The deficit in force generation is a measure of the magnitude of damage to sarcomeres caused by lengthening contractions of either single fibers or whole muscles. In addition, permeabilized single fibers may suffer breakages. Our goal was to understand the interaction between breakages and force deficits in 'young' and 'old' permeabilized single fibers from control muscles of young and old rats and 'conditioned' fibers from muscles that completed a six week program of in vivo lengthening contractions. Following single lengthening contractions of 'old-' compared with 'young-control' fibers, the two-fold greater force deficits at a 10% strain support the concept of an age-related increase in the susceptibility of fibers to mechanical damage. In addition, the much higher breakage rates for 'old' fibers at all strains tested indicate an increase with aging in the number of fibers at risk of being severely injured during any given stretch. Following the six week program of lengthening contractions, 'young-' and 'old-conditioned' fibers were not different with respect to force deficit or the frequency of breakages. A potential mechanism for the increased resistance to stretch-induced damage of 'old-conditioned' fibers is that, through intracellular damage and subsequent degeneration and regeneration, weaker sarcomeres were replaced by stronger sarcomeres. These data indicate that, despite the association of high fiber breakage rates and large force deficits with aging, the detrimental characteristics of 'old' fibers were improved by a conditioning program that altered both sarcomeric characteristics as well as the overall structural integrity the fibers.