Functional electrical stimulation in sheep calf muscles: analyzing frequency-related fatigue patterns

Abstract

Functional electrical stimulation is a promising therapy for restoring muscle motor function in patients suffering from paralysis due to spinal cord injury or other neurological conditions. However, the early onset of muscle fatigue is a significant drawback to its use. Several factors have been suggested as causes of early fatigue onset. One of the most accepted arguments is that the high frequency at which the stimulation pulses are delivered, and which is high to achieve fused contractions, has a significant impact on the onset of muscle fatigue. However, studies investigating this have only focused on a few frequencies, without providing solid results to deeply understand how frequency impacts the onset of muscle fatigue. The aim of this study is to explore how different frequencies in Functional Electrical Stimulation (FES) influence muscle fatigue through in vivo trials. Symmetrical biphasic pulses of 250 μs were administered to two different muscles in the sheep's leg (tibialis cranialis and extensor digitorum lateralis). Each stimulation sequence used a different stimulation frequency (5, 7.5, 10, 12, 15, 20, 30, 40, 50, 75, 100 Hz). The force generated by the isometric contraction was measured. The variables extracted from the stimulation included the slope of the force decline when fatigue appeared, the maximum force value, the instant when the force reached its maximum value, and the instant when the force decreased to 25% of the maximum value. Additionally, the fusion level of the contraction force was assessed using the signal power. Before and after the fatiguing stimulation, 10 pulses at 0.5 Hz were delivered to cause individual muscle contractions. These contractions were used to compare the change in force before and after the fatigue protocol. Both muscles showed an increase in fatigue as the stimulation frequency increased. In the frequency range between 40 and 100 Hz, fatigue appeared almost instantly at the start of the stimulation in both muscles. The extensor digitorum lateralis showed better fatigue resistance than the tibialis cranialis, despite having a lower percentage of Type I fibers, which are more fatigue-resistant. During the 180-second stimulation sequence, the lowest frequency did not generate fatigue in either muscle; fatigue appeared earlier as the frequency increased, but so did the force generated by the muscles. Force fusion increased with stimulation frequency, with both muscles showing fused force from 20 Hz. Individual contraction forces, measured before and after the stimulation sequences, were discarded as a tool to assess the degree of muscle fatigue, as the results were not consistent between the two stimulated muscles. This study has proven a strong relationship between the onset of fatigue and the stimulation frequency.