The lesson is that light exercise increases bone volume as much as the heavy training when they both performed to failure. Further, it states clearly that 3 sets built better than one set, and that heavy exercise improves the dynamic power more than light exercise. The question is whether the same is true for the upper body muscle volume. The legs're made ​​to run and go as they respond well to light training and many repetitions is not surprising. Upper body may very well behave differently.

In the study, as were 18 men aged around 21 years of strength training your legs to failure three times a week for 10 weeks. Participants' legs were randomly assigned two of three possible applications. 80% of 1RM in 1 set, 80% of 1RM in 3 sets and 30% of 1RM in 3 sets. Muscle volume was measured by magnetic resonance imaging before and after the 10 weeks and they took the same occasions also biopsies of the vastus lateralis.

Races gives you all stronger leg muscles and stronger butt. Butt getting stronger all if you run up and down hills. At the same time, you get stronger abs and back.
But be it running to build muscle is not quite the right way. Not in all cases, if you want to build more muscle volume. Then it is rather more weight training you should focus on. You can of course keep running because you seem to like it. Complete with some extra exercises to gain more muscle. Or start strength training on other days - if you go to the gym - you get even faster results.

To get more muscles in your legs, you can do squats (bending and stretching your legs). Either with your own body - then do more - or an extra weight in your hands. Weight can be a stone, a medicine ball or dumbbells. Also try to do a little faster squats so you descend more slowly in a squat and then extends dynamically, perhaps with a jump at the end. Exercise for 60 seconds, rest briefly and repeat another 60 seconds, now maybe with outcomes instead of squats (you are wooing position and bend and extend your knees up and down, or go forward with outcomes step). And you can make the outcome step with hope in between.