in satellite cell number seen with ageing. When the transcriptional activity of existing myonuclei reaches its maximum, daughter cells generated by satellite cell proliferation are involved in protein synthesis by enhancing the number of nuclear domains. Clearly, delineating the events and the mechanisms behind the activation of satellite cells both under physiological and pathological conditions in human skeletal muscles remains an important challenge.

Biomarkers↗
PubMed2005

Effects of 2 wk of GH administration on 24-h indirect calorimetry in young, healthy, lean men.

The present study was designed as a randomized, double-blind placebo (Plc)-controlled study to determine the effect of 2 wk of growth hormone administration (GH-adm.) on energy expenditure (EE) and substrate oxidation in healthy humans. Sixteen young healthy men were divided into two groups. The study consisted of two 24-h measurements (indirect calorimetry), separated by 2 wk of either Plc or GH injections (6 IU/day). At baseline, no significant differences were observed between the two groups in any of the measured anthropometric, hormonal, or metabolic parameters, neither did the parameters change over time in the Plc group. GH-adm. resulted in a 4.4% increase in 24-h EE (P < 0.05) and an increase in fat oxidation by 29% (P < 0.05). However, a decrease in the respiratory quotient was only observed in the postabsorptive phase after an overnight fast (0.84 +/- 0.1 to 0.79 +/- 0.1, P < 0.05). Furthermore, lean body mass (LBM) was increased by GH-adm. only [62.8 +/- 2.5 kg (baseline) vs. 64.7 +/- 2.4 kg (after), P < 0.001]. In conclusion, GH-adm. increases 24-h EE, which may be partly explained by increased LBM. Furthermore, GH-adm. stimulates fat combustion, especially in the postabsorptive state.

Adult↗
PubMed2005

No effect of growth hormone administration on substrate oxidation during exercise in young, lean men.

The purpose of this study was to examine the effects of increased fat availability induced by growth hormone (GH) administration on the oxidative metabolism during exercise. Seven well-trained males (age 25 +/- 2 years (mean +/- S.E.M.); peak oxygen consumption : 62 +/- 1 ml min(-1) kg(-1) (completed four randomised trials: 120 min bicycling at 55% 4 h after receiving either 7.5 IU (2.5 mg) GH or placebo (Plc), and during rest after receiving either GH or Plc. In all studies a standardized meal was given 2 h after GH or Plc injection. GH administration resulted in an approximately 60-fold increase in serum GH concentration at rest (P < 0.0001) and during exercise (P < 0.0001). The increase in serum GH was followed by an increase in circulating glycerol at rest (8%, P < 0.0001). When combined with exercise the increase in plasma glycerol was more pronounced (GH: 716% of baseline versus Plc: 328%, P < 0.0001). However, this increase in fat mobilization did not increase fat oxidation during exercise (indirect calorimetry). In conclusion, GH administration combined with aerobic exercise increased lipolytic parameters substantially more than exercise alone, but did not further augment whole body fat oxidation.

Adipose Tissue↗
PubMed2005

Coordinated collagen and muscle protein synthesis in human patella tendon and quadriceps muscle after exercise.

We hypothesized that an acute bout of strenuous, non-damaging exercise would increase rates of protein synthesis of collagen in tendon and skeletal muscle but these would be less than those of muscle myofibrillar and sarcoplasmic proteins. Two groups (n = 8 and 6) of healthy young men were studied over 72 h after 1 h of one-legged kicking exercise at 67% of maximum workload (W(max)). To label tissue proteins in muscle and tendon primed, constant infusions of [1-(13)C]leucine or [1-(13)C]valine and flooding doses of [(15)N] or [(13)C]proline were given intravenously, with estimation of labelling in target proteins by gas chromatography-mass spectrometry. Patellar tendon and quadriceps biopsies were taken in exercised and rested legs at 6, 24, 42 or 48 and 72 h after exercise. The fractional synthetic rates of all proteins were elevated at 6 h and rose rapidly to peak at 24 h post exercise (tendon collagen (0.077% h(-1)), muscle collagen (0.054% h(-1)), myofibrillar protein (0.121% h(-1)), and sarcoplasmic protein (0.134% h(-1))). The rates decreased toward basal values by 72 h although rates of tendon collagen and myofibrillar protein synthesis remained elevated. There was no tissue damage of muscle visible on histological evaluation. Neither tissue microdialysate nor serum concentrations of IGF-I and IGF binding proteins (IGFBP-3 and IGFBP-4) or procollagen type I N-terminal propeptide changed from resting values. Thus, there is a rapid increase in collagen synthesis after strenuous exercise in human tendon and muscle. The similar time course of changes of protein synthetic rates in different cell types supports the idea of coordinated musculotendinous adaptation.

