Standardizing fatigue-resistance testing during electrical stimulation of paralysed human quadriceps muscles, a practical approach

BackgroundRapid onset of muscular fatigue is still one of the main issues of functional electrical stimulation (FES). A promising technique, known as distributed stimulation, aims to activate sub-units of a muscle at a lower stimulation frequency to increase fatigue-resistance. Besides a general agreement on the beneficial effects, the great heterogeneity of evaluation techniques, raises the demand for a standardized method to better reflect the requirements of a practical application.MethodsThis study investigated the fatigue-development of 6 paralysed quadriceps muscles over the course of 180 dynamic contractions, evaluating different electrode-configurations (conventional and distributed stimulation). For a standardized comparison, fatigue-testing was performed at 40% of the peak-torque during a maximal evoked contraction (MEC). Further, we assessed the isometric torque for each electrode-configuration at different knee-extension-angles (70 degrees -170 degrees, 10 degrees steps).ResultsOur results showed no significant difference in the fatigue-index for any of the tested electrode-configurations, compared to conventional-stimulation. We conjecture that the positive effects of distributed stimulation become less pronounced at higher stimulation amplitudes. The isometric torque produced at different knee-extension angles was similar for most electrode-configurations. Maximal torque-production was found at 130 degrees -140 degrees knee-extension-angle, which correlates with the maximal knee-flexion-angles during running.ConclusionIn most practical applications, FES is intended to initiate dynamic movements. Therefore, it is crucial to assess fatigue-resistance by using dynamic contractions. Reporting the relationship between produced torque and knee-extension-angle can help to observe the stability of a chosen electrode-configuration for a targeted range-of-motion. Additionally, we suggest to perform fatigue testing at higher forces (e.g. 40% of the maximal evoked torque) in pre-trained subjects with SCI to better reflect the practical demands of FES-applications.

Automated summary

The study focused on investigating the fatigue development of paralyzed quadriceps muscles using different electrode configurations. The results showed no significant difference in fatigue index compared to conventional stimulation, suggesting the positive effects of distributed stimulation may decrease at higher stimulation amplitudes. It is recommended to perform fatigue testing at higher forces in pre-trained subjects with SCI to better reflect the practical demands of FES applications.