If you keep your workout the same, results will eventually plateau.
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There are situations where you can reduce volume and still make gains, but as a rule of thumb you should see an increase in demands placed over time to keep making progress. Keep in mind that more demanding workouts also places more demands on your ability to recover (eating right, sleeping enough and managing stress), which can become a limiting factor.
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Depending on your goal, you can either choose to add weight (for strength), or sets (for muscle mass). There is an overlap in the benefits of these methods, and periodically switching to lighter weights and doing high rep work can also benefit growth of other muscle fibers.
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Take home: Know what you want to get better at and track it! This way you can ensure that you are progressively overloading over time, even if you may need periods of reduced volume to recover properly.
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THE EVIDENCE:
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Hass, C. J., Feigenbaum, M. S., & Franklin, B. A. (2001). Prescription of resistance training for healthy populations. Sports medicine, 31(14), 953-964.
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Kraemer, W. J., Ratamess, N. A., & French, D. N. (2002). Resistance training for health and performance. Current sports medicine reports, 1(3), 165-171.