Changes in movement symmetry associated with strengthening and fatigue of agonist and antagonist muscles

The hypothesis that strengthening or fatiguing procedures applied on active muscles can affect the symmetry of rapid, discrete movements was tested. S ubjects (N = 12) performed rapid, consecutive elbow flexions and extensions between 2 targets before and after (a) applying a strength training progra m, (b) fatiguing elbow flexors, and (c) fatiguing elbow extensors. The resu lts demonstrated that an increase in strength of elbow extensors caused by applied strength training is associated with an increase in the symmetry ra tio (i.e., acceleration time divided by deceleration time) of elbow flexion movements. The symmetry ratio also increased and decreased in movements wh en agonists and antagonists were fatigued, respectively. Because the streng th training and fatiguing procedures are both known to affect muscle force, the data are interpreted as changes in muscles' ability to exert the force while acting as agonists or antagonists. Namely, muscles need equal impuls es of force (torque multiplied by time) to accelerate and, thereafter, to d ecelerate the limb while performing a rapid, discrete movement. The symmetr y ratio may therefore be changed so that more time will be provided for mus cles that become relatively weaker (compared with their antagonists) becaus e a strengthening or fatiguing procedure has been applied, whereas a shorte r time period should be sufficient for action of their stronger antagonists . Although, in the literature, the studied phenomenon has been discussed as a predominantly motor control phenomenon, the present data suggest that th e movement symmetry could also be related to agonists' and antagonists' abi lity to exert force, particularly while performing rapid, discrete movement s.