Running velocity at blood lactate threshold of boys aged 6-15 years compared with untrained and trained young males

Running velocities at the blood lactate threshold, which was estimated by the velocity just below 2 mmol X l-1 of blood lactate, were measured in sixty-six 6- to 15-year-old boys during a graded exercise stress test on the track and compared with those of twenty-three 16- to 18-year-old young males and eighteen 18- to 23-year-old untrained young males. The boys were divided into five age groups: 6-7, 8-9, 10-11, 12-13, and 14-15. The mean velocity at the lactate threshold of all age groups was significantly higher than that of untrained young males and not different from that of trained young males. There was no significant difference between boys of all age groups, but a significant negative correlation between the velocity at the lactate threshold and bone maturity score (RUS score) of more than 200 was found in boys. Heart rates and percent maximal heart rate at the lactate threshold were dependent on age. These were significantly inversely related to the chronological age and RUS score. These results suggest that maturation is one of the factors influencing lactate threshold.     

Effects of high-intensity strength training on cardiovascular function

Thirteen healthy, untrained males (age 44 +/- 1 yr, range 40-55 yr) were studied to determine the effects of 16 wk of high-intensity, variable-resistance, Nautilus strength training on cardiovascular function. A control group consisting of 10 untrained males (age 52 +/- 2 yr, range 40-64 yr) underwent the same evaluation procedures as the training group. Maximal oxygen uptake (VO2max), cardiac output during submaximal exercise, and body composition were determined before and after training. In addition, the physiological responses to an acute training session were evaluated. Muscular strength increased markedly, as evidenced by a 44% average increase in the "one-repetition maximum" in the various exercises. Body weight and percent body fat did not change with training, though fat-free weight did increase (66.9 +/- 2.6 vs 68.8 +/- 2.7 kg, P less than 0.05) significantly. Maximal oxygen uptake did not change significantly in either the training or the control group, and there were no changes in the hemodynamic responses to submaximal exercise after training. These findings indicate, therefore, that high-intensity, variable-resistance strength training produces no adaptative improvement in cardiovascular function. The physiological responses measured during a training session provide evidence that this lack of cardiovascular adaptation may be due to the low percentage of VO2max elicited by this form of exercise.     

Physiological training effects of playing youth soccer

The purpose of this investigation was to determine if a 9-wk youth soccer program had any effect on cardiorespiratory fitness (VO2max and VO2submax), peak knee torque, and flexibility. Subjects were 20 sixth grade boys, 11 of whom were members of a YMCA soccer team: 9 were normally active boys who were not participating in any organized sport during the study who served as a control group. Mean ages (+/- SD) were 11.8 +/- 0.34 and 11.5 +/- 0.60 yr for the soccer and control group, respectively. Initial VO2max values of 49.83 and 47.42 ml . kg-1 . min-1 for the soccer and the control group, respectively, are similar to those reported in the literature for untrained normal boys of this age. Results indicated that playing soccer three times weekly increased VEmax and reduced VO2 (ml . kg-1 . min-1 and 1 . min-1) at a submaximal running speed (all P's less than 0.05), while no change in VO2max was noted. No significant training effect was observed in peak knee torque or flexibility subsequent to soccer training. It is concluded that the effects of playing soccer in these subjects resulted in no change in cardiorespiratory fitness, peak knee torque, or flexibility.     

Skeletal muscle and hormonal adaptations to circuit weight training in untrained men

Twelve men either performed 10 weeks of timed circuit weight training 3 days week(-1) (CWT; n=8; X+/-SE; age=23.6+/-1.8 years), or were part of a sedentary control group (n=4; age=20.5+/-1.0 years). Significance was P<0.05 for all analyses. The CWT program significantly increased 1 repetition maximum (1 RM) strength for nine of 10 exercises (15-42%). Although no body composition measure significantly changed for the CWT group, low-to-moderate effect sizes were evident for body weight, lean body mass, and relative fat. CWT did not alter percent fiber type, but did increase cross-sectional areas for type IIA fibers (microm(2); pre=5988+/-323, post=7259+/-669). Relative (%) myosin heavy-chain (MHC) expression increased for MHC IIa (pre=42.5+/-2.7, post=50.1+/-2.6), and decreased for MHC IIb (pre=21.8+/-2.8, post=15.4+/-2.4) for the CWT group. Serum testosterone, cortisol, and the testosterone/cortisol ratio did not change at any time for the CWT group. None of the measured variables changed for the control group. These data indicate that for untrained subjects, CWT of the type used resulted in improved muscular strength and a tendency toward increased lean mass. Compared with other types of weight training, fewer adaptations of the muscle fibers were evident. This is likely due in part to the relatively low loads used with this type of resistance exercise.     

