BACKGROUND
the alkaline version of the single-cell gel  assay is considered as a method for quantifying dna damage  <cit> .
animal experiments have shown chronic and acute effects of exercise on dna damage as measured by the comet assay  <cit> .
selman et al.
<cit>  showed that short-term voluntary wheel running  does not lead to lymphocyte dna damage.
in addition, recently, siu et al.
<cit>  observed that after  <dig> and  <dig> weeks of habitual voluntary exercise  the percentage of dna content in the tail diminished by 21% and 45%, respectively, compared with sedentary control rats.
the authors suggested that elevated expression of antioxidant enzymes and dna-repairing enzymes play a fundamental role in the protective effect of habitual exercise on oxidant-induced lymphocyte dna damage.
for studying exercise physiology and sports medicine, besides the lack of lymphocyte dna damage, defining the chronic exercise load  and determining if this specific exercise leads to positive performance adaptations, such as development of aerobic capacity, are important.
studies on habitual voluntary exercise with wheel running  <cit>  provide no information regarding training load or performance development.
recently, martins et al.
<cit>  used a swimming training protocol proposed by gobatto et al.
<cit> , and observed that male adult wistar rats had diminished dna damage expressed as the mean percentage of tail dna in heart cells treated with doxorubicin, an anti-tumor drug that leads to cardiotoxicity  <cit> .
these authors also verified that trained rodents treated with saline do not have dna damage in cells of the heart, kidney, and liver compared with sedentary rodents  <cit> .
however, martins et al.’s investigation  <cit>  did not show results on changes in performance.
martins et al.’s study  <cit>  was the first investigation to determine the responses of dna damage as measured by the comet assay to a training protocol with intensity, volume and load clearly defined.
however, previous studies  <cit>  on the effects of exercise on dna damage did not examine whether an aerobic training protocol with intensity, volume and load clearly defined improves performance without leading to peripheral blood cell dna damage.
furthermore, although these previous studies investigated exercise-induced dna damage  <cit> , none of them provided information on dna damage in skeletal muscle cells.
in a treadmill running study, wierzba et al.
<cit>  observed that a 50-min single bout with intensity corresponding to 75–85% of maximal oxygen uptake  leads to lymphocyte dna damage compared with sedentary rodents.
notably, 33– <dig> min of the acute exercise bout were performed at an intensity above the lactate threshold.
therefore, the authors considered that a strenuous single bout of exercise is relevant for induction of lymphocyte dna damage  <cit> .
in addition, using the protocol described by hohl et al.
<cit> , dong et al.
<cit>  showed that overtrained rats had lymphocyte dna damage.
meeusen et al.
<cit>  defined functional overreaching  as a short-term performance decrement without severe psychological or other lasting negative symptoms that eventually leads to improvement in performance after days of recovery.
furthermore, the authors characterized nonfunctional overreaching  as a performance decrement that can be reversed after weeks or months of recovery, while a performance decrement in overtraining syndrome  can last months to years.
recently, we have developed a new overtraining protocol based on downhill running sessions that lead to nfor in 100% of mice  <cit> .
the present study aimed to examine the effects of aerobic training and overtraining protocols on dna damage to peripheral blood and skeletal muscle cells in swiss mice.
to examine possible changes in peripheral blood and skeletal muscle cell dna with oxidative stress, we measured reduced glutathione  levels in total blood, and gsh levels and lipid peroxidation in gastrocnemius samples in trained and overtrained mice.
RESULTS
body weight and food intake
body weight in the control, trained, and overtrained groups was not significantly different during the experimental weeks .
the control and trained groups showed a significant decrease in food intake compared with the overtrained group at the end of week  <dig> .
incremental load test
with regard to inter-group differences , the exhaustion time  and velocity  in the trained group were significantly higher compared with that in the control group at week  <dig>  and week  <dig> .
the exhaustion time and velocity in the overtrained group were significantly higher than those in the control group at week  <dig> .
furthermore, the exhaustion time and velocity in the overtrained group were significantly lower than those in the trained group at week  <dig> .
with regard to intra-group differences , exhaustion time and velocity in the trained group were increased at week  <dig>  and week  <dig>  compared with week  <dig> .
however, while exhaustion time and velocity in the overtrained group were increased at week  <dig>  compared with week  <dig> , these parameters were decreased at week  <dig>  compared with week  <dig> .
exhaustive test
with regard to inter-group differences , the time to exhaustion in the trained group was significantly higher than that in the control group at week  <dig>  and week  <dig> .
