Methodological issues with the interpolated twitch technique

A number of methodological issues in the use of the interpolated twitch technique were investigated for their effect on true maximum force (TMF) and activation (ACT): timing of control (pre- vs post-contraction) and superimposed twitches (first vs second); type of twitch stimulus (primarily magnitude); and the type of extrapolation utilised. On three occasions subjects performed a series of maximal and sub-maximal contractions of the knee extensors, with electrically evoked twitches delivered before, during and after each contraction. The twitch–voluntary force relationship was concave for all types of twitch stimuli, and extrapolation using this relationship typically calculated TMF 39 N (7%) higher, and ACT 7% lower than linear extrapolation. The timing of the control (2–4%) and superimposed twitches (∼4%) both influenced TMF and ACT. Despite the different twitch stimuli being a range of magnitudes (13–32% maximum voluntary force) they did not affect TMF and ACT. A novel finding was that prior potentiation changed the shape of the twitch–voluntary force relationship. For precise measurement of TMF and ACT it is recommended that: extrapolation is based on the twitch–voluntary force relationship of the experimental model; and post-contraction potentiated twitches be used, as the superimposed twitch on a high level contraction appears to be potentiated.