Effect of training intensity on insulin sensitivity as evaluated by insulin tolerance test

The purpose of this study was to evaluate the role of exercise intensity in the effect of physical training on insulin sensitivity. The insulin tolerance test (ITT) was applied to quantify insulin sensitivity. Eighteen healthy, young, untrained men and women participated in a 4-week, five times per week, 1-h per session bicycle-ergometer training program. Training consisted of 3-min bouts of cycling interspersed with 2 min at a lower exercise intensity. Intensities were 80 and 40% of pretraining maximal power output (W(max)) in the high-intensity (HI) and 40 and 20% W(max) in the low-intensity (LI) group. The insulin sensitivity index (IS(index)) was similar in the HI and LI group before the training intervention [mean (SD) -0.1898 (0.058) and -0.1892 (0.045), respectively]. After training, the IS(index) was -0.2358 (0.051) (P = 0.005 vs pretraining) in the HI group and -0.2050 (0.035) (P = 0. 099 against pretraining) in the LI group. We conclude that improvements in insulin sensitivity are more pronounced with high-intensity training, when exercise frequency and duration are kept similar. We further conclude that the ITT is suitable for use in intervention studies.     

Evaluation ofin vivo insulin action and glucose metabolism in milk-fed rats

Milk diet has long been recommended in the management of gastrointestinal pathologies. Since milk feeding represents a high fat-low carbohydrate diet and it is acknowledged that insulin resistance is one of the consequences of high fat feeding, it is important to know whether or not chronic milk feeding leads to an impairment of the insulin-mediated glucose metabolism. To examine this question, adult female rats were given raw cow's milk (50% of total calories as lipids) for 18 days. They were compared to rats raised in parallel and fed the standard laboratory diet (15% of total calories as lipids). At the end of the 18 day period, body weight, daily caloric intake, basal plasma glucose and insulin levels in the milk-fed rats were similar to those in the control rats.In vivo insulin action was assessed with the euglycemichyperinsulinemic clamp technique in anesthetized animals. These studies were coupled with the 2-deoxyglucose technique allowing a measurement of glucose utilization by individual tissues. In the milk fed rats: 1) the basal rate of endogenous glucose production was significantly (p<0.01) reduced (by 20%); 2) their hepatic glucose production was however normally suppressed by hyperinsulinemia; 3) their basal glucose utilization rate was significantly (p<0.01) reduced (by 20%); 4) their glucose utilization rate by the whole-body mass or by individual tissues was normally increased by hyperinsulinemia. These results indicate that insulin action in adult rats is not grossly altered after chronic milk-feeding, at least under the present experimental conditions.     

Benefits of caffeine ingestion on sprint performance in trained and untrained swimmers

The influence of specific training on benefits from caffeine (Caf) ingestion was examined during a sprint test in a group of highly trained swimmers (T) and compared with the response of a group of untrained occasional swimmers (UT). Seven T and seven UT subjects swam freestyle two randomly assigned 2 x 100 m distances, at maximal speed and separated by 20 min of passive recovery, once after Caf (250 mg) and once after placebo (Pla) ingestion. Anaerobic capacity was assessed by the mean velocity (meters per second) during each 100 m and blood was sampled from the fingertip just before and 1, 3, 5, 7, and 9 min after each 100 m for resting and maximal blood lactate concentration ([la-]b,max) determination. The [la-]bmax was significantly enhanced by Caf in both T and UT subjects (P less than 0.01). However, only T subjects exhibited significant improvement in their swimming velocity (P less than 0.01) after Caf or any significant impairment during the second 100 m. In light of these results, it appears that specific training is necessary to benefit from the metabolic adaptations induced by Caf during supramaximal exercise requiring a high anaerobic capacity.     

Swimming training induces liver adaptations to oxidative stress and insulin sensitivity in rats submitted to high-fat diet

