Fatigability during repetitive maximal knee extensions in 14-year-old boys.

The isokinetic forces, during 50 repeated maximal knee extensions with a constant velocity of 3.14 rad.s-1, and muscle cross-sectional area (CSA) of the quadriceps femoris muscles were measured for boys aged 14 years (n = 26) and young adult men (n = 26). As representative scores in the maximal session, the mean values of force (F) of every five consecutive and all trials were calculated. The CSA was measured by using a B-mode ultrasound technique at the midpoint of the thigh length (lt). The average values of F at the 1st-5th contractions were 193 (SEM 12) N for the boys and 303 (SEM 13) N for the young adults. The average decline of F with 50 contractions, expressed as a percentage of the value in the 1st-5th trial, was higher in the young adults than in the boys: F for the young adults was reduced by 48 (SEM 2.9)%, for the boys by 36 (SEM 3.1)%. The F of every five consecutive and all trials were significantly correlated to the product of CSA and lt (CSA.lt) in separate groups: for the boys r = 0.762-0.894 (P < 0.01), for the young adults r = 0.598-0.837 (P < 0.01). In a trial range between the 1st-5th and 11th-15th contractions, the young adults showed significantly higher values in the ratio of F to CSA.lt (F.CSA-1.lt-1) than the boys. However, the difference between groups of the ratio on and after the 16th-20th trial and for all trials became insignificant. Thus, at least for 50 maximal repeated knee extensions, the 14-year-old boys were inferior to the young adults in their ability to produce force during the earlier sessions even when the difference in muscle size was allowed for. The inferiority in the boys might be attributed to a lower reliance on glycolysis as pointed out in previous biochemical studies.