What "muscles" are you looking to work? I do alot of heavy hand walking, jogging and we sprint up some grassy hills in my local cemetery and I let her pull me. We also do some fetch, tug, and flirt pole. Genetics is going to determine how ripped your dog will be as well as excercise and diet. My girls chest and shoulders are real defined by just doing those activities. Look in my signature and you can see all her definition. Also, if your dog is under 18 months as mine is, I wouldn't work them to hard until they are two years of age.