In much the same way that your body does not need to continuously adapt to perform daily activities, if you perform the same exercise routine for long periods of time, your body will no longer need to make positive adaptations in response to the exercise. Let’s use bicep curls as an example. Say that when you first started exercising, performing 12 repetitions with 15 pounds was quite challenging. You were probably thrilled when you became stronger and reached the point where those 12 reps were quite easy with 15 pounds. If you do not increase the challenge to the muscles by either increasing the weight, reps, or both, your biceps will no longer need to adapt as they can now easily handle the stress you are placing on them. It’s easy to overlook that even reducing your rest between sets will add to the challenge.
Going through the motions with an exercise routine that does not challenge you is never going to produce the results you want.