Passive force enhancement is not abolished by shortening of single fibres.

Abstract

Passive force enhancement is defined as the increase in steady-state passive force following active muscle stretching compared to the corresponding passive force following passive muscle stretching. This mechanical property has been discovered just two decades ago (Herzog & Leonard, 2002), and has been associated with contributing to the residual force enhancement property, stability of sarcomeres, and preventing over-stretching during eccentric contractions. The molecular mechanisms underlying passive force enhancement development and abolishment remain unknown. An incidental observation on cat soleus muscle led to the speculation that passive force enhancement could be abolished instantaneously when the actively stretched muscle was deactivated and then passively shortened to its pre-stretched length (Herzog et al., 2003). We tested this hypothesis systematically using single skinned fibres from rabbit psoas. However, initial results led to the rejection of this hypothesis. Rather, we found that passive force enhancement increased following a shortening-stretch cycle of fibres to their pre-stretched length (2.4 µm), and back. Furthermore, we observed that the passive force enhancement increased by 70-106% when the shortening and subsequent stretch to the original length (3.6 µm) increased in duration (200 ms, 6 s, and 14 s). When reversing the order of the shortening-stretch experiments from slowest to fastest, the general result was the same as that of the primary experiment. These findings indicated that passive force enhancement increases during a shortening-stretch cycle, and that this increase is time-dependent. We propose that this increase in passive force enhancement may be caused by titin; specifically, with a refolding of titin's immunoglobulin domains that were unfolded during the active fibre stretching that produced the residual and passive force enhancement. Using fluorescent antibody labeling of selected titin segments, we will be testing this hypothesis in the near future.