Cellular adaptation to repeated eccentric exercise-induced muscle damage.

Eccentrically biased exercise results in skeletal muscle damage and stimulates adaptations in muscle, whereby indexes of damage are attenuated when the exercise is repeated. We hypothesized that changes in ultrastructural damage, inflammatory cell infiltration, and markers of proteolysis in skeletal muscle would come about as a result of repeated eccentric exercise and that gender may affect this adaptive response. Untrained male (n = 8) and female (n = 8) subjects performed two bouts (bout 1 and bout 2), separated by 5.5 wk, of 36 repetitions of unilateral, eccentric leg press and 100 repetitions of unilateral, eccentric knee extension exercises (at 120% of their concentric single repetition maximum), the subjects' contralateral nonexercised leg served as a control (rest). Biopsies were taken from the vastus lateralis from each leg 24 h postexercise. After bout 2, the postexercise force deficit and the rise in serum creatine kinase (CK) activity were attenuated. Women had lower serum CK activity compared with men at all times (P < 0.05), but there were no gender differences in the relative magnitude of the force deficit. Muscle Z-disk streaming, quantified by using light microscopy, was elevated vs. rest only after bout 1 (P < 0.05), with no gender difference. Muscle neutrophil counts were significantly greater in women 24 h after bout 2 vs. rest and bout 1 (P < 0.05) but were unchanged in men. Muscle macrophages were elevated in men and women after bout 1 and bout 2 (P < 0.05). Muscle protein content of the regulatory calpain subunit remained unchanged whereas ubiquitin-conjugated protein content was increased after both bouts (P < 0.05), with a greater increase after bout 2. We conclude that adaptations to eccentric exercise are associated with attenuated serum CK activity and, potentially, an increase in the activity of the ubiquitin proteosome proteolytic pathway.     

Dynamics of indirect symptoms of skeletal muscle damage after stretch-shortening exercise

Healthy untrained men (age 20.4 ± 1.7 years, n = 20) volunteered to participate in an experiment in order to establish dynamics of indirect symptoms of skeletal muscle damage (ISMD) (decrease in maximal isometric voluntary contraction torque (MVCT) and torque evoked by electrostimulation at different frequencies and at different quadriceps muscle length, height (H) of drop jump (DJ), muscle soreness and creatine kinase (CK) activity in the blood) after 100 DJs from 0.75 m height performed with maximal intensity with an interval of 20 s between the jumps (stretch-shortening exercise, SSE). All ISMDs remained even 72 h after SSE (P < 0.01–0.001). The muscle experienced greater decrease (P < 0.01) in torque evoked by electrostimulation (at low stimulation frequencies and at short muscle length in particular) after SSE than neuromuscular performance (MVCT and H of DJ) which demonstrated secondary decrease (P < 0.01) in neuromuscular performance during the first 48 h after SSE. Within 24–72 h after the SSE the subjects felt an acute muscle pain (5–7 points approximately) and the CK activity in the blood was significantly increased up to 1200 IU/L (P < 0.001). A significant correlation between decrease in MVCT and H of DJ 24–48 h after SSE on the one hand and muscle soreness registered within 24–48 h after SSE on the other was observed, whereas correlation between the other indirect symptoms of skeletal muscle damage was not significant.     

Acid-base balance during repeated cycling sprints in boys and men.

