The dependence of force enhancement on activation in human adductor pollicis.

It has been well recognized that the steady-state isometric force after active muscle/fiber stretch is greater than the corresponding isometric force for electrically stimulated muscles and maximal voluntary contractions (MVC). However, recent evidence obtained for sub-MVC suggests that force enhancement properties are different from those observed for electrically induced and MVC. Specifically, it appears that force enhancement is activation-dependent and that there is a subject-specific threshold for force enhancement in sub-MVC. To address these suggestions, the relationship between force enhancement and voluntary activation during stretch was investigated in 11 healthy subjects. Human adductor pollicis muscles were studied and force enhancement was measured while muscle activation during the steady-state isometric phase was controlled at a level of 30% of MVC. In order to study the effects of activation on force enhancement, subjects performed stretch contractions at 0, 10, 30, 60, and 100% of maximal voluntary effort while the steady-state isometric force after stretch, obtained at 30% of activation in all cases, was compared to the corresponding values measured in the isometric reference contractions. There was no force enhancement if muscle stretching occurred passively but all subjects showed force enhancement when muscle stretching occurred at maximal voluntary effort. When increasing the level of activation during the stretch phase, force enhancement increased, and the number of subjects who showed force enhancement increased as well. We conclude from these results that force enhancement during voluntary contractions is activation-dependent with a threshold that is subject-specific.