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The deficit of muscle-force production observed in children can be partly attributed to neural factors, such as an increased level of coactivation. This hypothesis, however, has not been thoroughly investigated under concentric and eccentric isokinetic conditions at different angular velocities. Thus, the purpose of this study was to examine whether prepubescent children present higher levels of activation of the antagonist knee muscles during isokinetic, concentric, and eccentric knee efforts compared with adults. Eighteen prepubertal and 13 young adult males (age: 10.9 ± 0.5 and 18.1 ± 0.1 years, respectively) performed maximal concentric and eccentric knee extensions and flexions at 45, 90, and 180 degrees/s. The vastus lateralis and biceps femoris electromyogram was recorded and the antagonist activation (coactivation) was calculated. Concentric contractions for both groups revealed significantly higher coactivation values (p < .05) compared with the eccentric conditions. Furthermore, increasing the angular velocity increased the level of coactivation significantly only during the concentric efforts for both groups. No significant difference in the antagonistic activity of the vastus lateralis and biceps femoris, however, was found between groups. Therefore, increased antagonist knee-muscle activation, which enhances joint stabilization during isokinetic concentric and eccentric effort, is similar in both prepubescent and adult males.

Purpose: To examine the effect of a 10-week plyometric training (PT) on the kinematic and kinetic properties of prepubescent girls during squat jump, countermovement jump, and drop jumps. Methods: Twenty-four untrained girls (aged 9–11 y) were assigned to a training group (TG) and a control group. The TG followed twice a week PT for 10 weeks. Squat jump, countermovement jump, and drop jumps performed from heights of 20, 35, and 50 cm were tested before and after PT. Jump height, kinematic, and kinetic parameters were evaluated using a motion analysis system and a force plate. Results: Jumping height in all jump types increased significantly after PT for the TG (P < .001). After training, the TG presented increased power (P < .001) and knee angular velocity (P < .001), higher knee flexion at the deepest point during the braking phase (P < .001), longer contact time (P < .001), and unchanged stiffness and reaction strength index (P > .05). No differences were observed in the control group (P > .05). Conclusion: These findings indicate that a 10-week PT positively affected jumping performance in prepubescent girls who improved their drop jump performance after training not by adopting a stiff/bouncing jumping style of short contact time and increased stiffness, but a compliant/absorbing style of prolonged contact time.

Purpose: This study examined the underlying mechanisms of postactivation potentiation and the time course of muscular- and neural-related variables. Methods: Fourteen trained males executed 4 sets of six 6-second maximum isometric conditioning plantar flexions, with 15 seconds and 2 minutes of interval between the contractions and sets, respectively. Peak twitch torque (TT), rate of torque development, time to peak torque, half relaxation time, and the neural-related variables of H-reflex and electromyogram, normalized to the maximum M-wave (H/M and RMS/M, respectively), were evaluated, as well as the level of the voluntary activation, assessed by the twitch interpolation technique. All neural-related variables were analyzed for the trial within each set when TT was maximal and for the trial within each set when the neural-related variable itself was maximal. Results: Compared with the baseline measures, TT and rate of torque development significantly increased in all sets (P < .001), whereas time to peak torque and half relaxation time significantly decreased in sets 1 to 4 and 2 to 4, respectively (P < .001). However, H/M and the RMS/M did not change for the repetition of each set for which the TT was maximal (P > .05). Interestingly, the within-set maximum H/M ratio of the lateral gastrocnemius muscle revealed a significant increase in all sets (P < .05), compared with the baseline measures. Conclusion: One set of 4 contractions with 6-second duration is sufficient to cause postactivation potentiation for most participants, whereas peak TT augmentation does not coincide with changes in the examined neural-related variables. Further experiments should consider the time lag on their maximal values and their inherent between-participants variability.

The aim of this study was to examine the fatigue and recovery in boys and men during a maximal intermittent isometric fatigue test of the knee extensor muscles, by evaluating the electromyogram of vastus lateralis, vastus medialis and biceps femoris. Thirteen boys (10.0 ± 0.8yrs) and 13 men (26.1 ± 4.2yrs) were fatigued until torque reached 50% of its initial value. Three and 6 min after, a maximal isometric knee extension test was assessed. Men had faster torque decline during fatigue and slower torque recovery compared with boys. Agonist activity declined in both groups during fatigue but men had greater extent of reduction. After 6 min boys recovered fully in respect to agonist EMG, whereas this was not the case for the men. The lower level of fatigue and faster recovery in boys could be attributed to the limited inhibition that was observed in the boys’ agonist muscles, whereas the antagonist activity does not seem to play a role in the fatigue or recovery differences between the groups.

This study examined the post-activation potentiation (PAP) effects on squat jump (SJ) performance and on peak rate of force development (RFD_peak) in preadolescent (10–12 y), adolescents (14–15 y) and adults (20–25 y) males and females. All participants performed a SJ with and without prior conditioning stimulus (PAP and control protocol, respectively), consisting of 3 × 3-second maximal isometric squats. Jump height and RFD_peak of the vertical ground reaction force during SJ were assessed before, and at 20 seconds and at 4 minutes following the conditioning stimulus. The results revealed a different pattern of age-effect on SJ performance within males and females. The RFD_peak significantly increased as a factor of age in both males and females (P < .05). Increase in SJ performance after conditioning stimulus occurred only in men (P < .05), with no effects in teen-males, boys, and female groups. There was a significant PAP effect on RFD_peak in both adult groups (P < .05) and teen-males, with no effects in children. In conclusion, the PAP effects on SJ performance and RFD_peak are age- and sex-dependent; that is PAP appears as a viable method for acutely enhancing SJ performance in men but not in pediatric population.

This study examines the biomechanical differences during different vertical jump tasks in 12 prepubescent and 12 adult males. The sagittal knee kinematics, vertical ground reaction force (vGRF) and electromyographic (EMG) activity of 5 lower extremity muscles were recorded. Compared with boys, men presented higher peak vGRF during the propulsive phase in all examined jumps, but lower values during the braking phase, even when related to body mass. Normalized EMG agonist activity in all phases was higher in men (p < .05), while antagonist coactivation was enhanced in boys (p < .05). The knee joint was on average 9 degrees more flexed at touchdown in men during drop jump tasks, but boys exhibited 12 degrees and 17 degrees higher knee flexion at the deepest point when performing drop jump from 20 and 40 cm, respectively. In conclusion, the performance deficit observed in boys in all jump types is a reflection of their immature technique, which could be partly attributed to the less efficient stiffness regulation and activation of their neuromuscular system.

Purpose: To examine the effect of drop height on vertical jumping performance in children with respect to sex and maturity status. Methods: Thirty-seven pre-pubertal, 71 circa-pubertal, and 69 post-pubertal boys and girls performed, in a randomized order, 2 squat jumps, 2 countermovement jumps, and 2 drop jumps (DJ) from heights of 20, 30, 40, 50, 60, and 70 cm. The trial with the best jump height in each test was used for analysis. Results: No significant sex × maturity status × jump type interaction for jump height was observed. However, on average, the children jumped higher in the countermovement jump than in squat jump and DJs (+1.2 and +1.6 cm, P < .001, respectively), with no significant differences between DJs and squat jumps or between DJs when increasing drop heights. Regarding DJs, 59.3% of the participants jumped higher from drop heights of 20 to 40 cm. Conclusions : Children, independent of sex and maturity status, performed best in the countermovement jump, and no performance gain was obtained by dropping from heights of 20 to 70 cm. During maturation, the use of drop heights between 20 and 40 cm may be considered in plyometric training, but the optimum height must be obtained individually.