The aim of this study was to investigate the acid-base balance during repeated cycling sprints in children and adults. Eleven boys (9.6 +/- 0.7 yr) and ten men (20.4 +/- 0.8 yr) performed ten 10-s sprints on a cycle ergometer separated by 30-s passive recovery intervals. To measure the time course of lactate ([La]), hydrogen ions ([H(+)]), bicarbonate ions ([HCO(3)(-)]), and base excess concentrations and the arterial partial pressure of CO(2), capillary blood samples were collected at rest and after each sprint. Ventilation and CO(2) output were continuously measured. After the 10th sprint, concentrations of boys vs. men were as follows: [La], 8.5 +/- 2.1 vs. 15.4 +/- 2.0 mmol/l; [H(+)], 43.8 +/- 1.3 vs. 66.9 +/- 9.9 nmol/l (P < 0.001). Significant correlations showed that, for a given [La], [H(+)] was lower in the boys compared with the men (P < 0.001). Significant relationships also indicated that, for a given [La], [HCO(3)(-)] and base excess concentration were similar in the boys compared with the men. Moreover, significant relationships revealed that, for a given [H(+)] or [HCO(3)(-)], arterial partial pressure of CO(2) was lower in the boys compared with the men (P < 0.001). The ventilation-to-CO(2) output ratio was higher in the boys during the first five rest intervals and was then higher in the men during the last five sprints. To conclude, during repeated sprints, the ventilatory regulation related to the change in acid-base balance induced by lactic acidosis was more important during the first rest intervals in the boys compared with the men.     

Post-exercise increase of lipid oxidation after a moderate exercise bout in untrained healthy obese men.

The aim of the study was to examine whether a moderate exercise increases the utilization of fatty acids during the recovery period in obese men. Six healthy obese participated in a randomized crossover investigation, one with exercise and one without exercise. At 8 a. m., the subjects had a standardized breakfast and they rested in a sitting position for 3 hours. The subjects were maintained in the sitting position for 4 additional hours in one session. In a second session, they exercised for 60 min at 50 % of their VO(2) max and then returned to the sitting position for 3 hours. Respiratory exchange ratio (RER) values were calculated by indirect calorimetry. During the resting session, plasma non-esterified fatty acids (NEFA) and glycerol concentrations rose progressively, whereas RER progressively decreased. During the exercise, plasma catecholamines, NEFA, glycerol, growth hormone and cortisol levels and RER increased while insulin decreased. During the recovery, plasma NEFA increased and glycerol decreased. During the first hour of recovery, RER values were lower and fatty acid utilization higher than during the same period of the resting session. The study shows that exercise induces modifications in hormonal factors promoting lipid mobilization and suggests that exercise provide substantial amounts of NEFA for muscle oxidation during recovery from an exercise bout in obese subjects.     

Gastrocnemius muscle specific force in boys and men.

The aim of this study was to assess whether the in vivo specific force and architectural characteristics of the lateral gastrocnemius (GL) muscle of early pubescent boys (n = 11, age = 10.9 +/- 0.3 yr, Tanner stage 2) differed from those of adult men (n = 12, age = 25.3 +/- 4.4 yr). Plantarflexor torque was 55% lower in the boys (77.4 +/- 21.4 N x m) compared with the adults (175.6 +/- 31.7 N x m, P < 0.01). Physiological cross-sectional area (PCSA), determined in vivo using ultrasonography and MRI, was 52% smaller in the boys (P < 0.01). No difference was found in pennation angle, or in the ratio of fascicle length (L(f)) to muscle length between the boys and men. Moment arm length was 25% smaller in the boys (P < 0.01). Antagonist coactivation, assessed using surface EMG on the dorsiflexors, was not different between the boys and men (11.8 +/- 6.7% and 13.5 +/- 5.8%, respectively). Surprisingly, GL force normalized to PCSA (specific force) was significantly higher (21%) in the boys than in the men (13.1 +/- 2.0 vs. 15.9 +/- 2.7 N/cm(2), P < 0.05). This finding could not be explained by differences in moment arm length, muscle activation, or architecture, and other factors, such as tendinous characteristics and/or changes in moment arm length with contraction, may be held responsible. These observations warrant further investigation.     

