Force enhancement at different levels of voluntary contraction in human adductor pollicis.

There is substantial evidence that the steady-state isometric force following active muscle stretch is greater than the corresponding isometric force for sub-maximal and maximal electrically induced contractions, and for maximal voluntary contractions (MVC). This so-called force enhancement (FE) has not been investigated systematically for sub-MVC. Recently, we found that FE only occurred in about half of all tested subjects at 30% of MVC in the adductor pollicis muscle, suggesting that FE for voluntary contractions might depend on the level of activation. Therefore, the purpose of this study was to determine if FE during voluntary contractions is associated with activation. We hypothesized that FE increases with increasing levels of activation, and thus increasing levels of effort. Subjects (n=12) performed voluntary contractions of the adductor pollicis using a custom-built dynamometer at 10 and 60% of maximal effort. There was consistent FE in 4 of 12 subjects at 10% and in 10 of 12 subjects at 60% of maximal voluntary effort. Furthermore, the absolute FE increased from 10 to 60% of maximal voluntary effort. The results of this study, together with previous results on the same subject group at 30% and other subjects at 100% of maximal voluntary effort, suggest that FE systematically increases with increasing levels of voluntary contraction. The mechanisms responsible for this increased FE remain unclear.