Fig. 1.

Force tracing and artifacts from magnetic field gradients during tag application and image acquisition. Image series began with 2 “dummy scans,” in which the imaging sequence was run but data were not acquired; the 2nd of these scans is shown. At ∼5 s into each contraction, spatial tags were applied and a series of 20 images were acquired as the subject maintained a steady force level, representing the position of the muscle during contraction. Each image acquisition required 70 ms, and acquisitions were separated by 0.26 s. At 10 s after the 1st set of tags was applied, a 2nd set of tags was applied. Then the subject relaxed and a 2nd set of 20 images, reflecting deformation of the tibialis anterior (TA) muscle due to relaxation, was acquired. Gray arrows indicate times of tag application; black arrows in middle and right sections of the trace indicate image analyzed for contraction and relaxation, respectively. Gray lines at 45% and 55% maximal voluntary contraction (MVC) indicate range of acceptable contraction intensities for tag application and contraction image acquisition. Gray lines at −5% and 5% MVC indicate acceptable range of forces for the image analyzed as the relaxation image.