Oxidative stress, physical inactivity and high-fat (FAT) diets are associated with hepatic disorders such as metabolic syndrome (MS). The therapeutic effects of physical training (PT) were evaluated in rats with MS induced by FAT diet for 13 weeks, on oxidative stress and insulin signaling in the liver, during the last 6 weeks. FAT-sedentary (SED) rats increased body mass, retroperitoneal fat, mean arterial pressure (MAP) and heart rate (HR), and total cholesterol, serum alanine aminotransferase, glucose and insulin. Livers of FAT-SED rats increased superoxide dismutase activity, thiobarbituric acid-reactive substances, protein carbonyl and oxidized glutathione (GSSG); and decreased catalase activity, reduced glutathione/GSSG ratio, and the mRNA expression of insulin receptor substrate 1 (IRS-1) and serine/threonine kinase 2. FAT-PT rats improved in fitness and reduced their body mass, retroperitoneal fat, and glucose, insulin, total cholesterol, MAP and HR; and their livers increased superoxide dismutase and catalase activities, the reduced glutathione/GSSG ratio and the expression of peroxisome proliferator-activated receptor gamma and insulin receptor compared to FAT-SED rats. These findings indicated adaptive responses to PT by restoring the oxidative balance and insulin signaling in the liver and certain biometric and biochemical parameters as well as MAP in MS rats.     

Effects of analogues of adenosine and methyl xanthines on insulin sensitivity in soleus muscle of the rat

The concentration of insulin that produces half-maximal stimulation of glycolysis by stripped soleus muscle preparations is markedly increased by the adenosine analogues, 2-chloroadenosine and N6-phenylisopropyladenosine, but is markedly decreased by the methyl xanthine analogue, 8-phenyltheophylline. 2-Chloroadenosine increases the concentration of insulin required to stimulate glycolysis half maximally, from about 100 to 2000 mu units/ml. 8-Phenyltheophylline decreases this concentration of insulin from about 100 to 10 mu units/ml, an effect which is similar to that produced either by addition of adenosine deaminase to the medium or to exercise-training of the donor animals for 4 weeks.     

Effect of muscle mass on lactate formation during exercise in humans

To elucidate the mechanisms of lactate formation during submaximal exercise, eight men were studied during one- (1-LE) and two-leg (2-LE) exercise (approximately 11-min cycling) using the catheterization technique and muscle biopsies (quadriceps femoris muscle). The absolute exercise intensity and thus the energy demand for the exercising limb was the same [mean 114 (SEM 7) W] during both 1-LE and 2-LE. At the end of exercise partial pressure of O₂ and O₂ saturation in femoral venous blood were lower and arterial adrenaline and noradrenaline were higher during 2-LE than during 1-LE. Mean arterial blood lactate concentration increased to 10.8 (SEM 0.8) (2-LE) and 5.2 (SEM 0.4) mmol · 1^–1 (1-LE) after 10 min of exercise. The intramuscular metabolic response to exercise was attenuated during 1-LE [mean, lactate = 49 (SEM 9); glucose 6-P = 3.3 (SEM 0.3); nicotinamide adenine dinucleotide, reduced = 0.17 (SEM 0.02); adenosine 5-diphosphate 2.7 (SEM 0.1) mmol · kg dry mass^–1] compared to 2-LE [76 (SEM 6); 6.1 (SEM 0.7); 0.21 (SEM 0.02); 3.0 (SEM 0.1) mmol · kg dry mass^–1, respectively]. To elucidate whether the lower plasma adrenaline concentration could contribute to the attenuated metabolic response, additional experiments were performed on four of the eight subjects with infusion of adrenaline during 1-LE (1-LEE). Average plasma adrenaline concentration was increased during 1-LEE and reached 2–4 times higher levels than during 2-LE. Post-exercise muscle lactate and glucose 6-P contents were higher during 1-LEE than during 1-LE and were similar to those during 2-LE. Also, leg lactate release was elevated during 1-LEE versus 1-LE. It was concluded that during submaximal dynamic exercise the intramuscular metabolic response not only depended on the muscle power output, but also on the total muscle mass engaged. Plasma adrenaline concentrations and muscle oxygenation were found to be dependent upon the working muscle mass and both may have affected the metabolic response during exercise.     

Effects of creatine supplementation on glucose tolerance and insulin sensitivity in sedentary healthy males undergoing aerobic training

Recent findings have indicated that creatine supplementation may affect glucose metabolism. This study aimed to examine the effects of creatine supplementation, combined with aerobic training, on glucose tolerance in sedentary healthy male. Subjects (n = 22) were randomly divided in two groups and were allocated to receive treatment with either creatine (CT) ( approximately 10 g . day over three months) or placebo (PT) (dextrose). Administration of treatments was double blind. Both groups underwent moderate aerobic training. An oral glucose tolerance test (OGTT) was performed and both fasting plasma insulin and the homeostasis model assessment (HOMA) index were assessed at the start, and after four, eight and twelve weeks. CT demonstrated significant decrease in OGTT area under the curve compared to PT (P = 0.034). There were no differences between groups or over time in fasting insulin or HOMA. The results suggest that creatine supplementation, combined with aerobic training, can improve glucose tolerance but does not affect insulin sensitivity, and may warrant further investigation with diabetic subjects.     

