This study examined the contribution of phosphocreatine (PCr) and aerobic metabolism during repeated bouts of sprint exercise. Eight male subjects performed two cycle ergometer sprints separated by 4 min of recovery during two separate main trials. Sprint 1 lasted 30 s during both main trials, whereas sprint 2 lasted either 10 or 30 s. Muscle biopsies were obtained at rest, immediately after the first 30-s sprint, after 3.8 min of recovery, and after the second 10- and 30-s sprints. At the end of sprint 1, PCr was 16.9 +/- 1.4% of the resting value, and muscle pH dropped to 6.69 +/- 0.02. After 3.8 min of recovery, muscle pH remained unchanged (6.80 +/- 0.03), but PCr was resynthesized to 78.7 +/- 3.3% of the resting value. PCr during sprint 2 was almost completely utilized in the first 10 s and remained unchanged thereafter. High correlations were found between the percentage of PCr resynthesis and the percentage recovery of power output and pedaling speed during the initial 10 s of sprint 2 (r = 0.84, P < 0.05 and r = 0.91, P < 0.01). The anaerobic ATP turnover, as calculated from changes in ATP, PCr, and lactate, was 235 +/- 9 mmol/kg dry muscle during the first sprint but was decreased to 139 +/- 7 mmol/kg dry muscle during the second 30-s sprint, mainly as a result of a approximately 45% decrease in glycolysis. Despite this approximately 41% reduction in anaerobic energy, the total work done during the second 30-s sprint was reduced by only approximately 18%. This mismatch between anaerobic energy release and power output during sprint 2 was partly compensated for by an increased contribution of aerobic metabolism, as calculated from the increase in oxygen uptake during sprint 2 (2.68 +/- 0.10 vs. 3.17 +/- 0.13 l/min; sprint 1 vs. sprint 2; P < 0.01). These data suggest that aerobic metabolism provides a significant part (approximately 49%) of the energy during the second sprint, whereas PCr availability is important for high power output during the initial 10 s.

Background: Many adolescent girls have low levels of physical activity and participation declines with age. This review identifies recent correlates of physical activity in adolescent girls.Methods: Systematic review of papers published 1999 to mid-2003. Papers (k = 51) reporting a measure of physical activity and at least one potential correlate of physical activity in adolescent girls were analyzed.Results: Demographics related to physical activity were female gender (–), non-white ethnicity (–), age (–), and socio-economic status (+). Psychological correlates positively associated with physical activity were enjoyment, perceived competence, self-efficacy, and physical self-perceptions. Behavioral correlates showed that smoking was associated with lower and organized sport involvement with greater activity. Physical activity was associated with parental and family support but we found no consistent trends for environmental variables. Effects were small-to-moderate.Conclusions: Modifiable correlates for adolescent girls clustered around “positive psychology,” organized sport involvement, and the family.

1. Muscle biopsy samples were obtained from the vastus lateralis of six healthy volunteers before and after 30 s of treadmill sprinting. A portion of each biopsy sample was used for mixed‐fibre metabolite analysis. Single fibres were dissected from the remaining portion of each biopsy and were used for ATP, phosphocreatine (PCr) and glycogen determination. 2. Before exercise, PCr and glycogen contents were higher in type II fibres (79.3 +/‐ 2.7 and 472 +/‐ 35 mmol (kg dry matter (DM)‐1, respectively) compared with type I fibres (71.3 +/‐ 3.0 mmol (kg DM)‐1, P < 0.01 and 375 +/‐ 25 mmol (kg DM)‐1, P < 0.001, respectively). 3. Peak power output was 885 +/‐ 66 W and declined by 65 +/‐ 3% during exercise. Phosphocreatine and glycogen degradation in type II fibres during exercise (74.3 +/‐ 2.5 and 126.3 +/‐ 15.8 mmol (kg DM)‐1, respectively) was greater than the corresponding degradation in type I fibres (59.1 +/‐ 2.9 mmol (kg DM)‐1, P < 0.001 and 77.0 +/‐ 14.3 mmol (kg DM)‐1, P < 0.01, respectively). The decline in ATP during exercise was similar when comparing fibre types (P > 0.05). 4. Compared with previous studies involving similar durations of maximal cycling exercise, isokinetic knee extension and intermittent isometric contraction, the rates of substrate utilization recorded in type I fibres were extremely high, being close to the rapid rates observed in this fibre type during intense contraction with limb blood flow occluded.

Background: It is well documented that heat acclimation of six or more sessions of at least 60 min duration prolongs the time to exhaustion during endurance walking, cycling and running in the heat. However, this type of acclimation is not specific to team sport activity and the effect of acclimation on prolonged high-intensity intermittent running has not yet been investigated. Objective: To assess the impact of an intermittent acclimation protocol on distance run during team sport activity. Methods: The impact of four short heat acclimation sessions (30–45 min of the Loughborough Intermittent Shuttle Test; LIST) on high-intensity intermittent running capacity (LIST) in the heat (30°C, 27% relative humidity (RH)), was examined. Seventeen female well-trained games players were split into three groups: an acclimation group (30°C, 24% RH), a moderate training group (18°C, 41% RH) and a control group who did not complete any training between the main trials (pre-acclimation and post-acclimation). The pre-acclimation (A) and post-acclimation (B) trials were separated by 28 days to control for menstrual phase and verified using hormonal analysis. The four acclimation or moderate training sessions utilising the LIST were completed with one or two rest days interspersed between each session in a 10-day period prior to the post-acclimation trial (B). Results: In the post-acclimation trial distance run was increased by 33% in the acclimation group (A: 7703 (SEM 1401) m vs B: 10215 (SEM 1746) m; interaction group × trial p<0.05), but was unchanged in the moderate and control groups. The acclimation group had a lower rectal temperature (interaction group × trial × time p<0.01) due to a lower rate of rise, and an increase in thermal comfort1 after acclimation (End A: 7 (SEM 2) vs 6 (SEM 2); interaction group × trial p<0.01). There was no difference in serum progesterone, aldosterone or cortisol concentrations following acclimation or between groups. Conclusion: Four 30–45 min sessions of intermittent exercise induced acclimation, and resulted in an improvement in intermittent running exercise capacity in female games players. A lower rectal temperature and a concomitant rise in thermal comfort may be partly responsible for the improvement in exercise capacity.