How quickly you’ll lose muscle once you stop training depends on different factors, but in general, you’ll notice losses in 3–4 weeks. Hence, the detrained athletes didn’t lose any muscle mass during a three week break from training (LaForgia et al., 1999). This type of fitness fades pretty quickly, unfortunately. Even if you have to stop working out at the gym completely, muscle atrophy isn't immediate. For cardio, it takes less time to break down your fitness; as stated above, it could take years to lose all your muscle. Some aspects of fitness last a month before beginning to decline. Does it matter in the long-term? But, their muscle glycogen stores did shrink. According to Harvard Health Publishing, after age 30 you begin to lose as much as three to five per cent of muscle mass per decade. As well as asking how long does it take to lose muscle, we should also consider how long this type of fitness hangs around. If you have to cut back on exercise for whatever reason, and you don’t want to lose any hard-earned muscle, you may be able to get away with doing only two strength workouts per week, according to Olson. However, let’s assume that 2-3 weeks detraining in beginners leads to muscle loss. The insitute says that most men will lose … Some … And the more muscle you have, the more you stand to lose. Even if you have to stop working out at the gym completely, muscle atrophy isn't immediate. A 2015 study found that active young adults lost one-third of their leg strength after just two weeks of inactivity.