Your body is always learning and like to make things as easy as possible for itself. Your muscles will learn your exercises and work out the easiest/most efficient way to perform the exercise. Need to mix it right up to shock the body. I found this the last two days. I've been doing lots of sprint training the last few weeks to build my speed so 1.5miles in 8 minutes, 2 miles in 12mins and 3 in 20mins as targets all on flat ground. Yesterday and today we did 5 mile runs through the wood tracks and hills and my quads and hamstrings are aching tonight. When I was doing the woods and hills 3-4 times a week my top legs NEVER hurt.