Adaptations in aerobic exercise responses as well as the relationship between aerobic exercise responses and running performance were examined in a group of previously trained adolescent runners (n = 9; 15.9 ± 1.0 years) over the course of a competitive cross-country season. Running economy (RE), submaximal blood lactate concentration [BLa] and VO2max were assessed before and immediately after the season. Five-km race time improved (P < 0.05) from 18.68 ± 1.10 min at the beginning of the season to 18.16 ± 1.11 min at the end of the season. Significant increases were observed in peak VO2 (61.6 ± 3.5 to 65.3 ± 2.9 mL·kg-1·min-1) and graded exercise test time (11.32 ± 1.56 to 12.22 ± 0.79 min). There was a tendency for RE (P = 0.051) to worsen slightly and for [BLa] (P = 0.057) to decline as a result of training. At the beginning of the season submaximal [BLa] at 14 km·hr-1 (r = 0.86) and graded exercise test time (r = -0.87) were significantly related to 5-km time. At the end of the season, RE (r = 0.78) and [BLa] (r = 0.77) at 14 km·hr-1 and graded exercise test time (r = -0.69) were significantly related to race time. In this well-trained group of runners, further training during the cross-country season increased peak VO2 and improved race time. Submaximal [BLa] and graded exercise test time appear to be the most robust predictors of performance, while RE became a significant predictor of race time at the end of the season.
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