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

Abstract

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 5 degrees of knee flexion between 55 degrees and 90 degrees (full extension = 0 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 +/- 63 N m) than women (177 +/- 60 N m), and greater for adults than children (boys 78 +/- 17 N m, girls 91 +/- 28 N m) (p < 0.01). The agonistic activation level was greater for adults (approximately 85%) than children (approximately 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 +/- 83, 229 +/- 70, 114.2 +/- 32 and 147 +/- 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 approximately 10 degrees (p < 0.01), the peak moment occurred at approximately 60 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.

Do neuromuscular adaptations occur in endurance-trained boys and men?

“…However, in both age groups and in all contraction modes, coactivation was very low and could not explain the higher size-normalized peak torque observed in our adult subjects. This is in agreement with previous studies that examined the isometric strength of untrained boys and men (Falk et al 2009b;Morse et al 2008;O'Brien et al 2009), although it disagrees with others (Hatzikotoulas et al 2008). …”

“…It has been suggested that the capacity of prepubertal boys to activate their neuromuscular systems is lower than that of adults (Belanger and McComas 1989;Grosset et al 2008;Hatzikotoulas et al 2008;O'Brien et al 2009;Pääsuke et al 2000), and that children are less capable of recruiting or utilizing their higher-threshold motor units (Asai and Aoki 1996;Falk and Dotan 2006;Falk et al 2009b). Although not all studies have demonstrated a lower motor-unit activation in children (Streckis et al 2007), Halin et al (2002) proposed that children's neuromuscular system may be more adaptive to a training stimulus, compared with that of adults.…”

Is There a Progressive Withdrawal of Physiological Protections against High-Intensity Exercise-Induced Fatigue during Puberty?

“…Although O'Brien et al [32] and Kluka et al [33] have shown no difference of optimal angle (i.e., the angle where force is maximal) between children and adults, Marginson and Eston [34] showed that the optimal angle corresponded to longer muscle lengths in children compared to adults. As all studies used the same angle to compare the fatigability between children and adults, their results could be affected since the children could exercise at a shorter relative length than adults.…”

Commentary on Child-Adult Differences in Muscle Activation–A Review

“…contractions than adults (1,4,10,22) and extends it to incorporate the fundamental physiological principle of recruitment order (11). The hypothesis that activation deficit (as one of the many factors affecting muscle strength) is due to differential motor-unit activation is very plausible when considering the activation factor in itself but there are a number of points raised throughout the manuscript which warrant further discussion.…”