The effects of agonist and antagonist muscle activation on the knee extension moment-angle relationship in adults and children

The present study examined the effect of agonist activation and antagonist co-activation on the shape of the knee extension moment-angle relationship in adults and children. Isometric knee extension maximum voluntary contractions (MVCs) were performed at every 5A degrees of knee flexion between 55A degrees and 90A degrees (full extension = 0A degrees) by ten men, ten women, ten boys and ten girls. For each trial, the knee extensors' voluntary activation level was quantified using magnetic stimulation and the level of antagonist co-activation was quantified from their electromyographical activity. Peak MVC moment was greater for men (264 +/- A 63 N m) than women (177 +/- A 60 N m), and greater for adults than children (boys 78 +/- A 17 N m, girls 91 +/- A 28 N m) (p < 0.01). The agonistic activation level was greater for adults (similar to 85%) than children (similar to 70%). Similarly, antagonist co-activation was greater for adults than children, but relative to the agonist moment there were no differences between groups (all groups 7-8%). Correcting the peak moment for agonist and antagonist activation levels resulted in moments produced by fully activated agonist muscles of 334 +/- A 83, 229 +/- A 70, 114.2 +/- A 32 and 147 +/- A 46 N m, for men, women, boys and girls, respectively. Although correcting for shifts in joint angle during contraction altered the angle of peak moment by similar to 10A degrees (p < 0.01), the peak moment occurred at similar to similar to 60A degrees for all groups. Changes in tendon stiffness, muscle size and architecture, and the pattern of the moment arm-angle relationship may in combination occur so that as children develop and mature into adults the shape of the moment-angle relationship is not altered.