Effect of exercise training on total daily physical activity in elderly humans

This study examined the effect of 12 weeks of exercise training on daily physical activity in elderly humans. Training consisted of a weekly group session and an individual session with cardio- and weight-stack machines. A group of 15 subjects served as the exercise group [EXER mean age 59 (SD 4) years], and 7 subjects as the controls [CONT mean age 57 (SD 3) years]. Physical activity and physical fitness were measured before the start of training (T), at week 6 and week 12 (T0, T6, T12 respectively) in EXER, and at T0 and T12 in CONT. Physical activity over 14 days was measured using a tri-axial accelerometer and physical fitness was measured during an incremental exercise test. At T12, mean maximal power output had significantly increased in EXER compared to CONT 8 (SD 12) vs -5 (SD 9) W; P < 0.02] and mean submaximal heart rate (at 100 W) had reduced [-10 (SD 7) vs -2 (SD 6) beats x min(-1); P < 0.05]. No differences or changes in physical activity were observed between EXER and CONT. At T6, physical activity on training days was significantly higher than on non-training days (P < 0.001). When the accelerometer output of the training session was subtracted from the accelerometer output on training days, at T12 non-training physical activity was significantly lower than on non-training days (P < 0.004). Accelerometer output of the individual training session at T12 had significantly increased compared to T6 (P < 0.05), whereas, accelerometer output of the group training session had remained unchanged. In conclusion, in elderly subjects an exercise training programme of moderate intensity resulted in an improved physical fitness but had no effect on total daily physical activity. Training activity was compensated for by a decrease in non-training physical activity.     

Effect of carbohydrate ingestion on glucose kinetics and muscle metabolism during intense endurance exercise.

There has been recent interest in the potential performance and metabolic effects of carbohydrate ingestion during exercise lasting approximately 1 h. In this study, 13 well-trained men ingested in randomized order either a 6% glucose solution (CHO trial) or a placebo (Con trial) during exercise to exhaustion at 83+/-1% peak oxygen uptake. In six subjects, vastus lateralis muscle was sampled at rest, at 32 min, and at exhaustion, and in six subjects, glucose kinetics was determined by infusion of [6,6-(2)H]glucose in both trials and ingestion of [6-(3)H]glucose in the CHO trial. Of the 84 g of glucose ingested during exercise in the CHO trial, only 22 g appeared in the peripheral circulation. This resulted in a small (12 g) but significant (P<0.05) increase in glucose uptake without influencing carbohydrate oxidation, muscle glycogen use, or time to exhaustion (CHO: 68.1+/-4.1 min; Con: 69.6+/-5.5 min). Decreases in muscle phosphocreatine content and increases in muscle inosine monophosphate and lactate content during exercise were similar in the two trials. Although endogenous glucose production during exercise was partially suppressed in the CHO trial, it remained significantly above preexercise levels throughout exercise. In conclusion, only 26% of the ingested glucose appeared in the peripheral circulation. Glucose ingestion increased glucose uptake and partially reduced endogenous glucose production but had no effect on carbohydrate oxidation, muscle metabolism, or time to exhaustion during exercise at 83% peak oxygen uptake.     

Effect of antioxidant supplementation on insulin sensitivity in response to endurance exercise training

While production of reactive oxygen and nitrogen species (RONS) is associated with some of the beneficial adaptations to regular physical exercise, it is not established whether RONS play a role in the improved insulin-stimulated glucose uptake in skeletal muscle obtained by endurance training. To assess the effect of antioxidant supplementation during endurance training on insulin-stimulated glucose uptake, 21 young healthy (age 29 ± 1 y, BMI 25 ± 3 kg/m(2)) men were randomly assigned to either an antioxidant [AO; 500 mg vitamin C and 400 IU vitamin E (α-tocopherol) daily] or a placebo (PL) group that both underwent a supervised intense endurance-training program 5 times/wk for 12 wk. A 3-h euglycemic-hyperinsulinemic clamp, a maximal oxygen consumption (Vo(2max)) and maximal power output (P(max)) test, and body composition measurements (fat mass, fat-free mass) were performed before and after the training. Muscle biopsies were obtained for determination of the concentration and activity of proteins regulating glucose metabolism. Although plasma levels of vitamin C (P < 0.05) and α-tocopherol (P < 0.05) increased markedly in the AO group, insulin-stimulated glucose uptake increased similarly in both the AO (17.2%, P < 0.05) and the PL (18.9%, P < 0.05) group in response to training. Vo(2max) and P(max) also increased similarly in both groups (time effect, P < 0.0001 for both) as well as protein content of GLUT4, hexokinase II, and total Akt (time effect, P ≤ 0.05 for all). Our results indicate that administration of antioxidants during strenuous endurance training has no effect on the training-induced increase in insulin sensitivity in healthy individuals.     

