Efficacy of high- versus moderate-intensity spatially distributed sequential stimulation in subjects with spinal cord injury: an isometric study

For producing isometric contractions, spatially distributed sequential stimulation (SDSS) has been demonstrated to be superior to conventional single electrode stimulation (SES) in terms of fatigue reduction and the power output produced by the muscle. However, the impact of stimulation parameters, particularly stimulation intensity, on the effectiveness of SDSS is not sufficiently understood. The aim of this work is to compare the fatigue-reducing capabilities of SDSS at two significantly different electrical stimulation intensities in individuals with lower-limb motor-complete spinal cord injuries, in order to understand the impact of stimulation intensity on the effectiveness of SDSS. Two experiments were conducted, focusing on isometric contractions of the quadriceps muscle group (Experiment 1) and the vastus lateralis muscle (Experiment 2). The effectiveness of high-intensity SDSS was compared to that of moderate-intensity SDSS, with SES serving as a reference. Seven subjects with spinal cord injuries participated in the study. Fatigue and force metrics, including time to fatigue (TTF) and force-time integral (FTI), were analyzed for both electrical stimulation intensity levels. Statistical analysis indicated that the advantages of SDSS over SES in reducing muscle fatigue and enhancing force generation were significantly diminished at high intensity compared to moderate intensity. These findings provide valuable scientific insights into the practical applications of SDSS and contribute to a deeper understanding of its mechanisms in mitigating muscle fatigue. Further research is recommended to explore the effects of various stimulation parameters to optimize SDSS for different muscle groups and functional tasks.