The effect of maintaining a constant preload or a constant degree of thumbabduction on the isometric twitch force of the thumb

Objective. To investigate the effects of maintaining a constant preload and of maintaining a constant degree of thumb abduction on the isometric twitc h force during mechanomyography of the thumb, we monitored neuromuscular fu nction in patients anaesthetized without the use of a neuromuscular blockin g agent. In addition, we studied the relationship between the degree of thu mb abduction, twitch force and preload. Methods.Fourteen patients were divi ded randomly into two groups, determining the sequence of the experiments i n respect of correcting the preload and maintaining a constant degree of ab duction after allowing the twitch forces to stabilize. Both experiments las ted 15 minutes. In both groups the relationship between the degree of thumb abduction, twitch force and preload was studied. Results. We found a progr essive increase in twitch force both with (6.6%; CI: 3.7-9.3%; p < 0.001) a nd without (1.9%; CI: 0.4-3.5%; p = 0.020) continuous correction of the thu mb preload. A significant greater increase in twitch force was seen when th e preload had been corrected than when it was not (4.7%; CI: 0.8-8.7%; p = 0.023). In addition, both twitch force and preload appeared to depend on th e degree of thumb abduction both before and at the time of measurement. Con clusions. Changes in length of the contracting muscle fibres and creep phen omena in the connective tissue of the muscles, both leading to changes in t he sarcomere length of the muscle fibres, may explain the observations in t his study. In general, a stabilized preload at a constant degree of abducti on seems to be required in order to obtain a stable twitch force.