Mapping of electrical muscle stimulation using MRI

Abstract

of electrical muscle stimulation using MRI. J. Appl. Physiol. 74(2): 532-537, 1993.-The purpose of this study was to map the pattern of muscle contractile activity elicited by electromyostimulation (EMS). A secondary interest was to determine whether EMS evoked a different pattern of contractile activity than voluntary (VOL) efforts. These objectives were addressed by examining the pattern and extent of contrast shift in magnetic resonance (MR) images after isometric actions of the left m. quadriceps of seven subjects had been elicited by EMS (1-s train of 500-µs sine wave pulses at 50 Hz) or by VOL means. For both conditions, five sets of 10 muscle actions were executed at each of the three force levels equal to 25, 50, and 75% of maximal VOL isometric torque. There were 1-s, 1.5-min, and 30-min rests between muscle actions, sets, and torque levels, respectively. Transaxial proton MR images (TR/TE = 2,000/30, 60) of m. quadriceps femoris were obtained with a 1.5-T imager at rest and after completion of the five sets of isometric actions at each force level. MR image contrast shift, as indicated by T 2 values> 1 SD above the mean resting muscle T 2 , was calculated per pixel. Torque declined ~18% (P < 0.05) during each EMS set independent of the preset relative force level but recovered between sets. EMS increased T 2 values above rest (29 ± 0.2 to 36 ± 0.5,

Vastus lateralis fatigue alters recruitment of musculus quadriceps femoris in humans

“…Fatigue of the VL necessitated greater use of the VM and RF, as reflected by contrast shift in MR images, to perform the submaximal KEE. The fact that exercise-induced contrast shifts in T2-weighted images correlate with integrated EMG activity (1), increase with exercise intensity (1,2,21,30,33,36), and relate to isometric torque with EMS (2) supports the notion of greater use of these two QF muscles for the second rather than the first bout of KEE. These responses are considered to be acute modifications of neuromuscular coordination among the synergistic muscles to perform the exercise.…”

“…There is little information concerning the pattern of skeletal muscle activation by surface EMS (2,24,41). Activation of the QF can be variable among subjects when low forces are evoked and electrodes are placed over the distal VM and proximal VL (2).…”

“…The KEE was performed at the MRI facility to permit image collection immediately after exercise (1,2,6,7,14,21,(33)(34)(35). Subjects performed four sets of 10 repetitions of concentric (extension phase)-eccentric (flexion phase) contractions with a load equal to 50% of the 4 ϫ 10 RM with the use of a plate-loaded knee extension machine (Badger-Magnum, Milwaukee, WI).…”

“…Standard spin-echo MR images of the left thigh were taken using a 1.5-T super-conducting magnet (Signa, General Electric, Milwaukee, WI) essentially as done previously (1,2,6,7,14,21,(33)(34)(35). Thirteen 10-mm-thick transaxial images (repetition time ϭ 1,500 ms; echo time ϭ 30 and 60 ms) of the left thigh spaced 10 mm apart were collected using 1.0 number of excitations with a whole body coil.…”

“…Whereas magnetic resonance (MR) imaging (MRI) has been used to acquire anatomical information (4,5,22), use of human skeletal muscle has also been assessed with exercise-induced contrast shift in transverse relaxation time (T2) of MR images (1-3, 6, 7, 14, 21, 33-35, 42). It has been demonstrated that this contrast shift is correlated with integrated EMG activity (1), related to isometric torque induced by electromyostimulation (EMS) (2), increases with exercise intensity (1,2,21,26,30,33,36) and the metabolic state of skeletal muscle (37,40,42), and can discriminate use among relatively small synergistic muscles (13,14) and neuromuscular compartments (3). Thus exerciseinduced contrast shift in MR images seemed ideal to assess neuromuscular plasticity among synergists.…”