There are so many variables you can change in a resistance training program to make it more challenging or just a little different to keep your muscles guessing. A study published in the Journal of Strength and Conditioning Research looked at how exercise order and number of reps per set influenced the rate of perceived exertion.
Scientists recruited groups of younger (20s) and older (70s) female subjects and had them perform 3 sets of 10 reps maximum to fatigue on bench press, shoulder press and triceps extension machines. Maximum oxygen uptake was always lowest for the first exercise and highest for the last in both groups. But the rate of perceived exhaustion was higher for older subjects when bench press was the last exercise. To stay fresh, older people should begin their weight training routine exercising large muscle groups, ending with movements for smaller muscle groups.