Physical training, growth hormone and testosterone levels and blood pressure in prepubertal, pubertal and adolescent boys

The purpose of this study was to determine the resting values of growth hormone (GH) and testosterone (T) within prepubertal, pubertal and adolescence ages and to compare the effects of physical training in each age, in 2 groups. The exercise group (A) consisted of 10 ten-year-old boys, 10 thirteen-year-old boys and 9 sixteen-year-old boys. The control group (B) consisted of approximately the same number of subjects of the same ages. Group A, in addition to the school activity, trained with intervals (3 months, 50 mid/day, 3 days/week). at high intensity (80–85% of max) and at light intensity (30–40% of max). Group B participated only in the school physical education program 2–3 times/week. Venous blood samples were taken at rest by intravenous catheter before and after training. Levels of GH and T in blood serum was determined by radioimmunoassay. Before training, the levels of T differed between the 3 ages, but there were no significant differences in GH (group A and B). After training, the levels of GH and T in group A were significantly higher in the 13-year-old boys and in the 16-year-old boys, but were unchanged in the 10-year-old boys. In group B the levels of GH and T were unchanged in all 3 ages. The conclusion is that neither high nor mild intensity training can change the levels of GH and T in prepubertal boys. However, high intensity training can be a stimulus for increasing GH and T levels in puberty and adolescence     

Single muscle fiber myosin heavy chain distribution in elite female track athletes

PURPOSE: Myosin heavy chain (MHC) characterization of tissue samples from the gastrocnemius muscle of six elite female athletes and 10 untrained females was performed using myosin ATPase histochemistry and gel electrophoresis. Athletes were of national and international caliber, whereas their untrained counterparts were healthy individuals not involved in a regular exercise program. METHODS: Muscle biopsies for the athletes were performed 14 wk into their training season and analyzed with sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and myosin ATPase techniques. RESULTS: Electrophoretic analysis of single muscle fibers from elite athletes revealed a MHC phenotype composition of 46 +/- 6% type I, 21 +/- 6% type IIa, and 0% type IIx, whereas 34% of the single fibers expressed multiple MHC isoforms. When compared with the elite women, untrained subjects demonstrated higher percentages of type I MHC and lower percentages of IIa MHC muscle fibers, 57 +/- 5 and 16 +/- 3%, respectively (P < 0.05). Similar to the female athletes, 27% of the fibers from untrained women possessed multiple myosin isoforms. Myosin ATPase staining demonstrated a greater percentage of type I fibers in untrained subjects versus the elite women (67 +/- 3 vs 41 +/- 2%, P< 0.05) (mean +/- SE), whereas the athletes had a higher percentage of type IIa fibers compared with the untrained women (49 +/- 5 vs 19 +/- 2%, P< 0.05). There were no differences in the percentage of IIb fibers between elite and untrained women (11 +/- 4 vs 14 +/- 2%, respectively). CONCLUSIONS: Whereas a preponderance of hybrid fibers is generally observed in untrained populations, the diverse MHC phenotype seen in these elite female athletes is uncommon. These unique findings are attributed to the chronic and varied nature of training in which these athletes were involved.     

北京市区与郊区12～14岁男女学生骨密度的调查

目的了解市区与郊区12～14岁男女骨密度(BMD)的变化规律。方法利用双能X线骨密度仪对613例健康中学生男女行全身扫描。其中,市区307例,郊区306例;男生296例,女生317例。结果市区男女生的全身总BMD值明显高于郊区男女生(P<0.01),市区男女生的身高比郊区男女生高2～5cm,市区男女生的体重比郊区男女生重2～10kg。女生BMD值高于同龄男生,女生BMD年增长速度快于同龄男生,女生BMD年增长2.7%～8.1%,男生BMD年增长2.1%～2.5%。结论该年龄段女生BMD值高于同龄男生,郊区男女生生长发育比市区同龄男女生晚约1～2年。     

Red blood cell variables in highly trained pubescent athletes: a comparative analysis

