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Figure 2. Mean heart rate responses of young and adult elite footballers during match-play.
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The under-15 years age group had a higher but not significantly different (P< 0.05) mean maximum heart rate (202±9 beats min−1) compared to the under-16 players (200±9 beats min−1). They also had higher but not significantly different (P<0.05) values for first half

mean HR (U15s; 88±2.1% HRmax-U16s; 87± 3.8% HRmax) for, second half mean HR (U15s; 85±2.2% HRmax-U16s; 84± 4% HRmax), full game mean HR (U15s; 87±1.9% HRmax-U16s; 86±3.4% HRmax) and heart rate range (U15s; 68-98% HRmax-U16s; 67-97% HRmax). The combined mean time spent below 85% HRmax was 27 min or 33% of the total time played and the mean time spent above 85% of HRmax was 53 min or 67% of the time played. The under-15 years age group spent 24 min (30% of total) below 85% of HRmax and 56 min (70% of total) above 85% HRmax compared to 31 min (39%) below and 49 min (61%) above 85% HRmax for the under-16 group (see Table 1).

Match-play statistics for adults based on estimations of energy expenditure from heart rate responses show that elite adult players spend on average 67% of the game exercising at an intensity below 85% HRmax or aerobically. The remaining 33% of the time during a game is spent exercising above 85% HRmax or using anaerobic energy systems. This gives a ratio of slightly greater than 2:1 in terms of aerobic over anaerobic energy expenditure. The results of this study (U15/U16 combined) displayed a mean HR below 85% HRmax for approximately 33% of the game, with the remaining 67% being played at a mean HR above 85% HRmax giving a ratio of 1:2 for aerobic to anaerobic energy expenditure (see Fig.4). This shows that elite adolescent players are exercising above 85% HRmax for twice the percentage of the game compared to elite adult players.