An Energetic Model of Low Frequency Isometric Neuromuscular Electrical Stimulation

The objective of this study was to evaluate whether an adapted Hill-type model of muscle energetics could account for the relatively high energy turnover observed during low frequency isometric Neuromuscular Electrical Stimulation (NMES). A previously validated Hill-based model was adapted to estimate the energy consumption due to muscle activation, force maintenance and internal shortening of the muscle during isometric NMES. Quadriceps muscle model parameters were identified for 10 healthy subjects based on the experimentally measured torque response to isometric stimulation at 8 Hz. Model predictions of torque and energy consumption rates across the stimulation range 1–12 Hz were compared with experimental data recorded from the same subjects. The model provided estimates in close agreement with the experimental values for the group mean energy consumption rate across the frequency range tested, ( R adj 2 = 0.98), although prediction of individual data points for all frequencies and all subjects was more variable, ( R adj 2 = 0.70). The model suggests that approximately one-third of the energy between 4 and 6 Hz is due to shortening heat. The model provides a means of identifying optimal therapeutic stimulation patterns for sustained incremental oxygen uptake at minimum torque output for a given muscle and provides insight into the energetic components involved.