BACKGROUND: A suboptimal haematological status has often been recorded in athletes involved in intensive physical activity. There have even been reports of "sports anaemia" associated with intensive physical exercise. However, studies on the effect of different types of exercise practiced over a long period of time on the red blood cell variables in pubescent athletes are very few. AIM: To assess the basic red blood cell variables in highly trained pubescent athletes from different sports and to compare the results with those for a control untrained group. Sex related differences in these variables were also assessed. METHODS: 876 highly trained athletes (559 boys and 317 girls) were included in the study. Their mean (SEM) age, weight, and duration of training were: 14.01 (0.06) years, 56.24 (0.52) kg, and 3.52 (0.07) years respectively. The control group consisted of 357 untrained subjects (171 boys and 186 girls) with mean (SEM) age and weight of 14.58 (0.09) years and 57.75 (0.67) kg. The group of athletes was divided into seven subgroups according to the sport practiced: athletics (105), swimming (107), rowing (230), wrestling (225), weight lifting (47), various team sports (92), and other sports (67). Venous blood samples were drawn from the cubital vein, and the red blood cell count, packed cell volume, haemoglobin concentration, and mean corpuscular volume were measured. Statistical indices were computed for each group and for each variable, and analysis of variance factorial analysis was performed to evaluate the statistical significance of the differences detected. RESULTS: The highly trained group was found to have lower red blood cell count, packed cell volume, and haemoglobin concentration (p<0.001) than the control untrained group (4.61 (0.01) x 10(12)/l v 4.75 (0.02) x 10(12)/l, 0.389 (0.001) v 0.404 (0.002) l/l, and 133.01 (0.38) v 139.9 (0.62) g/l respectively). These variables were lower for the boys of the trained group than for the boys of the control group (p<0.001), and similarly for the girls (p<0.001). The lowest red blood cell count, packed cell volume, and haemoglobin concentration were measured in blood samples from the boys of the swimming subgroup (4.54 (0.06) x 10(12)/l, 0.386 (0.006) l/l, and 129.38 (1.80) g/l respectively) and the rowing subgroup (4.66 (0.03) x 10(12)/l, 0.400 (0.003) l/l, and 136.21 (0.94) respectively). The same distribution was found for the girls: lowest in the rowing subgroup (4.32 (0.04) x 10(12)/l, 0.314 (0.003) l/l, and 124.27 (0.93) g/l) and the swimming subgroup (4.40 (0.05) x 10(12)/l, 0.375 (0.005) l/l, and 125.90 (1.30) g/l). No differences were found in the mean corpuscular volume. CONCLUSIONS: Continuous (more than one year) high intensity sports training (twice a day/five days a week) results in a decrease in the basic red blood cell variables in pubescent boys and girls, this being most pronounced in the submaximal sports.     

Effects of training and training cessation on insulin action.

Physical training has been shown to improve glucose tolerance and insulin action. In the present study, insulin action was determined using the euglycemic clamp technique in six trained male athletes compared with six untrained controls matched by age, sex, and weight at 14, 38, and 86 hours and at 6 days after cessation of exercise. The rate of insulin-mediated glucose uptake (glucose disposal) was 9.40 +/- 0.46 mg.kg-1.min-1 (mean +/- SEM) for the athletes at 14 h after the last exercise bout, compared with 6.80 +/- 0.86 mg.kg-1.min-1 obtained for the untrained controls (p less than 0.01). Glucose disposal was gradually decreased to 7.78 +/- 0.87 mg.kg-1.min-1 at 38 h, 6.82 +/- 0.49 mg.kg-1.min-1 at 86 h and to 7.11 +/- 1.00 mg.kg-1.min-1 at 6 days after cessation of physical training. At 38 h, 86 h, and 6 days of detraining, glucose disposal exhibited by training athletes did not differ significantly from untrained controls. These results suggest that physical training increases insulin action, and that this effect could be reversed to the control levels within 38 h after detraining.     

The specificity of energy utilisation by trained and untrained adolescent boys.

This study examined the relationship between estimates of alactacid anaerobic power, lactacid anaerobic power and aerobic power in a sample of trained swimmers (age 14.4 yr., n = 8) and a sample of untrained boys (age 13.7 yr., n = 13). The anaerobic power outputs were estimated using a modification of the Wingate Anaerobic Test and aerobic power was estimated using a continuous, incremental cycle ergometer test. In addition to leg power outputs the swimmers' arm power using each energy system was estimated and compared with the corresponding leg value. There was no relationship between the estimates of the power of the three energy systems with either the trained or untrained boys. Furthermore with the trained boys there was no relationship between estimates of the power of the same energy system utilised by different limbs. The data support a specificity hypothesis of energy utilisation during exercise with both trained and untrained adolescent boys.     

