The effect of stimulus anticipation on the interpolated twitch technique

Abstract

The objective of this study was to investigate the effect of expected and unexpected interpolated stimuli (IT) during a maximum voluntary contraction on quadriceps force output and activation. Two groups of male subjects who were either inexperienced (MI: no prior experience with IT tests) or experienced (ME: previously experienced 10 or more series of IT tests) received an expected or unexpected IT while performing quadriceps isometric maximal voluntary contractions (MVCs). Measurements included MVC force, quadriceps and hamstrings electromyographic (EMG) activity, and quadriceps inactivation as measured by the interpolated twitch technique (ITT). When performing MVCs with the expectation of an IT, the knowledge or lack of knowledge of an impending IT occurring during a contraction did not result in significant overall differences in force, ITT inactivation, quadriceps or hamstrings EMG activity. However, the expectation of an IT significantly (p ¼ 0.0001) reduced MVC force (9.5%) and quadriceps EMG activity (14.9%) when compared to performing MVCs with prior knowledge that stimulation would not occur. While ME exhibited non-significant decreases when expecting an IT during a MVC, MI force and EMG activity significantly decreased 12.4% and 20.9% respectively. Overall, ME had significantly (p ¼ 0.0001) higher force (14.5%) and less ITT inactivation (10.4%) than MI. The expectation of the noxious stimuli may account for the significant decrements in force and activation during the ITT. Key pointsA single orientation session may not be adequate for a valid estimation of muscle activation using the ITT.The expectation of an electrical stimulation whether delivered or not can impair performance.The validity of the ITT for estimating the extent of full muscle activation must be viewed with caution, since the expectation of IT discomfort may inhibit the individual's ability to exert maximum force, especially with inexperienced participants.

Keywords: Muscle activation; electromyography; evoked stimulation.

Figure 1.

The figure illustrates the interpolated twitch technique (ITT) which was used with a maximal voluntary contraction (MVC) to estimate the extent of inactivation during a voluntary contraction by comparing the amplitudes of evoked doublets superimposed upon a MVC, with the post-contraction potentiated doublet (interpolated doublet amplitude / potentiated doublet amplitude x 100 = percentage of muscle inactivation).