Age- and sex-associated differences in isokinetic knee muscle endurance between young children and adults.

The purpose of this study was to examine the age- and sex-associated differences of repeated isokinetic knee extension and flexion. Fifty one participants, 30 young children (16 boys and 14 girls; aged 11 and 12 years) and 21 adults (9 males and 12 females; aged 18-35 years), agreed to participate in the study. Isokinetic concentric peak knee extension (PET) and flexion (PFT) torque were measured using a calibrated Biodex System 3. Participants performed 4 concentric extension-flexion cycles with maximum effort; after a 2 min rest, 50 continuous concentric cycles were performed at 1.56 rad.s-1. Total work of the extensors (WKEX) and flexors (WKFL) for the complete 50 repetitions was recorded. Average peak torque and average work for the first and last 3 repetitions were calculated to represent the percentage decline in torque and work. There were no significant differences between groups in the peak torque generated during the pretrial and endurance task, suggesting that participants gave a maximal effort at the start of the endurance task. There was a significant interaction effect in the total work done for both extensors and flexors, with adult males producing the greatest amount of work (6622 and 3444 J, respectively). When total work was divided by body mass, there were no significant sex effects, only main effects for group. The percentage decline for PET (40% vs. 60%), PFT (50% vs. 65%), WKET (43% vs. 61%), and WKFL (60% vs. 69%) demonstrated significant main effects for group, with greater fatigue in adults. We found no significant sex effect for fatigue. This study concludes that females do not resist fatigue from repeated isokinetic muscle actions to a greater extent than males, and that the greater fatigue in adults than in children is probably a product of greater initial torque production and work performed.