Failure of target heart rate to accurately monitor intensity during aerobic dance

Fourteen untrained females (age 19 +/- 1, range 18-21) were studied to examine the heart rate-VO2 relationship during a single aerobic dance training session. These findings were used to help explain the changes in VO2max resulting from an aerobic dance training program. VO2max and body composition were determined before and after an 8 wk training period. In addition, the heart rate-VO2 responses to an aerobic dance training session were monitored and compared to the heart rate responses of treadmill jogging performed at the same VO2. The aerobic dance session elicited a significantly lower oxygen pulse than did treadmill exercise (7.2 +/- 0.3 vs 8.1 +/- 0.8 ml.beat-1; P less than 0.01). There were no significant changes in percent body fat, whereas VO2max increased by 11% (34.4 +/- 0.9 vs 38.1 +/- 0.8 ml.kg-1.min-1; P less than 0.05). No significant changes in any of the parameters tested were observed in 10 untrained controls. These findings indicate that the heart rate elicited from aerobic dance represents a lower relative exercise intensity (VO2) than that of running. Therefore, the assumption that aerobic dance training produces the same cardiovascular adaptations as running training when performed at the same target rate may be unwarranted.     

Effect of aerobic training quantity on the VO2 max of circumpubertal swimmers

Maximal oxygen uptake (VO2 max) was measured in 38 swimmers aged 10-14 years. Thirty of 38 boys participated in this study for at least 2 consecutive years. Group 1 consisted of 23 subjects (48 measures) who trained for 7 h/week while group 2 consisted of 15 subjects (27 measures) who trained for 14 h/week. In group 2, VO2 max normalized to body weight was significantly higher at 14 years of age than at 10, whereas the increase was nonsignificant during this period in group 1. The subjects of group 2 showed a large increase of VO2 max/kg body weight from the age of 13, which corresponded in this study to the age of peak height growth velocity. The differences between the two groups were statistically significant at both 13 (P less than 0.02) and 14 years of age (P less than 0.05). At 13 and 14, the most trained subjects also showed significantly higher (P less than 0.05) values of maximal oxygen pulse/kg body weight (VO2 max/kg/HR max). Maximal heart rate (HR max) was similar in the two groups between 10 and 14 years of age. Therefore, we conclude that an increase in a training program of the aerobic type induces a large increase in VO2 max from the age of peak height growth velocity. This is likely due to an increase in the stroke volume.     

Improving aerobic power in primary school boys: a comparison of continuous and interval training

The purpose of this study was to assess whether the magnitude of change in aerobic power was different in boys (mean age 10.25 +/- 0.50 y) who followed a high-intensity interval training protocol, compared to those who followed a moderate-intensity continuous training protocol. Boys were assigned to either a control group (n = 15), a continuous training group (n = 10), or an interval training group (n = 10). They completed peak oxygen uptake tests at baseline and following an 8-week training period. The control group continued with normal activity habits, whilst the continuous training group followed a 20-minute steady-state cycle protocol at 80-85 % of the maximal heart rate, and the interval training group completed 30-s sprints on a cycle ergometer, interspersed with active rest periods. The two training protocols were designed to incur similar cardiovascular work over the 20 minutes of each training session. Significant increases (p < 0.05) in peak oxygen uptake were noted for both the interval and continuous training groups. The interval training group showed marked pre- to post-increases in both peak oxygen pulse, oxygen pulse at the ventilatory threshold, and ventilatory threshold that were not apparent in the continuous group boys. It would appear that a high-intensity interval protocol confers a different training effect in comparison to continuous steady-state training in boys. Possible mechanisms that underpin these adaptations may include increased blood volume and a concomitant adjustment in stroke volume.     