the time to exhaustion in the overtrained group was significantly higher than that in the control group at week  <dig> .
in addition, the time to exhaustion in the overtrained group was significantly lower than that in the trained group at week  <dig> .
with regard to intra-group differences , the time to exhaustion was increased in the trained group at week  <dig> compared with week  <dig> .
however, the time to exhaustion was decreased in the overtrained group at week  <dig> compared with week  <dig> .
percentage of dna in the tail from peripheral blood and skeletal muscle cells
the overtrained group had a significantly higher percentage of dna in the tail compared with the control and trained groups in peripheral blood  and skeletal muscle cells .
independent from the experimental group, the comet assay scores measured in blood and skeletal muscle cells showed a significant correlation .
gsh levels in skeletal muscle cells and total blood, and thiobarbituric acid reactive substance  levels in skeletal muscle cells
gsh levels in skeletal muscle cells were significantly lower in the overtrained group than in the control and trained groups .
gsh levels in total blood in the trained group were significantly lower than those in the control group .
in addition, the overtrained group had significantly lower gsh levels in total blood than did the control and trained groups .
independent from the experimental group, gsh levels measured in skeletal muscle cells and total blood were significantly correlated .
the overtrained group had significantly higher levels of tbars in skeletal muscle cells than did the control and trained groups .
discussion
the main findings of the present investigation were that: 1) the training protocol proposed by ferreira et al.
<cit>  improved performance  in mice without leading to dna damage in peripheral blood and skeletal muscle cells or oxidative stress in skeletal muscle cells; and 2) the overtraining protocol based on downhill running sessions  <cit>  led to dna damage in peripheral blood and skeletal muscle cells, and to oxidative stress in skeletal muscle cells and total blood.
in the present study, we did not observe any significant differences in body weight between the groups during the experimental weeks.
however, mice in the overtrained group had significantly higher food intake at the end of week  <dig> compared with that in the control and trained groups.
food intake at week  <dig> represents the amount of food that mice eat from weeks  <dig> to  <dig>  based on significant correlations from percentage changes in food intake and body gain from weeks  <dig> to  <dig> , we hypothesize that these mice increased their food intake to compensate for energetic demand after the 3-week period of downhill running sessions.
although our experimental procedures were exactly the same as those by pereira et al.
<cit> , our findings of body weight and food intake were not similar.
armstrong and vanheest  <cit>  considered the loss of body weight as one of the signs and symptoms related to overtraining.
future research using these protocols needs to increase the number of days that body weight and food intake are recorded from one per week to three or five per week, and metabolic cages should be used to better control these specific variables.
in contrast to the discrepancies between the present study and pereira et al.’s study  <cit>  regarding body weight and food intake, our results on incremental load and exhaustive tests were similar in the trained and overtrained groups to their study.
after training during for  <dig> weeks in intensity corresponding to a maximal lactate steady state   <cit> , mice improved their exhaustion time, exhaustion velocity, and time to exhaustion by approximately  <dig> %,  <dig> %, and  <dig> %, respectively.
as previously demonstrated by pereira et al.
<cit> , the improvement in incremental load test parameters occurred mainly during the first  <dig> weeks of aerobic training, and these variables were not significantly different from weeks  <dig> to  <dig>
mlss represents the gold standard protocol for identification of the metabolic aerobic/anaerobic transition point during exercise  <cit> .
in our study, the intensity of the exhaustive test  was  <dig> % and  <dig> % higher than that corresponding to mlss in week  <dig>  and week  <dig> , respectively.
therefore, the energy that sustains the exhaustive test effort is predominantly from anaerobic metabolism.
consequently, even with aerobic characteristics, ferreira’s protocol  <cit>  improved aerobic and anaerobic performance parameters.
similar to pereira et al.’ study  <cit> , after  <dig> weeks of downhill running sessions at a grade of −14%, mice had diminished exhaustion time, exhaustion velocity, and time to exhaustion by approximately  <dig> %,  <dig> % and  <dig> %, respectively.
the major findings of the present investigation were dna damage in peripheral blood and skeletal muscle cells in trained and overtrained mice, and their relationship with gsh and tbars levels in skeletal muscle cells, and gsh levels in total blood.
interestingly, independent from the experimental group, we found significant correlations between the comet assay scores and gsh levels measured in blood and muscle cells.
these data are in accordance with other investigations  <cit>  showing that changes in muscle are reflected in the blood.
our study found that ferreira’s aerobic training protocol  <cit>  was able to improve performance in mice without leading to dna damage in peripheral blood and skeletal muscle cells.
furthermore, after  <dig> weeks of training, mice did not have significantly different levels of gsh and tbars in skeletal muscle cells compared with the control group.
however, trained mice had significantly lower levels of gsh in total blood compared with those in the control group.
using the same aerobic training protocol as in our study, ferreira et al.
