In general, running doesn't create big muscle gains. Large muscles are a hindrance to long- or middle-distance runners, who need to be lean and lightweight. If you're looking to build leg muscle, running probably isn't your best choice. However, that does depend somewhat on your starting size and shape, as well as the type of running you're doing. In certain conditions and at a relatively low level, running can build muscles in your legs. Consult your doctor before taking up running or any new exercise regimen.

Road Running
             Running for long distances is great for the heart and overall fitness levels. However, it might actually decrease the size of your leg muscles because it uses so much energy, according to Columbia University's Go Ask Alice! health services website. Where you may benefit from running is an increase in muscle mass as your legs become leaner and tighter. If you're thin or have little muscle definition to begin with, this can thus translate to a small but noticeable increase in your muscle density.

Anaerobic Running

 

 

 

 

 

 

 

             In some situations, your running exercise can be so demanding that it becomes more of an anaerobic workout than a cardiovascular exercise. For example, if you include sessions of all-out sprints in your routine or run up steep hills, you're straining your leg muscles more than during a steady long-distance jog. As fitness trainer Jason Spencer explains on the Ask The Trainer website, this type of activity can build more of the tougher, fast-twitch muscle fibers, leading to larger legs. 

Timing
             Ensuring that you don't do any cardiovascular training on the same day that you do resistance training can help you gain muscle faster, Go Ask Alice! explains. If you have scheduled both cardio and weight training, opt to perform the latter before you go out for your run, particularly if you're looking to gain muscle. Remember that running will work muscles in your legs, lower back, hips, abdomen and other areas of your body

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Considerations
            In his book "5K and 10K Training," Brian Clarke suggests that hill running does help build leg muscle- --- but warns that it also presents the risk of injury. Be careful when adding hill runs to your routine. The angle puts different pressures on your joints and ligaments, meaning you could more easily twist an ankle or pull a hamstring. Longer-distance runners tend to shorten their stride and pace when going up hill. This reduces the potential for muscle gain even further, though it helps prevent injury and keeps you within your physical limits.