Abstract
The interpolated twitch technique (ITT) is the gold standard to assess voluntary activation and central fatigue. Yet, its validity has been questioned. Here we studied how peripheral fatigue can affect the ITT. Repeated contractions at submaximal frequencies were produced by supramaximal electrical stimulations of the human adductor pollicis muscle in vivo and of isolated rat soleus fiber bundles; an extra stimulation pulse was given during contractions to induce a superimposed twitch. Human muscles fatigued by repeated 30-Hz stimulation trains (3 s on–1 s off) showed an ~80% reduction in the superimposed twitch force accompanied by a severely reduced EMG response (M-wave amplitude), which implies action potential failure. Subsequent experiments combined a less intense stimulation protocol (1.5 s on–3 s off) with ischemia to cause muscle fatigue, but which preserved M-wave amplitude. However, the superimposed twitch force still decreased markedly more than the potentiated twitch force; with ITT this would reflect increased “voluntary activation.” In contrast, the superimposed twitch force was relatively spared when a similar protocol was performed in rat soleus bundles. Force relaxation was slowed by >150% in fatigued human muscles, whereas it was unchanged in rat soleus bundles. Accordingly, results similar to those in the human muscle were obtained when relaxation was slowed by cooling the rat soleus muscles. In conclusion, our data demonstrate that muscle fatigue can confound the quantification of central fatigue using the ITT.
Highlights
The interpolated twitch technique (ITT) developed by Merton (1954) is considered the gold standard to evaluate non-invasively the ability to maximally activate motor units in healthy and clinical populations (Gandevia, 2001; Millet et al, 2012)
These intermittent contractions resulted in action potential impairments, which can contribute to the reduction in superimposed twitch force during fatigue
Our results showed a marked reduction in superimposed twitch force following fatigue in the human adductor pollicis muscle, even when membrane excitability was relatively well-preserved, whereas a small reduction in superimposed twitch force was observed in fatigued rat soleus muscle
Summary
The interpolated twitch technique (ITT) developed by Merton (1954) is considered the gold standard to evaluate non-invasively the ability to maximally activate motor units in healthy and clinical populations (Gandevia, 2001; Millet et al, 2012) It consists of electrically (or magnetically) stimulating a nerve trunk or axonal terminal branches during a maximal voluntary contraction. Place et al (2008) challenged the validity of ITT for central fatigue assessment with an in vitro model of isolated single fasttwitch fibers of mouse flexor digitorum brevis (FDB) muscles These fibers were fatigued by repeated tetani induced by direct electrical stimulation while introducing an extra stimulation pulse during contractions to mimic the ITT. In contrast to the results of Place et al (2008), they observed a reduction in the interpolated twitch force with fatigue development
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