<cit>  observed significantly higher levels of maximal citrate synthase activity in the gastrocnemius muscle, and a higher gsh/oxidized-glutathione  ratio and total glutathione levels compared with untrained mice  in plantaris and soleus muscles.
based on the present results in the trained group and in previous investigations  <cit> , we conclude that our trained mice did not present with dna damage in skeletal muscle cells because of a lack of significant changes in gsh and tbars levels in the same tissue.
however, we are not able to explain why trained mice did not have dna damage in peripheral blood cells, even with lower levels of gsh in total blood.
we speculate that lower gsh levels only are not sufficient to induce dna damage in blood cells.
however, overtrained mice had dna damage in peripheral blood and skeletal muscle cells, as well as lower gsh levels in skeletal muscle cells and total blood, and higher tbars levels in skeletal muscle cells compared with the control and trained groups.
the relationship between overtraining and oxidative stress has been previously studied in humans  <cit>  and rodents  <cit> .
margonis et al.
<cit>  observed that a 3-week overtraining period led to higher levels of tbars , catalase , and gssg , and to lower levels of gsh  and gsh/gssg  in  <dig> participants.
furthermore, another study on seven subjects who were severely overtrained concluded that increased oxidative stress plays an important role in the pathophysiology of overtraining syndrome  <cit> .
dong et al.
<cit>  studied the protocol described by hohl et al.
<cit>  and considered that overtraining can activate nicotinamide-adenine dinucleotide phosphate oxidase-mediated overproduction of reactive oxygen species, inducing increased lipid peroxidation.
zoppi and macedo  <cit>  analyzed the levels of tbars, reactive carbonylated derivates, gsh, catalase, citrate synthase, and stress protein hsp <dig> in soleus, extensor digital longus, and semitendinosus muscles of overreaching rats.
the authors found higher levels of tbars, reactive carbonylated derivates, and hsp <dig> only in soleus samples compared with rats submitted to 8-week endurance training, and concluded that the oxidative stress-induced overreaching protocol is fiber type dependent.
a previous study found that the red gastrocnemius muscle of wistar rats in nfor had significantly lower citrate synthase activity compared with for, but was similar to that of the controls  <cit> .
in addition, mitochondrial complex iv activity measured in nfor wistar rats was lower compared with that in control and for rats.
the authors considered that this impaired mitochondrial adaptation in the nfor group was associated with increased antioxidant enzyme activity and increased lipid peroxidation  compared with the for group and controls.
our data showing that nfor mice had dna damage in peripheral blood and skeletal muscle cells due to lower gsh levels and higher tbars levels in skeletal muscle cells, and lower gsh levels in total blood, are consistent with previous investigations  <cit> .
recently, pereira et al.
<cit>  showed that the current overtraining protocol based on downhill running sessions is linked to high concentrations of cytokines in serum, skeletal muscles, and the liver.
therefore, they concluded that the performance decrease induced by nfor is associated with muscle damage and inflammation  <cit> .
because muscle damage and inflammation are related to blood and skeletal muscle oxidative stress  <cit> , we speculate that our findings in the overtrained group of dna damage in peripheral blood muscle cells, and oxidative stress in skeletal muscle cells and total blood, may simply be responses to muscle damage and inflammation  <cit> .
CONCLUSIONS
in summary, aerobic and anaerobic performance can be improved with training at mlss during  <dig> weeks, without leading to dna damage in peripheral blood and skeletal muscle cells or to oxidative stress in skeletal muscle cells.
however, nfor induced by downhill running training sessions is associated with dna damage in peripheral blood and skeletal muscle cells, and to oxidative stress in skeletal muscle cells and total blood.

