Muscle balance is an important factor for decreasing injury risks among sports, as weak ago-nist-antagonist strength ratio may predispose to injuries. Hamstring-to-quadriceps (H/Q) strength ratio has been studied in several sports in order to describe the muscular balance of thighs. During pubertal longitudinal growth, hamstrings may be weaker due to the mechanical stress of growth. As both hamstring flexibility and knee extension strength are important for aesthetic group gymnastics (AGG), the H/Q strength ratio might be expected to be low among this sport. Describing muscle activation patterns of hamstrings and quadriceps femoris in AGG actions is efficient for determining the H and Q activities and relations in different movements. Neural effects of low-load hamstring strength training on these muscle activities were studied to determine if strength training, in order to improve muscle balance, affects performance or muscle activities in performance.
Two age groups of AGG gymna
sts (10–11 yrs old, n = 30; 13–14 yrs old, n = 30) were meas-ured cross-sectionally for the H/Q strength ratio and hamstring flexibility as finger-ground dis-tance (FGD). Subgroups (n = 8) were chosen from each group for the measurement of bal-ance, jump mechanics and biceps femoris, vastus lateralis and vastus medialis activities in gymnastic actions. A 9-week intervention period with low-load hamstring strength training was carried out to determine effects of strength training on four AGG movements (body swing, split leap, balance with leg in front, balance with leg behind).
No changes were observed for the FGD, split leap or body swing during the strength training period, or between the age groups. For the split leap, the time for take-off and RFD correlated negatively (p < 0.001) with flight time indicating a relation of shorter contact time, explosive take-off and longer flight time. The activation of biceps femoris (BF) differed in the two bal-ance movements; activation was higher (p < 0.01) in the balance with leg in front. Also, the activation of BF in this particular movement increased during the intervention period, but the increase was not statistically significant. However, the velocity moment decreased, especially for the right leg as supportive (p = 0.02), indicating improved balance and performance.
To conclude, the low-load hamstring strength training affected the balance with leg in front by improving the balance. As the intervention did not influence the FGD, strengthening of ham-strings may be suggested for gymnasts in order to improve the H/Q strength ratio. Because the strength training in the present study was synchronized into typical AGG training, it remains unknown whether the improvements on balance were directly related to the strength training.
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