In the fitness and running industries, the phrase "mix it up" is used a lot. Personal trainers, group exercise instructors and coaches consistently tell their clients and athletes to constantly change the training stimulus to ensure adaptation—to "mix up your workouts." In marketing their services, personal trainers say that when you train with them, "every workout is different" as if that's a selling point. Even Tony Horton, of P90X fame, talks about changing the stimulus for what he calls "muscle confusion."
Do you really need to constantly change your workouts to see results? No. Or at least not as often as you might think.
Progression in your training program in general and in your workouts in particular can and should be based on the degree to which a given workload is mastered; that is, handled with full physical and psychological confidence. You should control the workout rather than letting the workout control you.
All too often, people feel they're ready for the next progression when they've achieved a workout only once. Trainers are constantly changing their clients' training. This practice leads to a feeling of hurry and uncertainty, a lack of systematization, and an over-intensity of training. Running should be planned and systematic; nothing is done for the sheer sake of variety.
Once a certain goal is achieved—whether it be running 10 miles or completing an interval workout of 10 x 400 meters in 90 seconds with 90 seconds recovery between reps or completing 3 sets of 10 squats at 80 percent one rep max—it should be repeated several times during succeeding weeks, each time with greater relaxation and certainty of control.
When you do a workout, a strong signal is sent, mediated by hormones, to make specific adaptations via the activation of transcription factors involved in protein synthesis. Repeated workouts lead to a concerted accumulation of messenger RNAs that can be translated into a host of structural and functional proteins. If you repeat the same workout for a period of time, you continue to send signals to make adaptations until those adaptations are fully realized. After you have completed a specific workout a number of times, you become habituated to it, and the same workout no longer is enough of a stimulus to initiate further adaptations.