Effect of exercise training on insulin binding and glucose metabolism in mouse soleus muscle

Training stimulates glucose uptake and metabolism by muscles independent of a rise in serum glucose. Whether this increased insulin action is associated with enhanced insulin binding in muscles is unknown. We studied the effect of 6 weeks of treadmill running on insulin binding, uptake of 2-deoxy-D-glucose, glycolysis, and glycogenesis by the soleus muscle of Swiss Webster mice. Training was progressively increased. The in vitro studies using intact soleus preparations were done 48 h after the last exercise bout. Training increased insulin binding, insulin-stimulated uptake of 2-deoxy-D-glucose, and glycogenesis but not glycolysis in the soleus. Our data suggest that the enhanced glucose uptake and metabolism in muscles induced by exercise training are associated with an increase in insulin binding.     

Effects of exercise on insulin binding and glucose metabolism in muscle

To elucidate the mechanism of enhanced insulin sensitivity by muscle after exercise, we studied insulin binding, 2-deoxy-D-[1-14C]glucose (2-DOG) uptake and [5-3H]glucose utilization in glycolysis and glycogenesis in soleus and extensor digitorum longus (EDL) muscles of mice after 60 min of treadmill exercise. In the soleus, glycogenesis was increased after exercise (P less than 0.05) and remained sensitive to the action of insulin. Postexercise insulin-stimulated glycolysis was also increased in the soleus (P less than 0.05). In the EDL, glycogenesis was increased after exercise (P less than 0.05). However, this was already maximal in the absence of insulin and was not further stimulated by insulin (0.1-4 nM). The disposal of glucose occurred primarily via the glycolytic pathway (greater than 60%) in the soleus and EDL at rest and after exercise. The uptake of 2-DOG uptake was not altered in the soleus after exercise (4 h incubation at 18 degrees C). However, with 1-h incubations at 37 degrees C, a marked increase in 2-DOG uptake after exercise was observed in the soleus (P less than 0.05) in the absence (0 nM) and presence of insulin (0.2-4 nM) (P less than 0.05). A similar postexercise increase in 2-DOG uptake occurred in EDL. Despite the marked increase in glucose uptake and metabolism, no changes in insulin binding were apparent in either EDL or soleus at 37 degrees C or 18 degrees C. This study shows that the postexercise increase of glucose disposal does not appear to be directly attributable to increments in insulin binding to slow-twitch and fast-twitch muscles.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of lowered muscle temperature on the physiological response to exercise in men

The effect of low muscle temperature on the response to dynamic exercise was studied in six healthy men who performed 42 min of exercise on a cycle ergometer at an intensity of 70% of their maximal O2 uptake. Experiments were performed under control conditions, i.e. from rest at room temperature, and following 45 min standing with legs immersed in a water bath at 12 degrees C. The water bath reduced quadriceps muscle temperature (at 3 cm depth) from 36.4 (SD 0.5) degrees C to 30.5 (SD 1.7) degrees C. Following cooling, exercise heart rate was initially lower, the mean difference ranged from 13 (SD 4) beats.min-1 after 6 min of exercise, to 4 (SD 2) beats.min-1 after 24 min of exercise. Steady-state oxygen uptake was consistently higher (0.2 l.min-1). However, no difference could be discerned in the kinetics of oxygen uptake at the onset of exercise. During exercise after cooling a significantly higher peak value was found for the blood lactate concentration compared to that under control conditions. The peak values were both reached after approximately 9 min of exercise. After 42 min of exercise the blood lactate concentrations did not differ significantly, indicating a faster rate of removal during exercise after cooling. We interpreted these observations as reflecting a relatively higher level of muscle hypoxia at the onset of exercise as a consequence of a cold-induced vasoconstriction. The elevated steady-state oxygen uptake may in part have been accounted for by the energetic costs of removal of the extra lactate released into the blood consequent upon initial tissue hypoxia.     