 the alkaline version of the single-cell gel assay is considered as a method for quantifying dna damage. animal experiments have shown chronic and acute effects of exercise on dna damage as measured by the comet assay. selman et al. showed that short-term voluntary wheel running does not lead to lymphocyte dna damage. in addition, recently, siu et al. observed that after and weeks of habitual voluntary exercise the percentage of dna content in the tail diminished by 21% and 45%, respectively, compared with sedentary control rats. the authors suggested that elevated expression of antioxidant enzymes and dna-repairing enzymes play a fundamental role in the protective effect of habitual exercise on oxidant-induced lymphocyte dna damage. for studying exercise physiology and sports medicine, besides the lack of lymphocyte dna damage, defining the chronic exercise load and determining if this specific exercise leads to positive performance adaptations, such as development of aerobic capacity, are important. studies on habitual voluntary exercise with wheel running provide no information regarding training load or performance development. recently, martins et al. used a swimming training protocol proposed by gobatto et al., and observed that male adult wistar rats had diminished dna damage expressed as the mean percentage of tail dna in heart cells treated with doxorubicin, an anti-tumor drug that leads to cardiotoxicity. these authors also verified that trained rodents treated with saline do not have dna damage in cells of the heart, kidney, and liver compared with sedentary rodents. however, martins et al.’s investigation did not show results on changes in performance. martins et al.’s study was the first investigation to determine the responses of dna damage as measured by the comet assay to a training protocol with intensity, volume and load clearly defined. however, previous studies on the effects of exercise on dna damage did not examine whether an aerobic training protocol with intensity, volume and load clearly defined improves performance without leading to peripheral blood cell dna damage. furthermore, although these previous studies investigated exercise-induced dna damage, none of them provided information on dna damage in skeletal muscle cells. in a treadmill running study, wierzba et al. observed that a 50-min single bout with intensity corresponding to 75–85% of maximal oxygen uptake leads to lymphocyte dna damage compared with sedentary rodents. notably, 33– min of the acute exercise bout were performed at an intensity above the lactate threshold. therefore, the authors considered that a strenuous single bout of exercise is relevant for induction of lymphocyte dna damage. in addition, using the protocol described by hohl et al., dong et al. showed that overtrained rats had lymphocyte dna damage. meeusen et al. defined functional overreaching as a short-term performance decrement without severe psychological or other lasting negative symptoms that eventually leads to improvement in performance after days of recovery. furthermore, the authors characterized nonfunctional overreaching as a performance decrement that can be reversed after weeks or months of recovery, while a performance decrement in overtraining syndrome can last months to years. recently, we have developed a new overtraining protocol based on downhill running sessions that lead to nfor in 100% of mice. the present study aimed to examine the effects of aerobic training and overtraining protocols on dna damage to peripheral blood and skeletal muscle cells in swiss mice. to examine possible changes in peripheral blood and skeletal muscle cell dna with oxidative stress, we measured reduced glutathione levels in total blood, and gsh levels and lipid peroxidation in gastrocnemius samples in trained and overtrained mice.  body weight and food intake body weight in the control, trained, and overtrained groups was not significantly different during the experimental weeks. the control and trained groups showed a significant decrease in food intake compared with the overtrained group at the end of week. incremental load test with regard to inter-group differences, the exhaustion time and velocity in the trained group were significantly higher compared with that in the control group at week and week. the exhaustion time and velocity in the overtrained group were significantly higher than those in the control group at week. furthermore, the exhaustion time and velocity in the overtrained group were significantly lower than those in the trained group at week. with regard to intra-group differences, exhaustion time and velocity in the trained group were increased at week and week compared with week. however, while exhaustion time and velocity in the overtrained group were increased at week compared with week, these parameters were decreased at week compared with week. exhaustive test with regard to inter-group differences, the time to exhaustion in the trained group was significantly higher than that in the control group at week and week. the time to exhaustion in the overtrained group was significantly higher than that in the control group at week. in addition, the time to exhaustion in the overtrained group was significantly lower than that in the trained group at week. with regard to intra-group differences, the time to exhaustion was increased in the trained group at week compared with week. however, the time to exhaustion was decreased in the overtrained group at week compared with week. percentage of dna in the tail from peripheral blood and skeletal muscle cells the overtrained group had a significantly higher percentage of dna in the tail compared with the control and trained groups in peripheral blood and skeletal muscle cells. independent from the experimental group, the comet assay scores measured in blood and skeletal muscle cells showed a significant correlation. gsh levels in skeletal muscle cells and total blood, and thiobarbituric