Effect of exercise-induced muscle damage on the blood lactate response to incremental exercise in humans

Eccentric muscle actions are known to induce temporary muscle damage, delayed onset muscle soreness (DOMS) and muscle weakness that may persist for several days. The purpose of the present study was to determine whether DOMS-inducing exercise affects blood lactate responses to subsequent incremental dynamic exercise. Physiological and metabolic responses to a standardised incremental exercise task were measured two days after the performance of an eccentric exercise bout or in a control (no prior exercise) condition. Ten healthy recreationally active subjects (9 male, 1 female), aged 20 (SD 1) years performed repeated eccentric muscle actions during 40 min of bench stepping (knee high step; 15 steps · min⁻¹). Two days after the eccentric exercise, while the subjects experienced DOMS, they cycled on a basket loaded cycle ergometer at a starting work rate of 150 W, with increments of 50 W every 2 min until fatigue. The order of the preceding treatments (eccentric exercise or control) was randomised and the treatments were carried out 2 weeks apart. Two days after the eccentric exercise, all subjects reported leg muscle soreness and exhibited elevated levels of plasma creatine kinase activity (P < 0.05). Endurance time and peak V˙O₂ during cycling were unaffected by the prior eccentric exercise. Minute volume, respiratory exchange ratio and heart rate responses were similar but venous blood lactate concentration was higher (P < 0.05) during cycling after eccentric exercise compared with the control condition. Peak blood lactate concentration, observed at 2 min post-exercise was also higher [12.6 (SD 1.4) vs 10.9 SD (1.3) mM; P < 0.01]. The higher blood lactate concentration during cycling exercise after prior eccentric exercise may be attributable to an increased rate of glycogenolysis possibly arising from an increased recruitment of Type II muscle fibres. It follows that determination of lactate thresholds for the purpose of fitness assessment in subjects experiencing DOMS is not appropriate. Accepted: 27 September 1997     

MR measurements of muscle damage and adaptation after eccentric exercise.

The purposes of this study were, first, to clarify the long-term pattern of T2 relaxation times and muscle volume changes in human skeletal muscle after intense eccentric exercise and, second, to determine whether the T2 response exhibits an adaptation to repeated bouts. Six young adult men performed two bouts of eccentric biceps curls (5 sets of 10 at 110% of the 1-repetition concentric maximum) separated by 8 wk. Blood samples, soreness ratings, and T2-weighted axial fast spin-echo magnetic resonance images of the upper arm were obtained immediately before and after each bout; at 1, 2, 4, 7, 14, 21, and 56 days after bout 1; and at 2, 4, 7 and 14 days after bout 2. Resting muscle T2 [27.6 +/- 0.2 (SE) ms] increased immediately postexercise by 8 +/- 1 ms after both bouts. T2 peaked 7 days after bout 1 at 47 +/- 4 ms and remained elevated by 2.5 ms at 56 days. T2 peaked lower (37 +/- 4 ms) and earlier (2-4 days) after bout 2, suggesting an adaptation of the T2 response. Peak serum creatine kinase values, pain ratings, and flexor muscle swelling were also significantly lower after the second bout (P < 0.05). Total volume of the imaged arm region increased transiently after bout 1 but returned to preexercise values within 2 wk. The exercised flexor compartment swelled by over 40%, but after 2 wk it reverted to a volume 10% smaller than that before exercise and maintained this volume loss through 8 wk, consistent with partial or total destruction of a small subpopulation of muscle fibers.     

Smaller muscle ATP reduction in women than in men by repeated bouts of sprint exercise.

It was hypothesized that the reduction of high-energy phosphates in muscle after repeated sprints is smaller in women than in men. Fifteen healthy and physically active women and men with an average age of 25 yr (range of 19-42 yr) performed three 30-s cycle sprints (Wingate test) with 20 min of rest between sprints. Repeated blood and muscle samples were obtained. Freeze-dried pooled muscle fibers of types I and II were analyzed for high-energy phosphates and their breakdown products and for glycogen. Accumulation of plasma ATP breakdown products, plasma catecholamines, and blood lactate, as well as glycogen reduction in type I fibers, was all lower in women than in men during sprint exercise. Repeated sprints induced smaller reduction of ATP and smaller accumulation of IMP and inosine in women than in men in type II muscle fibers, with no gender differences in changes of ATP and its breakdown products during the bouts of exercise themselves. This indicates that the smaller ATP reduction in women than in men during repeated sprints was created during recovery periods between the sprint exercises and that women possess a faster recovery of ATP via reamination of IMP during these recovery periods.     

