Hindlimb suspension unloading (HSU) is a ground-based model simulating the effects of microgravity unloading on the musculoskeletal system. In this model, gravity causes the hind foot of the rat to drop, opening the front of the ankle to 90-105° plantarflexion at rest. As HSU proceeds, the normal weightbearing angle of 30° dorsiflexion is achieved progressively less, and the contraction range of soleus is abbreviated. We reported that 12 days of HSU caused central core-like lesions (CCLs) of myofibril breakdown (Riley et.al., JAP 69:58, 1990). The present study investigated whether daily stretch of the calf muscles prevents CCL formation. The soleus muscles of HSU Sprague Dawley male rats (~287g), were lengthened by unilateral ankle splinting at 30°. Compared to the non-splinted side, splinting for 10 or 20 min per day in awake rats significantly decreased CCLs in soleus by 88% and 91% respectively (p<0.01). Compared to control muscle wet weight, 20 min splinting reduced atrophy by 33%, whereas 10 min splinting ameliorated atrophy 17% (p<0.01). Bilateral soleus EMG recording revealed higher levels of contractile activity on the splinted side during splinting. To isolate the effects of stretch from isometric contractile activity, contractions were eliminated by whole animal anesthesia with isoflurane during 10 min daily splinting. The percentage of fibers with CCLs was reduced by 57%, and the average lesion size was 29% smaller in the stretched muscle (p<0.05). Soleus muscle wet weight and fiber area were unaltered by stretch alone. Loaded contractions during splinting are necessary to prevent muscle fiber atrophy. Passive muscle stretch acts to maintain myofibril structural integrity.