acid reactive substance levels in skeletal muscle cells gsh levels in skeletal muscle cells were significantly lower in the overtrained group than in the control and trained groups. gsh levels in total blood in the trained group were significantly lower than those in the control group. in addition, the overtrained group had significantly lower gsh levels in total blood than did the control and trained groups. independent from the experimental group, gsh levels measured in skeletal muscle cells and total blood were significantly correlated. the overtrained group had significantly higher levels of tbars in skeletal muscle cells than did the control and trained groups. discussion the main findings of the present investigation were that: 1) the training protocol proposed by ferreira et al. improved performance in mice without leading to dna damage in peripheral blood and skeletal muscle cells or oxidative stress in skeletal muscle cells; and 2) the overtraining protocol based on downhill running sessions led to dna damage in peripheral blood and skeletal muscle cells, and to oxidative stress in skeletal muscle cells and total blood. in the present study, we did not observe any significant differences in body weight between the groups during the experimental weeks. however, mice in the overtrained group had significantly higher food intake at the end of week compared with that in the control and trained groups. food intake at week represents the amount of food that mice eat from weeks to based on significant correlations from percentage changes in food intake and body gain from weeks to, we hypothesize that these mice increased their food intake to compensate for energetic demand after the 3-week period of downhill running sessions. although our experimental procedures were exactly the same as those by pereira et al., our findings of body weight and food intake were not similar. armstrong and vanheest considered the loss of body weight as one of the signs and symptoms related to overtraining. future research using these protocols needs to increase the number of days that body weight and food intake are recorded from one per week to three or five per week, and metabolic cages should be used to better control these specific variables. in contrast to the discrepancies between the present study and pereira et al.’s study regarding body weight and food intake, our results on incremental load and exhaustive tests were similar in the trained and overtrained groups to their study. after training during for weeks in intensity corresponding to a maximal lactate steady state, mice improved their exhaustion time, exhaustion velocity, and time to exhaustion by approximately %, %, and %, respectively. as previously demonstrated by pereira et al., the improvement in incremental load test parameters occurred mainly during the first weeks of aerobic training, and these variables were not significantly different from weeks to mlss represents the gold standard protocol for identification of the metabolic aerobic/anaerobic transition point during exercise. in our study, the intensity of the exhaustive test was % and % higher than that corresponding to mlss in week and week, respectively. therefore, the energy that sustains the exhaustive test effort is predominantly from anaerobic metabolism. consequently, even with aerobic characteristics, ferreira’s protocol improved aerobic and anaerobic performance parameters. similar to pereira et al.’ study, after weeks of downhill running sessions at a grade of −14%, mice had diminished exhaustion time, exhaustion velocity, and time to exhaustion by approximately %, % and %, respectively. the major findings of the present investigation were dna damage in peripheral blood and skeletal muscle cells in trained and overtrained mice, and their relationship with gsh and tbars levels in skeletal muscle cells, and gsh levels in total blood. interestingly, independent from the experimental group, we found significant correlations between the comet assay scores and gsh levels measured in blood and muscle cells. these data are in accordance with other investigations showing that changes in muscle are reflected in the blood. our study found that ferreira’s aerobic training protocol was able to improve performance in mice without leading to dna damage in peripheral blood and skeletal muscle cells. furthermore, after weeks of training, mice did not have significantly different levels of gsh and tbars in skeletal muscle cells compared with the control group. however, trained mice had significantly lower levels of gsh in total blood compared with those in the control group. using the same aerobic training protocol as in our study, ferreira et al. observed significantly higher levels of maximal citrate synthase activity in the gastrocnemius muscle, and a higher gsh/oxidized-glutathione ratio and total glutathione levels compared with untrained mice in plantaris and soleus muscles. based on the present results in the trained group and in previous investigations, we conclude that our trained mice did not present with dna damage in skeletal muscle cells because of a lack of significant changes in gsh and tbars levels in the same tissue. however, we are not able to explain why trained mice did not have dna damage in peripheral blood cells, even with lower levels of gsh in total blood. we speculate that lower gsh levels only are not sufficient to induce dna damage in blood cells. however, overtrained mice had dna damage in peripheral blood and skeletal muscle cells, as well as lower gsh levels in skeletal muscle cells and total blood, and higher tbars levels in skeletal muscle cells compared with the control and trained groups. the relationship between overtraining and oxidative stress has been previously studied in humans and rodents. margonis et al. observed that a 3-week overtraining period led to higher levels of tbars, catalase, and gssg, and to lower levels of gsh and gsh/gssg in participants. furthermore, another