Force enhancement following stretch of activated muscle: critical review and proposal for mechanisms.

Force enhancement is an acknowledged and well-accepted mechanical property of skeletal-muscle contraction. It occurs in isometric, steady-state contractions that are preceded by stretch of the activated muscle. Force enhancement increases with increasing magnitudes of stretch, is long lasting, but can be abolished by deactivation of a muscle just long enough for force to drop to zero, and is associated with an increase in passive force. The mechanisms underlying force enhancement are not known. One of the classic mechanisms for force enhancement is reviewed, and a new mechanism that is based on an active and a passive component is introduced. The passive component of force enhancement is tentatively associated with the molecular spring titin, and the active component is associated with an increase in the proportion of attached cross-bridges caused by a decrease in the cross-bridge detachment rate. The review is completed by the proposal of some questions and selected experiments that would test the proposed mechanisms in a rigorous way.