Adult↗
PubMed2005

The effect of dynamic knee-extension exercise on patellar tendon and quadriceps femoris muscle glucose uptake in humans studied by positron emission tomography.

Both tendon and peritendinous tissue show evidence of metabolic activity, but the effect of acute exercise on substrate turnover is unknown. We therefore examined the influence of acute exercise on glucose uptake in the patellar and quadriceps tendons during dynamic exercise in humans. Glucose uptake was measured in five healthy men in the patellar and quadriceps tendons and the quadriceps femoris muscle at rest and during dynamic knee-extension exercise (25 W) using positron emission tomography and [18F]-2-fluoro-2-deoxy-D-glucose ([18F]FDG). Glucose uptake index was calculated by dividing the tissue activity with blood activity of [18F]FDG. Exercise increased glucose uptake index by 77% in the patellar tendon (from 0.30 +/- 0.09 to 0.51 +/- 0.16, P = 0.03), by 106% in the quadriceps tendon (from 0.37 +/- 0.15 to 0.75 +/- 0.36, P = 0.02), and by 15-fold in the quadriceps femoris muscle (from 0.31 +/- 0.11 to 4.5 +/- 1.7, P = 0.005). The exercise-induced increase in the glucose uptake in neither tendon correlated with the increase in glucose uptake in the quadriceps muscle (r = -0.10, P = 0.87 for the patellar tendon and r = -0.30, P = 0.62 for the quadriceps tendon). These results show that tendon glucose uptake is increased during exercise. However, the increase in tendon glucose uptake is less pronounced than in muscle and the increases are uncorrelated. Thus tendon glucose uptake is likely to be regulated by mechanisms independently of those regulating skeletal muscle glucose uptake.

Adult↗
PubMed2005

Muscle performance during maximal isometric and dynamic contractions is influenced by the stiffness of the tendinous structures.

Contractile force is transmitted to the skeleton through tendons and aponeuroses, and, although it is appreciated that the mechanocharacteristics of these tissues play an important role for movement performance with respect to energy storage, the association between tendon mechanical properties and the contractile muscle output during high-force movement tasks remains elusive. The purpose of the study was to investigate the relation between the mechanical properties of the connective tissue and muscle performance in maximal isometric and dynamic muscle actions. Sixteen trained men participated in the study. The mechanical properties of the vastus lateralis tendon-aponeurosis complex were assessed by ultrasonography. Maximal isometric knee extensor force and rate of torque development (RTD) were determined. Dynamic performance was assessed by maximal squat jumps and countermovement jumps on a force plate. From the vertical ground reaction force, maximal jump height, jump power, and force-/velocity-related determinants of jump performance were obtained. RTD was positively related to the stiffness of the tendinous structures (r = 0.55, P < 0.05), indicating that tendon mechanical properties may account for up to 30% of the variance in RTD. A correlation was observed between stiffness and maximal jump height in squat jumps and countermovement jumps (r = 0.64, P < 0.05 and r = 0.55, P < 0.05). Power, force, and velocity parameters obtained during the jumps were significantly correlated to tendon stiffness. These data indicate that muscle output in high-force isometric and dynamic muscle actions is positively related to the stiffness of the tendinous structures, possibly by means of a more effective force transmission from the contractile elements to the bone.

