I do splits between upper body, lower body, and abs six days a week, and I mix it up so I'm not doing the exact same things over and over (to avoid plateaus). So why is it that sometimes my muscles are sore after a workout and sometimes they aren't? For example, I'll do Legs and Glutes one day and be sore the next day, and then I'll do it again next week and I won't feel a thing, and then another time I'll be sore again. When I'm not sore it makes me feel like I didn't work hard enough :-( Are the muscles still getting stronger even if they don't hurt?