>Introduction: Functional electrical stimulation is a common technique used in the rehabilitation of individuals with a spinal cord injury to produce functional movement of paralysed muscles. However, it is often associated with rapid muscle fatigue which limits its applications. >Methods: The objective of this study is to investigate the effects on the onset of fatigue of different multi-electrode patterns of stimulation via multiple pairs of electrodes using doublet pulses: Synchronous stimulation is compared to asynchronous stimulation patterns which are activated sequentially (AsynS) or randomly (AsynR), mimicking voluntary muscle activation by targeting different motor units. We investigated these three different approaches by applying stimulation to the gastrocnemius muscle repeatedly for 10 min (300 ms stimulation followed by 700 ms of no-stimulation) with 40 Hz effective frequency for all protocols and doublet pulses with an inter-pulse-interval of 6 ms. Eleven able-bodied volunteers (28 ± 3 years old) participated in this study. Ultrasound videos were recorded during stimulation to allow evaluation of changes in muscle morphology. The main fatigue indicators we focused on were the normalised fatigue index, fatigue time interval andpre-post twitch–tetanus ratio. >Results: The results demonstratethat asynchronous stimulation with doublet pulses gives a higher normalisedfatigue index (0.80 ± 0.08 and 0.87 ± 0.08) for AsynS and AsynR, respectively,than synchronous stimulation (0.62 ± 0.06). Furthermore, a longer fatigue timeinterval for AsynS (302.2 ± 230.9 s) and AsynR (384.4 ± 279.0 s) compared tosynchronous stimulation (68.0 ± 30.5 s) indicates that fatigue occurs laterduring asynchronous stimulation; however, this was only found to bestatistically significant for one of two methods used to calculate the groupmean. Although no significant difference was found in pre-post twitch–tetanusratio, there was a trend towards these effects. >Conclusion: In thisstudy, we proposed an asynchronous stimulation pattern for the application offunctional electrical stimulation and investigated its suitability for reducingmuscle fatigue compared to previous methods. The results show that asynchronousmulti-electrode stimulation patterns with doublet pulses may improve fatigueresistance in functional electrical stimulation applications in someconditions.
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