Muscle activation produces increases in magnetic resonance ( T(2)) signal intensity leading to recruitment images that demonstrate spatial patterns and intensity of muscle activation. These T(2) activation maps are useful for visualizing and quantifying various aspects of muscle function. Activity-dependent changes in T(2) result from an increase in the T(2) relaxation time of muscle water. The current state of investigation indicates that the mechanism of increased T(2) results from osmotically driven shifts of muscle water that increase the volume of the intracellular space and from intracellular acidification resulting from the end products of metabolism. Although the spatial resolution of magnetic resonance imaging is still insufficient to map territories of individual motor units, it is possible to demonstrate nonuniform activation between subregions or compartments of muscle. Taken together, the attributes of the T(2) mapping technique hold great potential for demonstrating aberrant muscle activation patterns in pathology and positive adaptation to exercise or rehabilitative intervention.