A method for classifying co-contraction of lumbar muscle activity

Concurrent activation of muscles on opposite sides of joints is a common ph enomenon. In simple planar mechanical systems, it is easy to identify such an electromyographic pattern as co-contraction of agonist and antagonist mu scles. In complex 3-D systems such as the lumbar spine, it is more difficul t to precisely identify whether EMG recordings represent co-contraction. Qu alitative definitions of antagonist muscles emphasize that their actions wh olly oppose the action of the prime movers. The qualitative definition of a ntagonist muscles was used to formulate a mathematical requirement for ther e to be co-contraction of agonists and antagonists. It was shown that the d efinition of co-contraction implies muscle activity beyond what is required to maintain equilibrium. The method was illustrated by classifying EMG rec ordings made of the lumbar region musculature during tasks involving combin ed torso extension and axial twisting loads. The method, which identified m uscle activity in excess of that required to maintain static equilibrium, c ould be used to identify conditions in which muscle activation is required for something other than merely maintaining moment equilibrium.