Becker muscular dystrophy (BMD) is a progressive dystrophinopathy in males but is typically less severe compared to its disease counterpart, Duchenne MD (DMD). BMD patients have in-frame mutations leading to some dystrophin expression in muscle, which allows them to retain more function over time compared to DMD patients. There are several well-characterized vertebrate animal models for DMD, including mice, rats, dogs, and pigs, but only a recently characterized mouse model exists for BMD. In this study, we sought to characterize the natural history of a naturally occurring BMD pig model with dystrophin insufficiency. We observed reduction in dystrophin expression in the vastus lateralis muscle at 4 months of age. Affected pigs gained comparable body weight over time as those of normal genotyped animals. Isometric flexion and extension force (body weight and foot length corrected) of the tibiotarsal joint muscles decreased over time in affected pigs. We created a fatigue protocol where we stimulated flexion and extension force with 30-40 consecutive replicates of tetanus in the same muscles. To induce eccentric contraction decrements, we developed a downhill running treadmill protocol at 10% decline while running 5 mph until exhaustion. Becker pigs that were treadmilled showed decreased overall flexor/extensor strength from the beginning to the end of the running regimen but reduced flexor and extensor fatigue by the end of the treadmill regimen (they gained endurance). Non-treadmilled, natural history pigs at the same age did not show progressive extensor or flexor force changes from 1-6 months of age, but showed slightly less fatigue from 3-6 months of age, but not to the extent of treadmilled BMD pigs. Muscle biopsies before the treadmill regimen demonstrated mild dystrophinopathy; after downhill treadmill exercise, there was evidence of increased muscle damage (central nuclei, increased fibrosis). Creatine kinase levels were increased in 3 months of age BMD pigs and further increased after treadmill exercise. No other biochemical abnormalities were evident. Conclusion: The BMD pigs studied here display a mild phenotype, which can be exacerbated to a certain degree with a downhill (eccentric) treadmill exercise regimen and an isometric fatigue protocol. This model may be used for in vivo studies to assess therapeutic safety and efficacy.