INTRODUCTION The critical velocity (CV) regression line its an equation type Y = ax + b. The a value (line sloop) has been used and accepted as an indicator of swimmers aerobic capacity. The b value (y-intercept) as been pointed out for Wakayoshi et al. (1993) as a possible indicator of individual swimmers anaerobic potential, but studies are needed to confirm this hypothesis. Knowing that the oxygen consumption and anaerobic potential are physiological parameters that rises with the growing child, it will be expected that a and b values rises from the infant to junior swimmers. Its our issue to study the variation of the a and b values in different age group swimmers. METHODS The study has been performed upon 64 Portuguese swimmers divided in four age groups with 8 males and 8 females each. The age groups, in years, of the male and female groups were, respectively: 12 and 11 (A group), 13/14 and 12/13 (B), 15/16 and 14 (C) and 17/18 and 15/16 (D). The regression line between competition distance and time was obtained using the official results of each swimmer, obtained in the same competition, for the following freestyle events: 50 m and 400 m (A group), 100 m and 800/1500 m (B and C) and 50 m and 800/1500 m (D). Mean values were compared through a ANOVA test (a=0.05). RESULTS The CV values seem to rise progressively with age, but statistical significant differences were only noticed between A and B groups in both genders. Gender differences were not significant. The mean b values for males were: A = 10.65±1.18m, B = 16.15±2.64m, C = 17.07±2.16m and D = 13.45±2.28m; for females: A = 11.29±1.52m, B = 14.31±1.06m, C = 12.84±3.24m and D = 9.55±0.78m, with statistical differences only between A and B male groups. The b values for female swimmers tended to be lower than those obtained for males, with statistical significant differences only for C groups. DISCUSSION The behaviour of CS with age, and gender differences can be explained by developmental physiology knowledge (Rowland, 1996). The b value don`t follow the same linear growth pattern, although several works describe a progressive increment of the anaerobic capacity with age (Rowland, 1996). This could be due either to maturation or methodological causes. In conclusion, the b variation with age seems to do not follow a expected behaviour, justifying further research.
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