Aerobic responses of prepubertal boys to two modes of training

OBJECTIVE: To investigate the effects of two contrasting eight week training programmes on the aerobic performance of 39 prepubescent boys (mean age 10.1 years). METHODS: All boys were volunteer subjects from three city schools and the schools were matched by a health related behaviour questionnaire. All of the boys were assessed as Tanner stage one for genitalia and pubic hair development. Criterion laboratory tests included peak VO2 as assessed by an incremental discontinuous treadmill test to voluntary exhaustion. Submaximal measurements of heart rate, minute ventilation (VE) and VO2 were also recorded during the treadmill test. One of the schools provided the control group (n = 14), and boys from the other schools followed two contrasting training programmes. The first was a sprint interval running programme (n = 12) comprising 10 second and 30 second sprints, and the second a continuous cycle ergometer programme (n = 13) maintaining a heart rate in the range 80-85% of maximum for 20 minutes on a Monark cycle ergometer. After eight weeks training three times a week, the three groups were retested. RESULTS: There were no significant differences in peak VO2 (p>0.05) with training in either of the groups. Neither were there significant changes in any of the submaximal variables VO2, VE, or heart rate (p>0.05). CONCLUSION: The findings of this study indicate that neither eight week sprint interval running nor continuous cycle ergometer training programmes significantly improve maximal or submaximal indicators of the aerobic performance of prepubertal boys.     

Mass and fiber cross-sectional area of soleus muscle grafts following training

This study tested the hypothesis that endurance training initiated 28 d following grafting of the soleus muscle would increase fiber cross-sectional area concomitant with an increase in mass. Nerve-implant orthotopic grafting operations were performed on 6-wk-old rats anesthetized with pentobarbital sodium. A cohort of animals began running 28 d later. Control muscles were from age-matched, untrained rats. Mass and fiber cross-sectional area of grafts were 37 and 66% less than the respective control values at both 56 and 112 d post-grafting. Training increased graft mass by 49% over the non-run graft value of 82 +/- 8 mg at 56 d post-grafting. Continued training did not increase mass further. The grafts of trained rats were 33% greater than untrained at day 112 due to growth of grafts in untrained rats. Running had no effect on fiber cross-sectional area of grafts through 56 d, but by 112 d the cross-sectional area of Type I fibers was 30% greater than the non-run graft value of 1,271 +/- 81 micron2. By 112 d fiber type profiles were not different between control muscle and grafts from trained and untrained rats. We conclude that there is a dissociation between mass and fiber cross-sectional area in grafts compared to control muscle, and training affects these variables by similar magnitudes but at different times.     

Mechanical muscle function, morphology, and fiber type in lifelong trained elderly

PURPOSE: Maximal muscle contraction force and muscle mass are both reduced during the natural aging process. Long-term training may be used to attenuate this age-related loss in muscle function and muscle size. METHODS: Maximum isometric quadriceps strength (MVC), rate of force development (RFD), and muscle fiber composition and size (CSA) were studied in elderly individuals (68-78 yr) chronically exposed (> 50 yr) to either endurance (E) or strength (S) training, and in age-matched, untrained (U) elderly group. RESULTS: E and S showed greater MVC than did U. Contractile RFD was elevated in S compared with U, and S also demonstrated greater type II fiber CSA than did U and E. The proportion of type I fibers was greater in E compared with U and S. CONCLUSIONS: Muscle fiber size and mechanical muscle performance, particularly RFD, were consistently elevated in aged individuals exposed to chronic (i.e., lifelong) strength training. This relative preservation in muscle morphology and function may provide an important physical reserve capacity to retain muscle mass and function above the critical threshold for independent living at old age.     

Effects of low- and high-repetition resistive training on lipoprotein-lipid profiles

Thirty-seven healthy untrained males (age = 21 +/- 1 yr; range = 19 to 35 yr) were studied to determine the effects of 10 wk of low- and high-repetition resistive training on lipoprotein-lipid profiles. Subjects were assigned to 1 of 3 groups: a low-repetition group (N = 15) that trained using 4 to 6 repetitions maximum or a high-repetition group (N = 14) that used 14 to 16 repetitions maximum in their training or to an inactive control group (N = 8). The number of sets was adjusted to equalize workloads. Muscular strength increased significantly in both training groups as indicated by the increase in the one-repetition maximum test (P less than 0.05). VO2max, body weight, and percent body fat did not change in either of these groups. However, fat-free weight increased significantly in both training groups (both P less than 0.05). The low-repetition training program resulted in no significant changes in the plasma concentrations of triglycerides (104 +/- 15 vs 89 +/- 8), total cholesterol (150 +/- 7 vs 141 +/- 6), high-density lipoprotein (HDL)-cholesterol (40 +/- 1 vs 41 +/- 2), and HDL2-cholesterol (7 +/- 1 vs 7 +/- 1). A similar pattern was observed for the high-repetition group ([i.e., no significant changes in the concentrations of triglycerides (87 +/- 10 vs 89 +/- 8), total cholesterol (148 +/- 6 vs 162 +/- 6), HDL-cholesterol (40 +/-2 vs 40 +/- 2), and HDL2-cholesterol (6 +/- vs 1 vs 7 +/- 2)]. All lipid values were expressed in milligrams per deciliter (mean +/- SE).(ABSTRACT TRUNCATED AT 250 WORDS)     