Adipose tissue lipolysis is increased during a repeated bout of aerobic exercise.

The goal of the study was to examine whether lipid mobilization from adipose tissue undergoes changes during repeated bouts of prolonged aerobic exercise. Microdialysis of the subcutaneous adipose tissue was used for the assessment of lipolysis; glycerol concentration was measured in the dialysate leaving the adipose tissue. Seven male subjects performed two repeated bouts of 60-min exercise at 50% of their maximal aerobic power, separated by a 60-min recovery period. The exercise-induced increases in extracellular glycerol concentrations in adipose tissue and in plasma glycerol concentrations were significantly higher during the second exercise bout compared with the first (P < 0.05). The responses of plasma nonesterified fatty acids and plasma epinephrine were higher during the second exercise bout, whereas the response of norepinephrine was unchanged and that of growth hormone lower. Plasma insulin levels were lower during the second exercise bout. The results suggest that adipose tissue lipolysis during aerobic exercise of moderate intensity is enhanced when an exercise bout is preceded by exercise of the same intensity and duration performed 1 h before. This response pattern is associated with an increase in the exercise-induced rise of epinephrine and with lower plasma insulin values during the repeated exercise bout.     

Oxidative stress and delayed-onset muscle damage after exercise

Reactive oxygen species (ROS) produced during exercise may be involved in delayed-onset muscle damage related to inflammation. To investigate this hypothesis, we studied whether oxidative stress increases nuclear translocation of nuclear factor-kappaB and chemokine expression in skeletal muscle using myotube L6 cells. We also assessed whether prolonged acute exercise could increase these parameters in rats. In L6 cells, H(2)O(2) induced nuclear translocation of p65 and increased the expression of cytokine-induced neutrophil chemoattractant-1 (CINC-1) and monocyte chemoattractant protein-1 (MCP-1), whereas preincubation with alpha-tocopherol limited the increase in these proteins. Sprague Dawley rats were divided into the following groups: rested control, exercised, rested with a high alpha-tocopherol diet, and exercised with a high alpha-tocopherol diet. After 3 weeks of acclimation, both exercise groups ran on a treadmill at 25 m/min for 60 min. Exercise increased nuclear p65, CINC-1, and MCP-1 in gastrocnemius muscle cells, but these changes were ameliorated by the high alpha-tocopherol diet. Increases in myeloperoxidase and thiobarbituric acid-reactive substrates were ameliorated by a high alpha-tocopherol diet, as were the histological changes. Neutrophil activity was not altered by either exercise or a high alpha-tocopherol diet. These results indicate that delayed-onset muscle damage induced by prolonged exercise is partly related to inflammation via phagocyte infiltration caused by ROS and that alpha-tocopherol (an antioxidant) can attenuate such inflammatory changes.     

Relationship between exercise-induced muscle damage and enzyme release in rats.

The relationship between the amount of exercise-induced muscle damage and the release of creatine kinase (CK), aspartate aminotransferase (AST), and lactate dehydrogenase (LD) was studied. Gender differences in enzyme release and histological damage were also studied. Serial pre- and postexercise blood samples were drawn from untrained male and female catheterized Wistar rats that ran 1.5 or 2.5 h on a treadmill (incline 10 degrees). Three days postexercise, muscle damage was quantified morphometrically in five different hindlimb and forearm muscles. The 1.5 and 2.5 h of exercise elicited histological damage only in the soleus muscle. Significant plasma CK, AST, and LD elevations were found immediately postexercise both in male and female rats. However, the enzyme release was significantly greater in males than in females. Part of this could be explained by differences in clearance rates between males and females. No gender difference in amount of histological damage was found. The actual volume of histological muscle damage was significantly less than the calculated muscle damage based on enzyme release. An increase in the exercise duration from 1.5 to 2.5 h resulted in a disproportional increase in both histological muscle damage and muscle enzyme release. From the present study it is concluded that muscle enzyme release is not clearly reflected in histological muscle damage.     

