Every few weeks, vary the number of repetitions and sets you do and the rest periods between sets. Or, if you usually use weight machines, try free weights or other types of strengthening activities such as exercises that use elastic tubing or your body as resistance. Healthy people should choose eight to 10 different exercises that involve major muscle groups, performing one set of each exercise two to three days a week, regardless of age.
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National Academy of Sports Medicine answered
There are a variety of ways you can vary your strength training routines. This includes switching from machines to free weights, from barbell to dumbbell exercises, high repetition to low repetition, upper body to lower body, push then pull programs, lifting at a slow tempo vs. a high tempo. Making any one of these variations or multiple variations can provide enough variety in your training to allow your body to continue to get results and improvements from your workouts.