Adult↗
PubMed2005

Gene expression of myogenic factors and phenotype-specific markers in electrically stimulated muscle of paraplegics.

The transcription factors myogenin and MyoD have been suggested to be involved in maintaining slow and fast muscle-fiber phenotypes, respectively, in rodents. Whether this is also the case in human muscle is unknown. To test this, 4 wk of chronic, low-frequency electrical stimulation training of the tibialis anterior muscle of paraplegic subjects were used to evoke a fast-to-slow transformation in muscle phenotype. It was hypothesized that this would result from an upregulation of myogenin and a downregulation of MyoD. The training evoked the expected mRNA increase for slow fiber-specific markers myosin heavy chain I and 3-hydroxyacyl-CoA dehydrogenase A, whereas an mRNA decrease was seen for fast fiber-specific markers myosin heavy chain IIx and glycerol phosphate dehydrogenase. Although the slow fiber-specific markers citrate synthase and muscle fatty acid binding protein did not display a significant increase in mRNA, they did tend to increase. As hypothesized, myogenin mRNA was upregulated. However, contrary to the hypothesis, MyoD mRNA also increased, although later than myogenin. The mRNA levels of the other myogenic regulatory factor family members, myogenic factor 5 and myogenic regulatory factor 4, and the myocyte enhancer factor (MEF) family members, MEF-2A and MEF-2C, did not change. The results indicate that myogenin is indeed involved in the regulation of the slow oxidative phenotype in human skeletal muscle fibers, whereas MyoD appears to have a more complex regulatory function.

Adaptation, Physiological↗
PubMed2005

The effect of resistance training combined with timed ingestion of protein on muscle fiber size and muscle strength.

Acute muscle protein metabolism is modulated not only by resistance exercise but also by amino acids. However, less is known about the long-term hypertrophic effect of protein supplementation in combination with resistance training. The present study was designed to compare the effect of 14 weeks of resistance training combined with timed ingestion of isoenergetic protein vs carbohydrate supplementation on muscle fiber hypertrophy and mechanical muscle performance. Supplementation was administered before and immediately after each training bout and, in addition, in the morning on nontraining days. Muscle biopsy specimens were obtained from the vastus lateralis muscle and analyzed for muscle fiber cross-sectional area. Squat jump and countermovement jump were performed on a force platform to determine vertical jump height. Peak torque during slow (30 degrees s-1) and fast (240 degrees s-1) concentric and eccentric contractions of the knee extensor muscle was measured in an isokinetic dynamometer. After 14 weeks of resistance training, the protein group showed hypertrophy of type I (18% +/- 5%; P < .01) and type II (26% +/- 5%; P < .01) muscle fibers, whereas no change above baseline occurred in the carbohydrate group. Squat jump height increased only in the protein group, whereas countermovement jump height and peak torque during slow isokinetic muscle contraction increased similarly in both groups. In conclusion, a minor advantage of protein supplementation over carbohydrate supplementation during resistance training on mechanical muscle function was found. However, the present results may have relevance for individuals who are particularly interested in gaining muscle size.

Adult↗
PubMed2005

Postexercise nutrient intake enhances leg protein balance in early postmenopausal women.

BACKGROUND: We investigated the effect of nutrient administration after a session of resistance exercise on muscle protein kinetics in six healthy, early postmenopausal women, in a crossover design of random and double-blinded administration of protein and carbohydrate (PC) or placebo (NON). METHODS: Fasted participants received a primed-constant infusion of L-[ring-2H5]-phenylalanine. After 90 minutes of rest, the participants performed leg-resistance exercises followed by the oral supplementation. During the following 4 hours, net protein balance (NB) and rate of disappearance and appearance of phenylalanine were calculated from arterial-venous blood samples and blood flow measurements. RESULTS: NB was elevated (p <.001) in the PC group compared to the NON group, and NB was not different from zero in the PC group, whereas it was negative in the NON group. Net balance results were supported by kinetic data from a reduced number of participants, showing that rate of disappearance was responsible for the initial (<1 hour) effect of PC, whereas a reduced rate of appearance enhanced the NB from 1.5 to 3 hours after training in the PC group. CONCLUSION: In early postmenopausal women, nutrient ingestion following resistance exercise improved anabolism by enhancing NB in skeletal muscle.

