Comparisons between automatic force based triggering and manually triggered methods of quantifying quadriceps voluntary activation assessment

Abstract

The interpolated twitch technique is a widely used method of assessing muscle voluntary activation. The approach involves delivering an electrical stimulus to determine the degree to which a voluntarily activated muscle is activated. If the muscle is not fully activated then additional force will be generated by this stimulus. The amount of additional force generated can be used to quantify the degree of activation. Conventionally, for this technique, stimuli are delivered either manually (when the force reaches a plateau) or after a set time period post onset of contractions. This study examined an approach for interpolated twitch that has recently been suggested to improve precision of the technique. For this method, stimuli were delivered once the force produced by participants reached 97% of their previously recorded maximum voluntary isometric contraction force. This method was used to examine quadriceps activation in 15 male volunteers. Muscle activation was determined using two different methods of stimulus delivery. One method involved the stimulus being automatically triggered when participant’s knee extension force reached 97% of their maximum voluntary isometric contraction. The other method involved the stimulus being manually delivered when the force tracing reached a plateau. The purpose of the study was to verify that this automatic force based triggering method improved the precision in delivering stimuli near peak force. Also, the quadriceps voluntary activation determined using both stimulus delivery approaches were compared. The findings indicated that the automatic force based triggering method reduced the stimulus delivery timing errors by 119% and increased voluntary activation levels by 3% on average. In conclusion, automatic force based triggering is suggested as an alternative approach to stimulus delivery when using interpolated twitch to assess voluntary isometric muscle activation.