A single bout of exercise activates matrix metalloproteinase in human skeletal muscle.

The aims of this study were 1) to characterize changes in matrix metalloproteinase (MMP), endostatin, and vascular endothelial growth factor (VEGF)-A expression in skeletal muscle in response to a single bout of exercise in humans; and 2) to determine if any exchange of endostatin and VEGF-A between circulation and the exercising leg is associated with a change in the tissue expression or plasma concentration of these factors. Ten healthy males performed 65 min of cycle exercise, and muscle biopsies were obtained from the vastus lateralis muscle at rest and immediately and 120 min after exercise. In the muscle biopsies, measurements of mRNA expression levels of MMP-2, MMP-9, MMP-14, and tissue inhibitor of metalloproteinase; VEGF and endostatin protein levels; and MMP activities were performed. Femoral arterial and venous concentrations of VEGF-A and endostatin were determined before, during, and 120 min after exercise. A single bout of exercise increased MMP-9 mRNA and activated MMP-9 protein in skeletal muscle. No measurable increase of endostatin was observed in the skeletal muscle or in plasma following exercise. A concurrent increase in skeletal muscle VEGF-A mRNA and protein levels was induced by exercise, with no signs of peripheral uptake from the circulation. However, a decrease in plasma VEGF-A concentration occurred following exercise. Thus 1) a single bout of exercise activated the MMP system without any resulting change in tissue endostatin protein levels, and 2) the increased VEGF-A protein levels are due to changes in the skeletal muscle tissue itself. Other mechanisms are responsible for the observed exercise-induced decrease in VEGF-A in plasma.     

Effects of step exercise on muscle damage and muscle Ca2+ content in men and women

Eccentric exercise often produces severe muscle damage, whereas concentric exercise of a similar load elicits a minor degree of muscle damage. The cellular events initiating muscle damage are thought to include an increase in cytosolic Ca. It was hypothesized that eccentric muscle activity in humans would lead to a larger degree of cell damage and increased intracellular Ca accumulation in skeletal muscle than concentric activity would. Furthermore, possible differences between men and women in muscle damage were investigated following step exercise. Thirty-three healthy subjects (18 men and 15 women) participated in a 30-minute step exercise protocol involving concentric contractions with 1 leg and eccentric contractions with the other leg. Muscle Ca content, maximal voluntary contraction (MVC), and muscle enzymes in the plasma were measured. In a subgroup of the subjects, T2 relaxation time was measured by magnetic resonance imaging. No significant changes were found in muscle Ca content in vastus lateralis biopsy specimens in women or in men. Following step exercise, MVC decreased in both legs of both genders. The women had a significantly larger strength decrease in the eccentric leg than the men had on postexercise day 2 (p < 0.01). Plasma creatine kinase increased following step exercise, with a sevenfold higher response in women than in men on day 3 (p < 0.001). The women, but not the men, had an increase in T2 relaxation time in the eccentrically working adductor magnus muscle, peaking on day 3 (75%) (p < 0.001). In conclusion, step exercise does not lead to Ca accumulation in the vastus lateralis but does induce muscle damage preferentially in the eccentrically working muscles, considerably more in women than in men. This indicates that gender-specific step training programs may be warranted to avoid excessive muscle damage.     

