Skeletal muscle is injured more easily in old animals than young animals; however, reports describing the effects of acute and potentially injurious exposures on the morphological response in young and old animals have varied substantially and reasons for this are not fully understood. To investigate the effects of a potentially injurious stretch-shortening cycle (SSC) exposure on the morphological response of skeletal muscle in young (12 wk, n = 6) and old (30 m, n = 5) rats. Training was performed on the dorsi-flexor muscles of hybrid Fischer-344 × Brown Norway rats in vivo. The testing protocol consisted of 150 total SSC contractions (15 sets of 10 contractions), which were conducted at 500 degrees/s throughout a 90 degrees-140 degrees range of motion. Ten days after treatment of SSCs, rats were anesthetized, weighed, and exsanguinated. The left (treated) and right (control) tibialis anterior muscle (LTA, RTA) was excised, weighed, sectioned, quick-frozen, and stored at -80 degrees C. Transverse sections (12 µm) were cut, mounted on pre-coated microscope slides, and stained using a routine procedure with Harris Hematoxylin & Eosin. Tissue sections were evaluated on a Leica DMLB microscope. Stereology was used to quantify the degree of inflammation and myofiber degeneration in muscle from each group and modifications to the interstitial space. Stereological analyses revealed that 10 days following exposure to SSCs the volume density of myofibers with normal morphological features (i.e., fibers without evidence of edema or degeneration) was greater in the young when compared with the old animals (p=0.0332) and greater in the RTA compared with the LTA (p=0.0432). Further analyses revealed that the average thickness of the normal myofibers was decreased in the young LTA compared to the young RTA. The volume density of non-cellular interstium (NCI), or edema, was increased in the old when compared with the young (p=0.0001). Also the volume density of cellular interstium (CI) was shown to have a significant main effect and was increased in the LTA when compared with the RTA (p=0.0088). No degenerative myofibers were detected in control or injured muscles from either age group. We observed an increase in the volume density of normal myofibers in the young compared with the old animals at 10 days. Conversely, the volume density of NCI and CI were significantly increased in the old animals. Thus, following an acute exposure to injurious SSC contractions, muscles in young animals, but not old animals, recover more quickly and do so without residual signs of inflammation.