In general, all kinds of running benefit the body with no negative effects overall. Nevertheless, many people are afraid that running will cause them to lose muscle mass. It always depends however what kind of muscle fibers are trained when running. That in turn depends on intensity and duration.
Sprints actually stimulate muscle growth as explosive movements at peak performance encourage the release of growth hormones and place extreme demands on muscle fibers, which increases muscular volume.
Endurance runs of five kilometers or more place demands upon red muscle fibers, as energy is primarily obtained aerobically over these distances. Nonetheless even endurance running won’t cause muscle wastage if sufficient alternative energy sources are available. The most important factor in building muscle is always a balanced diet that is rich in all necessary nutrients. Moreover exercises and workouts signal to the body that muscles are needed and the body will therefore ensure muscular tissue is preserved. The positive effects of endurance running are so strong that they cause a range of systems throughout the body to be optimized, thereby helping indirectly with gaining and maintaining muscle.