Role of brain IL-1beta on fatigue after exercise-induced muscle damage

Brain cytokines, induced by various inflammatory challenges, have been linked to sickness behaviors, including fatigue. However, the relationship between brain cytokines and fatigue after exercise is not well understood. Delayed recovery of running performance after muscle-damaging downhill running is associated with increased brain IL-1beta concentration compared with uphill running. However, there has been no systematic evaluation of the direct effect of brain IL-1beta on running performance after exercise-induced muscle damage. This study examined the specific role of brain IL-1beta on running performance (either treadmill or wheel running) after uphill and downhill running by manipulating brain IL-1beta activity via intracerebroventricular injection of either IL-1 receptor antagonist (ra; downhill runners) or IL-1beta (uphill runners). Male C57BL/6 mice were assigned to the following groups: uphill-saline, uphill-IL-1beta, downhill-saline, or downhill-IL-1ra. Mice initially ran on a motor-driven treadmill at 22 m/min and -14% or +14% grade for 150 min. After the run, at 8 h (wheel cage) or 22 h (treadmill), uphill mice received intracerebroventricular injections of IL-1beta (900 pg in 2 microl saline) or saline (2 microl), whereas downhill runners received IL-1ra (1.8 microg in 2 microl saline) or saline (2 microl). Later (2 h), running performance was measured (wheel running activity and treadmill run to fatigue). Injection of IL-1beta significantly decreased wheel running activity in uphill runners (P<0.01), whereas IL-1ra improved wheel running in downhill runners (P<0.05). Similarly, IL-1beta decreased and Il-1ra increased run time to fatigue in the uphill and downhill runners, respectively (P<0.01). These results support the hypothesis that increased brain IL-1beta plays an important role in fatigue after muscle-damaging exercise.     

Bdnf expression in rat skeletal muscle after acute or repeated exercise

Brain derived growth factor (BDNF) gene of rat has a complex structure: at least four 5' untranslated exons regulated by different promoters and one 3' exon containing the encoding region. BDNF is expressed by skeletal muscles in an activity-dependent manner. In this study, BDNF mRNA was analysed by RT-PCR in the soleus muscle following a single (acute) session of running or a training of five days of running (repetitive exercise). Moreover, the expression of the exons was quantitatively analysed by real time RT-PCR. Finally, muscle BDNF protein level was evaluated by western blotting. BDNF mRNA was found to increase over the second day after acute exercise; on the other hand, two peaks (2 and 24 hours after the last session, respectively) in BDNF mRNA level were found after repetitive exercise, but it was similar to that of controls 6 hours after the last session. BDNF protein level progressively increased also after the mRNA went back to the basal level, so suggesting that it cumulates within the cell after acute exercise, whereas it followed the mRNA level time course after repetitive exercise. These results point to the following conclusions: BDNF mRNA is up-regulated by activity, but this response is delayed to the second day after acute exercise; repetitive exercise transiently depresses the expression of BDNF mRNA, so that the over-expression due to the previous day's exercise completely disappears 6 hours after the last exercise session.     

Time course of insulin sensitivity and skeletal muscle glycogen synthase activity after a single bout of exercise in horses.

The time course of insulin sensitivity, skeletal muscle glycogen and GLUT4 content, and glycogen synthase (GS) activity after a single bout of intense exercise was examined in eight horses. On separate days, a euglycemic-hyperinsulinemic clamp (EHC) was undertaken at 0.5, 4, or 24 h after exercise or after 48 h of rest [control (Con)]. There was no increase in mean glucose infusion rate (GIR) with exercise (0.5-, 4-, and 24-h trials), and GIR was significantly decreased at 0.5 h postexercise (GIR: 8.6 +/- 2.7, 6.7 +/- 2.0, 9.0 +/- 2.0, and 10.6 +/- 2.2 mg.kg(-1).min(-1) for Con and at 0.5, 4, and 24 h, respectively). Before each EHC, muscle glycogen content (mmol glucosyl units/kg dry muscle) was higher (P < 0.05) for Con (565 +/- 102) than for other treatments (317 +/- 84, 362 +/- 79, and 382 +/- 74 for 0.5, 4, and 24 h, respectively) and muscle GLUT4 content was unchanged. Pre-EHC active-to-total GS activity ratio was higher (P < 0.05) at 0.5, 4, and 24 h after exercise than in Con. Post-EHC active GS and GS activity ratio were higher (P < 0.05) in Con and at 24 h. There was a significant inverse correlation (r = -0.43, P = 0.02) between glycogen content and GS activity ratio but no relationship between GS activity and GIR. The lack of increase in insulin sensitivity, determined by EHC, after exercise that resulted in a significant reduction in muscle glycogen content is consistent with the slow rate of muscle glycogen resynthesis observed in equine studies.     

