The effects of age on antagonist muscle activation during maximum voluntary efforts of the knee extensors and flexors

The purpose of this study was to test the hypothesis that children demonstrate greater antagonist activity levels compared to adults. Nine girls(age 12.7 ± 0.7 years) and nine (age 22.7 ± 2.3) females performed maximum eccentric and concentric efforts of knee extensors and flexors on a Biodex dynamometer at 30°·s-1. The electromyographic (EMG) activity of vastus lateralis and hamstrings was also recorded. The EMG was full-wave rectified and integrated (IEMG) over 10 ms intervals. The antagonist IEMG was normalized as a percentage of the IEMG activity of the same muscle group when acting as agonist, at the same angular velocity, angular position and muscle action. The mean and standard deviation of the normalized IEMG of the hamstrings in 23 years old was 16.8 ± 7.1% and 19.56 ±8.4% under eccentric and concentric conditions, respectively. The values for the 13 years old were 10.47 ± 3.0% and 14.43 ± 6.4%, respectively. The vastus medialis mean IEMG in 23 years old was 16.0± 5.2% and 18.74±8.1% under eccentric and concentric conditions, respectively. The respective values for the 13 years old females were 10.19 ± 4.6% and 15.75 ± 7.7%. A three way analysis of variance indicated no significant effects of age on antagonist IEMG of the hamstrings or vastus lateralis (p>0.05). The antagonist activity was significantly lower during eccentric compared with concentric conditions, irrespective of muscle and age (p<0.05). Based on the above results the hypothesis that children exhibit greater antagonist activity levels compared to adults was rejected. It is concluded that during isolated isokinetic maximum efforts of knee extensors and flexors, any strength differences between 13 year old children and adults are unlikely to be due to differences in antagonist muscle activity.