RIR

Do you need to train to failure for muscle growth? This article reviews current evidence on hypertrophy, RIR (reps in reserve), rep ranges, and strength adaptations to explain how load and effort impact results.

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That is pretty much the best way to improve cardio as well. There is some benefit to long slow cardio in that it induces the same mitochondrial growth and fitness improvement, but it requires more time. The injury risk is a little lower for low intensity work. Interval training is a proven quantity for cardio fitness as well, though. I don't think you can min-max cardio fitness much more than that. Mainly, just ensure you aren't doing anaerobic work (e.g. sprinting) if your goal is cardio improvement. But that is easy enough, if you really want to baseline it, you just run a mile all out and there are a bunch of calculators that are very good which give you good paces for VO2 Max / intervals. The goal is you want to sort of keep your HR elevated once warmed and stay in that high VO2 max zone sort of ending right at max HR on your final interval or two pushing yourself up to the limit. It ends up looking like the strength training wisdom at the end of the day. Do cardio. Make sure it is challenging and you aren't phoning it in if your min-maxing. Be consistent. For strength training, I think it comes down to what I captured in my blog post. For a given muscle, if you are challenging it and truly reaching 1 or 2 reps in reserve there isn't much else to do per set. Hit your desired training volumes and you are the happy place for time+gains. If the intensity is there and you are targeting your entire body at the right volume level every week it will come down to nutrition and other factors vs. what you are doing in the gym.

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