study on seven subjects who were severely overtrained concluded that increased oxidative stress plays an important role in the pathophysiology of overtraining syndrome. dong et al. studied the protocol described by hohl et al. and considered that overtraining can activate nicotinamide-adenine dinucleotide phosphate oxidase-mediated overproduction of reactive oxygen species, inducing increased lipid peroxidation. zoppi and macedo analyzed the levels of tbars, reactive carbonylated derivates, gsh, catalase, citrate synthase, and stress protein hsp in soleus, extensor digital longus, and semitendinosus muscles of overreaching rats. the authors found higher levels of tbars, reactive carbonylated derivates, and hsp only in soleus samples compared with rats submitted to 8-week endurance training, and concluded that the oxidative stress-induced overreaching protocol is fiber type dependent. a previous study found that the red gastrocnemius muscle of wistar rats in nfor had significantly lower citrate synthase activity compared with for, but was similar to that of the controls. in addition, mitochondrial complex iv activity measured in nfor wistar rats was lower compared with that in control and for rats. the authors considered that this impaired mitochondrial adaptation in the nfor group was associated with increased antioxidant enzyme activity and increased lipid peroxidation compared with the for group and controls. our data showing that nfor mice had dna damage in peripheral blood and skeletal muscle cells due to lower gsh levels and higher tbars levels in skeletal muscle cells, and lower gsh levels in total blood, are consistent with previous investigations. recently, pereira et al. showed that the current overtraining protocol based on downhill running sessions is linked to high concentrations of cytokines in serum, skeletal muscles, and the liver. therefore, they concluded that the performance decrease induced by nfor is associated with muscle damage and inflammation. because muscle damage and inflammation are related to blood and skeletal muscle oxidative stress, we speculate that our findings in the overtrained group of dna damage in peripheral blood muscle cells, and oxidative stress in skeletal muscle cells and total blood, may simply be responses to muscle damage and inflammation.  in summary, aerobic and anaerobic performance can be improved with training at mlss during weeks, without leading to dna damage in peripheral blood and skeletal muscle cells or to oxidative stress in skeletal muscle cells. however, nfor induced by downhill running training sessions is associated with dna damage in peripheral blood and skeletal muscle cells, and to oxidative stress in skeletal muscle cells and total blood. 
although some studies on chronic and acute effects of exercise on dna damage measured by the comet assay have been performed, it is unknown if an aerobic training protocol with intensity, volume, and load clearly defined will improve performance without leading to peripheral blood cell dna damage. to examine possible changes in these parameters with oxidative stress, we measured reduced glutathione levels in total blood, and gsh levels and lipid peroxidation in muscle samples. gsh levels were significantly lower in the otr group than in the c and tr groups. therefore, this study aimed to examine the effects of aerobic training and overtraining on dna damage in peripheral blood and skeletal muscle cells in swiss mice. aerobic and anaerobic performance parameters can be improved in training at maximal lactate steady state during weeks without leading to dna damage in peripheral blood and skeletal muscle cells or to oxidative stress in skeletal muscle cells. however, overtraining induced by downhill running training sessions is associated with dna damage in peripheral blood and skeletal muscle cells, and with oxidative stress in skeletal muscle cells and total blood. the otr group had significantly higher lipid peroxidation levels compared with the c and tr groups. the alkaline version of the single-cell gel assay is a useful method for quantifying dna damage. in addition, the effects of overtraining on dna damage are unknown. performance evaluations showed significant intra and inter-group differences.
 the alkaline version of the single-cell gel assay is a useful method for quantifying dna damage. although some studies on chronic and acute effects of exercise on dna damage measured by the comet assay have been performed, it is unknown if an aerobic training protocol with intensity, volume, and load clearly defined will improve performance without leading to peripheral blood cell dna damage. in addition, the effects of overtraining on dna damage are unknown. therefore, this study aimed to examine the effects of aerobic training and overtraining on dna damage in peripheral blood and skeletal muscle cells in swiss mice. to examine possible changes in these parameters with oxidative stress, we measured reduced glutathione levels in total blood, and gsh levels and lipid peroxidation in muscle samples.  performance evaluations showed significant intra and inter-group differences. the overtrained group showed a significant increase in the percentage of dna in the tail compared with the control and trained groups. gsh levels were significantly lower in the otr group than in the c and tr groups. the otr group had significantly higher lipid peroxidation levels compared with the c and tr groups.  aerobic and anaerobic performance parameters can be improved in training at maximal lactate steady state during weeks without leading to dna damage in peripheral blood and skeletal muscle cells or to oxidative stress in skeletal muscle cells. however, overtraining induced by downhill running training sessions is associated with dna damage in peripheral blood and skeletal muscle cells, and with oxidative stress in skeletal muscle cells and total blood. dna damageaerobic trainingovertrainingoxidative stress 