You don’t even need to lift a lot of heavy weights. Just pushing your limit a few times per session is enough to trigger your body into building new muscle.
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Yep. Resistance training is a relative thing. All you really need to be doing is challenging your muscles to a level higher than simple endurance exercise. The maximum number of reps that you can do is best indicator.
And building new muscle isn’t even necessarily the best reward, although having more functional strength is a good thing for almost anyone. There are metabolic and endocrine changes all over the body as a result of resistance work. The change can be systemic, and not just some targeted muscle groups.
Well put!