Loss of exercise-induced cardioprotection after cessation of exercise.

Endurance exercise provides cardioprotection against ischemia-reperfusion (I/R) injury. Exercise-induced cardioprotection is associated with increases in cytoprotective proteins, including heat shock protein 72 (HSP72) and increases in antioxidant enzyme activity. On the basis of the reported half-life of these putative cardioprotective proteins, we hypothesized that exercise-induced cardioprotection against I/R injury would be lost within days after cessation of exercise. To test this, male rats (4 mo) were randomly assigned to one of five experimental groups: 1). sedentary control, 2). exercise followed by 1 day of rest, 3). exercise followed by 3 days of rest, 4). exercise followed by 9 days of rest, and 5). exercise followed by 18 days of rest. Exercise-induced increases (P < 0.05) in left ventricular catalase activity and HSP72 were evident at 1 and 3 days postexercise. However, at 9 days postexercise, myocardial HSP72 and catalase levels declined to sedentary control values. To evaluate cardioprotection during recovery from I/R, hearts were isolated, placed in working heart mode, and subjected to 20.5 min of global ischemia followed by 30 min of reperfusion. Compared with sedentary controls, exercised animals sustained less I/R injury as evidenced by maintenance of a higher (P < 0.05) percentage of preischemia cardiac work during reperfusion at 1, 3, and 9 days postexercise. The exercise-induced cardioprotection vanished by 18 days after exercise cessation. On the basis of the time course of the loss of cardioprotection and the return of HSP72 and catalase to preexercise levels, we conclude that HSP72 and catalase are not essential for exercise-induced protection during myocardial stunning. Therefore, other cytoprotective molecules are responsible for providing protection during I/R.     

Gender differences in insulin action after a single bout of exercise.

Effects of a single exercise bout on insulin action were compared in men (n = 10) and women (n = 10). On an exercise day, subjects cycled for 90 min at 85% lactate threshold, whereas on a rest (control) day, they remained semirecumbent. The period of exercise, or rest, was followed by a 3-h hyperinsulinemic-euglycemic clamp (30 mU.m(-2).min(-1)) and indirect calorimetry. Glucose kinetics were measured isotopically by using an infusion of [6,6-2H2]glucose. Glucose infusion rate (GIR) during the clamp on the rest day was not different between the genders. However, GIR on the exercise day was significantly lower in men compared with women (P = 0.01). This was mainly due to a significantly lower glucose rate of disappearance in men compared with women (P = 0.05), whereas no differences were observed in the endogenous glucose rate of appearance. Nonprotein respiratory quotient (NPRQ) increased significantly during the clamp from preclamp measurements in men and women on the rest day (P < 0.01). Exercise abolished the increase in NPRQ seen during the clamp on the rest day and tended to decrease NPRQ in men. Our results indicate the following: 1) exercise abolishes the usual increase in NPRQ observed during a hyperinsulinemic-euglycemic clamp in both genders, 2) men exhibit relatively lower whole body insulin action in the 3-4 h after exercise compared with women, and 3) gender differences in insulin action may be explained by a lower glucose rate of disappearance in the men after acute exercise. Together, these data imply gender differences in insulin action postexercise exist in peripheral tissues and not in liver.