The purpose of this study was to compare symptoms of exercise-induced muscle damage after an initial and repeated bout of plyometric exercise in men and boys. Ten boys (9-10 yr) and 10 men (20-29 yr) completed two bouts of eight sets of 10 plyometric jumps, 2 wk apart. Perceived soreness (0-10, visual analog scale), isometric strength of the quadriceps at six knee flexion angles, and countermovement jump and squat jump height were assessed before and at 30 min, 24 h, 48 h, and 72 h after each bout. All variables followed the expected patterns of change in men, with soreness peaking at 24-48 h (5.8 +/- 1.7) and decrements in muscle function peaking at 30 min after the first bout (73-85% of baseline scores). Symptoms remained for 72 h after the first bout in men. In boys, symptoms were much less severe and peaked at 30 min (visual analog scale = 2.1 +/- 1.8, functional decrements 87-92% of baseline) and, with the exception of soreness, returned to baseline after 24 h. After the second bout of plyometric exercise, the level of soreness and decrements in countermovement jump, squat jump, and isometric strength were lower, although the effect was stronger in men, in all cases. The results of this study suggest that although children may experience symptoms of muscle damage after intensive plyometric exercise, they are much less severe. A prior bout of plyometric exercise also appears to provide children with some protection from soreness after a subsequent bout of plyometric exercise. Explanations for milder symptoms of exercise-induced muscle damage in children include greater flexibility leading to less overextension of sarcomeres during eccentric exercise, fewer fast-twitch muscle fibers, and greater and perhaps more varied habitual physical activity patterns.

Background: Exercise induced muscle damage (EIMD) from strenuous unaccustomed eccentric exercise is well documented. So too is the observation that a prior bout of eccentric exercise reduces the severity of symptoms of EIMD. This has been attributed to an increase in sarcomeres in series. Recent studies have suggested that prior concentric training increases the susceptibility of muscle to EIMD following eccentric exercise. This has been attributed to a reduction of sarcomeres in series, which decreases muscle compliance and changes the length-tension relation of muscle contraction. Objective: To assess the effects of prior concentric training on the severity of EIMD. Methods: Four men and four women (mean (SD) age 21.1 (0.8) years) followed a four week concentric training programme. The elbow flexor musculature of the non-dominant arm was trained at 60% of one repetition maximum dynamic concentric strength performance, three times a week, increasing to 70% by week 3. After three days of rest, participants performed 50 maximal isokinetic eccentric contractions on both arms. All participants gave written informed consent before taking part in this study, which was approved by the school ethics committee. Strength, relaxed arm angle (RAA), arm circumference, and soreness on active extension and flexion were recorded immediately before eccentric exercise, one hour after, and at 24 hour intervals for three days. Data were analysed with fully repeated measures analyses of variance. Results: Strength retention was significantly (p<0.01) greater in the control arm than the trained arm (84.0 (13.7)%, 90.4 (14.7)%, 95.2 (10.5)%, 103.5 (7.6)% v 75.5 (11.3)%, 77.6 (15.3)%, 80.1 (13.9)%, 80.9 (12.5)%) at one, 24, 48, and 72 hours respectively. Similarly, soreness was greater in the trained arm (0.7 (0.6), 3.1 (1.4), 3.0 (1.5), 1.9 (2.3)) than in the untrained arm (0 (0.2), 1.6 (1.3), 1.4 (0.6), 0.6 (0.4)) at one, 24, 48, and 72 hours respectively (p<0.05). Concentric training induced a significant reduction in RAA (165.2 (6.7)°v 157.3 (4.9)°) before the eccentric exercise bout (p<0.01). This was further reduced and remained lower in the trained arm at all time points after the eccentric exercise (p<0.01). The arm circumference of the concentrically trained arm was significantly greater than baseline (p<0.05) at 72 hours (30.3 (2.9) v 29.8 (3.3) cm). Conclusions: These findings extend the understanding of the effects of prior concentric training in increasing the severity of EIMD to an upper limb exercise model. The inclusion of concentric conditioning in rehabilitation programmes tends to exacerbate the severity of EIMD in subsequent unaccustomed exercise. However, where concentric conditioning is indicated clinically, the net effect of conditioning outcome and EIMD may still confer enhanced strength performance and capability to dynamically stabilise a joint system.

The aim of this study was to assess the effect of to isometic contraction durations during propiceptive neuromuscular facilitation stretching on gains inflexion at the hip. Forty-three women (M age = 20.0 years, SD = 1.3) were assigned to one of three groups: 5-s isometric contraction (5-IC), 10-IC, and control. Flexibility was assessed at baseline and Weeks 3 and 6. Analysis of covariance, controlling for pretest differences, showed a significant interaction, F(2, 33) = 44.1, p < .001. Flexibility was significant lower in the control group relative to the 5-IC and 10-IC groups and in the 5-IC group relative to the 10-IC group at 3 and 6 weeks (3 weeks = 101.2 +/- 1.4 degrees, 114.3 +/- 1.5 degrees, 120.5 +/- 1.3 degrees; 6 weeks = 103.0 +/- 1.4 degrees, 126.1 +/- 1.6 degrees, 133.3 +/- 1.4 degrees for control, 5-IC and 10-IC groups, respectively). A longer contraction time led to greater increases in flexibility.

The length-tension relationship of muscle contraction is well documented in adults. However, research on this relationship in children has been limited. The aim of this study was to compare differences in the torque-joint angle relationship of the quadriceps muscle in children and adults. Eight boys aged 8-10 years and eight men aged 20-26 years performed two maximal voluntary isometric contractions at six knee joint angles (20 degrees, 40 degrees, 60 degrees, 80 degrees, 90 degrees, 100 degrees). The mean of the two trials was used as the performance measure. Both groups demonstrated an expected increase in relative torque as the joint angle increased (P< 0.05). The men produced significantly greater relative torque at 20 degrees, 40 degrees and 60 degrees knee flexion (P < 0.05). The percentage of maximal torque at these angles for the men and boys respectively were: 35.2+/-4.3 vs 15.2+/-12%, 63.6+/-9.1 vs 51.8+/-16.8% and 93.6+/-6.5 vs 84.4+/-14.4%. There were no group differences at 80 degrees or 90 degrees. Peak torque was attained at 80 degrees in men, but decreased significantly (P< 0.05) at 90 degrees and 100 degrees. For boys, peak torque was attained at joint angles of 80 degrees and 90 degrees. The reduction in peak torque at 100 degrees was not statistically significant, but the relative torque at this angle was lower in men than in boys (77.9+/-13.7 vs 87.1+/-10.4%; P< 0.05). In conclusion, the relationship between torque and joint angle appears to be affected by age.