The effect of maintaining a constant preload or a constant degree of thumb abduction 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 twitch force during mechanomyography of the thumb, we monitored neuromuscular function in patients anaesthetized without the use of a neuromuscular blocking agent. In addition, we studied the relationship between the degree of thumb abduction, twitch force and preload. Methods.Fourteen patients were divided randomly into two groups, determining the sequence of the experiments in respect of correcting the preload and maintaining a constant degree of abduction after allowing the twitch forces to stabilize. Both experiments lasted 15 minutes. In both groups the relationship between the degree of thumb abduction, twitch force and preload was studied. Results. We found a progressive increase in twitch force both with (6.6%; CI: 3.7-9.3%; p < 0.001) and without (1.9%; CI: 0.4-3.5%; p = 0.020) continuous correction of the thumb preload. A significant greater increase in twitch force was seen when the 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 the degree of thumb abduction both before and at the time of measurement. Conclusions. Changes in length of the contracting muscle fibres and creep phenomena in the connective tissue of the muscles, both leading to changes in the sarcomere length of the muscle fibres, may explain the observations in this study. In general, a stabilized preload at a constant degree of abduction seems to be required in order to obtain a stable twitch force.