Contraction fatigability during interleaved neuromuscular electrical stimulation of the ankle dorsiflexors does not depend on contraction amplitude

Abstract

Introduction: Interleaved neuromuscular electrical stimulation (iNMES) involves alternating stimulus pulses between the tibialis anterior muscle and common peroneal nerve. 
 Purpose: The current investigation aimed to characterize the relationship between contraction amplitude, motor unit (MU) “overlap” and contraction fatigability during iNMES. It was hypothesized that as iNMES generates progressively larger contractions, more MUs would be recruited from both sites (i.e. more MU overlap), resulting in more fatigability for larger than smaller contractions. 
 Methods: Fourteen participants completed 3 sessions. Fatigability was assessed as the decline in torque over 180 contractions (0.3s “on”, 0.7s “off”) when iNMES was delivered to produce initial contractions of ~5, 15, or 30 percent of a maximal voluntary contraction. 
 Results: Although MU overlap increased significantly with contraction amplitude, the relative (percent) decline in torque was not different between the contraction amplitudes and torque declined on average by 23%. Contraction fatigability was not significantly correlated with either MU overlap or initial contraction amplitude. 
 Conclusion: iNMES can produce fatigue-resistant contractions across a functionally-meaningful range of contraction amplitudes for rehabilitation.
 NOVELTY BULLETS: 
 • Interleaved neuromuscular electrical stimulation progressively recruits motor units (MU) as contraction amplitude increases
 • However, the relative amount of fatigability of recruited MUs was not different as contraction amplitude increased
 • This suggests iNMES can be used effectively to produce fatigue-resistant and functionally-meaningful contractions

Intrinsic Neuromuscular Fatigability in Humans: The Critical Role of Stimulus Frequency

“…Furthermore, owing to a higher current density for smaller electrodes, participant discomfort was greater for eight cathodes compared with all other configurations (43). Interleaved NMES alternates pulses of the stimulus train between the muscle belly and nerve trunk (42,45,46). Similar to sequential NMES, this technique evenly divides the net frequency across sites and can recruit different MUs to asynchronously activate the muscle (16,42,46).…”

Enhancing Adaptations to Neuromuscular Electrical Stimulation Training Interventions

“…Interleaved NMES exploits the previously described spatial differences in the way NMES over the muscle and nerve recruit MUs. Interleaved NMES reduces fatigue by ~30%–40% compared with over-the-muscle or nerve alone during light tibialis anterior contractions (15% maximum voluntary contraction [MVC]) ( 17 ), with a similar effect observed in contractions up to at least 30% MVC ( 18 ). As contraction strength increases further, however, the “overlap” of MUs recruited by the muscle and nerve sites increases ( 10 ) and will theoretically eventually negate the advantage of the interleaved approach.…”