Effect of prolonged intermittent hypoxia and exercise training on glucose tolerance and muscle GLUT4 protein expression in rats

We compared the chronic effect of intermittent hypoxia and endurance training on the glucose tolerance and GLUT4 protein expression in rat skeletal muscle. Thirty-two Sprague-Dawley rats were matched for weight and assigned to one of the following four groups: control, endurance training, hypoxia, or hypoxia followed by endurance training. Hypoxic treatment consisted of breathing 14% O₂ for 12 h/day under normobaric conditions, and the training protocol consisted of making animals swim 2 times for 3 h/day. At the end of the 3rd week, an oral glucose tolerance test (OGTT) was performed 16 h after treatments. At the end of the 4th week, GLUT4 protein, mRNA, and glycogen storage in skeletal muscle were determined. Endurance training significantly improved OGTT results. Glycogen content and GLUT4 protein expression in the plantaris and red gastrocnemius, but not in the soleus or white gastrocnemius muscles, were also elevated. Chronic intermittent hypoxia also improved OGTT results, but did not alter GLUT4 protein expression. Additionally, hypoxia followed by exercise training produced significant increases in GLUT4 protein and mRNA in a greater number of muscles compared to endurance training alone. Both exercise training and hypoxia significantly reduced body mass, and an additive effect of both treatments was found. In conclusion, chronic intermittent hypoxia improved glucose tolerance in the absence of increased GLUT4 protein expression. This treatment facilitated the exercise training effect on muscle GLUT4 expression and glycogen storage. These new findings open the possibility of utilizing intermittent hypoxia, with or without exercise training, for the prevention and clinical treatment of type 2 diabetes or insulin resistance.     

Effect of insulin and contraction up on glucose transport in skeletal muscle

The major glucose transporter protein expressed in skeletal muscle is GLUT4. Both muscle contraction and insulin induce translocation of GLUT4 from the intracellular pool to the plasma membrane. The intracellular pathways that lead to contraction- and insulin-stimulated GLUT4 translocation seem to be different, allowing the attainment of a maximal effect when acting together. Insulin utilizes a phosphatidylinositol 3-kinase-dependent mechanism, whereas the exercise signal may be initiated by calcium release from the sarcoplasmic reticulum or from autocrine- or paracrine-mediated activation of glucose transport. During exercise skeletal muscle utilizes more glucose than when at rest. However, endurance training leads to decreased glucose utilization during sub-maximal exercise, in spite of a large increase in the total GLUT4 content associated with training. The mechanisms involved in this reduction have not been totally elucidated, but appear to cause the decrease of the amount of GLUT4 translocated to the plasma membrane by altering the exercise-induced enhancement of glucose transport capacity. On the other hand, the effect of resistance training is controversial. Recent studies, however, demonstrated the improvement in insulin sensitivity correlated with increasing muscle mass. New studies should be designed to define the molecular basis for these important adaptations to skeletal muscle. Since during exercise the muscle may utilize insulin-independent mechanisms to increase glucose uptake, the mechanisms involved should provide important knowledge to the understanding and managing peripheral insulin resistance.     

Effect of pre-cooling, with and without thigh cooling, on strain and endurance exercise performance in the heat

Body cooling before exercise (i.e. pre-cooling) reduces physiological strain in humans during endurance exercise in temperate and warm environments, usually improving performance. This study examined the effectiveness of pre-cooling humans by ice-vest and cold (3 degrees C) air, with (LC) and without (LW) leg cooling, in reducing heat strain and improving endurance performance in the heat (35 degrees C, 60% RH). Nine habitually-active males completed three trials, involving pre-cooling (LC and LW) or no pre-cooling (CON: 34 degrees C air) before 35-min cycle exercise: 20 min at approximately 65% VO2peak then a 15-min work-performance trial. At exercise onset, mean core (Tc, from oesophagus and rectum) and skin temperatures, forearm blood flow (FBF), heart rate (HR), and ratings of exertion, body temperature and thermal discomfort were lower in LW and LC than CON (P<0.05). They remained lower at 20 min [e.g. Tc: CON 38.4+/-0.2 (+/-S.E.), LW 37.9+/-0.1, and LC 37.8+/-0.1 degrees C; HR: 177+/-3, 163+/-3 and 167+/-3 b.p.m.), except that FBF was equivalent (P=0.10) between CON (15.5+/-1.6) and LW (13.6+/-1.0 ml.100 ml tissue(-1) x min(-1)). Subsequent power output was higher in LW (2.95+/-0.24) and LC (2.91+/-0.25) than in CON (2.52+/-0.28 W kg(-1), P=0.00, N=8), yet final Tc remained lower. Pre-cooling by ice-vest and cold air effectively reduced physiological and psychophysical strain and improved endurance performance in the heat, irrespective of whether thighs were warmed or cooled.     