Ventilatory threshold and VO2max changes in children following endurance training

There are conflicting data with regard to the effect of endurance training in children. On the basis of this information, the effects of 8 wk of run training on ventilatory threshold (VT) and VO2max of eight male children were investigated. Children ranged in age from 10 to 14 yr, with a mean age of 12.4 yr. All subjects were previously untrained. Training consisted of running 4 d.wk-1 for a period of 8 wk. Continuous running was performed 2 d.wk-1 for 10-30 min at 70-80% of VO2max. Interval running was performed the remaining 2 d.wk-1. Repeated intervals of 100-800 m at 90-100% of VO2max were used in this phase of the training. The total distance run for this type of training was 1.5-2.5 km. Incremental treadmill testing prior to and after the training period indicated a 19.4% increase in VT from 30.5 to 36.4 ml.kg-1.min-1 (P less than 0.05). When VT was expressed as a percentage of VO2max, there was a significant (P less than 0.05) increase from 66.6% to 73.8%. VO2max increased 7.5% from 45.9 to 49.4 ml.kg-2.min-2 (P less than 0.05). None of these changes was noted in eight age- and size-matched children who served as control subjects. The results of this study indicate that 8 wk of endurance running training which is of sufficient frequency, intensity, and duration can significantly improve VT and aerobic capacity in male children.     

Avoidance of soccer injuries with preseason conditioning

The effect of a preseason conditioning program was studied to evaluate its influence on the occurrence and severity of soccer injuries. Three hundred female soccer players (ages 14 to 18 years) were studied over a 1-year period. Forty-two of these players participated in a 7-week training program before the start of the season. The type, mechanism, and severity of the injury, when the injury occurred, the number of games or practices missed, and type of shoe worn were recorded. All injuries occurred in the lower extremities, with 61.2% occurring at the knee and ankle. Student's t-test evaluations revealed that the trained group experienced a significantly lower incidence of injury than the untrained group (P = 0.0085). Although not statistically significant, the trained group also had a lower percentage (2.4%) of anterior cruciate ligament injuries compared with the untrained group (3.1%). These results suggest that this type of conditioning has a significant influence on lowering the incidence of injury in female adolescent soccer players.     

Effects of aerobic exercise training on 24 hr profile of heart rate variability in female athletes

BACKGROUND: The aim of this study was to investigate the effects of exercise training on autonomic regulation of heart rate under daily life conditions. METHODS: Twenty-six healthy female athletes (age 24.5 +/- 1.9 yrs) involved in regular physical activity were recruited during a period of yearly rest and randomly assigned to a five-week aerobic exercise training program (n = 13) or to a non-exercise control group (n = 13). MEASURES: Before and after the five-week training, all subjects underwent a bycicle ergometer stress test and a 24-hour dynamic ECG monitoring. Autonomic regulation of heart rate has been investigated by means of both time and frequency domain analyses of heart rate variability (HRV). Spectral analysis of R-R interval variability (autoregressive algorithm) provided markers of sympathetic (low frequency, LF, 0.10 Hz) and parasympathetic (high frequency, HF, 0.25 Hz) modulation of the sinus node. RESULTS: Trained subjects showed a reduced heart rate response to submaximal workload. Before training there was no significant difference between the two groups. After training resting heart rate did not significantly differ between trained and untrained subjects. No significant differences were observed in the different time domain indexes of heart rate variability. The day-night difference in SD and SDRR were significantly less in the trained as compared to the untrained group. Normalized LF and HF components did not significantly differ between trained and untrained subjects, during the awake period. The decrease in the LF and the increase in the HF component during nighttime were significantly less in the trained group. The LF/HF ratio was significantly decreased during the night in the untrained group whereas it was not significantly different from the awake state in the trained group. CONCLUSIONS: These findings of the relative night-time increase in LF and the decrease in the day-night difference in time domain indexes of heart rate variability suggest that, in young female athletes, exercise training is able to induce an increase in the sympathetic modulation of the sinus node which may coexist with signs of relatively reduced, or unaffected, vagal modulation.