Abstract
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°–170°, 10° 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°–140° 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.
Highlights
Rapid onset of muscular fatigue is still one of the main issues of functional electrical stimulation (FES)
Maximal evoked contraction maximal evoked contraction (MEC) was comparable between the different subjects and ranged from 30 to 40 Nm
From the first session (CONV1) to the last session (CONV2) the MEC decreased from 36.2 ± 5.0 Nm to 33.6 ± 3.5 Nm, but there was no statistical difference between sessions (p = 0.458)
Summary
Rapid 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. FES-induced contractions are unable to replicate such a recruitment pattern—motor-neurons are rather activated based on an individual threshold defined by axon-size and distance to the electrode. The same motorunits are activated by every pulse, which causes a rapid onset of muscular fatigue. One strategy to counteract fatigue, known as distributed stimulation, is to reduce the number of stimulation pulses acting on a particular motor unit. The idea is to distribute stimulating pulses over multiple channels to activate different sub-units of a muscle, which was extensively described in [5]. Due to mechanical coupling inside of the muscle, distributed stimulation is able to generate strong fused contractions normally only achieved at higher frequencies
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