Age impairs the ability of skeletal muscle to adapt to loading even in the absence of fiber degeneration. Nevertheless, the mechanisms accounting for this reduced adaptation are not fully understood. Experiments using stretch-shortening contractions (SSC) provide a model to investigate skeletal muscle adaptation in response to non-injurious contractions in old animals. PURPOSE: To investigate the effect of a non-injurious SSC exposure on the temporal skeletal muscle response of young and old rodents. METHODS: Left dorsiflexor muscles of young (12 wks) and old (30 mo), male Fischer Brown Norway Hybrid rats were exposed to an acute protocol of 80 maximal SSC in vivo. The contra-lateral limb (right) served as an intra-animal control. Changes in muscle morphometry were quantified at 6, 24, 48, 72, and 120 hours post exposure. The left and right tibialis anterior muscles (LTA & RTA) were excised and weighed. Transverse sections (12 um) were prepared and stained using Harris Hematoxylin & Eosin. Tissue sections were assessed using a standardized quantitative morphometric point-counting method. RESULTS: LTA muscle wet-weight was increased compared to the RTA muscle wet-weight by 12% in the young rats at 6h recovery (p<0.05), and the LTA muscle wet-weight was increased compared to the RTA muscle wet-weight by 10% in old rats at 24h recovery (p<0.05). Stereological indices for cellular interstitium, indicative of cellular infiltrates, was increased at 72h in the young (p<0.0001) and old (p<0.05) LTA when compared to the RTA. Aging increased cellular interstitium, as cellular interstitium was greater in the old versus the young LTA at 72h (p<0.05). Non-cellular interstitium, indicative of edema, was increased in the LTA of old rats when compared to young rats at 120h recovery (p<0.05). There were no degenerative myofibers observed following the SSC exposure.CONCLUSIONS: Together these data indicate that muscles exposed to non-injurious SSCs have different characteristics than that typically reported for lengthening contraction-induced muscle injury. Nevertheless, muscle of old rodents has a delayed adaptation that may be influenced by infiltrating cells as well as non-cellular events, indicative of inflammation.