The elixir of muscle activity and kinesiology in a health perspective: Evidence of worksite tailored exercise training alleviating muscle disorders

Physical activity is known to benefit health while muscle activation and movements performed during occupational work in contrast may result in work-related musculoskeletal disorders. Therefore, we posed the research question: which mode of muscle activation may result in a reversal of work-related disorders? To address this, we performed electromyographic (EMG) and kinematic assessments of workers with diverse exposure categories: sedentary monotonous work, prolonged walking/standing, and physically heavy work. The various job-specific exposure variables could be categorized in terms of duration, intensity, repetition, static component, peak force etc. that were subsequently identified as risk factors. Based on sports science principles we developed tailored exercise programs to counteract job exposure. EMG activity during exercise training was monitored to identify principal differences between exercise training and job patterns. Evidence from more than 20 RCT studies including >4000 workers showed positive effects such as decreased muscle pain and increased workability. Finally, we identified plausible underlying mechanisms in muscle tissue – human and animal - that confirmed metabolic, morphological, and hormonal changes with e.g. repetitive work that were reversal to adaptations reported with exercise training. Progress has been made in developing intelligent physical exercise training, IPET, as the best complementary activity to job exposure and includes muscle activations and movements that limit work-related inactivity atrophy as well as overload injury.     

Temporal progress of muscle adaptation to endurance training in hind limb muscles of young rats

Summary The soleus, rectus femoris and gastrocnemius muscles of young rats were studied after 3, 6 and 12 weeks of treadmill training. The muscle fibers were characterized histochemically by their succinate dehydrogenase (SDH) and myofibrillar ATPase activity, and morphometrically by their cross-sectional areas, which were corrected for different body weights of control and trained animals.After 12 weeks of training the mean area of fibers in the muscles studied was not significantly different from the controls, as expected. In the soleus muscle the percentage of the fast-twitch fibers was decreased as a result of their transformation into slow-twitch fibers. Trained soleus muscles were the only muscles showing pathologically altered fibers, suggesting overload. The percentages of fiber types and their areas exhibited changes specific for the muscles and muscle regions studied.From these results it is concluded that the adaptation follows the sequence proportional adaptation of morphometrical parameters, disproportional adaptation of the areas of fiber types, and disproportional adaptation of the percentages and/or the areas of the fiber types. It is shown by comparison with the literature that this sequence may be generalized to a sequence of increasing expense necessary for the adaptation to increasing stimuli, and that the most decisive factors for adaptation are work load, frequency of exercise, period of training, and the age of the subject at the initiation of the training.     

Changes in membrane fluidity and lipid peroxidation of skeletal muscle mitochondria after exhausting exercise in rats

We studied the effects of exhausting exercise and exercise training on skeletal muscle mitochondrial membrane fluidity and lipid peroxidation in rats. The first part of the study involved 60 untrained rats divided into six equal groups. Of the total number 10 rats were sedentary and acted as controls. The remaining 50 rats exercised to exhaustion and were sacrificed at 0-h, 24-h, 48-h, 72-h, and 96-h post-exercise. The second part of the study involved 40 rats which were divided into four equal groups. Of these 10 rats were sedentary and acted as controls. The remaining 30 rats underwent 8 weeks of exercise training. They were then subjected to a single period of exhausting exercise and were sacrificed at 0-h, 24-h and 48-h post-exercise. Membrane fluidity was measured using the fluorescence polarization method. Lipid peroxidation was estimated by determining the thiobarbituric acid-reactive substances (TBARS) in mitochondria. In the untrained rats, mitochondrial fluorescence polarization and TBARS contents were significantly increased post-exercise compared with the sedentary controls (P < 0.05). They did not return to near control levels until 96 h and 48 h, respectively. In the trained rats, fluorescence polarization was raised compared with the sedentary controls but this was significantly lower than those measured at the same times of the untrained group post-exercise (P < 0.05). Exhausting exercise decreased membrane fluidity and increased lipid peroxidation in rat skeletal muscle mitochondria. These effects were relieved to some extent by exercise training.