The Effect of an Interphase Interval on Electrically Induced Dorsiflexion Force and Fatigue in Subjects With an Upper Motor Neuron Lesion

The study objective was to investigate the effects of an interphase interval (IPI) interposed between the two phases of a biphasic symmetric pulse, on electrically induced contraction (EIC) forces and fatigue during stimulation of the ankle dorsiflexors in individuals with an upper motor neuron lesion (UMNL). The dorsiflexor muscles of 20 subjects with UMNL routinely using functional electrical stimulation to correct a foot drop during ambulation, were electrically stimulated with biphasic pulses (250 µs phase duration and 35 Hz pulse frequency) using nine IPI durations (ranging from 0 to 400 µs). The induced muscle force and fatigue were measured. A significant positive correlation was found between IPI duration and induced muscle force. Introducing a 250 µs IPI significantly decreased rate of muscle fatigue compared to stimulation with no IPI (P < 0.05). Thus, the introduction of an IPI may reduce the current intensity required to achieve a specific force during functional electrical stimulation in individuals with UMNL. Reduction in muscle fatigue may shorten the conditioning period necessary for first time users of functional electrical stimulation.