Energy Metabolism↗
PubMed2005

Glucoregulation during exercise : the role of the neuroendocrine system.

Under normal healthy conditions, exercise initiates simultaneous elevations in hepatic glucose production (glucose R(a)) and glucose utilisation. As a result, circulating glucose levels are maintained at a relatively constant level. This relatively simple and effective relationship between the liver and the skeletal muscle is maintained by a complex interplay of circulating and locally released neuroendocrine controllers. In large part, exercise-induced changes in the pancreatic secretion of glucagon and insulin are primarily responsible for the stimulation of glucose R(a) during moderate exercise. However, exercise imposed on an additional metabolic stress (heavy exercise and poorly controlled diabetes mellitus) can increase sympathetic drive and has been suggested for decades to play a significant role in glucoregulation. In addition, blood-borne feedback and afferent reflex mechanisms may further modulate the glucose R(a) response to exercise. This article discusses new findings from novel animal and human experiments specifically designed to examine the regulatory components of the neuroendocrine system and their influence on glucoregulation during exercise.

Animals↗
PubMed2004

Increase in interstitial interleukin-6 of human skeletal muscle with repetitive low-force exercise.

Interleukin (IL)-6, which is released from muscle tissue during intense exercise, possesses important metabolic and probably anti-inflammatory properties. To evaluate the IL-6 response to low-intensity exercise, we conducted two studies: 1) a control study with insertion of microdialysis catheters in muscle and determination of interstitial muscle IL-6 response over 2 h of rest and 2) an exercise study to investigate the IL-6 response to 20 min of repetitive low-force exercise. In both studies, a microdialysis catheter (cutoff: 3,000 kDa) was inserted into the upper trapezius muscle of six male subjects, and the catheters were perfused with Ringer-acetate at 5 microl/min. Venous plasma samples were taken in the exercise study. The insertion of microdialysis catheters into muscle resulted in an increase in IL-6 from 8 +/- 0 to 359 +/- 171 and 484 +/- 202 pg/ml after 65 and 110 min, respectively (P < 0.001). Similarly, in the exercise study, IL-6 increased to 289 +/- 128 pg/ml after a 55-min rest (P < 0.001). During the subsequent repetitive low-force exercise, muscle IL-6 further increased to 1,246 +/- 461 pg/ml and reached 2,132 +/- 477 pg/ml after a 30-min recovery (all P < 0.001). In contrast to this, plasma IL-6 did not significantly change in response to exercise. We conclude that upper extremity, low-intensity exercise results in a substantial increase in IL-6 in the interstitium of the stabilizing trapezius muscle, whereas no change is seen for plasma IL-6.

Adult↗
PubMed2004

Training-induced changes in muscle CSA, muscle strength, EMG, and rate of force development in elderly subjects after long-term unilateral disuse.

The ability to develop muscle force rapidly may be a very important factor to prevent a fall and to perform other tasks of daily life. However, information is still lacking on the range of training-induced neuromuscular adaptations in elderly humans recovering from a period of disuse. Therefore, the present study examined the effect of three types of training regimes after unilateral prolonged disuse and subsequent hip-replacement surgery on maximal muscle strength, rapid muscle force [rate of force development (RFD)], muscle activation, and muscle size. Thirty-six subjects (60-86 yr) were randomized to a 12-wk rehabilitation program consisting of either 1) strength training (3 times/wk for 12 wk), 2) electrical muscle stimulation (1 h/day for 12 wk), or 3) standard rehabilitation (1 h/day for 12 wk). The nonoperated side did not receive any intervention and thereby served as a within-subject control. Thirty subjects completed the trial. In the strength-training group, significant increases were observed in maximal isometric muscle strength (24%, P < 0.01), contractile RFD (26-45%, P < 0.05), and contractile impulse (27-32%, P < 0.05). No significant changes were seen in the two other training groups or in the nontrained legs of all three groups. Mean electromyogram signal amplitude of vastus lateralis was larger in the strength-training than in the standard-rehabilitation group at 5 and 12 wk (P < 0.05). In contrast to traditional physiotherapy and electrical stimulation, strength training increased muscle mass, maximal isometric strength, RFD, and muscle activation in elderly men and women recovering from long-term muscle disuse and subsequent hip surgery. The improvement in both muscle mass and neural function is likely to have important functional implications for elderly individuals.

