In beginning, novice weight trainers seem to make progress no matter what they do. However, eventually it becomes increasingly difficult to make even the smallest gains because even the most productive exercises lose their effectiveness over time. Improvements will decrease and eventually halt, if the same exercises are continued for a prolonged period of time. This is known as Accommodation.
One way to attempt restimulating progress is by changing the exercise routinely executed for a particular muscle group. Incidentally, weight trainers commonly misattribute the effectiveness of a new exercise, or the workout routine, to some inherent characteristic or secret ingredient within the exercise or training principle itself. But, in truth, any significant change in one's routine can stimulate new progress.
When a new exercise has been introduced, the progress for the first few weeks is largely due to neural-motor adaptations (Komi 1986). Rapid increases of strength are attributed more to an improvement in neural firing pattern than an actual increase in muscular force output (Moritani 1979). After this initial 'learning, or re-learning' phase, subsequent strength increases become predominately morphological- muscles fibers increase in size (Sale 1988). Incidentally, some non-hypertrophy-related strength increases are possibly due to anatomical changes in connective tissue and angle of muscle fiber attachment to tendon (Jones & Rutherford; Narici et. al., 1989).
Changing exercises for a muscle group every workout is not necessary and may, in effect, not allow optimal neurological adaptation. In other words, you may be left with very few options in restimulating progress for future workouts if you are doing all the exercises in the book for a particular muscle group (see less is more). In addition, it becomes difficult to make incremental progress if you change your exercises every workout. Sticking to a program, or a group of selected exercises, for a longer time will decrease your chance of injury because you'll be more familiar with your limits regarding load and volume, and it will make it easier to practice and maintain good form. Your muscles, tendons, ligaments, joints, vascular elements and neural-motor units respond favorably with small increases in duration and intensity. (see SAID and Weight Training Specificity). If too much weight is used, form may suffer and injury is more likely. If too little weight is used, the body does not have to adapt to an overload (see Weight Training Log). It is very difficult to use the ideal resistance if you change your exercises every workout. Systematic increases of repetitions and resistance can easily be achieved by performing the same exercises for at least a few weeks.
Powerlifters, Olympic-style weightlifters, or other athletes who have to increase absolute strength on a few particular exercises throughout most of the year, may choose to change only their auxiliary exercise and continue to train on their essential lifts. Most commonly, these athletes implement some method of periodization which varies training volume and intensity throughout the year (Bompa, 1990) (see Sample Powerlifting Programs). Similarly, this wave has been used for Olympic athletic training (Naughton, 1991). In addition, periodic variations of the form (eg. wide to narrow stance squat, etc) of key exercises may also restimulate strength gains.
For beginners, the best exercises are those they are comfortable in performing. For more advanced trainees, the most effective exercises are those they are not as familiar with. For continued relative strength gains, keep an exercise in your weight training routine as long as you continue to make periodic gains, generally between four to eight weeks. After which point, exchange your workout with another basic exercise for each corresponding muscle group.
"Just when your body thinks it has all the answers, that's when you had better change the questions" - Louie Simmons, Westside Barbell