Functional electrical stimulation of long-term denervated, degenerated human skeletal muscle: estimating activation using T2-parameter magnetic resonance imaging methods.

Functional electrical stimulation (FES) of long-term denervated, degenerated human skeletal muscle has proven to be a suitable method for improving a number of physiological parameters. The underlying mechanisms of activation of a denervated muscle fiber can be described with suitably modified and extended Hodgin-Huxley type models, coupled with three-dimensional (3D) finite element models of the surrounding electrical field. Regions of activation within a muscle can be determined using a 3D computer model. However, simulation results have not yet been validated experimentally. During and immediately after exercise, muscle shows increased T2-relaxation times. It is thus possible to estimate muscle activation noninvasively and spatially resolved with the magnetic resonance imaging (MRI) method of T2 mapping, which was, therefore, chosen as a suitable validation approach. Six patients were scanned prior to FES training with a multislice multiecho MSME-sequence at 3 Tesla and then asked to perform one of their regular daily training-sessions (leg extensions). Subjects were then repositioned in the MR-scanner and two to five postexercise scans were recorded. Pre- and postexercise scans were coregistered and T2-parameter maps were calculated. Regions of interest (ROIs) were drawn manually around quadriceps femoris and its antagonists. Activation was detected in all patients. In well-trained patients, activation in the quadriceps was found to be considerably higher than in its antagonists. These experimental results will help further improve existing models of FES of denervated degenerated human skeletal muscle.