Aged↗
PubMed2004

Differential displacement of the human soleus and medial gastrocnemius aponeuroses during isometric plantar flexor contractions in vivo.

The human triceps surae muscle-tendon complex is a unique structure with three separate muscle compartments that merge via their aponeuroses into the Achilles tendon. The mechanical function and properties of these structures during muscular contraction are not well understood. The purpose of the study was to investigate the extent to which differential displacement occurs between the aponeuroses of the medial gastrocnemius (MG) and soleus (Sol) muscles during plantar flexion. Eight subjects (mean +/- SD; age 30 +/- 7 yr, body mass 76.8 +/- 5.5 kg, height 1.83 +/- 0.06 m) performed maximal isometric ramp contractions with the plantar flexor muscles. The experiment was performed in two positions: position 1, in which the knee joint was maximally extended, and position 2, in which the knee joint was maximally flexed (125 degrees ). Plantarflexion moment was assessed with a strain gauge load cell, and the corresponding displacement of the MG and Sol aponeuroses was measured by ultrasonography. Differential shear displacement of the aponeurosis was quantified by subtracting displacement of Sol from that of MG. Maximal plantar flexion moment was 36% greater in position 1 than in position 2 (132 +/- 20 vs. 97 +/- 11 N.m). In position 1, the displacement of the MG aponeurosis at maximal force exceeded that of the Sol (12.6 +/- 1.7 vs. 8.9 +/- 1.5 mm), whereas in position 2 displacement of the Sol was greater than displacement of the MG (9.6 +/- 1.0 vs. 7.9 +/- 1.2 mm). The amount and "direction" of shear between the aponeuroses differed significantly between the two positions across the entire range of contraction, indicating that the Achilles tendon may be exposed to intratendinous shear and stress gradients during human locomotion.

Adult↗
PubMed2004

Changes in satellite cells in human skeletal muscle after a single bout of high intensity exercise.

No studies to date have reported activation of satellite cells in vivo in human muscle after a single bout of high intensity exercise. In this investigation, eight individuals performed a single bout of high intensity exercise with one leg, the contralateral leg being the control. A significant increase in mononuclear cells staining for the neural cell adhesion molecule (N-CAM) and fetal antigen 1 (FA1) were observed within the exercised human vastus lateralis muscle on days 4 and 8 post exercise. In addition, a significant increase in the concentration of the FA1 protein was determined in intramuscular dialysate samples taken from the vastus lateralis muscle of the exercising leg (day 0: 1.89 +/- 0.82 ng ml(-1); day 2: 1.68 +/- 0.37 ng ml(-1); day 4: 3.26 +/- 1.29 ng ml(-1), P < 0.05 versus basal; day 8: 4.68 +/- 2.06 ng ml(-1), P < 0.05 versus basal and control). No change was noted in the control leg. Despite this increase in N-CAM- and FA1-positive mononuclear cells, an increased expression of myogenin and the neonatal isoform of the myosin heavy chain (MHCn) was not observed. Interestingly, myofibre lesions resulting from extensive damage to the proteins within the myofibre, particularly desmin or dystrophin, were not observed, and hence did not appear to induce the expression of either N-CAM or FA1. We therefore propose that satellite cells can be induced to re-enter the cell growth cycle after a single bout of unaccustomed high intensity exercise. However, a single bout of exercise is not sufficient for the satellite cell to undergo terminal differentiation.

Adult↗