Voluntary activation of the ankle dorsiflexors is overestimated using the interpolated twitch technique with...

The interpolated twitch technique (ITT) is often used to assess voluntary activation (VA); however, its effectiveness depends on the muscle tested. Using single pulse stimulation, VA of the dorsiflexors is overestimated. Therefore, the purpose of this study was to investigate whether using a doublet provides a more sensitive measure of VA in the dorsiflexors. Participants (n = 14, 7 females) performed isometric dorsiflexion contractions in a 45º plantar-flexed position from 10 to 100% maximal voluntary isometric contraction (MVIC) with fibular nerve stimulation superimposed using either a single pulse or doublet (100 Hz). Independent of sex, a single pulse induced greater estimates of VA at 20 and 30% MVIC compared to doublets (both P<0.001). Independent of pulse number, females demonstrated higher VA compared to males at 10 and 20% MVIC despite contracting at the same relative intensity (all P<0.01). With single pulses and doublets combined, VA was linearly correlated with MVIC (%) (r = 0.83, P < 0.001); however, this relationship was better fit to a negative exponential function (r² = 0.85, residual standard error =10.1 vs. r² = 0.69, residual standard error = 14.5). Furthermore, the exponential model identified saturation (95% VA) at only 73% MVIC. Therefore, VA was overestimated independent of pulse number and, at only 50% MVIC, estimated VA was ~90%. When assessed with the ITT, dorsiflexion VA is strikingly overestimated and artificially high at submaximal contraction intensities. This was observed independent of pulse number and sex, despite using a plantar-flexed position to bias larger superimposed twitches. These findings indicate that VA assessment in the dorsiflexors should be taken with caution to avoid overinterpretation.